The introduction of hydrogen modifies the energy barriers of these
reactions, albeit in different directions. For metallacycloheptane, the
energies required for possible reactions are generally raised upon
addition of hydrogen. The energy required for the agostic 3,7-H shift
seems to be reduced from 13.1 kcal/mol to 9.8 kcal/mol, but a
conformation change of 6.1 kcal/mol is required, making the overall
energy barrier higher than that without hydrogen. It is difficult to
determine exactly which pathway is applied to produce 1-hexene without
hydrogen, as the energy requirements of the two pathways are similar,
but it is clear that β -agostic 3,7-H shift becomes more favorable
among the three possible reaction pathways in the presence of hydrogen.
For metallacyclononane, the formation of 1-octene via β -agostic
hydrogen shift becomes much more favorable as the energy barrier is
greatly reduced from 15.8 kcal/mol to 10.7 kcal/mol with the
introduction of hydrogen, but the formation of metallacyclononane itself
becomes more difficult as the energy required for the ring expansion of
metallacycloheptane rises from 17.4 kcal/mol to 20.0 kcal/mol, making
hydrogen’s full significance to the reaction hard to ascertain.Thus, we
turned to explore how hydrogen might affect the formation of the active
center on the catalyst in hopes of finding a more conclusive influence.