Figure 1. Relative Gibbs free energy profiles (kcal/mol) to
produce 1-hexene from metallacycloheptane without hydrogen
Starting from the metallacycloheptane (M0), there are three possible
developments presented in Figure 1, from which two leads to the
formation of 1-hexene. 1-hexene could be produced via a one-stepβ -agostic interaction (M-TS0-1) due to ring flexibility with an
energy barrier of 13.1 kcal/mol, or via a two-step β -H transfer
incorporated with a conformation change, overcoming energy barriers of
12.4 kcal/mol (M-TS0-2) and 15.9 kcal/mol (M-TS2’-3) for each step. The
ring-expansion involves ethylene coordination from M0 to M4, followed by
insertion of ethylene to the M-C bond via M-TS4-5 with an energy barrier
of 17.1 kcal/mol. Note that both ethylene coordination and ring
expansion are endergonic, hence the overall energy barrier for ring
expansion without hydrogen should be the sum of the two, namely 17.4
kcal/mol.