Carbene-Bound Borane and Silane Adducts: A Comprehensive DFT Study on Their Stability and Propensity for Hydride-Mediated Ring Expansion

Abstract

The stability of a variety of borane (BH 3 and BH 2 NHMe) and silane (SiH n Ph 4– n, n = 0–4) adducts with diamino (NHC) and aminoalkyl (CAAC) carbenes has been carefully examined using M06-2X/cc-pVDZ computations, including natural bond orbital and atoms-in-molecules analyses. Moreover a potential mechanism for the hydride-mediated ring expansion of the carbene donors is reported. While the NHC adducts can undergo thermally induced ring-expansion chemistry, the CAAC adducts show increased stability due to a large energetic barrier for the insertion of boron (or silicon) atoms into the CAAC heterocycle. A series of substituted NHCs were investigated to further explore the roles of both electronic and steric effects on adduct stabilities and on their propensities for undergoing ring-expansion transformations.