Asymmetric synthesis via stereospecific C-N and C-O bond activation of alkyl amine and alcohol derivatives

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 Enantioconvergent Carboxylation Enabled by a Chiral 2,2'-Bipyridine Ligand

In contrast to previous approaches to chiral α-aryl carboxylic acids that based on reactions using hazardous gases, pressurized setup and mostly noble metal catalysts, in this work, a nickel-catalyzed general, efficient and highly enantioselective carboxylation reaction of racemic benzylic (pseudo)halides under mild conditions using atmospheric CO₂ has been developed. A unique chiral 2,2'-bipyridine ligand named Me-SBpy featuring compact polycyclic skeleton enabled both high reactivity and stereoselectivity. The utility of this method has been demonstrated by synthesis of various chiral α-aryl carboxylic acids (30 examples, up to 95 % yield and 99 : 1 er), including profen family anti-inflammatory drugs and transformations using the acids as key intermediates. Based on mechanistic experimental results, a plausible catalytic cycle involving Ni-complex/radical equilibrium and Lewis acid-assisted CO₂ activation has been proposed.

Enantioselective Deaminative Alkylation of Amino Acid Derivatives with Unactivated Olefins

Herein, we report the first Ni-catalyzed enantioselective deaminative alkylation of amino acid and peptide derivatives with unactivated olefins. Key for success was the discovery of a new sterically encumbered bis(oxazoline) ligand backbone, thus offering a de novo technology for accessing enantioenriched sp3-sp3 linkages via sp3 C-N functionalization. Our protocol is distinguished by its broad scope and generality across a wide number of counterparts, even in the context of late-stage functionalization. In addition, an enantioselective deaminative remote hydroalkylation reaction of unactivated internal olefins is within reach, thus providing a useful entry point for forging enantioenriched sp3-sp3 centers at remote sp3 C-H sites.

Benzyl thioether formation merging copper catalysis

A novel copper-catalyzed thioetherification reaction has been developed to afford benzyl thioethers in moderate to excellent yields. Under the mild and easy-to-operate conditions, a variety of thioethers are efficiently prepared from readily available benzyl alcohols (primary, secondary, and tertiary) and thiols in the presence of Cu(OTf)2 as the Lewis acid catalysis. This C-S bond formation protocol furnishes exceptional chemoselectivity, and the preliminary mechanism studies show that the reaction should proceed through a Lewis-acid-mediated SN1-type nucleophilic attack of the carbocations formed in situ.

Nickel-Catalyzed Kumada Cross-Coupling Reactions of Benzylic Sulfonamides

Herein, we report a Kumada cross-coupling reaction of benzylic sulfonamides. The scope of the transformation includes acyclic and cyclic sulfonamide precursors that cleanly produce highly substituted acyclic fragments. Preliminary data are consistent with a stereospecific mechanism that allows for a diastereoselective reaction.

Regioselective Silylations of Propargyl and Allyl Pivalates through Ca-Promoted Reductive C(sp3)-O Bond Cleavage

A practical protocol for the regioselective preparation of 3-phenylpropargylsilanes and 3-phenylallylsilanes in yields of 36-77 and 48-86%, respectively, from readily accessible 3-phenylpropargyl and 1-phenylallyl pivalates was developed through reductive C(sp³)-O bond cleavage. This method represents the first example of the direct application of vastly abundant calcium granules to a reductive coupling reaction. A broad range of propargylsilanes and allylsilanes are simply prepared using easy-to-handle pivalates and chlorotrimethylsilane under mild catalyst-free and additive-free conditions.

Chirality memory of α-methylene-π-allyl iridium species

Chirality is one of the most important types of steric information in nature. In addition to central chirality, axial chirality has been catching more and more attention from scientists. However, although much attention has recently been paid to the creation of axial chirality and the chirality transfer of allenes, no study has been disclosed as to the memory of such an axial chirality. The reason is very obvious: the chiral information is stored over three carbon atoms. Here, the first example of the memory of chirality (MOC) of allenes has been recorded, which was realized via an optically active alkylidene-π-allyl iridium intermediate, leading to a highly stereoselective electrophilic allenylation with amines. Specifically, we have established the transition metal-mediated highly stereoselective 2,3-allenylation of amines by using optically active 2,3-allenyl carbonates under the catalysis of a nonchiral iridium(iii) complex. This method is compatible with sterically bulky and small substituents on both amines and 2,3-allenyl carbonates and furnishes the desired optically active products with a high efficiency of chirality transfer. Further mechanistic experiments reveal that the isomerization of the optically active alkylidene-π-allyl iridium intermediate is very slow.

Chirality is one of the most important types of steric information in nature.

Synthesis of α-substituted indolylacetamide using acetonitriles as acetamide enolate equivalents through O-transfer reactions

We introduce readily available ammonium hemiaminals as O-transfer reagents and commercially available acetonitriles as a primary amide enolate precursor. The combination serves as an amide enolate equivalent, thereby providing one-pot access to α-substituted indolylacetamides. A broad substrate scope and good functional group tolerance as well as gram-scale synthesis make this protocol highly attractive. Mechanistic experiments suggest that the cyano group is trapped by a hydroxy group of hemiaminals en route to the desired primary amides under metal-free conditions.

Dissection of Alkylpyridinium Structures to Understand Deamination Reactions

Via conversion to Katritzky pyridinium salts, alkyl amines can now be used as alkyl radical precursors for a range of deaminative functionalization reactions. The key step of all these methods is single electron reduction of the pyridinium ring, which triggers C-N bond cleavage. However, little has been done to understand how the precise nature of the pyridinium influences these events. Using a combination of synthesis, computation, and electrochemistry, this study delineates the steric and electronic effects that substituents have on the canonical steps and the overall process. Depending on the approach taken, consideration of both the reduction and the subsequent radical dissociation may be necessary. Whereas the electronic effects on these steps work in opposition to each other, the steric effects are synergistic, with larger substituents favoring both steps. This understanding provides a framework for future design of pyridinium salts to match the mode of catalysis or activation.

Mechanism and Selectivity Control in Ni- and Pd-Catalyzed Cross-Couplings Involving Carbon-Oxygen Bond Activation

Transition-metal-catalyzed C-O bond activation provides a useful strategy for utilizing alcohol- and phenol-derived electrophiles in cross-coupling reactions, which has become a research field of active and growing interest in organic chemistry. The synergy between computation and experiment elucidated the mechanistic model and controlling factors of selectivities in these transformations, leading to advances in innovative C-O bond activation and functionalization methods.Toward the rational design of C-O bond activation, our collaborations with the Jarvo group bridged the mechanistic models of C(sp²)-O and C(sp³)-O bond activations. We found that the nickel catalyst cleaves the benzylic and allylic C(sp³)-O bonds via two general mechanisms: the stereoinvertive S_N2 back-side attack model and the stereoretentive chelation-assisted model. These two models control the stereochemistry in a wide array of stereospecific Ni-catalyzed cross-coupling reactions with benzylic or allylic alcohol derivatives. Because of the catalyst distortion, the ligands can differentiate the competing stereospecific C(sp³)-O bond activations. The PCy₃ ligand interacts with nickel mainly through σ-donation, and the Ni(PCy₃) catalyst can undergo facile bending of the substrate-nickel-ligand angle, which favors the stereoretentive benzylic C-O bond activation. The N-heterocyclic carbene SIMes ligand has additional d(metal)-p(ligand) back-donation with nickel, which leads to an extra energy penalty for the same angle bending. This results in the preference of stereoinvertive benzylic C-O bond activation under Ni/SIMes catalysis. In addition to ligand control, a Lewis acid can increase the selectivity for stereoinvertive C(sp³)-O activation by stabilizing the S_N2 back-side attack transition state. The oxygen leaving group complexes with the MgI₂ Lewis acid in the stereoinvertive activation, leading to the exclusive stereoinvertive Kumada coupling of benzylic ethers. We also identified that the competing C(sp³)-O bond activation models have noticeable differences in charge separation. This leads to the solvent polarity control of the stereospecificity in C(sp³)-O activations. Low-polarity solvents favor the neutral stereoretentive C-O bond activation, while high-polarity solvents favor the zwitterionic stereoinvertive cleavage.In sharp contrast to the nickel catalysts, the C(sp²)-O bond activation under palladium catalysis mainly proceeds via the classic three-membered ring oxidative addition mechanism instead of the chelation-assisted mechanism. This is due to the lower oxophilicity of palladium, which disfavors the oxygen coordination in the chelation-assisted-type activation. The three-membered ring activation model selectively cleaves the weak C-O bond, resulting in the exclusive chemoselectivity of acyl C-O bond activation in Pd-catalyzed cross-coupling reactions with aryl carboxylic acid derivatives. This explains the overall acylation in the Pd-catalyzed Suzuki-Miyaura coupling with aryl esters. In collaboration with the Szostak group, we revealed that the three-membered ring model applies in the Pd-catalyzed C-O bond activation of carboxylic acid anhydride, which stimulated the development of a series of Pd-catalyzed decarbonylative functionalizations of aryl carboxylic acids.

Nickel-Catalyzed, Stereospecific C-C and C-B Cross-Couplings via C-N and C-O Bond Activation

Highly enantioenriched benzylic and allylic amines and alcohols are readily available via asymmetric synthesis and in complex natural products. The development of mild, nickel-catalyzed cross-couplings of their derivatives has advanced the tools available for the preparation of a range of highly enantioenriched products, including those with quaternary stereocenters. This perspective focuses on cross-couplings with convenient and functional group-tolerant organoboron reagents and highlights the discoveries of activating groups and conditions that have led to high-yielding and highly stereospecific reactions. Emphasis is placed on mechanistic understanding, particularly with regards to controlling inversion vs. retention pathways. Limitations and opportunities for future developments are also highlighted.

Deaminative metal-free reaction of alkenylboronic acids, sodium metabisulfite and Katritzky salts

A convenient and efficient approach to (E)-alkylsulfonyl olefins via a metal/light-free three-component reaction of alkenylboronic acids, sodium metabisulfite and Katritzky salts is described. This alkylsulfonylation proceeds smoothly with a broad substrate scope, leading to diverse (E)-alkylsulfonyl olefins in moderate to good yields. During the process, excellent functional group tolerance is observed and sodium metabisulfite is used as the source of sulfur dioxide. Mechanistic studies show that the alkyl radical generated in situ from Katritzky salt via a single electron transfer with alkenylboronic acid or DIPEA is the key step for providing an alkyl radical intermediate, which undergoes further alkylsulfonylation with sulfur dioxide.

Deaminative carbonylative thioesterification of activated alkylamines with thiophenols under transition-metal-free conditions

A transition-metal-free radical carbonylation of activated alkylamines with thiophenols has been successfully developed. Various thioesters were selectively produced with moderate to good yields.

Nickel-catalyzed C-O/N-H, C-S/N-H, and C-CN/N-H annulation of aromatic amides with alkynes: C-O, C-S, and C-CN activation

The Ni-catalyzed reaction of ortho-phenoxy-substituted aromatic amides with alkynes in the presence of LiO^tBu as a base results in C–O/N–H annulation with the formation of 1(2H)-isoquinolinones. The use of a base is essential for the reaction to proceed. The reaction proceeds, even in the absence of a ligand, and under mild reaction conditions (40 °C). An electron-donating group on the aromatic ring facilitates the reaction. The reaction was also applicable to carbamate (C–O bond activation), methylthio (C–S bond activation), and cyano (C–CN bond activation) groups as leaving groups.

The Ni-catalyzed reaction of ortho-phenoxy-substituted aromatic amides with alkynes in the presence of LiO^tBu as a base results in C–O/N–H annulation with the formation of 1(2H)-isoquinolinones.

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.

Room-Temperature Negishi Reaction of Trisubstituted Vinyl Phosphates for the Synthesis of Tetrasubstituted Alkenes

The present study investigated the ability of bromovinyl phosphates to react with organozinc reagents at room temperature during palladium-catalyzed reactions. It was determined that both the bromine atom and the phosphate group were successfully substituted by means of the reaction with the organozinc reagents, thereby allowing for the synthesis of cyclic and acyclic tetrasubstituted double bonds. The low stability of the organozinc compounds in an acidic environment was exploited to accomplish the synthesis of alkenes using a one-pot, two-step experimental setup.

Palladium-Catalyzed Regioselective and Stereospecific Ring-Opening Cross-Coupling of Aziridines: Experimental and Computational Studies

Aziridines, i.e., the smallest saturated N-heterocycles, serve as useful building blocks in synthetic organic chemistry. Because of the release of the large ring strain energy accommodated in the small ring, (ca. 27 kcal/mol), aziridines undergo ring-opening reactions with a variety of nucleophiles. Therefore, among the synthetic reactions utilizing aziridines, regioselective ring-opening substitutions of aziridines with nucleophiles, such as heteroatomic nucleophiles (e.g., amines, alcohols, and thiols) and carbonaceous nucleophiles (e.g., carbanions, organometallic reagents, and electron-rich arenes), constitute a useful synthetic methodology to synthesize biologically relevant β-functionalized alkylamines. However, the regioselection in such traditional ring-opening substitutions of aziridines is highly dependent on the substrate combination, and stereochemical control is challenging to achieve, especially in the case of Lewis acid-promoted variants. Therefore, the development of robust catalytic ring-opening functionalization methods that enable precise prediction of regioselectivity and stereochemistry is desirable. In this direction, our group focused on the highly regioselective and stereospecific nature of the stoichiometric oxidative addition elementary step of 2-substituted aziridines into Pd(0) complexes in an S_N2 fashion. In conjunction with the recent advancements in transition-metal-catalyzed cross-coupling reactions of alkyl pseudohalides containing a C(sp³)-Q (Q = O, N, S, etc.) bond, aziridines can be used as nonclassical alkyl pseudohalides in regioselective and stereospecific cross-couplings.In this Account, starting from the background of transition-metal-catalyzed ring-opening functionalization of aziridines, our contributions to the palladium-catalyzed regioselective and stereoinvertive cross-couplings of aziridines with organoboron reagents to form C(sp³)-C, C(sp³)-B, and C(sp³)-Si bonds have been compiled. The developed methods allow the syntheses of medicinally important amine compounds, e.g., enantioenriched β-phenethylamines, β-amino acids, and their boron and silyl surrogates, from readily available enantiopure aziridine substrates. Notably, the regioselectivity of the ring opening can be switched by appropriate selection of the catalyst (i.e., Pd/NHC vs Pd/PR₃ systems). Computational studies rationalized the detailed mechanisms of the full catalytic cycle and the regioselectivity and stereospecificity of the reactions. The computational results suggested that the interactions operating between the Pd catalyst and aziridine substrate play important roles in determining the regioselection of the aziridine ring-opening event (i.e., oxidative addition). Also, the computational results rationalized the role of water molecules in promoting the transmetalation step through the formation of a Pd-hydroxide active intermediate. This Account evidences the benefits of synergistic collaborations between experimental and computational methods in developing novel transition-metal-catalyzed cross-coupling reactions.

Nickel-Catalyzed Formal Aminocarbonylation of Secondary Benzyl Chlorides with Isocyanides

Phenylacetamides represent versatile feedstocks in synthetic chemistry, widely existing in drug molecules and natural products. Herein, we disclose a nickel-catalyzed formal aminocarbonylation of secondary benzyl chlorides with isocyanides yielding α-substituted phenylacetamide with steric hindrance, which is synthetically challenging via palladium-catalyzed aminocarbonylation. The reaction features wide functional group tolerance under mild conditions, highlighted by the tolerance of various aromatic halide (-Cl, -Br, -I) and heteroaromatic rings (pyridine and pyrazine).

Sequential C-O decarboxylative vinylation/C-H arylation of cyclic oxalates via a nickel-catalyzed multicomponent radical cascade

A selective, sequential C-O decarboxylative vinylation/C-H arylation of cyclic alcohol derivatives enabled by visible-light photoredox/nickel dual catalysis is described. This protocol utilizes a multicomponent radical cascade process, i.e. decarboxylative vinylation/1,5-HAT/aryl cross-coupling, to achieve efficient, site-selective dual-functionalization of saturated cyclic hydrocarbons in one single operation. This synergistic protocol provides straightforward access to sp3-enriched scaffolds and an alternative retrosynthetic disconnection to diversely functionalized saturated ring systems from the simple starting materials.

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.

Iron-Catalyzed Reductive Vinylation of Tertiary Alkyl Oxalates with Activated Vinyl Halides

We present herein a rare and efficient method for the creation of vinylated all carbon quaternary centers via Fe-catalyzed cross-electrophile coupling of vinyl halides with tertiary alkyl methyl oxalates. The reaction displays excellent functional group tolerance and broad substrate scope, which allows cascade radical cyclization and vinylation to afford complex bicyclic and spiral structural motifs. The reaction proceeds via tertiary alkyl radicals, and the putative vinyl-Br/Fe complexation appears to be crucial for activating the alkene and enabling a possibly concerted radical addition/C-Fe forming process.

Formation of allylated quaternary carbon centers via C-O/C-O bond fragmentation of oxalates and allyl carbonates

Disclosed herein emphasizes Fe-promoted cross-electrophile allylation of tertiary alkyl oxalates with allyl carbonates that generates all C(sp3)-quaternary centers. The reaction involves fragmentation of tertiary alkyl oxalate C-O bonds to give tertiary alkyl radical intermediates, addition of the radicals to less hindered alkene terminals, and subsequent cleavage of the allyl C-O bonds. Allylation with 2-aryl substituted allyl carbonates was mediated by Zn/MgCl2, and Fe is used to promote the radical addition efficiency. By introduction of activated alkenes, a three-component radical cascade reaction took place.

Chemical transformations of quaternary ammonium salts via C–N bond cleavage

The reaction of quaternary ammonium salts via C–N bond cleavage to construct C–C, C–H and C–heteroatom bonds is summarized.

Deaminative carbonylative coupling of alkylamines with styrenes under transition-metal-free conditions

Transition-metal-free deaminative carbonylation through C–N bonds activation via Katritzky salts has been successful developed. Various α,β-unsaturated ketones were obtained in moderate to good yields with alkylamines and styrenes as the substrates.

A metal-, oxidant-, and fluorous solvent-free synthesis of α-indolylketones enabled by an umpolung strategy

We have developed the first transition metal-, oxidant-, and fluorous solvent-free α-indolization of ketones using ammonium salts and enamines.

A Unified Explanation for Chemoselectivity and Stereospecificity of Ni-Catalyzed Kumada and Cross-Electrophile Coupling Reactions of Benzylic Ethers: A Combined Computational and Experimental Study

Ni-catalyzed C(sp³)-O bond activation provides a useful approach to synthesize enantioenriched products from readily available enantioenriched benzylic alcohol derivatives. The control of stereospecificity is key to the success of these transformations. To elucidate the reversed stereospecificity and chemoselectivity of Ni-catalyzed Kumada and cross-electrophile coupling reactions with benzylic ethers, a combined computational and experimental study is performed to reach a unified mechanistic understanding. Kumada coupling proceeds via a classic cross-coupling mechanism. Initial rate-determining oxidative addition occurs with stereoinversion of the benzylic stereogenic center. Subsequent transmetalation with the Grignard reagent and syn-reductive elimination produce the Kumada coupling product with overall stereoinversion at the benzylic position. The cross-electrophile coupling reaction initiates with the same benzylic C-O bond cleavage and transmetalation to form a common benzylnickel intermediate. However, the presence of the tethered alkyl chloride allows a facile intramolecular S_N2 attack by the benzylnickel moiety. This step circumvents the competing Kumada coupling, leading to the excellent chemoselectivity of cross-electrophile coupling. These mechanisms account for the observed stereospecificity of the Kumada and cross-electrophile couplings, providing a rationale for double inversion of the benzylic stereogenic center in cross-electrophile coupling. The improved mechanistic understanding will enable design of stereoselective transformations involving Ni-catalyzed C(sp³)-O bond activation.

Nickel-catalyzed reductive defunctionalization of esters in the absence of an external reductant: activation of C–O bonds

The nickel-catalyzed reductive cleavage of esters in the absence of an external reductant, which involves the cleavage of an inert acyl C–O bond in O-alkyl esters is reported.