Recently, Professor Xie Jin, Zhu Chengjian and Li Shuhua's research group (School of Chemistry and Chemical Engineering, Nanjing University) have cooperated to completed the high regioselective synthesis b-borylated amides via visible-light induced radical hydroboration. The result was published online at Angew. Chem. Int. Ed. (10.1002/anie.202005749) under the title Photoredox Controlled b-Regioselective Radical Hydroboration of Activated Alkenes with NHC-Boranes. The first author is Zhu Congjun, a doctoral student of 2018, and Professor Xie Jin, Zhu Chengjian and Li Shuhua are co-corresponding authors.
The organoboron are versatile building blocks for cross-coupling in synthetic chemistry and also important structural motifs in medicine and materials. The direct hydroboration of alkenes with boranes has been esteemed as a direct and efficient method for the construction of C-B bonds. In general, the regio-selectivity for hydroboration of olefin should meet the electronic demand in the transition states, in which the boron motif adds to the relatively less electronic-rich position. N-heterocyclic carbene (NHC) boranes are air-stable and easily handling organoborane reagents. During the past decade, many groups have successfully shown the promising chemical space in the exploration of NHC-boryl radicals in both organic synthesis and main-group element radical chemistry. In 2018, Prof. Xie and Zhu realized reverse hydroboration of imine with NHC-borane (Angew. Chem. Int. Ed. 2018, 57, 3990) and trifluoromethylthiolation of tertiary ethers(Angew. Chem. Int. Ed. 2018, 57, 10357) through synergistic organo-photoredox catalysis and organocatalysis.With their continuous efforts in radical hydroboration and visible-light chemistry, they accomplished reverse radical b-hydroboration for electron-deficient olefin, providing synthetic valuable b-boronated products with excellent selectivity. This visible-light-induced hydroboration shows opposite regio-selectivity (b-hydroboration) compared with thermal conditions (a-hydroboration).
DFT result indicates that theb-addition pathway under photoredox condition is both thermodynamically and kinetically favorable, while the a-addition path is thermodynamically unfavorable. In contrast, under thermal condition, the b-addition step is slightly favorable than the a-addition step, but the subsequent hydrogen-atom transfer (HAT) of the a-addition and the formation of a-addition product is kinetically more favorablewhich reasonably explained the root cause of the regioselectivity of radical hydroboration addition under photoredox and thermal conditions. In the course of our work, Professor Wang Yifeng's group at the University of Science and Technology of China realized the a-regioselective radical hydroboration of unsaturated olefins under thermal conditions (Nature Commun.2019, 10, 1934).
We thank the National Natural Science Foundation of China (21971108, 21971111, 21702098, 21732003, 21672099, 21833002 and 21673110), the Natural Science Foundation of Jiangsu Province (Grant No. BK20190006), “Innovation & Entrepreneurship Talents Plan” of Jiangsu Province, “Jiangsu Six Peak Talent Project” and start-up funds from Nanjing University for financial support. Professor Yong Liang at Nanjing University, Professor Xiang-Ai Yuan at Qufu Normal University, Mr. Jie Han at Theoretical and Computational Chemistry Deparment of Heidelberg University and Dr. Guoqiang Wang at Technische Universität Berlin are warmly acknowledged for their helpful suggestion and discussion on DFT calculation section.