Evaluation of Enantiomerically Pure Binaphthol-Based Molybdenum Catalysts for Asymmetric Olefin Metathesis Reactions that Contain 3,3‘-Diphenyl- or 3,3‘-Dimesityl-Substituted Binaphtholate Ligands. Generation and Decomposition of Unsubstituted Molybdacyclobutane Complexes

Enantiomerically pure molybdenum imido alkylidene complexes were prepared that contain binaphtholate ligands substituted in the 3 and 3‘ positions with either phenyl ( 4; Mo(NAr)(CHCMe 2 Ph)[Ph](THF)) or 2,4,6-trimethylphenyl groups ( 5; Mo(NAr)(CHCMe 2 Ph)[Mes](THF); Ar = 2,6-i-Pr 2 C 6 H 3 ). Compound 5 was found to be an anti trigonal bipyramidal complex in an X-ray study. Asymmetric metatheses employing complex 5 were found to proceed to high conversion and to give high product % ee, while reactions that employed complex 4 were found to give lower conversion and lower % ee. One ring-closing reaction with 5 was shown to take place at a rate that was proportional to catalyst and substrate. Molybdacyclobutane complexes were observed in solution during ring-closing reactions. The molybdacyclobutane complex generated by treating 5 with ethylene was found to be a trigonal bipyramidal species which decomposed to yield some propylene; the metal-containing product or products of decomposition could not be identified. Molybdacyclobutane complexes were also formed upon adding ethylene to the analogous complex containing a 2,4,6-triisopropylphenyl-substituted binaphtholate ligand, or to 4 . The detection of propylene as a product of decomposition of the molybdacyclobutane complexes examined here suggests that a decomposition pathway that consists of β hydride rearrangement can compete with bimolecular decomposition of methylene complexes under some conditions.