Metal Halide Exchange in Benzylborane NHC-Adducts: The Effect of Backbone Alkylation in the N-Heterocyclic Carbene

Abstract

Among other main group elements, the chemistry of boron has been stimulated by N-heterocyclic carbenes (NHCs) as important ligands, the nature of which can significantly control the outcome of reactions. In four-coordinate benzyl boranes of type (NHC)BH2CH2(o-BrC6H4), we demonstrate this effect at the example of a lithiation silylation procedure. For NHC=IiPr lithiation of the arene ring by lithium-bromide exchange with tBuLi ultimately leads to backbone substitution with proton rearrangement, as demonstrated by trapping with trimethylsilyl chloride. In contrast, such process is suppressed for NHC=IiPrMe, which undergoes lithiation and subsequent trimethyl-silylation at the arene moiety.