Photoinduced Dehydrogenative Borylation via Dihydrogen Bond Bridged Electron Donor and Acceptor Complexes**

Abstract

Air-stable amine-and phosphine-boranes are discovered as donors to integrate with pyridinium acceptor for generating photoactive electron-donor-acceptor (EDA) complexes. Experimental results and DFT calculations suggest a dihydrogen bond bridging the donor and acceptor. Irradiating the EDA complex enables an intra-complex single electron transfer to give a boron-centered radical for dehy-drogenative borylation with no need of external photosensitizer and radical initiator. The deprotonation of Whelandlike radical intermediate rather than its generation is believed to determine the good ortho-selectivity based on DFT calculations. A variety of α-borylated pyridine derivatives have been readily synthesized with good functional group tolerance.

Selective multifunctionalization ofN-heterocyclic carbene boranesviathe intermediacy of boron-centered radicals

“…However, relevant investigation on boryl radicals remains relatively underdeveloped. 12 In this connection, we chose 4-vinylpyridine 2 as the model substrate, in view of the unsuccessful application of previous methods on this type of coupling partner (see Table S1† in the ESI for details). 8 In the presence of TBADT (1 mol%) and triphenylmethanethiol (50 mol%), the treatment of NHC–borane 1 with 2 in DMA (0.1 M) under light irradiation (390 nm) at room temperature for 48 h afforded the desired disubstituted product 4 in an isolated yield of 61% (Table 1, entry 1).…”

“…Boryl-radical mediated borylation has emerged as transition-metal-free access to various organoboron compounds. 8 The active boron-centered radical is reported to attack electron-deficient alkenes, 9 alkynes, 10 imines, 11 (hetero)arenes, 12 and isocyanides 13 for the construction of new boron–carbon (B–C) bonds. However, the developed strategy has shown limited success in B–H multi-functionalization, probably because the original catalytic system was not competent for the generation of boron-centered radicals from mono-functionalized boranes, the product of the first B–H activation (Scheme 1d).…”

Formation of C–B, C–C, and C–X Bonds from Nonstabilized Aryl Radicals Generated from Diaryl Boryl Radicals

“…Radicals, which can be generated from many feedstock chemicals, are among the most fundamental intermediates in synthetic chemistry and have become useful tools for developing novel methodologies. − With the development of organoboron chemistry, boron-centered radicals have become more and more attractive, but their synthetic applications remain limited. − For example, the applications of neutral boryl radicals, which are three-center–five-electron radicals, are limited because of their extreme electron deficiency (Figure A). In contrast, four-center–seven-electron boryl radicals ligated with a Lewis base (usually a carbene, a phosphine, or an amine) are relatively stable and have been extensively studied. − These Lewis base-based boryl radicals are known to react with alkenes and heteroaromatic rings, and such reactions have been used to modify drug molecules. − In recent years, the groups of Wang and Li have reported some elegant uses of amine-based boron free radicals as catalysts. , In 2022, Xia’s group reported a method for alkyl radical generation by direct splitting of the C–O bonds of alcohol–boron radical intermediates; in these reactions, various alcohols were successfully used as alkyl radical precursors (Figure B).…”

C4-Selective C–H Borylation of Pyridinium Derivatives Driven by Electron Donor–Acceptor Complexes

“…Although NMR experiments revealed that the Ph 3 P–BH 3 complex effectively produced the borylated product, the isolated product was not chemically stable, and decomposition occurred on silica during column chromatography (entry 7). To our delight, we observed that after borylation, the replacement of PPh 3 with a tertiary amine as a Lewis base resulted in the formation of a more stable amine-borylated product, thus expanding the scope of the Lewis base of the borylated pyridines (entry 8). As expected, a trace quantity of the desired product was detected without light irradiation (entry 9).…”