Synthesis of N-heterocyclic carbene boranes via silver N-heterocyclic carbene complexes

The reaction of N-heterocyclic carbene (NHC) complexes of silver with BH3 or BEt3 was investigated. On treatment of l(i)Pr.AgCl (1-AgCI) ((IPr)-Pr-i = 1,3-diisopropylimidazol-2-ylidene) with two equivalents of NaBH4-free BH3.THE (purified BH3.THF) in THF at room temperature for 2 h, (IPr)-Pr-i.BH3 complex (1-BH3) was obtained in excellent yield (96%). In the reaction of various silver NHC complexes with purified BH3.THF, the corresponding NHC.BH3 complexes were obtained in moderate to good yields. The reaction of 1-AgCl with two equivalents of BEt3, instead of BH3, afforded (IPr)-Pr-i.BEt3 complex (1-BEt3) in 86% yield. This procedure enabled the preparation of BEt3-adducts of imidazol- and benzimidazol-2-ylidenes. (C) 2017 Elsevier Ltd. All rights reserved.