Coordination chemistry and FLP reactivity of 1,1- and 1,2-bis-boranes

Abstract

Coordination chemistry and frustrated Lewis pair (FLP) chemistry have been most commonly studied using monodentate Lewis acids. In this paper, we examine the corresponding reactions employing the 1,1- and 1,2-bis-boranes, PhCH₂CH(B(C₆F₅)₂)₂1 and Me₃SiCH(B(C₆F₅)₂)CH₂B(C₆F₅)₂2, respectively. Coordination of isocyanide to these species results in the formation of the products RCH(B(C₆F₅)₂CNtBu)CH₂(B(C₆F₅)₂CNtBu) (R = Ph 3, Me₃Si 4). The rearrangement of 1 to give the 1,2-bis-borane adduct 3 was probed and attributed to a donor-induced retrohydroboration and subsequent hydroboration. The analogous reaction of 1 is evident in efforts to use the Gutman–Beckett method to assess its Lewis acidity. However, in combination with tBu₃P, bis-boranes 1 and 2 form FLPs and react with H₂ to give [tBu₃PH][PhCH₂CH(B(C₆F₅)₂)₂(μ-H)] 5a and [tBu₃PH][Me₃SiCH(B(C₆F₅)₂)CH₂(B(C₆F₅)₂)(μ-H)] 6, respectively. Reactions of 1 and 2 with various donors and PhCCH were shown to give deprotonation and addition products, depending on the nature of the base. However, in the case of 1, products resulting from retrohydroboration, and subsequent hydroboration are evident. Several of these alkyne products are crystallographically characterized.