Ti-Catalyzed Radical Alkylation of Secondary and Tertiary Alkyl Chlorides Using Michael Acceptors

Electrochemical Selenized Reaction of N-Arylbicyclo[1.1.0]butane-1-carboxamides: Access to 3-(Arylselanyl)spiro[cyclobutane-1,3'-indolin]-2'-one Derivatives

A novel selenized reaction of N-arylbicyclo [1.1.0]butane-1-carboxamides with diselenide for the synthesis of polycyclic indoline derivatives is developed under electrochemical conditions. The synthesis is achieved by the bicyclo[1.1.0]butane strain-release reaction and intramolecular cyclization process. In addition, this approach features a wide range of substrates, good group tolerance, shorter reaction time, and mild conditions.

Construction of quaternary alkyl motifs through palladium-catalyzed oxidative coupling of 1,3-dicarbonyl compounds with alkenes followed by C-C bond cleavage

A palladium-catalyzed coupling reaction has been developed for the generation of tertiary alkylation products by reacting olefins with diversely functionalized 1,3-dicarbonyls. The reaction involves the tertiary C-H alkylation of 1,3-dicarbonyls with olefins to produce a tertiary alcohol, followed by C-C bond cleavage.

Strategies and Mechanisms of First-Row Transition Metal-Regulated Radical C-H Functionalization

Radical C-H functionalization represents a useful means of streamlining synthetic routes by avoiding substrate preactivation and allowing access to target molecules in fewer steps. The first-row transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are Earth-abundant and can be employed to regulate radical C-H functionalization. The use of such metals is desirable because of the diverse interaction modes between first-row transition metal complexes and radical species including radical addition to the metal center, radical addition to the ligand of metal complexes, radical substitution of the metal complexes, single-electron transfer between radicals and metal complexes, hydrogen atom transfer between radicals and metal complexes, and noncovalent interaction between the radicals and metal complexes. Such interactions could improve the reactivity, diversity, and selectivity of radical transformations to allow for more challenging radical C-H functionalization reactions. This review examines the achievements in this promising area over the past decade, with a focus on the state-of-the-art while also discussing existing limitations and the enormous potential of high-value radical C-H functionalization regulated by these metals. The aim is to provide the reader with a detailed account of the strategies and mechanisms associated with such functionalization.

Titanium in photocatalytic organic transformations: current applications and future developments

Titanium, as an important transition metal, has garnered extensive attention in both industry and academia due to its excellent mechanical properties, corrosion resistance, and unique reactivity in organic synthesis. In the field of organic photocatalysis, titanium-based compounds such as titanium dioxide (TiO2), titanocenes (Cp2TiCl2, CpTiCl3), titanium tetrachloride (TiCl4), tetrakis(isopropoxy)titanium (Ti(OiPr)4), and chiral titanium complexes have demonstrated distinct reactivity and selectivity. This review focuses on the roles of these titanium compounds in photocatalytic organic reactions, and highlights the reaction pathways such as photo-induced single-electron transfer (SET) and ligand-to-metal charge transfer (LMCT). By systematically surveying the latest advancements in titanium-involved organic photocatalysis, this review aims to provide references for further research and technological innovation within this fast-developing field.

Oxidative Difunctionalization of N-Aryl Bicyclobutyl Amides with Aldehydes: Divergent Synthesis of Acylated and Alkylated 3-Spirocyclobutyl Oxindoles

An efficient and operationally simple oxidative radical difunctionalization of N-aryl bicyclobutyl (BCB) amides with aldehydes is described. It was found that acylated 3-spirocyclobutyl oxindoles were generated from the coupling of BCB-amides and aromatic aldehydes, while reactions gave exclusively decarbonylative alkylarylation products using alkyl aldehydes as radical precursors.

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.

Kinetics of sulfur-transfer from titanocene (poly)sulfides to sulfenyl chlorides: rapid metal-assisted concerted substitution

The kinetics of sulfur transfer from titanocene (poly)sulfides (RCp2TiS5, Cp2TiS4CMe2, Cp2Ti(SAr)2, Cp2TiCl(SAr)) to sulfenyl chlorides (S2Cl2, RSCl) have been investigated by a combination of stopped-flow UV-Vis/NMR reaction monitoring, titration assays, numerical kinetic modelling and KS-DFT calculations. The reactions are rapid, proceeding to completion over timescales of milliseconds to minutes, via a sequence of two S-S bond-forming steps (k 1, k 2). The archetypical polysulfides Cp2TiS5 (1a) and Cp2TiS4C(Me2) (2a) react with disulfur dichloride (S2Cl2) through rate-limiting intermolecular S-S bond formation (k 1) followed by a rapid intramolecular cyclization (k 2, with k 2 ≫ k 1 [RSCl]). The monofunctional sulfenyl chlorides (RSCl) studied herein react in two intermolecular S-S bond forming steps proceeding at similar rates (k 1 ≈ k 2). Reactions of titanocene bisthiophenolates, Cp2Ti(SAr)2 (5), with both mono- and di-functional sulfenyl chlorides result in rapid accumulation of the monothiophenolate, Cp2TiCl(SAr) (6) (k 1 > k 2). Across the range of reactants studied, the rates are relatively insensitive to changes in temperature and in the electronics of the sulfenyl chloride, moderately sensitive to the electronics of the titanocene (poly)sulfide (ρ (Ti-(SAr)) ≈ -2.0), and highly sensitive to the solvent polarity, with non-polar solvents (CS2, CCl4) leading to the slowest rates. The combined sensitivities are the result of a concerted, polarized and late transition state for the rate-limiting S-S bond forming step, accompanied by a large entropic penalty. Each substitution step {[Ti]-SR' + Cl-SR → [Ti]-Cl + RS-SR'} proceeds via titanium-assisted Cl-S cleavage to generate a transient pentacoordinate complex, Cl-[Cp2TiX]-S(R')-SR, which then undergoes rapid Ti-S dissociation.

Catalytic tert-alkylation of enamides via C-C bond cleavage under photoredox conditions

Efficient C-C bond cleavage is recognized as a persistent challenge in the field of synthetic methodology. In this study, we found that tertiary alkyl radicals are smoothly formed from tertiary alkylated dienones (BHT adducts) via SET, using PDI as a photocatalyst. Resulting tert-alkyl radicals could be applied to the tert-alkylation of enamides. The driving force of this C-C bond cleavage reaction is the mesolytic cleavage of the BHT adducts. The mechanistic study revealed that PDI anion radical is the key active species during the catalytic cycle.

Overcoming a Radical Polarity Mismatch in Strain-Release Pentafluorosulfanylation of [1.1.0]Bicyclobutanes: An Entryway to Sulfone- and Carbonyl-Containing SF5-Cyclobutanes

The first assortment of achiral pentafluorosulfanylated cyclobutanes (SF5-CBs) are now synthetically accessible through strain-release functionalization of [1.1.0]bicyclobutanes (BCBs) using SF5Cl. Methods for both chloropentafluorosulfanylation and hydropentafluorosulfanylation of sulfone-based BCBs are detailed herein, as well as proof-of-concept that the logic extends to tetrafluoro(aryl)sulfanylation, tetrafluoro(trifluoromethyl)sulfanylation, and three-component pentafluorosulfanylation reactions. The methods presented enable isolation of both syn and anti isomers of SF5-CBs, but we also demonstrate that this innate selectivity can be overridden in chloropentafluorosulfanylation; that is, an anti-stereoselective variant of SF5Cl addition across sulfone-based BCBs can be achieved by using inexpensive copper salt additives. Considering the SF5 group and CBs have been employed individually as nonclassical bioisosteres, structural aspects of these unique SF5-CB "hybrid isosteres" were then contextualized using SC-XRD. From a mechanistic standpoint, chloropentafluorosulfanylation ostensibly proceeds through a curious polarity mismatch addition of electrophilic SF5 radicals to the electrophilic sites of the BCBs. Upon examining carbonyl-containing BCBs, we also observed rare instances whereby radical addition to the 1-position of a BCB occurs. The nature of the key C(sp3)-SF5 bond formation step - among other mechanistic features of the methods we disclose - was investigated experimentally and with DFT calculations. Lastly, we demonstrate compatibility of SF5-CBs with various downstream functionalizations.

Umpolung reactivity of strained C-C σ-bonds without transition-metal catalysis

Umpolung is an old and important concept in organic chemistry, which significantly expands the chemical space and provides unique structures. While, previous research focused on carbonyls or imine derivatives, the umpolung reactivity of polarized C-C σ-bonds still needs to explore. Herein, we report an umpolung reaction of bicyclo[1.1.0]butanes (BCBs) with electron-deficient alkenes to construct the C(sp3)-C(sp3) bond at the electrophilic position of C-C σ-bonds in BCBs without any transition-metal catalysis. Specifically, this transformation relies on the strain-release driven bridging σ-bonds in bicyclo[1.1.0]butanes (BCBs), which are emerged as ene components, providing an efficient and straightforward synthesis route of various functionalized cyclobutenes and conjugated dienes, respectively. The synthetic utilities of this protocol are performed by several transformations. Preliminary mechanistic studies including density functional theory (DFT) calculation support the concerted Alder-ene type process of C-C σ-bond cleavage with hydrogen transfer. This work extends the umpolung reaction to C-C σ-bonds and provides high-value structural motifs.

Directing-Group-Assisted Non-Strained Ether C-O Bond Homolysis Mediated by Low-Valent Titanium

Ether C-O bonds are typical constituents of organic molecules that are seldom regarded as reactive functional groups except when highly strained. With the assistance of appropriate directing groups, low-valent titanium was found to homolytically cleave non-strained C-O bonds. In particular, a newly designed catechol monoether directing group rendered a route toward the activation of non-benzylic C(sp3)-O bonds. This method has been applied to conventional radical addition reactions to alkenes.

Photoredox-Catalyzed Alkylarylation of N-Aryl Bicyclobutyl Amides with α-Carbonyl Alkyl Bromides: Access to 3-Spirocyclobutyl Oxindoles

A visible-light-induced radical alkylarylation of N-aryl bicyclobutyl amides with α-carbonyl alkyl bromides for the synthesis of functionalized 3-spirocyclobutyl oxindoles is described in which β-selective radical addition of the alkyl radical to N-aryl bicyclobutyl amides forms a key radical intermediate followed by interception with intrinsic arene functional group. This approach can be applicable to a wide range of α-carbonyl alkyl bromides, including primary, secondary, and tertiary α-bromoalkyl esters, ketones, nitriles, and nitro compounds.

Strain-enabled radical spirocyclization cascades: rapid access to spirocyclobutyl lactones and - lactams

Spirocyclobutane derivatives have gained significant attention in drug discovery programs due to their broad spectrum of biological activities and clinical applications. Ring-strain in organic molecules is a powerful tool to promote reactivity by releasing strain energy, allowing the construction of complex molecules selectively and efficiently. Herein, we report the first strain-enabled radical spirocyclization cascades for the synthesis of functionalized spirocyclobutyl lactones and - lactams, which are finding increasing applications in medicinal chemistry. The reaction of interelement compounds with bicyclobutane (BCB) allyl esters and - amides proceeds with high chemoselectivity under simple, catalyst-free conditions using blue light irradiation. The reaction has been successfully extended to synthesize bis-spirocycles. To introduce a more diverse set of functional groups, we have developed a dual photoredox/nickel catalytic system capable of mediating the carbosulfonylation of BCB allyl amides. The reaction shows broad applicability across various (hetero)aryl halides, aryl sulfinates, and BCB allyl amides, operates under mild conditions and demonstrates excellent functional group compatibility. The functional groups introduced during the cascade reactions served as versatile handles for further synthetic elaboration.

Cobalt-Catalyzed Electroreductive Alkylation of Unactivated Alkyl Chlorides with Conjugated Olefins

Reactions of unactivated alkyl chlorides under mild and sustainable conditions are rare compared to those of alkyl bromides or iodides. As a result, synthetic methods capable of modifying the vast chemical space of chloroalkane reagents, wastes, and materials are limited. We report the cobalt-catalyzed reductive addition of unactivated alkyl chlorides to conjugated alkenes. Co-catalyzed activation of alkyl chlorides is performed under electroreductive conditions, and the resulting reactions constitute formal alkyl-alkyl bond formation. In addition to developing an operationally simple methodology, detailed mechanistic studies provide insights into the elementary steps of a proposed catalytic cycle. In particular, we propose a switch in the mechanism of C-Cl bond activation from nucleophilic substitution to halogen atom abstraction, which is critical for efficiently generating alkyl radicals. These mechanistic insights were leveraged in designing ligands that enable couplings of primary, secondary, and tertiary alkyl chlorides.

Photochemical α-selective radical ring-opening reactions of 1,3-disubstituted acyl bicyclobutanes with alkyl halides: modular access to functionalized cyclobutenes

Although ring-opening reactions of bicyclobutanes bearing electron-withdrawing groups, typically with β-selectivity, have evolved as a powerful platform for synthesis of cyclobutanes, their application in the synthesis of cyclobutenes remains underdeveloped. Here, a novel visible light induced α-selective radical ring-opening reaction of 1,3-disubstituted acyl bicyclobutanes with alkyl radical precursors for the synthesis of functionalized cyclobutenes is described. In particular, primary, secondary, and tertiary alkyl halides are all suitable substrates for this photocatalytic transformation, providing ready access to cyclobutenes with a single all-carbon quaternary center, or with two contiguous centers under mild reaction conditions.

Recent Advances in First-Row Transition Metal-Catalyzed Reductive Coupling Reactions for π-Bond Functionalization and C-Glycosylation

ConspectusEfficient construction of ubiquitous carbon-carbon bonds between two electrophiles has garnered interest in recent decades, particularly if it is mediated by nonprecious, first-row transition metals. Reductive coupling has advantages over traditional cross-coupling by obviating the need for stoichiometric air- and moisture-sensitive organometallic reagents. By harnessing transition metal-catalyzed reductive coupling as a powerful tool, intricate molecular architectures can be readily assembled through the installation of two C-C bonds across π systems (alkenes/alkynes) via reaction with two appropriate electrophiles. Despite advances in reductive alkene difunctionalization, there remains significant potential for the discovery of novel reaction pathways. In this regard, development of reductive protocols that enable the union of challenging alkyl/alkynyl electrophiles in high regio- and chemoselectivity remains a highly sought-after goal.Apart from π-bond functionalization, reductive coupling has found application in carbohydrate chemistry, particularly in the synthesis of valuable C-glycosyl compounds. In this vein, suitable glycosyl donors can be used to generate reactive glycosyl radical intermediates under reductive conditions. Through elaborately designed reactions, these intermediates can be trapped to furnish pharmaceutically relevant glycoconjugates. Consequently, diversification in C-glycosyl compound synthesis using first-row transition metal catalysis holds strong appeal.In this Account, we summarize our efforts in the development of first-row transition metal-catalyzed reductive coupling reactions for applications in alkene/alkyne functionalization and C-glycosylation. We will first discuss the nickel (Ni)-catalyzed reductive difunctionalization of alkenes, aided by an 8-aminoquinoline (AQ) directing auxiliary. Next, we highlight the Ni-catalyzed hydroalkylation of alkenyl amides tethered with a similar AQ-derived directing auxiliary. Lastly, we discuss an efficient synthesis of 1,3-enynes involving site- and stereoselective reductive coupling of terminal alkynes with alkynyl halides and NHPI esters.Beyond alkene dicarbofunctionalization, we extended the paradigm of transition metal-catalyzed reductive coupling toward the construction of C-glycosidic linkages in carbohydrates. By employing an earth-abundant iron (Fe)-based catalyst, we show that useful glycosyl radicals can be generated from glycosyl chlorides under reductive conditions. These intermediates can be captured in C-C bond formation to furnish valuable C-aryl, C-alkenyl, and C-alkynyl glycosyl compounds with high diastereoselectivity. Our Ni-catalyzed multicomponent union of glycosyl chlorides, aryl/alkyl iodides, and isobutyl chloroformate under reductive conditions led to the stereoselective synthesis of C-acyl glycosides. In addition to Fe and Ni, we discovered a Ti-catalyzed/Mn-promoted synthetic route to access C-alkyl and C-alkenyl glycosyl compounds, through the reaction of glycosyl chlorides with electron-deficient alkenes/alkynes. We further developed an electron donor-acceptor (EDA) photoactivation system leveraging decarboxylative and deaminative strategies for C-glycosylation under Ni catalysis. This approach has been demonstrated to selectively activate carboxyl and amino motifs to furnish glycopeptide conjugates. Finally, through two distinct catalytic transformations of bench-stable heteroaryl glycosyl sulfones, we achieved stereodivergent access to both α- and β-anomers of C-aryl glycosides, one of which involves a Ni-catalyzed reductive coupling with aryl iodides.The findings presented in this Account are anticipated to have far-reaching implications beyond our research. We foresee that these results will pave the way for new transformations founded on the concept of reductive coupling, leading to the discovery of novel applications in the future.

Electrochemical halogen-atom transfer alkylation via α-aminoalkyl radical activation of alkyl iodides

Alkyl halides, widely recognized as important building blocks and reagents in organic synthesis, can serve as versatile alkyl radical precursors in radical-based transformations. However, generating alkyl radicals directly from unactivated alkyl halides under mild conditions remains a challenge due to their extremely low reduction potentials. To address this issue, α-aminoalkyl radicals were employed as efficient halogen-atom transfer (XAT) reagents in the photoredox activation of unactivated alkyl halides. Here, we report an effective electrooxidation strategy for generating alkyl radicals from unactivated alkyl iodides via an electrochemical halogen-atom transfer (e-XAT) process under mild conditions. The α-aminoalkyl radicals generated by anodic oxidation are demonstrated to be efficient XAT reagents in these transformations. This facile electricity-driven strategy obviates the need for sacrificial anodes and external chemical oxidants. The method successfully applies to a wide variety of alkyl iodides, including primary, secondary, and tertiary, as well as structurally diverse olefins, exhibiting excellent functional group tolerance. Moreover, we further demonstrate the utility of this strategy by rapidly functionalizing complex molecules and biomolecules.

C(sp2)-H cyclobutylation of hydroxyarenes enabled by silver-π-acid catalysis: diastereocontrolled synthesis of 1,3-difunctionalized cyclobutanes

Ring-opening of bicyclo[1.1.0]butanes (BCBs) is emerging as a powerful strategy for 1,3-difunctionalized cyclobutane synthesis. However, reported radical strain-release reactions are typically plagued with diastereoselectivity issues. Herein, an atom-economic protocol for the highly chemo- and diastereoselective polar strain-release ring-opening of BCBs with hydroxyarenes catalyzed by a π-acid catalyst AgBF4 has been developed. The use of readily available starting materials, low catalyst loading, high selectivity (up to >98 : 2 d.r.), a broad substrate scope, ease of scale-up, and versatile functionalizations of the cyclobutane products make this approach very attractive for the synthesis of 1,1,3-trisubstituted cyclobutanes. Moreover, control experiments and theoretical calculations were performed to illustrate the reaction mechanism and selectivity.

Observing the Entry Events of a Titanium-Based Photoredox Catalytic Cycle in Real Time

Titanium-based catalysis in single electron transfer (SET) steps has evolved into a versatile approach for the synthesis of fine chemicals and first attempts have recently been made to enhance its sustainability by merging it with photo-redox (PR) catalysis. Here, we explore the photochemical principles of all-Ti-based SET-PR-catalysis, i.e. in the absence of a precious metal PR-co-catalyst. By combining time-resolved emission with ultraviolet-pump/mid-infrared-probe (UV/MIR) spectroscopy on femtosecond-to-microsecond time scales we quantify the dynamics of the critical events of entry into the catalytic cycle; namely, the singlet-triplet interconversion of the do-it-all titanocene(IV) PR-catalyst and its one-electron reduction by a sacrificial amine electron donor. The results highlight the importance of the PR-catalyst's singlet-triplet gap as a design guide for future improvements.

Photoredox catalysis harvesting multiple photon or electrochemical energies

Photoredox catalysis (PRC) is a cutting-edge frontier for single electron-transfer (SET) reactions, enabling the generation of reactive intermediates for both oxidative and reductive processes via photon activation of a catalyst. Although this represents a significant step towards chemoselective and, more generally, sustainable chemistry, its efficacy is limited by the energy of visible light photons. Nowadays, excellent alternative conditions are available to overcome these limitations, harvesting two different but correlated concepts: the use of multi-photon processes such as consecutive photoinduced electron transfer (conPET) and the combination of photo- and electrochemistry in synthetic photoelectrochemistry (PEC). Herein, we review the most recent contributions to these fields in both oxidative and reductive activations of organic functional groups. New opportunities for organic chemists are captured, such as selective reactions employing super-oxidants and super-reductants to engage unactivated chemical feedstocks, and scalability up to gram scales in continuous flow. This review provides comparisons between the two techniques (multi-photon photoredox catalysis and PEC) to help the reader to fully understand their similarities, differences and potential applications and to therefore choose which method is the most appropriate for a given reaction, scale and purpose of a project.

Formal γ-C-H Functionalization of Cyclobutyl Ketones: Synthesis of cis-1,3-Difunctionalized Cyclobutanes

1,3-Difunctionalized cyclobutanes are an emerging scaffold in medicinal chemistry that can confer beneficial pharmacological properties to small-molecule drug candidates. However, the diastereocontrolled synthesis of these compounds typically requires complicated synthetic routes, indicating a need for novel methods. Here, we report a sequential C-H/C-C functionalization strategy for the stereospecific synthesis of cis-γ-functionalized cyclobutyl ketones from readily available cyclobutyl aryl ketones. Specifically, a bicyclo[1.1.1]pentan-2-ol intermediate is generated from the parent cyclobutyl ketone via an optimized Norrish-Yang procedure. This intermediate then undergoes a ligand-enabled, palladium-catalyzed C-C cleavage/functionalization to produce valuable cis-γ-(hetero)arylated, alkenylated, and alkynylated cyclobutyl aryl ketones, the benzoyl moiety of which can subsequently be converted to a wide range of functional groups including amides and esters.

Synthesis and Applications of Bicyclo[1.1.0]butyl and Azabicyclo[1.1.0]butyl Organometallics

The use of metalated (aza)bicyclo[1.1.0]butanes in synthesis is currently experiencing a renaissance, as evidenced by the numerous reports in the last 5 years that have relied on such intermediates to undergo unique transformations or generate novel fragments. Since their discovery, these species have been demonstrated to participate in a wide range of reactions with carbon and heteroatom electrophiles, as well as metal complexes, to facilitate the rapid diversification of (aza)bicyclo[1.1.0]butane-containing compounds. Key to this is the relative acidity of the bridgehead C-H bonds which promotes facile deprotonation and subsequent functionalization of an unsubstituted position on the carbon framework via the intermediacy of a metalated (aza)bicyclo[1.1.0]butane. Additionally, the late-stage incorporation of deuterium atoms in strained fragments has led to the elucidation of numerous reaction mechanisms that involve strained bicycles. The continued investigation into the inimitable reactivity of metalated bicycles will cement their importance within the field of organometallic chemistry.

Aminals as powerful XAT-reagents: activation of fluorinated alkyl chlorides

Readily available 1,3,5-trimethyl-1,3,5-triazinane serves as an efficient reagent for halogen atom transfer. Under photocatalytic conditions, the triazinane generates an α-aminoalkyl radical, which can activate the C-Cl bond of fluorinated alkyl chlorides. The hydrofluoroalkylation reaction between fluorinated alkyl chlorides and alkenes is described. The efficiency of the diamino-substituted radical derived from the triazinane is associated with stereoelectronic effects defined by a six-membered cycle forcing the anti-periplanar arrangement of the radical orbital and lone pairs of adjacent nitrogen atoms.

Rapid and Scalable Halosulfonylation of Strain-Release Reagents

Sulfonylated aromatics are commonplace motifs in drugs and agrochemicals. However, methods for the direct synthesis of sulfonylated non-classical arene bioisosteres, which could improve the physicochemical properties of drug and agrochemical candidates, are limited. Here we report a solution to this challenge: a one-pot halosulfonylation of [1.1.1]propellane, [3.1.1]propellane and bicyclo[1.1.0]butanes that proceeds under practical, scalable and mild conditions. The sulfonyl halides used in this chemistry feature aryl, heteroaryl and alkyl substituents, and are conveniently generated in situ from readily available sulfinate salts and halogen atom sources. This methodology enables the synthesis of an array of pharmaceutically and agrochemically relevant halogen/sulfonyl-substituted bioisosteres and cyclobutanes, on up to multidecagram scale.

Bicyclobutanes as unusual building blocks for complexity generation in organic synthesis

Bicyclobutanes are among the most highly strained isolable organic compounds and their associated low activation barriers to reactivity make them intriguing building-blocks in organic chemistry. In recent years, numerous creative synthetic strategies exploiting their heightened reactivity have been presented and these discoveries have often gone hand-in-hand with the development of more practical routes for their synthesis. Their proclivity as strain-release reagents through their weak central C-C bond has been harnessed in a variety of addition, rearrangement and insertion reactions, providing rapid access to a rich tapestry of complex molecular scaffolds. This review will provide an overview of the different options available for bicyclobutane synthesis, the main classes of compounds that can be prepared from bicyclobutanes, and the associated modes of reactivity used.

How mono- and diphosphine ligands alter regioselectivity of the Rh-catalyzed annulative cleavage of bicyclo[1.1.0]butanes

Rh(I)-catalyzed cycloisomerizations of bicyclo[1.1.0]butanes provide a fruitful approach to cyclopropane-fused heterocycles. Products and stereochemical outcome are highly dependent on catalyst. The triphenylphosphine (PPh3) ligand provides pyrrolidines, placing substituents anti to the cyclopropyl group. The 1,2-bis(diphenylphosphino)ethane (dppe) ligand yields azepanes with substituents syn to the cyclopropyl group. In this work, quantum mechanical DFT calculations pinpoint a reversal of regio- and diastereoselectivity, suggesting a concerted (double) C−C bond cleavage and rhodium carbenoid formation, driven by strain-release. The ligand-influenced cleavage step determines the regioselectivity of carbometalation and product formation, and suggests new applications of bicyclobutanes.

C(sp3)-C(sp3) coupling of non-activated alkyl-iodides with electron-deficient alkenes via visible-light/silane-mediated alkyl-radical formation

Here, we present a remarkably mild and general initiation protocol for alkyl-radical generation from non-activated alkyl-iodides. An interaction between a silane and an alkyl iodide is excited by irradiation with visible light to trigger carbon-iodide bond homolysis and form the alkyl radical. We show how this method can be developed into an operationally simple and general Giese addition reaction that can tolerate a range of sensitive functionalities not normally explored in established approaches to this strategically important transformation. The new method requires no photocatalyst or other additives and uses only commerical tris(trimethylsilyl)silane and visible light to effectively combine a broad range of alkyl halides with activated alkenes to form C(sp3)-C(sp3) bonds embedded within complex frameworks.

Bicyclobutanes: from curiosities to versatile reagents and covalent warheads

The unique chemistry of small, strained carbocyclic systems has long captivated organic chemists from a theoretical and fundamental standpoint. A resurgence of interest in strained carbocyclic species has been prompted by their potential as bioisosteres, high fraction of sp3 carbons, and limited appearance in the patent literature. Among strained ring systems, bicyclo[1.1.0]butane (BCB) stands apart as the smallest bicyclic carbocycle and is amongst the most strained carbocycles known. Despite the fact that BCBs have been synthesized and studied for well over 50 years, they have long been regarded as laboratory curiosities. However, new approaches for preparing, functionalizing, and using BCBs in "strain-release" transformations have positioned BCBs to be powerful synthetic workhorses. Further, the olefinic character of the bridgehead bond enables BCBs to be elaborated into various other ring systems and function as covalent warheads for bioconjugation. This review will discuss the recent developments in the synthesis and functionalization of BCBs as well as the applications of these strained rings in synthesis and drug discovery. An overview of the properties and the historical context of this interesting structure will be provided.

Reductive Cross-Coupling of Unreactive Electrophiles

Transition-metal-catalyzed reductive coupling of electrophiles has emerged as a powerful tool for the construction of molecules. While major achievements have been made in the field of cross-couplings between organic halides and pseudohalides, an increasing number of reports demonstrates reactions involving more readily available, low-cost, and stable, but unreactive electrophiles. This account summarizes the recent results in our laboratory focusing on this topic. These findings typically include deoxygenative C-C coupling of alcohols, reductive alkylation of alkenyl acetates, reductive C-Si coupling of chlorosilanes, and reductive C-Ge coupling of chlorogermanes.The reductive deoxygenative coupling of alcohols with electrophiles is synthetically appealing, but the potential of this chemistry remains to be disclosed. Our initial study focused on the reaction of allylic alcohols and aryl bromides by the combination of nickel and Lewis acid catalysis. This method offers a selectivity that is opposite to that of the classic Tsuji-Trost reactions. Further investigation on the reaction of benzylic alcohols led to the foundation of a dynamic kinetic cross-coupling strategy with applications in the nickel-catalyzed reductive arylation of benzylic alcohols and cobalt-catalyzed enantiospecific reductive alkenylation of allylic alcohols. The titanium catalysis was later established to produce carbon radicals directly from unactivated tertiary alcohols via C-OH cleavage. The development of their coupling reactions with carbon fragments delivers new methods for the construction of all-carbon quaternary centers. These reactions have shown high selectivity for the functionalization of tertiary alcohols, leaving primary and secondary alcohols intact. Alkenyl acetates are inexpensive, stable, and environmentally friendly and are considered the most attractive alkenyl reagents. The development of reductive alkylation of alkenyl acetates with benzyl ammoniums and alkyl bromides offers mild approaches for the conversion of ketones into aliphatic alkenes.Extensive studies in this field have enabled us to extend the cross-electrophile coupling from carbon to silicon and germanium chemistry. These reactions harness the ready availability of chlorosilanes and chlorogermanes but suffer from the challenge of their low reactivity toward transition metals. Under reductive nickel catalysis, a broad range of alkenyl and aryl electrophiles couple well with vinyl- and hydrochlorosilanes. The use of alkyl halides as coupling partners led to the formation of functionalized alkylsilanes. The C-Ge coupling seems less substrate-dependent, and various common chlorogermanes couple well with aryl, alkenyl, and alkyl electrophiles. In general, functionalities such as Grignard-sensitive groups (e.g., acid, amide, alcohol, ketone, and ester), acid-sensitive groups (e.g., ketal and THP protection), alkyl fluoride and chloride, aryl bromide, alkyl tosylate and mesylate, silyl ether, and amine are tolerated. These methods provide new access to organosilicon and organogermanium compounds, some of which are challenging to obtain otherwise.

Mechanism of a Luminescent Dicopper System That Facilitates Electrophotochemical Coupling of Benzyl Chlorides via a Strongly Reducing Excited State

Photochemical radical generation has become a modern staple in chemical synthesis and methodology. Herein, we detail the photochemistry of a highly reducing, highly luminescent dicopper system [Cu2] (Eox* ≈ -2.7 V vs SCE; τ0 ≈ 10 μs) within the context of a model reaction: single-electron reduction of benzyl chlorides. The dicopper system is mechanistically well defined. As we show, it is the [Cu2]* excited state that serves as the outer-sphere photoreductant of benzyl chloride substrates; the ground-state oxidized byproduct, [Cu2]+, is electrochemically recycled, demonstrating a catalytic electrophotochemical C-C coupling process.

Boryl Radicals Enabled a Three-Step Sequence to Assemble All-Carbon Quaternary Centers from Activated Trichloromethyl Groups

The construction of diversely substituted all-carbon quaternary centers has been a longstanding challenge in organic synthesis. Methods that add three alkyl substituents to a simple C(sp³) atom rely heavily on lengthy multiple processes, which usually involve several preactivation steps. Here, we describe a straightforward three-step sequence that uses a range of readily accessible activated trichloromethyl groups as the carbon source, the three C-Cl bonds of which are selectively functionalized to introduce three alkyl chains. In each step, only a single C-Cl bond was cleaved with the choice of an appropriate Lewis base-boryl radical as the promoter. A vast range of diversely substituted all-carbon quaternary centers could be accessed directly from these activated CCl₃ trichloromethyl groups or by simple derivatizations. The use of different alkene traps in each of the three steps enabled facile collections of a large library of products. The utility of this strategy was demonstrated by the synthesis of variants of two drug molecules, whose structures could be easily modulated by varying the alkene partner in each step. The results of kinetic and computational studies enabled the design of the three-step reaction and provided insights into the reaction mechanisms.

Titanium Radical Redox Catalysis: Recent Innovations in Catalysts, Reactions, and Modes of Activation

Radical chemistry has emerged as a cornerstone in modern organic synthesis, providing chemists with numerous new tools to rapidly expand reactivity and chemical space in academic and industrial research. In this regard, titanium complexes have been recognized as an attractive class of catalysts owing to their rich redox activities in addition to the abundance and low toxicity of this early transition metal. Traditionally employed for the activation of epoxides and carbonyl compounds, Ti radical redox catalysis has broken into new grounds in recent years, giving rise to a diverse repertoire of useful transformations. In this Perspective, we highlight recent developments in the area of TiIII/IV catalysis with respect to the activation of different types of chemical bonds. Furthermore, we discuss future opportunities in integrating Ti radical chemistry with other catalytic systems as well as with emerging new technologies such as photochemistry and electrochemistry.

A Catalytic Alkylation of Ketones via sp3 C-H Bond Activation

We identified a ternary hybrid catalyst system composed of an acridinium photoredox catalyst, a thiophosphoric imide (TPI) catalyst, and a titanium complex catalyst that promoted an intermolecular addition reaction of organic molecules with various ketones through sp³ C-H bond activation. The thiyl radical generated via single-electron oxidation of TPI by the excited photoredox catalyst abstracted a hydrogen atom from organic molecules such as toluene, benzyl alcohol, alkenes, aldehydes, and THF. The thus-generated carbon-centered radical species underwent addition to ketones and aldehydes. This intrinsically unfavorable step was promoted by single-electron reduction of the intermediate alkoxy radical by catalytically generated titanium(III) species. This reaction provided an efficient and straightforward route to a broad range of tertiary alcohols and was successfully applied to late-stage functionalization of drugs or their derivatives. The proposed mechanism was supported by both experimental and theoretical studies.

Diastereoselective and enantioselective photoredox pinacol coupling promoted by titanium complexes with a red-absorbing organic dye

The pinacol coupling reaction, a reductive coupling of carbonyl compounds that proceeds through the formation of ketyl radicals in the presence of an electron donor, affords the corresponding 1,2-diols in one single step. The photoredox version of this transformation has been accomplished using different organic dyes or photoactive metal complexes in the presence of sacrificial donors such as tertiary amines or Hantzsch's ester. Normally, the homo-coupling of such reactive ketyl radicals is neither diastereo- nor enantio-selective. Herein, we report a highly diastereoselective pinacol coupling reaction of aromatic aldehydes promoted by 5 mol% of the non-toxic, inexpensive and available Cp₂TiCl₂ complex. The key feature that allows the complete control of diastereoselectivity is the employment of a red-absorbing organic dye in the presence of a redox-active titanium complex. Taking advantage of the well-tailored photoredox potential of this organic dye, the selective reduction of Ti(iv) to Ti(iii) is achieved. These conditions enable the formation of the d,l (syn) diastereoisomer as the favored product of the pinacol coupling (d.r. > 20 : 1 in most of the cases). Moreover, employing a simply prepared chiral SalenTi complex, the new photoredox reaction gave a complete diastereoselection for the d,l diastereoisomer, and high enantiocontrol (up to 92% of enantiomeric excess).

A metallaphotoredox, diastereoselective and enantioselective pinacol coupling reaction promoted by titanium complexes with the use of a red-absorbing organic dye was developed.

Photoredox Activation of Inert Alkyl Chlorides for the Reductive Cross-Coupling with Aromatic Alkenes

The inertness of chloroalkanes has precluded them as coupling partners for cross-coupling reactions. Herein we disclose a general strategy for the activation of inert alkyl chlorides through photoredox catalysis and their use as coupling partners with alkenes. The catalytic system is formed by [Ni(OTf)(Py₂ ^Ts tacn)](OTf) (1_Ni ), which is responsible for the Csp³ -Cl bond activation, and [Ir(^NMe2 bpy)(ppy)₂ ]PF_6, (PC_Ir ^NMe2 ), which is the photoredox catalyst. Combined experimental and theoretical studies show an in situ photogenerated Ni^I intermediate ([Ni(Py₂ ^Ts tacn)]⁺ ) which is catalytically competent for the Csp³ -Cl bond cleavage via a S_N 2 mechanism for primary alkyl chlorides, forming carbon-centered free radicals, which react with the olefin leading to the formation of the Csp³ -Csp³ bond. These results suggest inert alkyl chlorides can be electrophiles for developing new intermolecular strategies in which low-valent aminopyridine nickel complexes act as key catalytic species.

One-Pot Synthesis of Strain-Release Reagents from Methyl Sulfones

Sulfone-substituted bicyclo[1.1.0]butanes and housanes have found widespread application in organic synthesis due to their bench stability and high reactivity in strain-releasing processes in the presence of nucleophiles or radical species. Despite their increasing utility, their preparation typically requires multiple steps in low overall yield. In this work, we report an expedient and general one-pot procedure for the synthesis of 1-sulfonylbicyclo[1.1.0]butanes from readily available methyl sulfones and inexpensive epichlorohydrin via the dialkylmagnesium-mediated formation of 3-sulfonylcyclobutanol intermediates. Furthermore, the process was extended to the formation of 1-sulfonylbicyclo[2.1.0]pentane (housane) analogues when 4-chloro-1,2-epoxybutane was used as the electrophile instead of epichlorohydrin. Both procedures could be applied on a gram scale with similar efficiency and are shown to be fully stereospecific in the case of housanes when an enantiopure epoxide was employed, leading to a streamlined access to highly valuable optically active strain-release reagents.

Direct and Unified Access to Carbon Radicals from Aliphatic Alcohols by Cost-Efficient Titanium-Mediated Homolytic C-OH Bond Cleavage

Low-valent Ti-mediated homolytic C-O bond cleavage offers unified access to carbon radicals from ubiquitous non-activated tertiary, secondary, and even primary alcohols. In contrast to the representative Ti reagents, which were ineffective for this purpose, "TiCl₂ (cat)"/Zn (cat=catecholate) was found to be specifically active. This method was applied to the addition reactions of radicals to alkenes and exhibited high generality and yields. More than 50 combinations were examined. The excellent cost-efficiency and accessibility of "TiCl₂ (cat)"/Zn further enhance its applicability. Control experiments proved the presence of a carbon radical intermediate and excluded the pathway via alkyl chlorides. Further mechanistic study indicated that the 1 : 2 complex of alkoxide (R-O^- ) and Ti^III is an active species in the C-O cleavage.

Titanocene(III)-Catalyzed Precision Deuteration of Epoxides

We describe a titanocene(III)-catalyzed deuterosilylation of epoxides that provides β-deuterated anti-Markovnikov alcohols with excellent D-incorporation, in high yield, and often excellent diastereoselectivity after desilylation. The key to the success of the reaction is a novel activation method of Cp2 TiCl2 and (tBuC5 H4 )2 TiCl2 with BnMgBr and PhSiD3 to provide [(RC5 H4 )2 Ti(III)D] without isotope scrambling. It was developed after discovering an off-cycle scrambling with the previously described method. Our precision deuteration can be applied to the synthesis of drug precursors and highlights the power of combining radical chemistry with organometallic catalysis.

Diastereoselective synthesis of 1,1,3,3-tetrasubstituted cyclobutanes enabled by cycloaddition of bicyclo[1.1.0]butanes

A diastereoselective cycloaddition of bicyclo[1.1.0]butanes (BCBs) with triazolinedione or nitrosoarenes was developed to access multi-substituted cyclobutanes.

Synthesis and Applications of Polysubstituted Bicyclo[1.1.0]butanes

Bicyclo[1.1.0]butanes (BCBs) are valuable substrates in the "strain release" synthesis of polysubstituted four-membered ring systems, with applications including bioconjugation agents. The introduction of substituents onto the BCB bridges is challenging due to limitations in current methods for the preparation of this bicyclic scaffold, typically necessitating linear syntheses with limited functional group tolerance and/or substituent scope. Here, we report the synthesis of tri- and tetrasubstituted BCBs via directed metalation of readily accessed BCB amides; this straightforward "late stage" approach generates a wide variety of bridge-substituted BCBs that can be easily converted into other useful small ring building blocks. Access to a monodeuterated BCB afforded unprecedented insight into the mechanism of dihalocarbene insertion into BCBs to afford bicyclo[1.1.1]pentanes (BCPs).