Regiodivergent Copper Catalyzed Borocyanation of 1,3-Dienes

Ligand-Controlled Regiodivergent Cyanoboration of Internal Allenes by Copper Catalysis

The first copper-catalyzed regiodivergent cyanoboration of internal allenes with B₂ pin₂ (bis(pinacolato)diboron) and NCTS (N-cyano-N-phenyl-p-toluenesulfonamide) derivatives is reported. The β,γ- and α,β-cyanoborylated products were synthesized with high regio- and stereo-selectivity. Computational studies revealed that nucleophilic addition of allylcopper or related intermediates on cyanation reagent is the regio- and stereo-determining step, while transmetalation with B₂ pin₂ is the rate-determining step. The nucleophilic addition step proceeds via inner-sphere mechanism in the Cu^I /P(o-tol)₃ and Cu^I /Xantphos (P(o-tol)₃ =tris(o-methylphenyl)phosphine, Xantphos=4,5-bis(diphenylphosphino)-9,9-dimethylxanthene) catalytic systems and via outer-sphere mechanism in the Cu^II /Xantphos catalytic system, respectively.

Synthesis of Cyclic Allylborates from 1,3-Dienes and a Diboron Reagent

The synthesis of cyclic allylborates was achieved by the reaction of 1,3-dienes and B₂ pin₂ using a copper catalyst. Several 1,3-dienes were converted to the corresponding cyclic allylborates in moderate to high yields. The cyclic allylborate obtained could be used in several organic transformations such as allylation of electrophiles and Suzuki-Miyaura coupling.

Copper-Catalyzed Highly Selective Protoboration of CF3 -Containing 1,3-Dienes

The copper-catalyzed highly selective protoboration of CF₃ -containing conjugated diene with proton source and B₂ Pin₂ has been developed. This chemistry could suppress the well-known defluorination and provide borated reagents with an intact CF₃ -group. Further studies indicated that the functional group tolerance of this chemistry is very well, and the products could be used as versatile precursors for different types of transformations. Importantly, using chiral diphosphine ligand, we have developed the first example for using such starting material to synthesis allylic boron-reagents which bearing a CF₃ -containing chiral center. Notably, the reaction mechanism was intensively studied by DFT calculations, which could reveal the reason that defluorination was inhibited.

Copper(I)-Catalyzed Asymmetric Conjugate 1,6-, 1,8-, and 1,10-Borylation

Catalytic asymmetric remote conjugate borylation is challenging as the control of regioselectivity is not trivial, the electrophilicity of remote sites is extenuated, and the remote asymmetric induction away from the carbonyl group is difficult. Herein, catalytic asymmetric conjugate 1,6-, 1,8- and 1,10-borylation was developed with excellent regioselectivity, which delivered α-chiral boronates in moderate to high yields with high enantioselectivity. The produced chiral boronate smoothly underwent oxidation, cross-coupling, and one-carbon homologation to give synthetically versatile chiral compounds in moderate yields with excellent stereoretention. Furthermore, a stereomechanistic analysis was conducted using DFT calculations, which provides insights into the origins of the regioselectivity. Finally, the present 1,6-borylation was successfully applied in an efficient one-pot asymmetric synthesis of (-)-7,8-dihydrokavain.

Palladium-Catalyzed Difunctionalization of 1,3-Diene with Amine and Disilane under a Mild Re-oxidation System

A highly regioselective and stereoselective difunctionalization reaction of 1,3-diene with amine and disilane to form C-N and C-Si bonds via a one-step Pd/Cu/O₂ system is disclosed. The difunctionalization reaction affords allylic silanes, including the allylic amine moiety, in up to 92 % yield in the absence of any acid, base, or external ligand. The developed synthetic methodology can be scaled to 100 g in high yield with high Z-selectivity, which demonstrates the feasibility of the reaction for industrial applications.

Orthogonal Regulation of Nucleophilic and Electrophilic Sites in Pd-Catalyzed Regiodivergent Couplings between Indazoles and Isoprene

Depending on the reactant property and reaction mechanism, one major regioisomer can be favored in a reaction that involves multiple active sites. Herein, an orthogonal regulation of nucleophilic and electrophilic sites in the regiodivergent hydroamination of isoprene with indazoles is demonstrated. Under Pd-hydride catalysis, the 1,2- or 4,3-insertion pathway with respect to the electrophilic sites on isoprene could be controlled by the choice of ligands. In terms of the nucleophilic sites on indazoles, the reaction occurs at either the N¹ - or N² -position of indazoles is governed by the acid co-catalysts. Preliminary experimental studies have been performed to rationalize the mechanism and regioselectivity. This study not only contributes a practical tool for selective functionalization of isoprene, but also provides a guide to manipulate the regioselectivity for the N-functionalization of indazoles.

Synthesis of Nitrile Bearing Acyclic Quaternary Centers through Co(III)-Catalyzed Sequential C-H Bond Addition to Dienes and N-Cyanosuccinimide

Herein we disclose a three-component strategy to access quaternary centers bearing nitriles by cobalt-catalyzed C-H bond activation and sequential addition to internally substituted 1,3-dienes and an electrophilic cyanating reagent with high regio and stereocontrol. 2-Aryl and alkyl monosubstituted dienes provide α-aryl and α-alkyl α-methyl-substituted nitriles, respectively. An even wider variety of functionality can be installed at the quaternary carbon by using 1,2-disubstituted dienes. The synthetic utility of the nitrile products was successfully demonstrated by various transformations, including conversions to γ-lactones and tetrazoles. The observed connectivity in the products along with studies with deuterium labeled reactants provide insight into the mechanism. Formation of a 7-membered cobaltacycle by C-H activation and migratory insertion of the diene is followed by β-hydride elimination and hydride reinsertion to give a 6-membered cobaltacycle that then reacts with the cyanating agent.

Ligand-Controlled Copper-Catalyzed Regiodivergent Carbonylative Synthesis of α-Amino Ketones and α-Boryl Amides from Imines and Alkyl Iodides

Regioselective transformation is among the long-standing challenges in organic synthesis. In this communication, a copper-catalyzed selectivity controlled regiodivergent borocarbonylation of imines with alkyl iodides has been developed. Various α-amino ketones and α-boryl amides were produced in moderate to good yields from the same substrates. The choice of the ligand is key for the regioselectivity control: α-amino ketones were produced selectively in good yields with (p-CF₃ C₆ H₄ )₃ P as the ligand, whereas the corresponding α-boryl amides were obtained with high regioselectivities when using ^Me IMes as the ligand.

Copper-Catalyzed Borylative Couplings with C-N Electrophiles

Copper-catalyzed borylative multicomponent reactions (MCRs) involving olefins and C-N electrophiles are a powerful tool to rapidly build up molecular complexity. The products from these reactions contain multiple functionalities, such as amino, cyano and boronate groups, that are ubiquitous in medicinal and process chemistry programs. Copper-catalyzed MCRs are particularly attractive because they use a relatively abundant and non-toxic catalyst to selectively deliver high-value products from simple feedstocks such as olefins. In this Minireview, we explore this rapidly emerging field and survey the borylative union of allenes, dienes, styrenes and other olefins, with imines, nitriles and related C-N electrophiles.

Enantio- and Diastereoselective Synthesis of Homopropargyl Amines by Copper-Catalyzed Coupling of Imines, 1,3-Enynes, and Diborons

An efficient, enantio- and diastereoselective, copper-catalyzed coupling of imines, 1,3-enynes, and diborons is reported. The process shows broad substrate scope and delivers complex, chiral homopropargyl amines; useful building blocks on the way to biologically-relevant compounds. In particular, functionalized homopropargyl amines bearing up to three contiguous stereocenters can be prepared in a single step.

Transition-Metal-Catalyzed 1,2-Carboboration of Alkenes: Strategies, Mechanisms, and Stereocontrol

During the past decade, many research groups have described catalytic methods for 1,2-carboboration, allowing access to structurally complex organoboronates from alkenes. Various transition metals, especially copper, palladium, and nickel, have been widely used in these reactions. This review summarizes advances in this field, with a special focus on the catalytic cycles involved in different metal-catalyzed carboboration reactions, as well as the regio- and stereochemical consequences of the underlying mechanisms. 1,2-Carboboration of other unsaturated systems, such as alkynes and allenes, are outside of the scope of this review.

The Mechanism of Copper-Catalyzed Trifunctionalization of Terminal Allenes

A highly selective copper-catalyzed trifunctionalization of allenes has been established based on diborylation/cyanation with bis(pinacolato)diboron (B₂ pin₂ ) and N-cyano-N-phenyl-p-toluenesulfonamide (NCTS). The Cu-catalyzed trifunctionalization of terminal allenes is composed of three catalytic reactions (first borocupration, electrophilic cyanation, and second borocupration) that provide a densely functionalized product with regio-, chemo- and diastereoselectivity. Allene substrates have multiple reaction-sites, and the selectivities are determined by the suitable interactions (e.g., electronic and steric demands) between the catalyst and substrates. We employed DFT calculations to understand the cascade copper-catalyzed trifunctionalization of terminal allenes, providing densely-functionalized organic molecules with outstanding regio-, chemo- and diastereoselectivity in high yields. The selectivity challenges presented by cumulated π-systems are addressed by systematic computational studies; these give insight to the catalytic multiple-functionalization strategies and explain the high selectivities that we see for these reactions.

Ligand-Controlled Regiodivergent Enantioselective Rhodium-Catalyzed Alkene Hydroboration

Regiocontrol in the rhodium-catalyzed boration of vinyl arenes is typically dominated by the presence of the conjugated aryl substituent. However, small differences in TADDOL-derived chiral monophosphite ligands can override this effect and direct rhodium-catalyzed hydroboration of β-aryl and β-heteroaryl methylidenes by pinacolborane to selectively produce either chiral primary or tertiary borated products. The regiodivergent behavior is coupled with enantiodivergent addition of the borane. The nature of the TADDOL backbone substituents and that of the phosphite moiety function synergistically to direct the sense and extent of regioselectivity and enantioinduction. Twenty substrates are shown to undergo each reaction mode with regioselectivity values reaching greater than 20:1 and enantiomer ratios reaching up to 98:2. A variety of subsequent transformations illustrate the potential utility of each product.

Racemic Vinylallenes in Catalytic Enantioselective Multicomponent Processes: Rapid Generation of Complexity through 1,6-Conjugate Additions

Racemic vinylallenes are shown to be effective substrates for catalytic multicomponent diastereo- and enantioselective 1,6-conjugate addition of multifunctional allyl moieties to easily accessible α,β,γ,δ-unsaturated diesters. Reactions may be catalyzed by 5.0 mol % of a readily accessible NHC-Cu complex at ambient temperature, and other than a vinylallene, involve B₂ (pin)₂ and an α,β,γ,δ-unsaturated diester. A variety of vinylallenes were converted to products bearing a Z-trisubstituted alkenyl-B(pin) moiety, a vinyl group, a β,γ-unsaturated diester unit, and vicinal stereogenic centers in up to 67 % yield, 87:13 Z/E ratio, >98:2 d.r., and 98:2 e.r. Chemoselective modifications involving the alkenyl-B(pin), the vinyl, or the 1,2-disubstituted olefin moieties were carried out to demonstrate versatility and utility. Stereochemical models, based on mechanistic and DFT studies, demonstrate the dynamic behavior of intermediated Cu-allyl species and account for various selectivity profiles.