Stereospecific cross-coupling of alpha-(thiocarbamoyl)organostannanes with alkenyl, aryl, and heteroaryl iodides

Photoredox Nickel-Catalysed Stille Cross-Coupling Reactions

The Stille cross-coupling reaction is one of the most common strategies for the construction of C-C bonds. Despite notable strides in the advancement of the Stille reaction, persistent challenges persist in hindering its greener evolution. These challenges encompass multiple facets, such as the high cost of precious metals and ligands, the demand for various additives, and the slow reaction rate. In comparison to the dominant palladium-catalysed Stille reactions, cost-effective nickel-catalysed systems lag behind, and enantioconvergent Stille reactions of racemic stannanes remain undeveloped. Herein, we present a pioneering instance of nickel-catalysed enantioconvergent Stille cross-coupling reactions of racemic stannane reagents, resulting in the formation of C-C bonds in good to high yields with excellent stereoselectivity. This strategy provides a practical, scalable, and operationally straightforward method for the synthesis of C(sp3 )-C(sp3 ), C(sp3 )-C(sp2 ), and C(sp3 )-C(sp) bonds under exceptionally mild conditions (without additives and bases, ambient temperature). The innovative use of synergistic photoredox/nickel catalysis enables a novel single-electron transmetalation process of stannane reagents, providing a new research paradigm of Stille reactions.

Transition Metal Free Stannylation of Alkyl Halides: The Rapid Synthesis of Alkyltrimethylstannanes

A transition metal free stannylation reaction of alkyl bromides and iodides with hexamethyldistannane has been developed. This protocol is operationally convenient and features a rapid reaction and good functional group tolerance. A wide range of functionalized primary and secondary alkyl and benzyl trimethyl stannanes are prepared in moderate to excellent yields. The success of the gram-scale procedure and tandem Stille coupling reaction has allowed this protocol to demonstrate potential for application in organic synthesis. Both experimental and theoretical studies reveal the mechanistic details of this stannylation reaction.

Stereoselectivity in Pd-catalysed cross-coupling reactions of enantioenriched nucleophiles

Advances in Pd-catalysed cross-coupling reactions have facilitated the development of stereospecific variants enabling the use of configurationally stable, enantioenriched, main-group organometallic nucleophiles to form C(sp ³)-C(sp ²) bonds. Such stereospecific cross-coupling reactions constitute a powerful synthetic approach to attaining precise 3D control of molecular structure, allowing new stereogenic centres to be readily introduced into molecular architectures. Examples of stereospecific, Pd-catalysed cross-coupling reactions have been reported for isolable enantioenriched alkylboron, alkyltin, alkylgermanium and alkylsilicon nucleophiles. In these reactions, a single, dominant stereospecific pathway of transmetallation to palladium is required to effect efficient chirality transfer to the cross-coupled product. However, the potential for competing stereoretentive and stereoinvertive pathways of transmetallation complicates the stereochemical transfer in these processes and impedes the rational development of new stereospecific cross-coupling variants. In this Review, we describe the use of enantioenriched organometallic nucleophiles in stereospecific, Pd-catalysed cross-coupling reactions. We focus on systems involving well-defined, isolable, enantioenriched nucleophiles in which a clear stereochemical course of transmetallation is followed. Specific modes of electronic activation that influence the reactivity of alkylmetal nucleophiles are described and presented in the context of their impact on the stereochemical course of cross-coupling reactions. We expect that this Review will serve as a valuable resource to assist in deconvoluting the many considerations that potentially impact the stereochemical outcome of Pd-catalysed cross-coupling reactions.

Ligand-Free Copper(I)-Mediated Cross-Coupling Reactions of Organostannanes with Sulfur Electrophiles

The synthesis of aryl thioether through the cross-coupling of C-S bond is a highly attractive area of research due to the prevalence of aryl thioether in bioactive natural products, functional materials, agrochemicals, and pharmaceutically active compounds. Herein, we report a ligand-free Cu(I) mediated electrophilic thiolation of organostannanes with sulfur electrophiles. A selective transfer of alkyl groups was achieved in reactions with alkyl carbastannatranes affording congested thioethers. This study offers a unified method to access diaryl and aryl alkyl thioethers and was demonstrated in the context of late-stage modifications..

A General Approach to Stereospecific Cross-Coupling Reactions of Nitrogen-Containing Stereocenters

A novel strategy employing cyclohexyl spectator ligands in Stille cross-coupling reactions has been developed as a general solution to the long-standing challenge of conducting stereospecific cross-coupling reactions at nitrogen-containing stereocenters. This method enables direct access to enantioenriched products that are difficult (or impossible) to obtain via alternative preparative methods. Selective and predictable transfer of a single secondary alkyl unit can be achieved under reaction conditions that exploit subtle electronic differences between activated and unactivated alkyl units. Through this approach, enantioenriched α-stannylated nitrogen-containing stereocenters undergo Pd-catalyzed arylation and acylation reactions with exceptionally high stereofidelity in all instances investigated. We demonstrate this process by using α-stannylated pyrrolidine, azetidine, and open-chain (benzylic and non-benzylic) nucleophiles in stereospecific reactions. This process will facilitate rapid and reliable access to enantioenriched compounds possessing nitrogen-substituted stereocenters, which constitute ubiquitous structural motifs in biologically active compounds emerging from the drug-discovery process.

Iodine–DMSO-promoted divergent reactivities of arylacetylenes

An unprecedented set of efficient, economical, atom-economic and exceedingly selective I₂–DMSO-promoted methods is described for the generation of different structures.

Rethinking Carbohydrate Synthesis: Stereoretentive Reactions of Anomeric Stannanes

In this Concept article, recent advances are highlighted in the synthesis and applications of anomeric nucleophiles, a class of carbohydrates in which the C1 carbon bears a carbon-metal bond. First, the advantages of exploiting the carboanionic reactivity of carbohydrates and the methods for the synthesis of mono- and oligosaccharide stannanes are discussed. Second, recent developments in the glycosyl cross-coupling method resulting in the transfer of anomeric configuration from C1 stannanes to C-aryl glycosides are reviewed. These highly stereoretentive processes are ideally suited for the preparation of carbohydrate-based therapeutics and were demonstrated in the synthesis of antidiabetic drugs. Next, the application of the glycosyl cross-coupling method to the preparation of Se-glycosides and to glycodiversification of small molecules and peptides are highlighted. These reactions proceed with exclusive anomeric control for a broad range of substrates and tolerate carbohydrates with free hydroxyl groups. Taken together, anomeric nucleophiles have emerged as powerful tools for the synthesis of oligosaccharides and glycoconjugates and their future applications will open new possibilities to incorporate saccharides into small molecules and biologics.

Stereospecific Stille Cross-Couplings Using Mn(II)Cl2

Cross-coupling reactions are a staple in organic synthesis, especially for C-C bond formation with sp- and sp²-carbon electrophiles. In recent years, the range of accessible C-C bonds has been extended to stereogenic centers which expedites access to greater molecular complexity. However, these reactions predominantly depend upon late transition metal (LTM) catalysts whose cost, toxicity, and/or environmental impact have come under increasing scrutiny and governmental regulation. Here, we report Mn(II)Cl₂ complexes alone, or with assistance from copper, catalyze the stereospecific cross-coupling of α-alkoxyalkylstannanes with organic electrophiles with complete retention of configuration.

Glycosyl Cross-Coupling of Anomeric Nucleophiles: Scope, Mechanism, and Applications in the Synthesis of Aryl C-Glycosides

Stereoselective manipulations at the C1 anomeric position of saccharides are one of the central goals of preparative carbohydrate chemistry. Historically, the majority of reactions forming a bond with anomeric carbon has focused on reactions of nucleophiles with saccharide donors equipped with a leaving group. Here, we describe a novel approach to stereoselective synthesis of C-aryl glycosides capitalizing on the highly stereospecific reaction of anomeric nucleophiles. First, methods for the preparation of anomeric stannanes have been developed and optimized to afford both anomers of common saccharides in high anomeric selectivities. We established that oligosaccharide stannanes could be prepared from monosaccharide stannanes via O-glycosylation with Schmidt-type donors, glycal epoxides, or under dehydrative conditions with C1 alcohols. Second, we identified a general set of catalytic conditions with Pd₂(dba)₃ (2.5 mol%) and a bulky ligand (JackiePhos, 10 mol%) controlling the β-elimination pathway. We demonstrated that the glycosyl cross-coupling resulted in consistently high anomeric selectivities for both anomers with mono- and oligosaccharides, deoxysugars, saccharides with free hydroxyl groups, pyranose, and furanose substrates. The versatility of the glycosyl cross-coupling reaction was probed in the total synthesis of salmochelins (siderophores) and commercial anti-diabetic drugs (gliflozins). Combined experimental and computational studies revealed that the β-elimination pathway is suppressed for biphenyl-type ligands due to the shielding of Pd(II) by sterically demanding JackiePhos, whereas smaller ligands, which allow for the formation of a Pd-F complex, predominantly result in a glycal product. Similar steric effects account for the diminished rates of cross-couplings of 1,2-cis C1-stannanes with aryl halides. DFT calculations also revealed that the transmetalation occurs via a cyclic transition state with retention of configuration at the anomeric position. Taken together, facile access to both anomers of various glycoside nucleophiles, a broad reaction scope, and uniformly high transfer of anomeric configuration make the glycosyl cross-coupling reaction a practical tool for the synthesis of bioactive natural products, drug candidates, allowing for late-stage glycodiversification studies with small molecules and biologics.

Stereospecific Palladium-Catalyzed Acylation of Enantioenriched Alkylcarbastannatranes: A General Alternative to Asymmetric Enolate Reactions

We report the development of a Pd-catalyzed process for the cross coupling of unactivated primary, secondary, and tertiary alkylcarbastannatrane nucleophiles with acyl electrophiles. Reactions involving optically active alkylcarbastannatranes occur with exceptional stereofidelity and with net retention of absolute configuration. Because the stereochemistry of the resulting products is entirely reagent-controlled, this process may be viewed as a general, alternative approach to the preparation of products typically accessed via asymmetric enolate methodologies. Additionally, we report a new method for the preparation of optically active alkylcarbastannatranes, which should facilitate their future use in stereospecific reactions.

Iodine(iii)-induced regioselective carbocyclization of terminal alkynes: a facile approach to prepare 1,1-diiodomethylene substituted cyclic compounds

An iodine(iii)-induced regioselective carbocyclization of functionally substituted terminal alkynes to afford 1,1-diiodomethylene substituted cyclic compounds is reported.

Configurationally Stable, Enantioenriched Organometallic Nucleophiles in Stereospecific Pd-Catalyzed Cross-Coupling Reactions: An Alternative Approach to Asymmetric Synthesis

Several research groups have recently developed methods to employ configurationally stable, enantioenriched organometallic nucleophiles in stereospecific Pd-catalyzed cross-coupling reactions. By establishing the absolute configuration of a chiral alkyltin or alkylboron nucleophile prior to its use in cross-coupling reactions, new stereogenic centers may be rapidly and reliably generated with preservation of the known initial stereochemistry. While this area of research is still in its infancy, such stereospecific cross-coupling reactions may emerge as simple, general methods to access diverse, optically active products from common enantioenriched organometallic building blocks. This minireview highlights recent progress towards the development of general, stereospecific Pd-catalyzed cross-coupling reactions using configurationally stable organometallic nucleophiles.

SmI2-mediated dimerization of indolylbutenones and synthesis of the myxobacterial natural product indiacen B

The synthesis and reactivity of indole derivatives substituted in the benzene section was studied. Starting materials 4- and 6-iodoindole were conveniently prepared via the Batcho–Leimgruber route and purified by sublimation. Novel vicinally indolyl-substituted cyclopentanols with unexpected cis-configuration were formed by SmI₂-mediated reductive dimerization of a 4-(indol-6-yl)butenone, obtained by Heck reaction. The two indolyl units appear to chelate Sm(II)/(III) leading to a gauche-type arrangement at the newly formed bond between the two β-carbons. Through a sequence of Sonogashira cross coupling and Meyer–Schuster rearrangement 6-prenoylindole was synthesized and reductively dimerized to a cyclopentane in a [3 + 2] cycloaddition by treatment with SmI₂ in THF. From 4-iodoindole, the natural product indiacen B from the myxobacterium Sandaracinus amylolyticus was synthesized for the first time, confirming its antimicrobial activity. The E-configuration of the chloroalkene moiety of indiacen B was confirmed by X-ray analysis.

Catalytic Synthesis of N-Unprotected Piperazines, Morpholines, and Thiomorpholines from Aldehydes and SnAP Reagents

Commercially available SnAP (stannyl amine protocol) reagents allow the transformation of aldehydes and ketones into a variety of N-unprotected heterocycles. By identifying new ligands and reaction conditions, a robust catalytic variant that expands the substrate scope to previously inaccessible heteroaromatic substrates and new substitution patterns was realized. It also establishes the basis for a catalytic enantioselective process through the use of chiral ligands.

Photoredox Cross-Coupling: Ir/Ni Dual Catalysis for the Synthesis of Benzylic Ethers

Single-electron transmetalation has emerged as an enabling paradigm for the cross-coupling of Csp(3) hybridized organotrifluoroborates. Cross-coupling of α-alkoxymethyltrifluoroborates with aryl and heteroaryl bromides has been demonstrated by employing dual catalysis with a combination of an iridium photoredox catalyst and a Ni cross-coupling catalyst. The resulting method enables the alkoxymethylation of diverse (hetero)arenes under mild, room-temperature conditions.

Copper-catalyzed asymmetric three-component borylstannation: enantioselective formation of C-Sn bond

In summary, a first copper-catalyzed synthesis of α-aryl-β-borylstannane compounds was accomplished through three-component borylstannation of aryl-substituted alkenes. In the exploration of an asymmetric variant, chiral sulfinylphosphine ligands proved advantageous in controlling stereochemistry of B-Cu addition and in promoting transmetalation of enantioenriched alkyl-Cu species. The stereochemical outcome supported a sequential syn-borylcupration and configuration-retentive transmetalation mechanism. Moreover, α-chiral β-borylstannanes were easily transformed into a diverse array of secondary alkylstannanes and triarylethane with high enantiomeric purity. The applications of chiral sulfinylphosphine ligands to other tandem Cu-B addition reactions are currently under investigation in our group.

Carbastannatranes: a powerful coupling mediators in Stille coupling

Carbastannatranes are better coupling mediators in Stille coupling due to easy transfer of axial Sn-bound hydrocarbyl substituent during transmetallation step.

Stereospecific pd-catalyzed cross-coupling reactions of secondary alkylboron nucleophiles and aryl chlorides

We report the development of a Pd-catalyzed process for the stereospecific cross-coupling of unactivated secondary alkylboron nucleophiles and aryl chlorides. This process tolerates the use of secondary alkylboronic acids and secondary alkyltrifluoroborates and occurs without significant isomerization of the alkyl nucelophile. Optically active secondary alkyltrifluoroborate reagents undergo cross-coupling reactions with stereospecific inversion of configuration using this method.

Enantiospecific sp(2)-sp(3) coupling of secondary and tertiary boronic esters

The cross-coupling of boronic acids and related derivatives with sp(2) electrophiles (the Suzuki-Miyaura reaction) is one of the most powerful C-C bond formation reactions in synthesis, with applications that span pharmaceuticals, agrochemicals and high-tech materials. Despite the breadth of its utility, the scope of this Nobel prize-winning reaction is rather limited when applied to aliphatic boronic esters. Primary organoboron reagents work well, but secondary and tertiary boronic esters do not (apart from a few specific and isolated examples). Through an alternative strategy, which does not involve using transition metals, we have discovered that enantioenriched secondary and tertiary boronic esters can be coupled to electron-rich aromatics with essentially complete enantiospecificity. As the enantioenriched boronic esters are easily accessible, this reaction should find considerable application, particularly in the pharmaceutical industry where there is growing awareness of the importance of, and greater clinical success in, creating biomolecules with three-dimensional architectures.

Stereoretentive Pd-catalysed Stille cross-coupling reactions of secondary alkyl azastannatranes and aryl halides

The development of transition metal-catalysed cross-coupling reactions has greatly influenced the manner in which the synthesis of complex organic molecules is approached. A wide variety of methods are now available for the formation of C(sp(2))-C(sp(2)) bonds, and more recent work has focused on the use of C(sp(3)) electrophiles and nucleophiles. The use of secondary and tertiary alkyl nucleophiles in cross-coupling reactions remains a challenge because of the propensity of these alkyl groups to isomerize under the reaction conditions. Here, we report the development of a general Pd-catalysed process for the stereoretentive cross-coupling of secondary alkyl azastannatrane nucleophiles with aryl chlorides, bromides, iodides and triflates. Coupling partners with a wide range of electronic characteristics are well tolerated. The reaction occurs with minimal isomerization of the secondary alkyltin nucleophile, and with retention of absolute configuration. This process constitutes the first general method to use secondary alkyltin reagents in cross-coupling reactions.

Inversion or retention? Effects of acidic additives on the stereochemical course in enantiospecific Suzuki-Miyaura coupling of α-(acetylamino)benzylboronic esters

The stereochemical course of the stereospecific Suzuki-Miyaura coupling of enantioenriched α-(acetylamino)benzylboronic esters with aryl bromides can be switched by the choice of acidic additives in the presence of a Pd/XPhos catalyst system. Highly enantiospecific, invertive C-C bond formation takes place with the use of phenol as an additive. In contrast, high enantiospecificity for retention of configuration is attained in the presence of Zr(Oi-Pr)(4)·i-PrOH as an additive.

Stereocontrolled synthesis of α-amino-α'-alkoxy ketones by a copper-catalyzed cross-coupling of peptidic thiol esters and α-alkoxyalkylstannanes

A stereocontrolled synthesis of α-amino-α'-alkoxy ketones is described. This pH-neutral copper(I) thiophene-2-carboxylate (CuTC)-catalyzed cross-coupling of amino acid thiol esters and chiral nonracemic α-alkoxyalkylstannanes gives α-amino-α'-alkoxy ketones in good to excellent yields with complete retention of configuration at the α-amino- and α-alkoxy-substituted stereocenters.

Mobilizing Cu(I) for carbon-carbon bond forming catalysis in the presence of thiolate. Chemical mimicking of metallothioneins

Copper(I) is rendered catalytically viable in the presence of thiolate by the design of a small molecule chemical analogue of the metallothionein system in which an N-O reactant serves the same conceptual purpose of the S-S reactant of the biological system.

Pd-catalyzed cross-coupling of α-(acyloxy)-tri-n-butylstannanes with alkenyl, aryl, and heteroaryl electrophiles

Racemic and scalemic α-(acyloxy)-tri-n-butylstannanes undergo Pd-catalyzed cross-couplings with alkenyl/aryl/heteroaryl iodides, bromides, and triflates in moderate to good yields in THF at 45 °C. Simple aryl iodides and unprotected aza-arenes, two classes of electrophiles that typically react sluggishly, are also good substrates. Cross-couplings proceed with retention of configuration at the alkenyl and stannyl-substituted stereocenters.

Stereospecific Suzuki cross-coupling of alkyl alpha-cyanohydrin triflates

Scalemic alpha-cyanohydrin triflates undergo Pd-catalyzed cross-coupling with aryl, heteroaryl, and vinyl boronic acids under mild conditions. Coupling proceeds with complete inversion of configuration at the stereogenic carbon. The resultant nitrile can be easily converted into a variety of alternative functional groups of value in organic synthesis and thus achieves a higher level of molecular complexity than the products of traditional Suzuki reactions.

The smallest and one of the brightest. Efficient preparation and optical description of the parent borondipyrromethene system

Fully unsubstituted 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) 1 was prepared. Reaction of 8-thiomethylbodipy 2 with triethylsilane in the presence of a catalytic amount of Pd and a stoichiometric amount of copper(I) thienyl-2-carboxylate (CuTC) in THF at 55 degrees C gave compound 1 in nearly quantitative yield. This compound displays high quantum yields (up to 93%) in polar solvents including water. Its optical properties and crystal structure are discussed.

Potassium trifluoroborate salts as convenient, stable reagents for difficult alkyl transfers

The past decade has witnessed the emergence of potassium organotrifluoroborates as a new class of nucleophilic boron reagents for Suzuki-Miyaura cross-coupling and Rh-catalyzed addition reactions. Potassium organotrifluoroborates are easily prepared, and most are indefinitely stable to air and moisture. Recent advances have focused on the utility of alkyltrifluoroborates in appending alkyl groups selectively and conveniently onto appropriate molecular substructures. This review describes strategies employing these reagents as nucleophilic substrates.

Organotrifluoroborates and monocoordinated palladium complexes as catalysts--a perfect combination for Suzuki-Miyaura coupling

Monocoordinated palladium catalysts derived from sterically hindered, electron-rich phosphines or N-heterocyclic carbenes have revolutionized the Suzuki-Miyaura coupling reaction. The emergence of organotrifluoroborates has provided important new perspectives for the organoboron component of these reactions. In combination, these two components prove to be extraordinarily powerful partners for cross-coupling reactions.