Cross-metathesis of Allenes. Mechanistic Analysis and Identification of a Ru-CAAC as the Most Effective Catalyst

The first examples of cross-metathesis between two different allenes is disclosed. First- and second-generation Ru complexes were found to be ineffective, at most affording only oligomeric products. The exception was a first-generation complex bearing a bidentate phenyl isopropoxy ligand (i.e., PCy₃ is not released upon initiation), reactions with which afforded a 1,3-disubstituted allenyl boronate in 22% yield. On the basis of mechanistic studies designed to gain deeper understanding of the reasons for the ineffectiveness of different Ru catalysts, it was discovered that phosphine-free Ru-CAAC complexes have the steric and electronic attributes to be highly effective. The results of these investigations pave the way for development of additional olefin metathesis reactions that generate allenes.