Site-Selective Functionalization of Carboranes at the Electron-Rich Boron Vertex: Photocatalytic B-C Coupling via a Carboranyl Cage Radical

Photoinduced Selective B-H Activation of nido-Carboranes

The development of new synthetic methods for B-H bond activation has been an important research area in boron cluster chemistry, which may provide opportunities to broaden the application scope of boron clusters. Herein, we present a new reaction strategy for the direct site-selective B-H functionalization of nido-carboranes initiated by photoinduced cage activation via a noncovalent cage···π interaction. As a result, the nido-carborane cage radical is generated through a single electron transfer from the 3D nido-carborane cage to a 2D photocatalyst upon irradiation with green light. The resulting transient nido-carborane cage radical could be directly probed by an advanced time-resolved EPR technique. In air, the subsequent transformations of the active nido-carborane cage radical have led to efficient and selective B-N, B-S, and B-Se couplings in the presence of N-heterocycles, imines, thioethers, thioamides, and selenium ethers. This protocol also facilitates both the late-stage modification of drugs and the synthesis of nido-carborane-based drug candidates for boron neutron capture therapy (BNCT).

Charge Transfer Complex-Enabled Synthesis of (Hetero)arylated m-Carboranes from m-Carborane Phosphonium Salts

A photoinduced charge transfer complex (CTC)-enabled photoreduction of carborane phosphonium salts for the cage carbon (hetero)arylation of carboranes was developed. It offers a convenient approach for introducing a wide range of aryl and heteroaryl groups, such as pyrroles, thiophenes, indoles, thianaphthenes, benzofurans, pyridines, and benzenes, into carboranes. This strategy offers operational simplicity, mild reaction conditions, and a broad substrate scope, making it highly advantageous.

Lewis Base-Boryl Radicals Enabled Borylation Reactions and Selective Activation of Carbon-Heteroatom Bonds

ConspectusThe past decades have witnessed tremendous progress on radical reactions. However, in comparison with carbon, nitrogen, oxygen, and other main group element centered radicals, the synthetic chemistry of boron centered radicals was less studied, mainly due to the high electron-deficiency and instability of such 3-center-5-electron species. In the 1980s, Roberts and co-workers found that the coordination of a Lewis base (amines or phosphines) with the boron center could form 4-center-7-electron boryl radicals (Lewis base-boryl radicals, LBRs) that are found to be more stable. However, only limited synthetic applications were developed. In 2008, Curran and co-workers achieved a breakthrough with the discovery of N-heterocyclic carbene (NHC) boryl radicals, which could enable a range of radical reduction and polymerization reactions. Despite these exciting findings, more powerful and valuable synthetic applications of LBRs would be expected, given that the structures and reactivities of LBRs could be easily modulated, which would provide ample opportunities to discover new reactions. In this Account, a summary of our key contributions in LBR-enabled radical borylation reactions and selective activation of inert carbon-heteroatom bonds will be presented.Organoboron compounds have shown versatile applications in chemical society, and their syntheses rely principally on ionic borylation reactions. The development of mechanistically different radical borylation reactions allows synthesizing products that are inaccessible by traditional methods. For this purpose, we progressively developed a series of NHC-boryl radical mediated chemo-, regio-, and stereoselective radical borylation reactions of alkenes and alkynes, by which a wide variety of structurally diverse organoboron molecules were successfully prepared. The synthetic utility of these borylated products was also demonstrated. Furthermore, we disclosed a photoredox protocol for oxidative generation of NHC-boryl radicals, which enabled useful defluoroborylation and arylboration reactions.Selective bond activation is an ideal way to convert simple starting materials to value-added products, while the cleavage of inert chemical bonds, in particular the chemoselectivity control when multiple identical bonds are present in similar chemical environments, remains a long-standing challenge. We envisaged that finely tuning the properties of LBRs might provide a new solution to address this challenge. Recently, we disclosed a 4-dimethylaminopyridine (DMAP)-boryl radical promoted sequential C-F bond functionalization of trifluoroacetic acid derivatives, in which the α-C-F bonds were selectively snipped via a spin-center shift mechanism. This strategy enables facile conversion of abundantly available trifluoroacetic acid to highly valuable mono- and difluorinated molecules. Encouraged by this finding, we further developed a boryl radical enabled three-step sequence to construct all-carbon quaternary centers from a range of trichloromethyl groups, where the three C-Cl bonds were selectively cleaved by the rational choice of suitable boryl radical precursors in each step. Furthermore, a boryl radical promoted dehydroxylative alkylation of α-hydroxy carboxylic acid derivatives was achieved, allowing for the efficient conversion of some biomass platform molecules to high value products.

Iridium-Catalyzed Selective B(4)-H Amination of o-Carboranes with Anthranils

We report here a catalytic selective cage B4-H amination of o-carboranes employing an Ir(III) complex as a catalyst and anthranils as aminating agents, leading to a large class of B4-aminated o-carboranes with very high yields and a broad substrate scope under mild conditions without any oxidants. In these reactions, the carboxyl group serves as a traceless directing unit to determine the site selectivity and degree of substitution.