Additionally, the barrier for isomerization from h 3 bound BAC-III-A (BAC-TS-III-A, 19.5 kcal/mol) is moderately higher than the corresponding barrier for 7 (IMes-TS-III-A, 13.9 kcal/mol). Taken together, The above analysis suggests that the fumarate ligands of IMes precatalyst 7 and BAC precatalyst 11 react via different mechanisms. With this knowledge in hand, we then hypothesized that this difference can explain why 7 is a competent catalyst, but 11 is not. To evaluate this hypothesis, we followed the progression of path A in 7 and path B in 11 along the free energy surface. In the case of catalyst 7, path A provides a means to release a potential active catalyst. Figure details the pathway for catalyst release. Seven-membered metallacycle IMes-IV-A can ligate to an aldehyde (IMes-V-A, Figure ), and can then undergo an aldol reaction (IMes-TS-V-A) to yield complex The quantum chemical results shown in Table and and can be used to explain the competency of catalyst 7 in coupling of 12 and 13. Despite simple ligand exchange being thermodynamically unfeasible, catalyst 7 is competent in the production of allylic alcohol 17. This implies the catalyst activation route involves consumption of the fumarate, and the catalytic activity is predicated on the formation of a byproduct such as 18 (Scheme 1). This observation motivated us to experimentally isolate compound 18 to provide a test of the fumarate consumption hypothesis. A feasible reaction pathway leading to 18 is outlined in Figures and.