Aqueous Hydroboration of Alkynes via Nonclassical Generation of N-Heterocyclic Carbenes

Sheng Tao, Yang Wang, Qianxiu Pan, Ji-Xing Zhao, Qingqing Bu, Fei Chen, Jichang Liu, Bin Dai*, Donghui Wei* and Ning Liu*.  Green Chem., 2023, 25, 6704–6716.

Abstract
The hydroboration of alkynes with bis(pinacolato)diboron (B₂pin₂) using H₂O or untreated H₂O as a solvent is a challenging task. This work reports the aqueous N-heterocyclic carbene (NHC)-catalyzed hydroboration of alkynes with B₂pin₂, in which a wide range of alkynes with different functional groups are tolerated. The proposed NHC active species are formed through a nonclassical strategy in contrast to the traditional method involving deprotonation of the imidazolium salts in the presence of a base. The proposed NHC-catalyzed hydroboration process can be performed using untreated H₂O as a solvent under an air atmosphere in the absence of exogenous bases. Mechanistic investigations, including control experiments, Fourier transform infrared spectroscopy, and density functional theory calculations revealed that the NHC generated in situ is the true active species. Moreover, an in situ generated diboron-NHC adduct, which acts as a reservoir of NHC and is critical for the stability of NHC in aqueous media, was isolated and fully characterized.