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