Dynamic Behavior of N‑Heterocyclic Carbene Boranes: Boron–Carbene Bonds in B,B‑Disubstituted N,N‑Dimethylimidazol-2-ylidene Boranes Have Substantial Rotation Barriers

Dynamic NMR spectroscopy has been used to measure rotation barriers in five B,B-disubstituted 1,3-dimethylimidazol-2-ylidene boranes. The barriers are attributed to the sp²–sp³ bond between C(1) of the N-heterocyclic carbene ring and the boron atom. Bonds to boron atoms bearing a thexyl (1,1,2-trimethylpropyl) group show especially high barriers, ranging from 75–86 kJ mol^–1. 2-Isopropyl-1,3,5-trimethylbenzene is used as a comparable to help understand the nature and magnitude of the barriers.