Figure 4. Relative Gibbs free energy profiles (kcal/mol) to produce 1-octene from metallacyclononane with hydrogen
Upon addition of hydrogen, one-step agostic interaction via N-TS0”-5-H2 requires an energy barrier of 7.8 kcal/mol. The overall energy barrier required for this agostic interaction includes the necessary conformation change from N0-H2 to N0”-H2, and thus rise to 10.7 kcal/mol, which is still significantly lower than that without hydrogen (15.8 kcal/mol). The two-step β -H transfer starts with an exergonic conformation change from N0-H2 to N0’-H2, enabling the transfer via transition state N-TS0’-3-H2 and N-TS3-4-H2 with energy barriers of 12.8 kcal/mol and 19.8 kcal/mol respectively. It is difficult to compare how hydrogen affects this two-step reaction, as one step becomes more favorable yet the other more unfavorable, but since the agostic interaction is generally easier to occur, the reaction might well proceed via the one-step agostic route. The pathway of β -H transfer after hydrogenolysis is also calculated on metallacyclononane. The hydrogen insertion from N0-H2 to N1-H2 via N-TS0-1-H2 requires 8.9 kcal/mol, and the followingβ -H transfer via N-TS1’-2-H2 after an endergonic conformation change of 4.1 kcal/mol has an energy barrier of 15.8 kcal/mol.
The energy required for conformation change and coordination is necessary to take into consideration, especially when the transformation is endergonic, as the reaction needs the overcoming of these barriers to occur. Here we present the relevant energy barrier figures with or without hydrogen in Table 1, in which endergonic conformation change are labeled in brackets.
Table 1. ΔG of the reactions calculated regarding metallacycloheptane and metallacyclononane with and without hydrogen, figures in brackets indicate energy required for necessary conformation change or ethylene coordination