Accessing Both Retention and Inversion Pathways in Stereospecific, Nickel-Catalyzed Miyaura Borylations of Allylic Pivalates

Optimization of Metal-Support Cooperation for Boosting the Performance of Supported Gold Catalysts for the Borylation of C-O and C-N Bonds

The cooperation of multiple catalytic components is a powerful tool for intermolecular bond formation, specifically, cross-coupling reactions. Supported metal catalysts have interfacial sites between metal nanoparticles and their supports where multiple catalytic elements can work in cooperation to efficiently promote intermolecular reactions. Hence, the establishment of novel guidelines for designing active interfacial sites of supported metal catalysts is indispensable for heterogeneous catalysts which enable efficient cross-coupling reactions. In this article, we performed kinetic and theoretical studies to elucidate the effect of metal-support cooperation for the borylation of C-O bonds by supported gold catalysts and revealed that the Lewis acid density of the supports determined the number of active sites at which metal nanoparticles (NPs) and Lewis acid at the surface of the supports work in cooperation. Furthermore, DFT calculations revealed that strong adsorption of diborons at the interface between Au NPs and supports and a decrease in the LUMO level of adsorbed diboron were responsible for efficient C-O bond borylation. Supported Au catalysts with the optimized metal-metal oxide cooperation sites, namely, Au/α-Fe2O3 catalyst, showed excellent activity for C-O bond borylation, and also enabled the synthesis of organoboron compounds by using continuous-flow reactions. Furthermore, Au/α-Fe2O3 showed high activity for direct C-N bond borylation without the transformation of amino groups to ammonium cations. The results described herein suggest that the optimization of metal-metal oxide cooperation is beneficial for taking full advantage of the potential performance of supported metal catalysts for intermolecular reactions.

Visible Light Induced B-H Bond Insertion Reaction with Diazo Compounds

A protocol induced by visible light for the direct insertion of α-carbonyl carbenes into the B-H bond of amine-borane adducts has been developed under conditions that are free of metal and photocatalyst. This approach provides a straightforward route to various organoboron compounds from diazo compounds and amine-borane adducts with moderate to good yields. Mechanistic investigations reveal that this photoinduced reaction proceeds through concerted carbene insertion into the B-H bond, and the photoinduced generation of free carbene from α-diazo esters may be the rate-determining step.

Chemoselective and Stereoselective Allylation of Bis(alkenyl)boronates

Bis(alkenyl)boronates react with optically active Ir(π-allyl) species in a process that involves allylation of the more substituted olefin and 1,2-metalate shift of the less substituted olefin. The method constructs valuable enantioenriched tertiary allylic boronic esters with high chemoselectivity, enantioselectivity and diastereoselectivity. Allylic functionalization reactions transform the 1,3-stereodiad to 1,5- and 1,6-stereochemical relationships.

Enantio- and Regioconvergent Nickel-Catalyzed Etherification of Phenols by Allylation to Access Chiral C(sp3)-O Allyl Aryl Ethers

An enantio- and regioconvergent allylation of phenols under nickel catalysis with an α-/γ-regioisomeric mixture of racemic silylated/germylated allylic chlorides is reported. The silyl/germyl group governs the regioselectivity, and the transformation affords enantiomerically enriched unsymmetrical 1,3-disubstituted allyl aryl ethers with excellent regiocontrol in good yields and excellent enantioselectivities. Notably, no nickel-mediated C-O bond activation is observed at room temperature. The synthetic value of these densely functionalized silicon-containing building blocks is demonstrated in a series of chemoselective transformations, including a [3,3]-sigmatropic rearrangement for the construction of an α-chiral silane.

Borylation of Stable C(sp3)-O Bonds of Alkyl Esters over Supported Au Catalysts

We report herein that supported gold catalysts efficiently promote the borylation of stable C(sp3)-O bonds of alkyl esters. The use of a disilane as an electron source and gold nanoparticles as a single-electron transfer catalyst is the key to generating alkyl radicals via the homolysis of stable C(sp3)-O bonds, thereby enabling cross-coupling between bis(pinacolato)diboron and linear and cyclic alkyl esters to afford the diverse alkyl boronates.

Nickel-catalyzed electrophiles-controlled enantioselective reductive arylative cyclization and enantiospecific reductive alkylative cyclization of 1,6-enynes

Transition metal-catalyzed asymmetric cyclization of 1,6-enynes is a powerful tool for the construction of chiral nitrogen-containing heterocycles. Despite notable achievements, these transformations have been largely limited to the use of aryl or alkenyl metal reagents, and stereoselective or stereospecific alkylative cyclization of 1,6-enynes remains unexploited. Herein, we report Ni-catalyzed enantioselective reductive anti-arylative cyclization of 1,6-enynes with aryl iodides, providing enantioenriched six-membered carbo- and heterocycles in good yields with excellent enantioselectivities. Additionally, we have realized Ni-catalyzed enantiospecific reductive cis-alkylative cyclization of 1,6-enynes with alkyl bromides, furnishing chiral five-membered heterocycles with high regioselectivity and stereochemical fidelity. Mechanistic studies reveal that the arylative cyclization of 1,6-enynes is initiated by the oxidative addition of Ni(0) to aryl halides and the alkylative cyclization is triggered by the oxidative addition of Ni(0) to allylic acetates. The utility of this strategy is further demonstrated in the enantioselective synthesis of the antiepileptic drug Brivaracetam.

A general approach to stereospecific Pd-catalyzed cross-coupling reactions of benzylic stereocenters

We have developed a general process for the formation of enantioenriched benzylic stereocenters via stereospecific Pd-catalyzed cross-coupling reactions of enantioenriched benzylic tricyclohexyltin nucleophiles. This process proceeds with excellent stereospecificity for a remarkably broad scope of electrophilic coupling partners including aryl and heteroaryl halides and triflates, acid chlorides, thioesters, chloroformates, and carbamoyl chlorides. Thus, enantioenriched 1,1-diarylalkanes as well as formal products of asymmetric enolate arylation are readily accessed using this approach. We additionally provide the first demonstration of a Sn-selective cross-coupling reaction using a vicinal alkylborylstannane nucleophile. In these reactions, the presence of cyclohexyl spectator ligands on tin is essential to ensure selective transfer of the secondary benzylic unit from tin to palladium.

Nickel-Catalyzed Stereoselective Coupling Reactions of Benzylic and Alkyl Alcohol Derivatives

ConspectusNickel-catalyzed reactions of alkyl alcohol derivatives leverage the high prevalence of hydroxyl groups in natural products, medicinal agents, and synthetic intermediates to provide access to C(sp3)-rich frameworks. This Account describes our laboratory's development of stereospecific and stereoconvergent C-C bond forming reactions employing C(sp3)-O and C(sp3)-N electrophiles. In the context of development of new transformations, we also define fundamental characteristics of the nickel catalysts.Part I details the nickel-catalyzed cross-coupling reactions developed by our group which hinges on stereospecific formation of stable π-benzyl intermediates. Acyclic and cyclic ethers, esters, carbamates, lactones, and sulfonamides undergo Kumada-, Suzuki-, and Negishi-type coupling reactions to produce enantioenriched products with high fidelity of stereochemical information. We describe extension to include ring-opening reactions of saturated heterocycles to afford acyclic 1,3-fragments in high diastereomeric ratios. We also describe our advances in stereospecific nickel-catalyzed cross-electrophile coupling reactions. Tethered C-O and C-X electrophiles proved fruitful for construction of a variety of carbocyclic frameworks. We report an intramolecular cross-electrophile coupling of benzylic pivalates with aryl bromides for the synthesis of indanes and tetralins. We found that 4-halotetrahydropyrans and 4-halopiperidines readily undergo stereospecific ring contraction to afford substituted cyclopropanes. Mechanistic investigations are consistent with closed-shell intermediates, a Ni(0)/Ni(II) cycle, and an intramolecular SN2-type reaction of a key organonickel intermediate to form the cyclopropane. Building toward more complex cascade reactions, we have demonstrated that 2-alkynyl piperidines incorporate MeMgI in a dicarbofunctionalization of the alkyne to afford highly substituted vinyl cyclopropanes.In Part II we present our development of stereoconvergent reactions of alkyl alcohol derivatives. In order to expand the utility of the intramolecular XEC reaction, we sought to employ unactivated alkyl electrophiles. Specifically, alkyl dimesylates engage in intramolecular XEC reactions to form alkyl cyclopropanes. In contrast to our previous work, these reactions proceed through open-shell intermediates and favor stereoconvergent formation of the trans-cyclopropane. Enantioselective aldol reactions can be employed in syntheses of 1,3-diols which furnish enantioenriched cyclopropanes in high ee. Experimental and computational evidence reveals that MeMgI mediates formation of alkyl iodides in situ. The coupling reaction initiates with halogen atom abstraction at the secondary alkyl iodide. The alkyl Ni(II) complex then proceeds through a stereospecific SN2-type ring closure to form cyclopropane. In an effort to increase functional group compatibility in the synthesis of cyclopropanes from alkyl dimesylates we developed a zinc-mediated reaction of 1,3-dimesylates prepared from medicinal analogues. In challenging nickel-catalyzed intramolecular cross-electrophile coupling we were also able to show that vicinal carbocycles can be prepared under similar conditions, affording vicinal cyclopentyl-cyclopropyl motifs in high yield.In Part III we discuss our recent findings on the role of ligand identity in catalyst selectivity for stereospecific vs stereoablative mechanisms for oxidative addition. We demonstrate multivariable control of mechanism, where the choice of substrate and ligand work together to promote open- or closed-shell intermediates. In divergent reactions of 4-halotetrahydropyrans we observe distinct ligand preference for reactions at the C(sp3)-O center or the C(sp3)-Cl center. These findings are the source of continued investigations in our laboratory.

Enantioselective Hydroalkoxylation of 1,3-Dienes via Ni-Catalysis

As an advance in hydrofunctionalization, we herein report that alcohols add to 1,3-dienes with high regio- and enantioselectivity. Using Ni-DuPhos, we access enantioenriched allylic ethers. Through the choice of solvent-free conditions, we control the reversibility of C-O bond formation. This work showcases a rare example of methanol as a reagent in asymmetric synthesis.

A General Strategy for the Asymmetric Preparation of α-Stereogenic Allyl Silanes, Germanes, and Boronate Esters via Dual Copper Hydride- and Palladium-Catalysis

α-Stereogenic allyl metalloids are versatile synthetic intermediates which can undergo various stereocontrolled transformations. Most existing methods to prepare α-stereogenic allyl metalloids involve multi-step sequences that curtail the number of compatible substrates and are limited to the synthesis of boronates. Here, we report a general method for the enantioselective preparation of α-stereogenic allyl metalloids utilizing dual CuH- and Pd-catalysis. This approach leverages a stereoretentive Cu-to-Pd transmetalation of an in situ generated alkyl copper species to allow access to enantioenriched allyl silanes, germanes, and boronate esters with broad functional group compatibility.

Solvent coordination to palladium can invert the selectivity of oxidative addition

Reaction solvent was previously shown to influence the selectivity of Pd/P t Bu3-catalyzed Suzuki-Miyaura cross-couplings of chloroaryl triflates. The role of solvents has been hypothesized to relate to their polarity, whereby polar solvents stabilize anionic transition states involving [Pd(P t Bu3)(X)]- (X = anionic ligand) and nonpolar solvents do not. However, here we report detailed studies that reveal a more complicated mechanistic picture. In particular, these results suggest that the selectivity change observed in certain solvents is primarily due to solvent coordination to palladium. Polar coordinating and polar noncoordinating solvents lead to dramatically different selectivity. In coordinating solvents, preferential reaction at triflate is likely catalyzed by Pd(P t Bu3)(solv), whereas noncoordinating solvents lead to reaction at chloride through monoligated Pd(P t Bu3). The role of solvent coordination is supported by stoichiometric oxidative addition experiments, density functional theory (DFT) calculations, and catalytic cross-coupling studies. Additional results suggest that anionic [Pd(P t Bu3)(X)]- is also relevant to triflate selectivity in certain scenarios, particularly when halide anions are available in high concentrations.

Stereoselective Dehydroxyboration of Allylic Alcohols to Access (E)-Allylboronates by a Combination of C-OH Cleavage and Boron Transfer under Iron Catalysis

Iron-catalyzed direct S_N2' dehydroxyboration of allylic alcohols has been developed to access (E)-stereoselective allylboronates. Allylic alcohols with diverse structures and functional groups, especially derived from natural products, underwent smooth transformation. The six-membered ring transition state formed by allylic alcohols and iron-boron intermediate was indicated to be the key component involved in transfer of the boron group, activation of the C-OH bond, and control of the stereoselectivity.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

Nickel/Photo-Cocatalyzed C(sp2)-H Allylation of Aldehydes and Formamides

Herein we report a nickel/photo-cocatalyzed C(sp²)-H allylation of aldehydes and formamides wherein both allyl acetates and allyl alcohols can be used as the allylating agents. In this reaction, radical-type umpolung of the formyl moiety is enabled by tetrabutylammonium decatungstate as a hydrogen-atom-transfer photocatalyst, whereas nickel serves to cleave the C-O bond of allyl acetates or allyl alcohols. The synergistic effect of these two catalysts provides new access to various β,γ-unsaturated ketones and amides with high selectivities.

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.

Nickel-Catalyzed Ipso-Borylation of Silyloxyarenes via C-O Bond Activation

The conversion of silyloxyarenes to boronic acid pinacol esters via nickel catalysis is described. In contrast to other borylation protocols of inert C-O bonds, the method is competent in activating the carbon-oxygen bond of silyloxyarenes in isolated aromatic systems lacking a directing group. The catalytic functionalization of benzyl silyl ethers was also achieved under these conditions. Sequential cross-coupling reactions were achieved by leveraging the orthogonal reactivity of silyloxyarenes, which could then be functionalized subsequently.

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.

Boron insertion into alkyl ether bonds via zinc/nickel tandem catalysis

Mild methods to cleave the carbon-oxygen (C-O) bond in alkyl ethers could simplify chemical syntheses through the elaboration of these robust, readily available precursors. Here we report that dibromoboranes react with alkyl ethers in the presence of a nickel catalyst and zinc reductant to insert boron into the C-O bond. Subsequent reactivity can effect oxygen-to-nitrogen substitution or one-carbon homologation of cyclic ethers and more broadly streamline preparation of bioactive compounds. Mechanistic studies reveal a cleavage-then-rebound pathway via zinc/nickel tandem catalysis.

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.

Overcoming Kinetic and Thermodynamic Challenges of Classic Cope Rearrangements

Systematic evaluation of 1,5-dienes bearing 3,3-electron-withdrawing groups and 4-methylation results in the discovery of a Cope rearrangement for Meldrum's acid-containing substrates that have unexpectedly favorable kinetic and thermodynamic profiles. The protocol is quite general due to a concise and convergent synthesis from abundant starting materials. Furthermore, products with an embedded Meldrum's acid moiety are prepared, which, in turn, can yield complex amides under neutral conditions. We have now expanded the scope of the reductive Cope rearrangement, which, via chemoselective reduction, can promote thermodynamically unfavorable [3,3] sigmatropic rearrangements of 3,3-dicyano-1,5-dienes to form reduced Cope rearrangement products. The Cope rearrangement is found to be stereospecific and can yield enantioenriched building blocks when chiral, nonracemic 1,3-disubstituted allylic electrophiles are utilized. We expand further the use of Cope rearrangements for the synthesis of highly valuable building blocks for complex- and drug-like molecular synthesis.

Nickel-catalyzed alkenylboration of alkenylarenes to access homoallylic boronic esters

A practical nickel-catalyzed alkenylboration of alkenylarenes with excellent chemo- and regio-selectivity has been developed.

Nickel-Catalyzed Highly Selective Hydroalkenylation of Alkenyl Boronic Esters to Access Allyl Boron

Allyl boron derivatives are valuable building blocks in the synthesis of natural products and bioactive molecules. Herein, a practical strategy of nickel-catalyzed highly selective hydroalkenylation of alkenyl boronic esters was developed. Under the mild reaction conditions, a variety of allyl boronic esters were accessed with excellent chemo- and regioselectivity. The mechanism of this transformation was illustrated by control experiments and kinetic studies.

Methodologies and Strategies for Selective Borylation of C-Het and C-C Bonds

Organoborons have emerged as versatile building blocks in organic synthesis to achieve molecular diversity and as carboxylic acid bioisosteres with broad applicability in drug discovery. Traditionally, these compounds are prepared by the substitution of Grignard/lithium reagents with electrophilic boron species and Brown hydroboration. Recent developments have provided new routes for the efficient preparation of organoborons by applying reactions using chemical feedstocks with leaving groups. As compared to the previous methods that used organic halides (I, Br, and Cl), the direct borylation of less reactive C-Het and C-C bonds has become highly important to get efficiency and functional-group compatibility. This Review aims to provide a comprehensive overview of this topic, including (1) C-F bond borylation, (2) C-O bond borylation, (3) C-S bond borylation, (4) C-N bond borylation, and (5) C-C bond borylation. Considerable attention is given to the strategies and mechanisms involved. We expect that this Review will inspire chemists to discover more efficient transformations to expand this field.

Nickel-Catalyzed Homo- and Cross-Coupling of Allyl Alcohols via Allyl Boronates

A nickel-catalyzed homo- and cross-coupling of allylic alcohols to 1,5-dienes in the presence of B₂pin₂ with excellent regioselectivity has been developed. Mechanistic studies indicate that the reaction proceeds via sequential nickel-catalyzed borylation of allyl alcohols followed by cross-coupling of the resulting allyl boronates with allyl alcohols. The method was effectively applied to nickel-catalyzed allylation of aldehydes using allylic alcohols directly.

Synthesis of Allylboranes via Cu(I)-Catalyzed B-H Insertion of Vinyldiazoacetates into Phosphine-Borane Adducts

Cu(I) catalysts enable C-B bond formation via direct insertion of vinyldiazoacetates into B-H bonds of borane-phosphine Lewis adducts to form phosphine-protected allylboranes under mild conditions. The resulting allylborane-phosphine Lewis adducts can be used in the diastereoselective allylation of aldehydes directly without the need for removal of the phosphine. The allylation reaction proceeds with high diastereoselectivity and yields 5,6-disubstituted dihydropyranones after treatment with an appropriate acid.

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.

Kinetic Resolution of Secondary Allyl Boronates and Their Application in the Synthesis of Homoallylic Amines

Highly enantioenriched, chromatographically-stable secondary allyl boronates featuring a 1,1,2,2-tetraethyl-1,2-ethanediol fragment (Epin) were obtained by kinetic resolution of their racemic mixtures. The Epin group at boron considerably improved stability of allyl boronates allowing them to be readily isolated by chromatography on silica. The resolved reagents were applied in stereoselective synthesis of homoallylic amines with an internal double bond employing unprotected imines formed in situ from aldehydes and ammonia. The reactions proceeded with an excellent transfer of chirality.

Stereoselective Homocrotylation of Aldehydes: Enantioselective Synthesis of Allylic-Substituted Z/ E-Alkenes

Cyclopropanated allyl- and crotylboron reagents participate in homoallylation and homocrotylation reactions that enable enantioselective access to motifs that otherwise require many steps to synthesize. In this study, we investigated the effect of substituents α- to boron, predicted either to counteract or reinforce the 1,3- selectivity of the parent reagents. We then investigated the transformation of the substituted homocrotylation products in intramolecular photocycloadditions to produce stereochemically complex natural-product-like scaffolds, finding that flow conditions enhanced the regioselectivity and yield.

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

This perspective showcases our development of benzylic and allylic amine and alcohol derivatives as electrophiles for stereospecific, nickel-catalyzed cross-coupling reactions, as well as the prior art that inspired our efforts. The success of our effort has relied on the use of benzyl ammonium triflates as electrophiles for cross-couplings via C-N bond activation and benzylic and allylic carboxylates for cross-couplings via C-O bond activation. Our work, along with others' exciting discoveries, has demonstrated the potential of stereospecific, nickel-catalyzed cross-couplings of alkyl electrophiles in asymmetric synthesis, and enables efficient generation of both tertiary and quaternary stereocenters.

Asymmetric remote C-H borylation of internal alkenes via alkene isomerization

Recent years have witnessed the growing interest in the remote functionalization of alkenes for it offers a strategy to activate the challenging C–H bonds distant from the initiation point via alkene isomerization/functionalization. However, the catalytic enantioselective isomerization/functionalization with one single transition metal catalyst remains rare. Here we report a highly regio- and enantioselective cobalt-catalyzed remote C–H bond borylation of internal alkenes via sequential alkene isomerization/hydroboration. A chiral ligand featured twisted pincer, anionic, and non-rigid characters is designed and used for this transformation. This methodology, which is operationally simple using low catalyst loading without additional activator, shows excellent enantioselectivity and can be used to convert various internal alkenes with regio- and stereoisomers to valuable chiral secondary organoboronates with good functional group tolerance.

Sequential alkene isomerization/functionalization enables enantioselective transformations of remote C–H bonds. Here, the authors report a chiral cobalt catalytic system for the highly enantioselective, remote C–H borylation of internal alkenes via an isomerization/hydroboration sequence.

Stereodivergent Catalysis

This review covers diastereo- and enantiodivergent catalyzed reactions in acyclic and cyclic systems using metal complexes or organocatalysts. Among them, nucleophilic addition to carbon-carbon and carbon-nitrogen double bonds, α-functionalization of carbonyl compounds, allylic substitutions, and ring opening of oxiranes and aziridines are considered. The diastereodivergent synthesis of alkenes from alkynes is also included. Finally, stereodivergent intramolecular and intermolecular cycloadditions and other cyclizations are also reported.

Nickel-Catalyzed Hydrogenolysis and Conjugate Addition of 2-(Hydroxymethyl)pyridines via Organozinc Intermediates

2-Hydroxymethylpyridines undergo nickel-catalyzed hydrogenolysis upon activation with a chlorophosphate. Reactions employ diethylzinc and are proposed to proceed through secondary benzylzinc reagents. Quenching with deuteromethanol provides straightforward incorporation of a deuterium label in the benzylic position. Intramolecular conjugate additions with α,β-unsaturated esters are also demonstrated and support the intermediacy of a benzylzinc complex.