In short, catalyst 7 is competent because it can undergo a reaction that removes its (strongly bound) fumarates from solution. This observation also provides a putative reason as to why BAC catalyst 11 is incompetent in similar reductive couplings to form 12 or 13. Computational investigation of the activation pathways for 14 suggest that a ketene-first path is preferred, in contrast to the aldol-first path preferred by 11. As a result of this change, catalyst 11 is can form highly stabilized complex BAC-IV-B (Figure ).