Diverse Synthesis of C-Glycosides by Stereoselective Ni-Catalyzed Carboboration of Glycals

C-Glycosides are important structures that are common to natural products and pharmaceutical agents. Established methods for their synthesis involve the reaction of an activated anomeric carbon. In this study, we report a conceptually new approach that involves the stereoselective Ni-catalyzed carboboration of glycals. In these reactions, not only is a C-C bond formed at the anomeric carbon, but a synthetically useful C-B bond is also installed. Upon C-B oxidation, differentially protected C-glycosides to be formed. In addition, stereospecific manipulation of the C-B bond leads to diverse C-glycosides. Finally, we report the application of this method in the synthesis of established C-glycosides, such as C-glycosyl amino acids, as well as a strategy to make all possible diastereomers at C1 and C2.

Mechanism and Origins of Regio- and Stereoselective Alkylboration of Endocyclic Olefins Enabled by Nickel Catalysis

The Ni-catalyzed alkylboration of endocyclic olefins is a stereo- and regioselective approach for the synthesis of boron-containing compounds. We report a detailed density functional theory (DFT) study to elucidate the mechanism and origins of the stereo-, chemo-, and regioselectivity of alkylboration of endocyclic olefins enabled by nickel catalysis. The alkylboration proceeds via the migratory insertion of alkenes, β-H elimination of the Ni(II) complex, subsequent migratory insertion leading to a new Ni(II) complex, combined with an alkyl radical, and reductive eliminations. The electronic effects of the endocyclic olefins synergistically control the regioselectivity toward the C1- and C2-position boration. In C1-position boration, a more electron-deficient carbon atom tends to combine with an electron-rich -Bpin group and leads to C1-position boration products. The stereoselectivity is influenced by the solvent effect, and the interaction between the substrate and Ni-catalyzed groups, the low-polarity solvent 1,4-dioxane, and a favorable steric hindrance effect result in the cis-alkylboration product. Chemoselectivity toward 1,3-alkylboration results from the steric hindrance effects of the -Bpin group.

Recent progress in nickel-catalyzed carboboration of alkenes

Alkenes represent one of the most useful building blocks for organic synthesis, owing to their abundance and versatile reactivity. Transition metal (Pd, Cu, Co, Ni, Fe, etc.) catalyzed difunctionalization of alkenes provides efficient access to substituted molecules from readily available alkenes by installing functional groups across their carbon-carbon double bonds. Particularly, Nickel-based catalytic complexes have attracted a great deal of attention. This is because they are prone to undergoing oxidative addition and slow β-hydride elimination, and can access both two-electron and radical pathways. Numerous elegant Ni-catalyzed cross-coupling methods, e.g., (hetero)arylboration, alkenylboration, alkylboration and alkynylboration of alkenes, have been developed with broad scopes and a high tolerance to a variety of functional groups. Therefore, the Ni-catalyzed carboboration of alkenes has become an efficient synthetic protocol to deliver substituted compounds by the cross-coupling of alkenes, electrophiles, and B₂Pin₂. Despite this progress, a number of challenging issues remaining in the field include broadening the types of carboboration reactions, especially the asymmetric ones, diversifying electrophile types (which is limited to halogens for now) and gaining profound insight into the reaction mechanisms. This review summarizes the recent progress in this emerging field from the literature published since 2018. It will provide the scientific community with convenience to access collective information and to accelerate their further research in order to broaden the scope of methodology and application in drug discovery programs.

Nickel-Catalyzed Dearomative Arylboration of Indoles: Regioselective Synthesis of C2- and C3-Borylated Indolines

Indole dearomatization is an important strategy to access indolines: a motif present in a variety of natural products and biologically active molecules. Herein, a method for transition-metal catalyzed regioselective dearomative arylboration of indoles to generate diverse indolines is presented. The method accomplishes intermolecular dearomatization of simple indoles through a migratory insertion pathway on substrates that lack activating or directing groups on the C2- or C3-positions. Synthetically useful C2- and C3-borylated indolines can be accessed through a simple change in N-protecting group in high regio- and diastereoselectivities (up to >40:1 rr and >40:1 dr) from readily available starting materials. Additionally, the origin of regioselectivity was explored experimentally and computationally to uncover the remarkable interplay between carbonyl orientation of the N-protecting group on indole, electronics of the C2-C3 π-bond, and sterics. The method enabled the first enantioselective synthesis of (-)-azamedicarpin.

Nickel-catalyzed enantioselective 1,2-vinylboration of styrenes

A novel nickel-catalyzed asymmetric 1,2-vinylboration reaction has been developed to afford benzylic alkenylboration products with high yields and excellent enantioselectivities by using a chiral bisoxazoline ligand. Under optimized conditions, a wide variety of chiral 2-boryl-1,1-arylvinylalkanes are efficiently prepared from readily available olefins and vinyl halides in the presence of bis(pinacolato)diboron as the boron source in a mild and easy-to-operate manner. This three-component cascade protocol furnishes exceptional chemo- and stereoselectivity, and its usefulness is illustrated by its application in asymmetric modifications of several structurally complex natural products and pharmaceuticals.

A novel nickel-catalyzed asymmetric 1,2-vinylboration reaction has been developed to afford benzylic alkenylboration products with high yields and excellent enantioselectivities by using a chiral bisoxazoline ligand.

Nickel-Catalyzed Reductive Cross-Coupling of Alkyl Bromides and Chlorosilanes

A novel nickel-catalyzed highly selective reductive cross-coupling of alkyl bromides and chlorosilanes to construct the C-Si bond has been developed. Under benign reaction conditions, a series of structurally interesting organosilanes can be accessed without Ni-catalyzed isomerization. The utility of this chemistry is illustrated by further transformations of the product. Moreover, the radical mechanism of the reaction is illustrated by control experiments.