Transition metal-catalyzed alkyl-alkyl bond formation: Another dimension in cross-coupling chemistry

Mechanistic insights into facilitating reductive elimination from Ni(II) species

Reductive elimination is a key step in Ni-catalysed cross-couplings, which is often considered to result in new covalent bonds. Due to the weak oxidizing ability of Ni(II) species, reductive eliminations from Ni(II) centers are challenging. A thorough mechanistic understanding of this process could inspire the rational design of Ni-catalysed coupling reactions. In this article, we give an overview of recent advances in the mechanistic study of reductive elimination from Ni(II) species achieved by our group. Three possible models for reductive elimination from Ni(II) species were investigated and discussed, including direct reductive elimination, electron density-controlled reductive elimination, and oxidation-induced reductive elimination. Notably, the direct reductive elimination from Ni(II) species often requires a high activation energy in some cases. In contrast, the electron density-controlled and oxidation-induced reductive elimination pathways can significantly enhance the driving force for reductive elimination, accelerating the formation of new covalent bonds. The intricate reaction mechanisms for each of these pathways are thoroughly discussed and systematically summarized in this paper. These computational studies showcase the characteristics of three models for reductive elimination from Ni(II) species, and we hope that it will spur the development of mechanistic studies of cross-coupling reactions.

Ni-Catalyzed Kumada-Corriu Cross-Coupling Reactions of Tertiary Grignard Reagents and Bromostyrenes

The development of protocols for the construction of congested quaternary centers is highly sought-after. Herein, we report a method for the cross-coupling of C(sp3) tertiary Grignard reagents with C(sp2) styrenyl bromides using readily available nickel precatalysts. We identified conditions that afford the products in practical yield for several combinations of electrophiles and nucleophiles, including sensitive α-magnesiated Grignard reagents. Dependent upon the nature of their substituents, regiodivergency was observed when α-vinyl bromides were employed.

Zinc Promoted Cross-Electrophile Sulfonylation to Access Alkyl-Alkyl Sulfones

The transition metal-catalyzed multi-component cross-electrophile sulfonylation, which incorporates SO2 as a linker within organic frameworks, has proven to be a powerful, efficient, and cost-effective means of synthesizing challenging alkyl-alkyl sulfones. Transition metal catalysts play a crucial role in this method by transferring electrons from reductants to electrophilic organohalides, thereby causing undesirable side reactions such as homocoupling, protodehalogenation, β-hydride elimination, etc. It is worth noting that tertiary alkyl halides have rarely been demonstrated to be compatible with current methods owing to various undesired side reactions. In this work, a zinc-promoted cross-electrophile sulfonylation is developed through a radical-polar crossover pathway. This approach enables the synthesis of various alkyl-alkyl sulfones, including 1°-1°, 2°-1°, 3°-1°, 2°-2°, and 3°-2° types, from inexpensive and readily available alkyl halides. Various functional groups are well tolerated in the work, resulting in yields of up to 93%. Additionally, this protocol has been successfully applied to intramolecular sulfonylation and homo-sulfonylation reactions. The insights gained from this work shall be useful for the further development of cross-electrophile sulfonylation to access alkyl-alkyl sulfones.

Dual Nickel- and Photoredox-Catalyzed Asymmetric Reductive Cross-Couplings: Just a Change of the Reduction System?

ConspectusIn recent years, nickel-catalyzed asymmetric coupling reactions have emerged as efficient methods for constructing chiral C(sp3) carbon centers. Numerous novel approaches have been reported to rapidly construct chiral carbon-carbon bonds through nickel-catalyzed asymmetric couplings between electrophiles and nucleophiles or asymmetric reductive cross-couplings of two different electrophiles. Building upon these advances, our group has been devoted to interrogating dual nickel- and photoredox-catalyzed asymmetric reductive cross-coupling reactions.In our endeavors over the past few years, we have successfully developed several dual Ni-/photoredox-catalyzed asymmetric reductive cross-coupling reactions involving organohalides. While some probably think that this system is just a change of the reduction system from traditional metal reductants to a photocatalysis system, a question that we also pondered at the beginning of our studies, both the achievable reaction types and mechanisms suggest a different conclusion: that this dual catalysis system has its own advantages in the chiral carbon-carbon bond formation. Even in certain asymmetric reactions where the photocatalysis regime functions only as a reducing system, the robust reducing capability of photocatalysts can effectively accelerate the regeneration of low-valent nickel species, thus expanding the selectable scope of chiral ligands. More importantly, in many transformations, besides reducing nickel catalysts, the photocatalysis system can also undertake the responsibility of alkyl radical formation, thereby establishing two coordinated, yet independent catalytic cycles. This catalytic mode has been proven to play a crucial role in achieving diverse asymmetric coupling reactions with great challenges.In this Account, we elucidate our understanding of this system based on our experience and findings. In the Introduction, we provide an overview of the main distinctions between this system and traditional Ni-catalyzed asymmetric reductive cross-couplings with metal reductants and the potential opportunities arising from these differences. Subsequently, we outline various chiral carbon-carbon bond-forming types obtained by this dual Ni/photoredox catalysis system and their mechanisms. In terms of chiral C(sp3)-C(sp2) bond formation, extensive discussion focuses on the asymmetric arylations of α-chloroboronates, α-trifluoromethyl alkyl bromides, α-bromophosphonates, and so on. In the realm of chiral C(sp3)-C(sp) bond formation, asymmetric alkynylations of α-bromophosphonates and α-trifluoromethyl alkyl bromides have been presented herein. Regarding C(sp3)-C(sp3) bond formation, we take the asymmetric alkylation of α-chloroboronates as a compelling example to illustrate the great efficiency of this dual catalysis system. This summary would enable a better grasp of the advantages of this dual catalysis system and clarify how the photocatalysis regime facilitates enantioselective transformations. We anticipate that this Account will offer valuable insights and contribute to the development of new methodologies in this field.

Synergistic Copper-Aminocatalysis for Direct Tertiary α-Alkylation of Ketones with Electron-Deficient Alkanes

In this study, a novel approach for the tertiary α-alkylation of ketones using alkanes with electron-deficient C─H bonds is presented, employing a synergistic catalytic system combining inexpensive copper salts with aminocatalysis. This methodology addresses the limitations of traditional alkylation methods, such as the need for strong metallic bases, regioselectivity issues, and the risk of over alkylation, by providing a high reactivity and chemoselectivity without the necessity for pre-functionalized substrates. The dual catalytic strategy enables the direct functionalization of C(sp3)─H bonds, demonstrating remarkable selectivity in the presence of conventional C(sp3)─H bonds that are adjacent to heteroatoms or π systems, which are typically susceptible to single-electron transfer processes. The findings contribute to the advancement of alkylation techniques, offering a practical and efficient route for the construction of C(sp3)─C(sp3) bonds, and paving the way for further developments in the synthesis of complex organic molecules.

Bis(pinacolato)diboron-Enabled Ni-Catalyzed Reductive Arylation/Vinylation of Alkyl Electrophiles

Herein, the use of economically and environmentally friendly bis(pinacolato)diboron (B2Pin2) is described as a non-metallic reductant in mediating Ni-catalyzed C(sp3)-C(sp2) reductive cross-coupling of alkyl electrophiles with aryl/vinyl halides. This method exhibits excellent suitability for heteroaryl halides and alkyl halides/Katritzky salts. The present study is compatible with an in situ halogenation of alcohol method, allowing for selective mono-functionalization of diols and bio-relevant alcohols (e.g., carbohydrates). The use of B2Pin2 shows potential for easy scalability without introducing additional metal impurities into the products. It is observed for the first time in the realm of cross-electrophile coupling chemistry that B2Pin2 can sever as a reductant to reduce NiII to Ni0. This mechanistic insight may inspire the development of new reductive bond-forming methodologies that can otherwise be difficult to achieve with a metal reductant.

Direct C-H alkylation of 3,4-dihydroquinoxaline-2-ones with N-(acyloxy)phthalimides via radical-radical cross coupling

We present an organophotoredox-catalyzed direct Csp3-H alkylation of 3,4-dihydroquinoxalin-2-ones employing N-(acyloxy)pthalimides to provide corresponding products in good yields. A broad spectrum of NHPI esters (1°, 2°, 3°, and sterically encumbered) participates in the photoinduced alkylation of a variety of 3,4-dihydroquinoxalin-2-ones. In general, mild conditions, broad scope with good functional group tolerance, and scalability are the salient features of this direct alkylation process.

Triple Radical Sorting: Aryl-Alkylation of Alkenes

The cross-coupling of aryl bromides with alkenes can provide access to diverse combinatorial chemical space. Two-component couplings between these partners are well-known, but three-component aryl-functionalizations of unactivated alkenes remain underdeveloped. In particular, the aryl-alkylation of unactivated alkenes would allow for rapid construction of molecular complexity and the expedient exploration of a pharmaceutically relevant and C(sp3)-rich structural landscape. Herein, we report a general approach toward the aryl-alkylation of alkenes through a triple radical sorting mechanism. Over the course of the reaction, a high energy aryl radical, a primary radical, and a hindered alkyl radical are simultaneously formed. Through mediation by a nickel-based catalyst, the three radicals are sorted into productive bond-forming pathways toward the efficient aryl-alkylation of alkenes. A wide range of electronically and sterically differentiated alkenes and aryl radical precursors can be used to access complex scaffolds. This method was further applied to the synthesis of highly substituted semisaturated fused heterocycles.

Ni/Photoredox-Catalyzed Enantioselective Acylation of α-Bromobenzoates with Aldehydes: A Formal Approach to Aldehyde-Aldehyde Cross-Coupling

The catalytic cross-coupling of identical or similar functional groups is a cornerstone strategy for carbon-carbon bond formation, as exemplified by renowned methods, such as olefin cross-metathesis, Kolbe electrolysis, and various cross-electrophile couplings. However, similar methodologies for coupling aldehydes─fundamental building blocks in organic synthesis─remain underdeveloped. While the benzoin-type condensation, first reported in 1832, offers a reliable route for aldehyde dimerization, the chemo- and enantioselective cross-coupling of nonidentical yet similar aldehydes remains an unsolved challenge. Herein, we report a unified platform enabling highly chemo- and enantioselective cross-coupling of aldehydes. By leveraging nickel photoredox catalysis in tandem with discrete activation strategies for each aldehyde, this mechanistically distinct approach facilitates the enantioselective union of an aldehyde-derived α-oxy radical with an acyl radical, photocatalytically generated from a distinct aldehyde. This novel strategy enables modular access to enantioenriched α-oxygenated ketones with two minimally differentiated aliphatic substituents, a feat not achievable with existing chemocatalytic or biocatalytic techniques. The synthetic utility of this method is demonstrated by its application in the streamlined asymmetric synthesis of various medicinally relevant molecules. Additionally, mechanistic investigations rationalize the versatility of nickel photoredox catalysis to exploit new pathways for addressing long-standing synthetic challenges.

Three-Component Radical Cross-Coupling: Asymmetric Vicinal Sulfonyl-Esterification of Alkenes Involving Sulfur Dioxide

A novel catalytic system for radical cross-coupling reactions based on copper and chiral Pyridyl-bis(imidazole) (PyBim) ligands is described. It overcomes the challenges of chemoselectivity and enantioselectivity, achieving a highly enantioselective vicinal sulfonyl-esterification reaction of alkenes involving sulfur dioxide. This strategy involves the use of earth-abundant metal catalyst, mild reaction conditions, a broad range of substrates (84 examples), high yields (up to 97% yield), and exceptional control over enantioselectivity. The reaction system is compatible with different types of radical precursors, including O-acylhydroxylamines, cycloketone oxime esters, aryldiazonium salts, and drug molecules. Chiral ligand PyBim is identified as particularly effective in achieving the desired high enantioselectivity. Mechanistic studies reveal that copper/PyBim system plays a vital role in C─O coupling, employing an outer-sphere model. In addition, the side arm effect of ligand is observed.

From Quaternary Carbon to Tertiary C(sp3)-Si and C(sp3)-Ge Bonds: Decyanative Coupling of Malononitriles with Chlorosilanes and Chlorogermanes Enabled by Ni/Ti Dual Catalysis

Transition-metal-catalyzed C-Si/Ge cross-coupling offers promising avenues for the synthesis of organosilanes/organogermanes, yet it is fraught with long-standing challenges. A Ni/Ti-catalyzed strategy is reported here, allowing the use of disubstituted malononitriles as tertiary C(sp3) coupling partners to couple with chlorosilanes and chlorogermanes, respectively. This method enables the catalytic cleavage of the C(sp3)-CN bond of the quaternary carbon followed by the formation of C(sp3)-Si/C(sp3)-Ge bonds from ubiquitously available starting materials. The efficiency and generality are showcased by a broad scope for both of the coupling partners, therefore holding the potential to synthesize structurally diverse quaternary organosilanes and organogermanes that were difficult to access previously.

CuH-Catalyzed Regio- and Enantioselective Formal Hydroformylation of Vinyl Arenes

A highly enantioselective formal hydroformylation of vinyl arenes enabled by copper hydride (CuH) catalysis is reported. Key to the success of the method was the use of the mild Lewis acid zinc triflate to promote the formation of oxocarbenium electrophiles through the activation of diethoxymethyl acetate. Using the newly developed protocol, a broad range of vinyl arene substrates underwent efficient hydroacetalization reactions to provide access to highly enantioenriched α-aryl acetal products in good yields with exclusively branched regioselectivity. The acetal products could be converted to the corresponding aldehydes, alcohols, and amines with full preservation of the enantiomeric purity. Density functional theory studies support that the key C-C bond-forming event between the alkyl copper intermediate and the oxocarbenium electrophile takes place with inversion of configuration of the Cu-C bond in a backside SE2-type mechanism.

A Rationally Designed Iron(II) Catalyst for C(sp3)-C(sp2) and C(sp3)-C(sp3) Suzuki-Miyaura Cross-Coupling

Despite the paramount importance of the Suzuki-Miyaura coupling (SMC) in academia and industry, and the great promise of iron to offer sustainable catalysis, iron-catalyzed SMC involving sp3-hybridized partners is still in its infancy. We herein report the development of a versatile, well-defined electron-deficient anilido-aldimine iron(II) catalyst. This catalyst effectively performed C(sp3)-C(sp2) and C(sp3)-C(sp3) SMC of alkyl halide electrophiles and (hetero)aryl boronic ester and alkyl borane nucleophiles respectively, in the presence of a lithium amide base. These couplings operated under mild reaction conditions and displayed wide functional group compatibility including various medicinally relevant N-, O- and S-based heterocycles. They also tolerated primary, secondary and tertiary alkyl halides (Br, Cl, I), electron-neutral, -rich and -poor boronic esters and primary and secondary alkyl boranes. Our methodology could be directly and efficiently applied to synthesize key intermediates relevant to pharmaceuticals and a potential drug candidate. For C(sp3)-C(sp2) couplings, radical probe experiments militated in favor of a carbon-centered radical derived from the electrophile. At the same time, reactions run with a pre-formed activated boron nucleophile coupled to competition experiments supported the involvement of neutral, rather than an anionic, (hetero)aryl boronic ester in the key transmetalation step.

Simultaneous Stereoinvertive and Stereoselective C(sp3)-C(sp3) Cross-Coupling of Boronic Esters and Allylic Carbonates

With increasing interest in constructing more three-dimensional entities, there has been growing interest in cross-coupling reactions that forge C(sp3)-C(sp3) bonds, which leads to additional challenges as it is not just a more difficult bond to construct but issues of stereocontrol also arise. Herein, we report the stereocontrolled cross-coupling of enantioenriched boronic esters with racemic allylic carbonates enabled by iridium catalysis, leading to the formation of C(sp3)-C(sp3) bonds with single or vicinal stereogenic centers. The method shows broad substrate scope, enabling primary, secondary, and even tertiary boronic esters to be employed, and can be used to prepare any of the four possible stereoisomers of a coupled product with vicinal chiral centers. The new method, which combines the simultaneous enantiospecific reaction of a chiral nucleophile with the enantioselective reaction of a chiral electrophile in a single process, offers a solution for stereodivergent cross-coupling of two C(sp3) fragments.

Enantioselective synthesis of chiral α,α-dialkyl indoles and related azoles by cobalt-catalyzed hydroalkylation and regioselectivity switch

General, catalytic and enantioselective construction of chiral α,α-dialkyl indoles represents an important yet challenging objective to be developed. Herein we describe a cobalt catalyzed enantioselective anti-Markovnikov alkene hydroalkylation via the remote stereocontrol for the synthesis of α,α-dialkyl indoles and other N-heterocycles. This asymmetric C(sp3)-C(sp3) coupling features high flexibility in introducing a diverse set of alkyl groups at the α-position of chiral N-heterocycles. The utility of this methodology has been demonstrated by late-stage functionalization of drug molecules, asymmetric synthesis of bioactive molecules, natural products and functional materials, and identification of a class of molecules exhibiting anti-apoptosis activities in UVB-irradiated HaCaT cells. Ligands play a vital role in controlling the reaction regioselectivity. Changing the ligand from bi-dentate L6 to tridentate L12 enables CoH-catalyzed Markovnikov hydroalkylation. Mechanistic studies disclose that the anti-Markovnikov hydroalkylation involves a migratory insertion process while the Markovnikov hydroalkylation involves a MHAT process.

Effect of 6,6'-Substituents on Bipyridine-Ligated Ni Catalysts for Cross-Electrophile Coupling

A family of 4,4'-tBu2-2,2'-bipyridine (tBubpy) ligands with substituents in either the 6-position, 4,4'-tBu2-6-Me-bpy (tBubpyMe), or 6 and 6'-positions, 4,4'-tBu2-6,6'-R2-bpy (tBubpyR2; R = Me, iPr, sBu, Ph, or Mes), was synthesized. These ligands were used to prepare Ni complexes in the 0, I, and II oxidation states. We observed that the substituents in the 6 and 6'-positions of the tBubpy ligand impact the properties of the Ni complexes. For example, bulkier substituents in the 6,6'-positions of tBubpy better stabilized (tBubpyR2)NiICl species and resulted in cleaner reduction from (tBubpyR2)NiIICl2. However, bulkier substituents hindered or prevented coordination of tBubpyR2 ligands to Ni0(cod)2. In addition, by using complexes of the type (tBubpyMe)NiCl2 and (tBubpyR2)NiCl2 as precatalysts for different XEC reactions, we demonstrated that the 6 or 6,6' substituents lead to major differences in catalytic performance. Specifically, while (tBubpyMe)NiIICl2 is one of the most active catalysts reported to date for XEC and can facilitate XEC reactions at room temperature, lower turnover frequencies were observed for catalysts containing tBubpyR2 ligands. A detailed study on the catalytic intermediates (tBubpy)Ni(Ar)I and (tBubpyMe2)Ni(Ar)I revealed several factors that likely contributed to the differences in catalytic activity. For example, whereas complexes of the type (tBubpy)Ni(Ar)I are low spin and relatively stable, complexes of the type (tBubpyMe2)Ni(Ar)I are high-spin and less stable. Further, (tBubpyMe2)Ni(Ar)I captures primary and benzylic alkyl radicals more slowly than (tBubpy)Ni(Ar)I, consistent with the lower activity of the former in catalysis. Our findings will assist in the design of tailor-made ligands for Ni-catalyzed transformations.

Late-Stage Saturation of Drug Molecules

The available methods of chemical synthesis have arguably contributed to the prevalence of aromatic rings, such as benzene, toluene, xylene, or pyridine, in modern pharmaceuticals. Many such sp2-carbon-rich fragments are now easy to synthesize using high-quality cross-coupling reactions that click together an ever-expanding menu of commercially available building blocks, but the products are flat and lipophilic, decreasing their odds of becoming marketed drugs. Converting flat aromatic molecules into saturated analogues with a higher fraction of sp3 carbons could improve their medicinal properties and facilitate the invention of safe, efficacious, metabolically stable, and soluble medicines. In this study, we show that aromatic and heteroaromatic drugs can be readily saturated under exceptionally mild rhodium-catalyzed hydrogenation, acid-mediated reduction, or photocatalyzed-hydrogenation conditions, converting sp2 carbon atoms into sp3 carbon atoms and leading to saturated molecules with improved medicinal properties. These methods are productive in diverse pockets of chemical space, producing complex saturated pharmaceuticals bearing a variety of functional groups and three-dimensional architectures. The rhodium-catalyzed method tolerates traces of dimethyl sulfoxide (DMSO) or water, meaning that pharmaceutical compound collections, which are typically stored in wet DMSO, can finally be reformatted for use as substrates for chemical synthesis. This latter application is demonstrated through the late-stage saturation (LSS) of 768 complex and densely functionalized small-molecule drugs.

C(sp3)-C(sp3) Coupling of Cycloalkanes and Alkyl Halides via Dual Photocatalytic Hydrogen Atom Transfer and Nickel Catalysis

Functionalization of C(sp3)-H bonds represents the most straightforward and atom-economical transformation in organic synthesis. An innovative approach integrating photocatalytic hydrogen atom transfer (HAT) and transition metal catalysis has made significant progress in the coupling of α-heterosubstituted C-H bonds with alkyl halides. However, unactivated alkanes were ineffective as a result of the preponderance of byproduct formation. Herein, we demonstrate direct HAT and nickel catalysis in the coupling of cycloalkanes and benzyl bromides/primary alkyl iodides. Additionally, tetrabutylammonium decatungstate (TBADT) was recovered and recycled.

A Bimolecular Homolytic Substitution-Enabled Platform for Multicomponent Cross-Coupling of Unactivated Alkenes

The construction of C(sp3)-C(sp3) bonds remains one of the most difficult challenges in cross-coupling chemistry. Here, we report a photoredox/nickel dual catalytic approach that enables the simultaneous formation of two C(sp3)-C(sp3) linkages via trimolecular cross-coupling of alkenes with alkyl halides and hypervalent iodine-based reagents. The reaction harnesses a bimolecular homolytic substitution (SH2) mechanism and chemoselective halogen-atom transfer (XAT) to orchestrate the regioselective addition of electrophilic and nucleophilic alkyl radicals across unactivated alkenes without the need for a directing auxiliary. Utility is highlighted through late-stage (fluoro)alkylation and (trideutero)methylation of C═C bonds bearing different substitution patterns, offering straightforward access to drug-like molecules comprising sp3-hybridized carbon scaffolds.

Selective Hydrofunctionalization of Alkenyl Fluorides Enabled by Nickel-Catalyzed Hydrogen Atoms and Group Transfer: Reaction Development and Mechanistic Study

Due to the unique effect of fluorine atoms, the efficient construction of high-value alkyl fluorides has attracted significant interest in modern drug development. However, enantioselective catalytic strategies for the efficient assembly of highly functionalized chiral C(sp3)-F scaffolds from simple starting materials have been underutilized. Herein, we demonstrate a nickel-catalyzed radical transfer strategy for the efficient, modular, asymmetric hydrogenation and hydroalkylation of alkenyl fluorides with primary, secondary, and tertiary alkyl halides under mild conditions. The transformation provides facile access to various structurally complex secondary and tertiary α-fluoro amide products from readily available starting materials with excellent substrate compatibility and distinct selectivity. Furthermore, the utility of this method is demonstrated by late-stage modifications and product derivatizations. Detailed mechanistic studies and DFT calculations have been conducted, showing that the rate-determining step for asymmetric hydrogenation reaction is NiH-HAT toward alkenyl fluorides and the stereo-determining step is alcohol coordination to Ni-enolates followed by a barrierless protonation. The mechanism for the asymmetric hydroalkylation reaction is also delivered in this investigation.

Fully Automated Flow Protocol for C(sp3)-C(sp3) Bond Formation from Tertiary Amides and Alkyl Halides

Herein, we present a novel C(sp3)-C(sp3) bond-forming protocol via the reductive coupling of abundant tertiary amides with organozinc reagents prepared in situ from their corresponding alkyl halides. Using a multistep fully automated flow protocol, this reaction could be used for both library synthesis and target molecule synthesis on the gram-scale starting from bench-stable reagents. Additionally, excellent chemoselectivity and functional group tolerance make it ideal for late-stage diversification of druglike molecules.

Access to Stereoblock Polyesters via Irreversible Chain-Transfer Ring-Opening Polymerization (ICT-ROP)

Precise control over polymer microstructure can enable the molecular tunability of material properties and represents a significant challenge in polymer chemistry. Stereoblock copolymers are some of the simplest stereosequenced polymers, yet the synthesis of stereoblock polyesters from prochiral or racemic monomers outside of "simple" isotactic stereoblocks remains limited. Herein, we report the development of irreversible chain-transfer ring-opening polymerization (ICT-ROP), which overcomes the fundamental limitations of single catalyst approaches by using transmetalation (e.g., alkoxide-chloride exchange) between two catalysts with distinct stereoselectivities as a means to embed temporally controlled multicatalysis in ROP. Our combined small-molecule model and catalytic polymerization studies lay out a clear molecular basis for ICT-ROP and are exploited to access the first examples of atactic-syndiotactic stereoblock (at-sb-st) polyesters, at-sb-st polyhydroxyalkanoates (PHAs). We achieve high levels of control over molecular weight, tacticity, monomer composition, and block structures in a temporally controlled manner and demonstrate that stereosequence control leads to polymer tensile properties that are independent of thermal properties.

Enantioselective Construction of C-SCF3 Stereocenters via Nickel Catalyzed Asymmetric Negishi Coupling Reaction

The construction of the SCF3-containing 1,1-diaryl tertiary carbon stereocenters with high enantioselectivities is reported via a nickel-catalyzed asymmetric C-C coupling strategy. This method demonstrates simple operations, mild conditions and excellent functional group tolerance, with newly designed SCF3-containing synthon, which can be easily obtained from commercially available benzyl bromide and trifluoromethylthio anion in a two-step manner. Further substrate exploration indicated that the reaction system could be extended to diverse perfluoroalkyl sulfide (SC2F5, SC3F7, SC4F9, SCF2CO2Et)-substituted 1,1-diaryl compounds with excellent enantioselectivities. The synthetic utility of this transformation was further demonstrated by convenient derivatization to optical SCF3-containing analogues of bioactive compounds without an apparent decrease in enantioselectivity.

Enantioselective Construction of Quaternary Stereocenters via Cooperative Photoredox/Fe/Chiral Primary Amine Triple Catalysis

The catalytic and enantioselective construction of quaternary (all-carbon substituents) stereocenters poses a formidable challenge in organic synthesis due to the hindrance caused by steric factors. One conceptually viable and potentially versatile approach is the coupling of a C-C bond through an outer-sphere mechanism, accompanied by the realization of enantiocontrol through cooperative catalysis; however, examples of such processes are yet to be identified. Herein, we present such a method for creating different compounds with quaternary stereocenters by photoredox/Fe/chiral primary amine triple catalysis. This approach facilitates the connection of an unactivated alkyl source with a tertiary alkyl moiety, which is also rare. The scalable process exhibits mild conditions, does not necessitate the use of a base, and possesses a good functional-group tolerance. Preliminary investigations into the underlying mechanisms have provided valuable insights into the reaction pathway.

Copper-Catalyzed Enantioconvergent Radical N-Alkylation of Diverse (Hetero)aromatic Amines

The 3d transition metal-catalyzed enantioconvergent radical cross-coupling provides a powerful tool for chiral molecule synthesis. In the classic mechanism, the bond formation relies on the interaction between nucleophile-sequestered metal complexes and radicals, limiting the nucleophile scope to sterically uncongested ones. The coupling of sterically congested nucleophiles poses a significant challenge due to difficulties in transmetalation, restricting the reaction generality. Here, we describe a probable outer-sphere nucleophilic attack mechanism that circumvents the challenging transmetalation associated with sterically congested nucleophiles. This strategy enables a general copper-catalyzed enantioconvergent radical N-alkylation of aromatic amines with secondary/tertiary alkyl halides and exhibits catalyst-controlled stereoselectivity. It accommodates diverse aromatic amines, especially bulky secondary and primary ones to deliver value-added chiral amines (>110 examples). It is expected to inspire the coupling of more nucleophiles, particularly challenging sterically congested ones, and accelerate reaction generality.

Alkene dialkylation by triple radical sorting

The development of bimolecular homolytic substitution (SH2) catalysis has expanded cross-coupling chemistries by enabling the selective combination of any primary radical with any secondary or tertiary radical through a radical sorting mechanism1-8. Biomimetic9,10 SH2 catalysis can be used to merge common feedstock chemicals-such as alcohols, acids and halides-in various permutations for the construction of a single C(sp3)-C(sp3) bond. The ability to sort these two distinct radicals across commercially available alkenes in a three-component manner would enable the simultaneous construction of two C(sp3)-C(sp3) bonds, greatly accelerating access to complex molecules and drug-like chemical space11. However, the simultaneous in situ formation of electrophilic and primary nucleophilic radicals in the presence of unactivated alkenes is problematic, typically leading to statistical radical recombination, hydrogen atom transfer, disproportionation and other deleterious pathways12,13. Here we report the use of bimolecular homolytic substitution catalysis to sort an electrophilic radical and a nucleophilic radical across an unactivated alkene. This reaction involves the in situ formation of three distinct radical species, which are then differentiated by size and electronics, allowing for regioselective formation of the desired dialkylated products. This work accelerates access to pharmaceutically relevant C(sp3)-rich molecules and defines a distinct mechanistic approach for alkene dialkylation.

Ni-catalyzed enantioconvergent deoxygenative reductive cross-coupling of unactivated alkyl alcohols and aryl bromides

Transition metal-catalyzed enantioconvergent cross-coupling of an alkyl precursor presents a promising method for producing enantioenriched C(sp3) molecules. Because alkyl alcohol is a ubiquitous and abundant family of feedstock in nature, the direct reductive coupling of alkyl alcohol and aryl halide enables efficient access to valuable compounds. Although several strategies have been developed to overcome the high bond dissociation energy of the C - O bond, the asymmetric pattern remains unknown. In this report, we describe the realization of an enantioconvergent deoxygenative reductive cross-coupling of unactivated alkyl alcohol (β-hydroxy ketone) and aryl bromide in the presence of an NHC activating agent. The approach can accommodate substituents of various sizes and functional groups, and its synthetic potency is demonstrated through a gram scale reaction and derivatizations into other compound families. Finally, we apply our convergent method to the efficient asymmetric synthesis of four β-aryl ketones that are natural products or bioactive compounds.

Nickel-Catalyzed Carbonylative Negishi Cross-Coupling of Unactivated Secondary Alkyl Electrophiles with 1 atm CO Gas

We describe a nickel-catalyzed carbonylative cross-coupling of unactivated secondary alkyl electrophiles with the organozinc reagent at atmospheric CO gas, thus allowing the expedient construction of unsymmetric dialkyl ketones with broad functional group tolerance. The leverage of a newly developed NN2-pincer type ligand enables the chemoselective three-component carbonylation by overcoming the competing Negishi coupling, the undesired β-hydride elimination, and dehalogenation of alkyl iodides side pathways. Both alkyl iodides and alkyl tosylates are compatible in the single electron transfer involved mechanism.

Alcohol-alcohol cross-coupling enabled by SH2 radical sorting

Alcohols represent a functional group class with unparalleled abundance and structural diversity. In an era of chemical synthesis that prioritizes reducing time to target and maximizing exploration of chemical space, harnessing these building blocks for carbon-carbon bond-forming reactions is a key goal in organic chemistry. In particular, leveraging a single activation mode to form a new C(sp3)-C(sp3) bond from two alcohol subunits would enable access to an extraordinary level of structural diversity. In this work, we report a nickel radical sorting-mediated cross-alcohol coupling wherein two alcohol fragments are deoxygenated and coupled in one reaction vessel, open to air.

Nickel-Catalyzed Deaminative Alkyl-Alkyl Cross-Coupling of Katritzky Salts with Cyclopropanols: Merging C-N and C-C Bond Activation

Herein, we report a general and practical nickel-catalyzed deaminative alkylation of Katritzky salts with cyclopropyl alcohols via merging C-N and C-C bond activation. This protocol enables the formation of an alkyl-alkyl bond along with the generation of a versatile ketone functional group in a single operation, thus providing a convenient approach for accessing β-alkyl ketones. This reaction is distinguished by its high functional group tolerance, broad substrate scope, and efficient late-stage derivatization of complex bioactive molecules.

Copper-Catalyzed Enantioselective C(sp3 )-SCF3 Coupling of Carbon-Centered Benzyl Radicals with (Me4 N)SCF3

In contrast with the well-established C(sp2 )-SCF3 cross-coupling to forge the Ar-SCF3 bond, the corresponding enantioselective coupling of readily available alkyl electrophiles to forge chiral C(sp3 )-SCF3 bond has remained largely unexplored. We herein disclose a copper-catalyzed enantioselective radical C(sp3 )-SCF3 coupling of a range of secondary/tertiary benzyl radicals with the easily available (Me4 N)SCF3 reagent. The key to the success lies in the utilization of chiral phosphino-oxazoline-derived anionic N,N,P-ligands through tuning electronic and steric effects for the simultaneous control of the reaction initiation and enantioselectivity. This strategy can successfully realize two types of asymmetric radical reactions, including enantioconvergent C(sp3 )-SCF3 cross-coupling of racemic benzyl halides and three-component 1,2-carbotrifluoromethylthiolation of arylated alkenes under mild reaction conditions. It therefore provides a highly flexible platform for the rapid assembly of an array of enantioenriched SCF3 -containing molecules of interest in organic synthesis and medicinal chemistry.

Copper-Catalyzed Asymmetric Remote C(sp3)-H Alkylation of N-Fluorocarboxamides with Glycine Derivatives and Peptides

Saturated hydrocarbon bonds are ubiquitous in organic molecules; to date, the selective functionalization of C(sp3)-H bonds continues to pose a notorious difficulty, thereby garnering significant attention from the synthetic chemistry community. During the past several decades, a wide array of powerful new methodologies has been developed to enantioselectively modify C(sp3)-H bonds that is successfully applied in asymmetric formation of diverse bonds, including C-C, C-N, and C-O bonds; nevertheless, the asymmetric C(sp3)-H alkylation is elusive and, therefore, far less explored. In this work, we report a direct and robust strategy to construct highly valuable enantioenriched unnatural α-amino acid (α-AA) cognates and peptides by a copper-catalyzed enantioselective remote C(sp3)-H alkylation of N-fluorocarboxamides and readily accessible glycine esters under ambient conditions. The key to success lies in the optically active Cu catalyst generated through the coordination of glycine derivatives to enantiopure bisphosphine/Cu(I) species, which is beneficial to the single electronic reduction of N-fluorocarboxamides and the subsequent stereodetermining alkylation. More importantly, all types (primary, secondary, tertiary, and even α-oxy) of δ-C(sp3)-H bonds could be site- and stereospecifically activated by the kinetically favored 1,5-hydrogen atom transfer (1,5-HAT) step.

Interception of Transient Allyl Radicals with Low-Valent Allylpalladium Chemistry: Tandem Pd(0/II/I)-Pd(0/II/I/II) Cycles in Photoredox/Pd Dual-Catalytic Enantioselective C(sp3)-C(sp3) Homocoupling

We present comprehensive computational and experimental studies on the mechanism of an asymmetric photoredox/Pd dual-catalytic reductive C(sp3)-C(sp3) homocoupling of allylic electrophiles. In stark contrast to the canonical assumption that photoredox promotes bond formation via facile reductive elimination from high-valent metal-organic species, our computational analysis revealed an intriguing low-valent allylpalladium pathway that features tandem operation of Pd(0/II/I)-Pd(0/II/I/II) cycles. Specifically, we propose that (i) the photoredox/Pd system enables the in situ generation of allyl radicals from low-valent Pd(I)-allyl species, and (ii) effective interception of the fleeting allyl radical by the chiral Pd(I)-allyl species results in the formation of an enantioenriched product. Notably, the cooperation of the two pathways highlights the bifunctional role of Pd(I)-allyl species in the generation and interception of transient allyl radicals. Moreover, the mechanism implies divergent substrate-activation modes in this homocoupling reaction, suggesting a theoretical possibility for cross-coupling. Combined, the current study offers a novel mechanistic hypothesis for photoredox/Pd dual catalysis and highlights the use of low-valent allylpalladium as a means to efficiently intercept radicals for selective asymmetric bond constructions.

Aminative Suzuki-Miyaura coupling

The Suzuki-Miyaura and Buchwald-Hartwig coupling reactions are widely used to form carbon-carbon (C-C) and carbon-nitrogen (C-N) bonds, respectively. We report the incorporation of a formal nitrene insertion process into the Suzuki-Miyaura reaction, altering the products from C-C-linked biaryls to C-N-C-linked diaryl amines and thereby joining the Suzuki-Miyaura and Buchwald-Hartwig coupling pathways to the same starting-material classes. A combination of a bulky ancillary phosphine ligand on palladium and a commercially available amination reagent enables efficient reactivity across aryl halides and pseudohalides, boronic acids and esters, and many functional groups and heterocycles. Mechanistic insights reveal flexibility on the order of bond-forming events, suggesting potential for expansion of the aminative cross-coupling concept to encompass diverse nucleophiles and electrophiles as well as four-component variants.

Photoinduced Triphenylphosphine and Iodide Salt Promoted Reductive Decarboxylative Coupling

The transient electron donor-acceptor (EDA) complex has been an emerging area in the photoinduced organic synthesis field, generating radicals without exogenous transition-metal or organic dye-based photoredox catalysts. The catalytic platform to form suitable photoactive EDA complexes for photochemical reduction reactions remains underdeveloped. Herein, a general photoinduced reductive alkylation via the EDA complex strategy is described. A simple yet multifunctional system, triphenylphosphine and iodide salt, promotes the photoinduced decarboxylative hydroalkylation, and reductive defluorinative decarboxylative alkylation of trifluoromethyl alkenes, to access trifluoromethyl alkanes and gem-difluoroalkenes. Moreover, decarboxylative hydroalkylation can be applied to more kinds of electron-deficient alkenes as a general Giese addition reaction.

Congested C(sp3)-rich architectures enabled by iron-catalysed conjunctive alkylation

Catalytic cross-coupling by transition metals has revolutionized the formation of C-C bonds in organic synthesis. However, the challenge of forming multiple alkyl-alkyl bonds in crowded environments remains largely unresolved. Here, we report the regioselective functionalization of olefins with sp3-hybridized organohalides and organozinc reagents using a simple (terpyridine)iron catalyst. Aliphatic groups of various sizes are successfully installed on either olefinic carbon, furnishing a diverse array of products with congested cores featuring C- or heteroatom-substituted stereocenters. The method enables access to valuable but synthetically challenging C(sp3)-rich molecules, including alicyclic compounds bearing multiple contiguous stereocenters through annulation cascades. Mechanistic and theoretical studies suggest a stepwise iron-mediated radical carbometallation pathway followed by outer-sphere C-C bond formation, which potentially opens the door to a broader scope of transformations and new chemical space.

A general copper-catalysed enantioconvergent C(sp3)-S cross-coupling via biomimetic radical homolytic substitution

Although α-chiral C(sp3)-S bonds are of enormous importance in organic synthesis and related areas, the transition-metal-catalysed enantioselective C(sp3)-S bond construction still represents an underdeveloped domain probably due to the difficult heterolytic metal-sulfur bond cleavage and notorious catalyst-poisoning capability of sulfur nucleophiles. Here we demonstrate the use of chiral tridentate anionic ligands in combination with Cu(I) catalysts to enable a biomimetic enantioconvergent radical C(sp3)-S cross-coupling reaction of both racemic secondary and tertiary alkyl halides with highly transformable sulfur nucleophiles. This protocol not only exhibits a broad substrate scope with high enantioselectivity but also provides universal access to a range of useful α-chiral alkyl organosulfur compounds with different sulfur oxidation states, thus providing a complementary approach to known asymmetric C(sp3)-S bond formation methods. Mechanistic results support a biomimetic radical homolytic substitution pathway for the critical C(sp3)-S bond formation step.

Metal-free photocatalytic cross-electrophile coupling enables C1 homologation and alkylation of carboxylic acids with aldehydes

In contemporary drug discovery, enhancing the sp3-hybridized character of molecular structures is paramount, necessitating innovative synthetic methods. Herein, we introduce a deoxygenative cross-electrophile coupling technique that pairs easily accessible carboxylic acid-derived redox-active esters with aldehyde sulfonyl hydrazones, employing Eosin Y as an organophotocatalyst under visible light irradiation. This approach serves as a versatile, metal-free C(sp3)-C(sp3) cross-coupling platform. We demonstrate its synthetic value as a safer, broadly applicable C1 homologation of carboxylic acids, offering an alternative to the traditional Arndt-Eistert reaction. Additionally, our method provides direct access to cyclic and acyclic β-arylethylamines using diverse aldehyde-derived sulfonyl hydrazones. Notably, the methodology proves to be compatible with the late-stage functionalization of peptides on solid-phase, streamlining the modification of intricate peptides without the need for exhaustive de-novo synthesis.

Ligand-Controlled On-Off Switch of a Silicon-Tethered Directing Group Enabling the Regiodivergent Hydroalkylation of Vinylsilanes under Ni-H Catalysis

A regiodivergent Ni-H-catalyzed hydroalkylation of vinylsilanes is described. Depending on the ancillary ligand at the nickel catalyst, the regioselectivity can be steered by a directing group attached to the silicon atom. The mild protocols allow for the selective synthesis of either branched or linear alkylsilanes. An example of a vinylgermane is also reported. The method features a broad scope with high functional-group tolerance and follows a radical mechanism.

Ligand-Controlled Cobalt-Catalyzed Regio-, Enantio-, and Diastereoselective Oxyheterocyclic Alkene Hydroalkylation

Metal-hydride-catalyzed alkene hydroalkylation has been developed as an efficient method for C(sp3)-C(sp3) coupling with broad substrate availability and high functional group compatibility. However, auxiliary groups, a conjugated group or a chelation-directing group, are commonly required to attain high regio- and enantioselectivities. Herein, we reported a ligand-controlled cobalt-hydride-catalyzed regio-, enantio-, and diastereoselective oxyheterocyclic alkene hydroalkylation without chelation-directing groups. This reaction enables the hydroalkylation of conjugated and unconjugated oxyheterocyclic alkenes to deliver C2- or C3-alkylated tetrahydrofuran or tetrahydropyran in uniformly good yields and with high regio- and enantioselectivities. In addition, hydroalkylation of C2-substituted 2,5-dihydrofuran resulted in the simultaneous construction of 1,3-distereocenters, providing convenient access to polysubstituted tetrahydrofuran with multiple enantioenriched C(sp3) centers.

Unveiling Pre-Transmetalation Intermediates in Base-Free Suzuki-Miyaura Cross-Couplings: A Computational Study

The pre-transmetalation intermediates are critically important in Suzuki-Miyaura cross-coupling (SMC) reactions and have become a hot spot of the current research. However, the pre-transmetalation intermediates under base-free conditions have not been clear. Herein, a comprehensive theoretical study is performed on the base-free Pd-catalyzed desulfonative SMC reaction. The fragile coordination feature and the acceleration role of the RuPhos chelate ligand are revealed. The hydrogen-bond complex between the Pd-F complex and aryl boronic acid is identified as an important pre-transmetalation intermediate, which increases the energy span to 32.5 kcal/mol. The controlling factor for the formation of the hydrogen-bond complexes is attributed to the electronegativities of halogen atoms in the metal halide complexes. What is more, other reported SMC reaction systems involving metal halide complexes and aryl boronic acids are reconsidered and suggest that the hydrogen-bond complexes widely exist as stable pre-transmetalation intermediates with influencing the catalytic activities. The earth-abundant Ni-catalyzed desulfonative SMC reaction is further designed and predicted to have a higher activity than the original Pd-catalyzed SMC reaction.

Decarboxylative Radical Sulfilimination via Photoredox, Copper, and Brønsted Base Catalysis

Sulfilimines, the aza-variants of sulfoxides, are key structural motifs in natural products, pharmaceuticals, and agrochemicals; and sulfilimine synthesis is therefore important in organic chemistry. However, methods for radical sulfilimination remain elusive, and as a result, the structural diversity of currently available sulfilimines is limited. Herein, we report the first protocol for decarboxylative radical sulfilimination reactions between sulfenamides and N-hydroxyphthalimide esters of primary, secondary, and tertiary alkyl carboxylic acids, which were achieved via a combination of photoredox, copper, and Brønsted base catalysis. This novel protocol provided a wide variety of sulfilimines, in addition to serving as an efficient route for the synthesis of S-alkyl/S-aryl homocysteine sulfilimines and S-(4-methylphenyl) homocysteine sulfoximine. Moreover, it could be used for late-stage introduction of a sulfilimine group into structurally complex molecules, thereby avoiding the need to preserve labile organosulfur moieties through multistep synthetic sequences. A mechanism involving photocatalytic substrate transformation and copper-mediated C(sp3 )-S bond formation is proposed.

Eight-Membered Palladacycle Intermediate Enabled Synthesis of Cyclic Biarylphosphonates

Transition-metal-catalyzed coupling reactions that involve the direct functionalization of insert C-H bond represent one of the most efficient strategies for forming carbon-carbon bonds. Herein, a palladium-catalyzed intramolecular C-H bond arylation of triaryl phosphates is reported to access seven-membered cyclic biarylphosphonate targets. The reaction is achieved via a unique eight-membered palladacyclic intermediate and shows good functional group compatibility. Meanwhile, the product can be readily converted into other valuable phosphate compounds.

Ni-Catalyzed 1,2-Alkyl Borylation and Silylation of Allenes En Route to [1,3]-Bis-Organometallic Reagents

A nickel-catalyzed multicomponent reaction that rapidly and reliably accesses [1,3]-bis-organometallic reagents from allenes is reported. The protocol exhibits a predictable regioselectivity pattern that enables the incorporation of B,B(Si) fragments across the allene backbone under mild conditions, thus offering a complementary platform for accessing polyorganometallic reagents possessing both sp2 and sp3 hybridization from readily available precursors.

Catalytic Asymmetric α-Alkylation of Ketones with Unactivated Alkyl Halides

A catalytic, enantioselective method for direct α-alkylation of ketones with unactivated alkyl halides is realized by employing an α-enolizable ketone in a nickel-catalyzed C(sp3)-C(sp3) cross-coupling reaction. The key to the success is attributed to a unique bimetallic ligand. A variety of acyclic ketones and unactivated alkyl iodides can serve as suitable substrates under mild conditions to generate chiral ketones with α-quaternary carbon stereocenters in high yields with good enantioselectivities. A range of transformations based on the ketone moiety are also demonstrated to show the potential application of this method. Preliminary mechanistic studies support a dinickel-catalyzed cross-coupling mechanism.

Redox-Active Thiocarbonyl Auxiliaries in Ni-Catalyzed Cross-Couplings of Aliphatic Alcohols

ConspectusThe Barton-McCombie deoxygenation reaction first established the use of O-alkyl thiocarbonyl derivatives as powerful redox-active agents for C(sp3)-O reduction. In recent years, first-row transition metals capable of engaging with alkyl radical intermediates generated from O-alkyl thiocarbonyl derivatives using alternative stoichiometric radical precursors have been developed. Given the ability of select Ni catalysts to both participate in single-electron oxidative addition pathways and intercept alkyl radical intermediates, our group has investigated the use of O-alkyl thiocarbonyl derivatives as electrophiles in novel cross-coupling reactions. After describing related work in this area, this Account will first summarize our entry point into this field. Here, we used the cyclopropane ring as a reporter of leaving group reactivity to aid in the design and optimization of a novel redox-active O-thiocarbamate leaving group for C(sp3)-O arylation. Motivation for this pursuit was driven by the propensity of the cyclopropane ring to undergo ring opening under polar (2e) oxidative addition pathways or to be maintained under single-electron (1e) conditions. Using these guiding principles, we developed a method for the deoxygenative arylation of cyclopropanol derivatives using a Ni catalyst without the need for a stoichiometric external reductant or photocatalyst. We next summarize our evaluation of an alternative redox-active O-thiocarbonyl imidazole auxiliary in a related deoxygenative cross-coupling. This work demonstrated an extension of our initial approach to the deoxygenative arylation of primary and secondary aliphatic alcohol derivatives. A brief mechanistic investigation revealed that this reaction likely proceeds via a distinct mechanism involving direct homolytic C(sp3)-O bond cleavage. We conclude this Account with a summary of work aimed toward a unique approach for thiocarboxylic acid derivative synthesis. This project was inspired by the efficiency of thionoester generation under most of the reaction conditions evaluated in our prior investigations. Using alcohol, amine, or thiol starting materials, which were activated with convenient thiocarbonyl sources in a single step, we optimized for a Ni-catalyzed cross-coupling capable of providing access to a range of thionoester, thioamide, or dithioester products. In summary, our work has revealed the potential of redox-active thiocarbonyl auxiliaries in Ni-catalyzed cross-couplings with C(sp3)-O electrophiles. We anticipate that the continued investigation of aliphatic thiocarbonyl derivatives as radical precursors with alternative single-electron inputs will be an area of continued growth in the years to come.

Nickel-Catalyzed Radical Mechanisms: Informing Cross-Coupling for Synthesizing Non-Canonical Biomolecules

Nickel excels at facilitating selective radical chemistry, playing a pivotal role in metalloenzyme catalysis and modern cross-coupling reactions. Radicals, being nonpolar and neutral, exhibit orthogonal reactivity to nucleophilic and basic functional groups commonly present in biomolecules. Harnessing this compatibility, we delve into the application of nickel-catalyzed radical pathways in the synthesis of noncanonical peptides and carbohydrates, critical for chemical biology studies and drug discovery.We previously characterized a sequential reduction mechanism that accounts for chemoselectivity in cross-electrophile coupling reactions. This catalytic cycle begins with nickel(I)-mediated radical generation from alkyl halides, followed by carbon radical capture by nickel(II) complexes, and concludes with reductive elimination. These steps resonate with mechanistic proposals in nickel-catalyzed cross-coupling, photoredox, and electrocatalytic reactions. Herein, we present our insights into each step involving radicals, including initiation, propagation, termination, and the nuances of kinetics, origins of selectivity, and ligand effects.Radical generation from C(sp3) electrophiles via one-electron oxidative addition with low-valent nickel radical intermediates provides the basis for stereoconvergent and cross-electrophile couplings. Our electroanalytical studies elucidate a concerted halogen atom abstraction mechanism, where electron transfer is coupled with halide dissociation. Using this pathway, we have developed a nickel-catalyzed stereoselective radical addition to dehydroalanine, facilitating the synthesis of noncanonical peptides. In this application, chiral ligands modulate the stereochemical outcome through the asymmetric protonation of a nickel-enolate intermediate.The capture of the alkyl radical by nickel(II) expands the scope of cross-coupling, promotes reductive elimination through the formation of high-valent nickel(III) species, and governs chemo- and stereoselectivity. We discovered that nickel(II)-aryl efficiently traps radicals with a barrier ranging from 7 to 9 kcal/mol, followed by fast reductive elimination. In contrast, nickel(II)-alkyl captures radicals to form a nickel(III) species, which was characterized by EPR spectroscopy. However, the subsequent slow reductive elimination resulted in minimal product formation. The observed high diastereoselectivity of radical capture inspired investigations into C-aryl and C-acyl glycosylation reactions. We developed a redox auxiliary that readily couples with natural carbohydrates and produces glycosyl radicals upon photoredox activation. Nickel-catalyzed cross-coupling of the glycosyl radical with bromoarenes and carboxylic acids leads to diverse non-natural glycosides that can facilitate drug discovery.Stoichiometric studies on well-defined d8-nickel complexes have showcased means to promote reductive elimination, including ligand association, oxidation, and oxidative addition.In the final section, we address the influence of auxiliary ligands on the electronic structure and redox activity of organonickel intermediates. Synthesis of a series of low-valent nickel radical complexes and characterization of their electronic structures led us to a postulate that ligand redox activity correlates with coordination geometry. Our data reveal that a change in ligand redox activity can shift the redox potentials of reaction intermediates, potentially altering the mechanism of catalytic reactions. Moreover, coordinating additives and solvents may stabilize nickel radicals during catalysis by adjusting ligand redox activity, which is consistent with known catalytic conditions.

Copper-Catalyzed Benzylic C-H Cross-Coupling Enabled by Redox Buffers: Expanding Synthetic Access to Three-Dimensional Chemical Space

ConspectusCross-coupling methods are the most widely used synthetic methods in medicinal chemistry. Existing reactions are dominated by methods such as amide coupling and arylation reactions that form bonds to sp2-hybridized carbon atoms and contribute to the formation of "flat" molecules. Evidence that three-dimensional structures often have improved physicochemical properties for pharmaceutical applications has contributed to growing demand for cross-coupling methods with sp3-hybridized reaction partners. Substituents attached to sp3 carbon atoms are intrinsically displayed in three dimensions. These considerations have led to efforts to establish reactions with sp3 cross-coupling partners, including alkyl halides, amines, alcohols, and carboxylic acids. As C(sp3)-H bonds are much more abundant that these more conventional coupling partners, we have been pursuing C(sp3)-H cross-coupling reactions that achieve site-selectivity, synthetic utility, and scope competitive with conventional coupling reactions.In this Account, we outline Cu-catalyzed oxidative cross-coupling reactions of benzylic C(sp3)-H bonds with diverse nucleophilic partners. These reactions commonly use N-fluorobenzenesulfonimide (NFSI) as the oxidant. The scope of reactivity is greatly improved by using a "redox buffer" that ensures that the Cu catalyst is available in the proper redox state to promote the reaction. Early precedents of catalytic Cu/NFSI oxidative coupling reactions, including C-H cyanation and arylation, did not require a redox buffer, but reactions with other nucleophiles, such as alcohols and azoles, were much less effective under similar conditions. Mechanistic studies show that some nucleophiles, such as cyanide and arylboronic acids, promote in situ reduction of CuII to CuI, contributing to successful catalytic turnover. Poor reactivity was observed with nucleophiles, such as alcohols, that do not promote CuII reduction in the same manner. This insight led to the identification of sacrificial reductants, termed "redox buffers", that support controlled generation of CuI during the reactions and enable successful benzylic C(sp3)-H cross-coupling with diverse nucleophiles. Successful reactions include those that feature direct coupling of (hetero)benzylic C-H substrates with coupling partners (alcohols, azoles) and sequential C(sp3)-H functionalization/coupling reactions. The latter methods feature generation of a synthetic linchpin that can undergo subsequent reaction with a broad array of nucleophiles. For example, halogenation/substitution cascades afford benzylic amines, (thio)ethers, and heterodiarylmethane derivatives, and an isocyanation/amine-addition sequence generates diverse benzylic ureas.Collectively, these Cu-catalyzed (hetero)benzylic C(sp3)-H cross-coupling reactions rapidly access diverse molecules. Analysis of their physicochemical and topological properties highlights the "drug-likeness" and enhanced three-dimensionality of these products relative to existing bioactive molecules. This consideration, together with the high benzylic C-H site-selectivity and the broad scope of reactivity enabled by the redox buffering strategy, makes these C(sp3)-H cross-coupling methods ideally suited for implementation in high-throughput experimentation platforms to explore novel chemical space for drug discovery and related applications.

Pyrimidine-morpholine hybrids as potent druggable therapeutics for Alzheimer's disease: Synthesis, biochemical and in silico analyses

The identification of effective and druggable cholinesterase inhibitors to treat progressive neurodegenerative Alzheimer's disorder remains a continuous drug discovery hunt. In this perspective, the present study investigates the design and discovery of pyrimidine-morpholine hybrids (5a-l) as potent cholinesterase inhibitors. Palladium-catalyzed Suzuki-Miyaura cross-coupling reaction was employed to introduce the structural diversity on the pyrimidine heterocyclic core. A range of commercially available boronic acids was successfully coupled showing a high functional group tolerance. In vitro cholinesterase inhibitory potential using Ellman's method revealed significantly strong potency. Compound 5h bearing a meta-tolyl substituent at 2-position of pyrimidine ring emerged as a lead candidate against AChE with an inhibitory potency of 0.43 ± 0.42 µM, ∼38-fold stronger value than neostigmine (IC50 = 16.3 ± 1.12 µM). Compound 5h also showed the lead inhibition against BuChE with an IC50 value of 2.5 ± 0.04 µM. The kinetics analysis of 5h revealed the non-competitive mode of inhibition against AChE whereas computational modelling results of potent leads depicted diverse contacts with the binding site amino acid residues. Molecular dynamics simulations revealed the stability of biomolecular system, while, ADME analysis demonstrated druglikeness behaviour of potent compounds. Overall, the investigated pyrimidine-morpholine scaffold presented a remarkable potential to be developed as efficacious anti-Alzheimer's drugs.

Asymmetric Radical Oxyboration of β-Substituted Styrenes via Late-Stage Stereomutation

Herein, we report an unprecedented copper-catalyzed highly enantio- and diastereoselective radical oxyboration of β-substituted styrenes. The lynchpin of success is ascribed to the development of a late-stage stereomutation strategy, which enables enantioenriched cis-isomers among a couple of early-generated diastereomers to be converted into trans-isomer counterparts, thus fulfilling high diastereocontrol; while the degree of enantio-differentiation is determined by the borocupration process of the C=C bond. This reaction provides an efficient protocol to access enantioenriched trans-1,2- dioxygenation products. The value of this method has further been highlighted in a diversity of follow-up stereospecific transformations and further modifying complex molecules.