Figure 2. Relative Gibbs free energy profiles (kcal/mol) to produce 1-hexene from metallacycloheptane with hydrogen.
When hydrogen is introduced into the system, an additional pathway is included into the calculation, and all energies are referred to M0-H2, the Gibbs free energy of which is set to 0 for analysis convenience. The one-step β -agostic interaction via M-TS0’-1-H2 to liberate 1-hexene has an energy barrier of 9.8 kcal/mol, but an endergonic conformation change of 6.1 kcal/mol from M0-H2 to M0’-H2 is necessary to enable the reaction, thus the overall energy barrier rise to 15.9 kcal/mol, which is actually higher than that without hydrogen (13.1 kcal/mol). The two-step β -H transfer occur via two transition states M-TS0-4-H2 and M-TS4’-5-H2, overcoming energy barriers of 13.7 kcal/mol and 16.9 kcal/mol respectively. The hydrogenolysis occur via M-TS0-2-H2 to give an alkane chain, overcoming an energy barrier of 15.8 kcal/mol. An exergonic conformation change from M2-H2 to M2’-H2 occurred to enable further β -H transfer to liberate 1-hexene via M-TS2’-3-H2, with an energy barrier of 11.8 kcal/mol. The ring expansion occur through first an endergonic ethylene coordination of 2.4 kcal/mol, followed by an ethylene insertion of 17.6 kcal/mol, both figures higher than those without hydrogen, indicating a higher overall energy barrier.