Insertion of Group 12-16 Hydrides into NHCs: A Theoretical Investigation

The endocyclic ring expansion of N-heterocyclic carbene (NHC) rings by transition metal (group 12) and main group (group 13–16) element hydrides has been investigated in a computational study. In addition to previously reported insertion reactivity with Si, B, Be and Zn, similar reactivity is predicted to be feasible for heavier group 13 elements (Al, Ga, In, Tl), with the reaction barriers for Al–Tl calculated to be lower than for boron. Insertion is not expected with group 15–16 element hydrides, as the initial adduct formation is thermodynamically unfavorable. The reaction pathway with group 12 hydrides is calculated to be more favorable with two NHCs rather than a single NHC (analogous to Be), however hydride ring insertion with metal dihydrides is not feasible, but rather a reduced NHC is thermodynamically favored. For group 14, ring-insertion reactivity is predicted to be feasible with the heavier dihydrides. Trends in reactivity of element hydrides may be related to the protic or hydridic character of the element hydrides.