Mechanistic investigations on E-N bond-breaking and ring expansion for N-heterocyclic carbene analogues containing the group 14 elements (E).

Abstract

The potential-energy surfaces for the ring-expansion reactions (i)Pr N(CH)2N((i)Pr)E :(Rea-E) + SiH2Ph2 → six-membered ring heterocyclcic product (E = C, Si, Ge, Sn, and Pb) and (i)Pr N(CH)2N((i)Pr)C :(Rea-C) + EH2Ph2 → six-membered ring heterocyclcic product are studied at the M06-2X/Def2-TZVP level of theory. These theoretical investigations suggest that for a given SiH2Ph2, the relative reactivity of Rea-E toward the ring-ring expansion reaction decreases as the atomic weight of the central atom E increases, that is, in the order Rea-C ≫ Rea-Si > Rea-Ge > Rea-Sn > Rea-Pb. However, for a given Rea-C, these theoretical observations demonstrate that the reactivity of the EH2Ph2 molecule that undergoes the ring-expansion reaction decreases in the order SiH2Ph2 ≈ GeH2Ph2 ≈ SnH2Ph2 > PbH2Ph2 ≫ CH2Ph2. This theoretical study indicates that both the electronic structure and steric effects play a crucial role in determining their reactivities. The model conclusions are consistent with available experimental findings. Furthermore, a valence bond state correlation diagram model can be used to rationalize the computational results. These results allow a number of predictions to be made.