Synthesis of Molybdenum Imido Alkylidene Complexes That Contain 3,3‘-Dialkyl-5,5‘,6,6‘-tetramethyl-1,1‘-biphenyl-2,2‘-diolates (Alkyl = t-Bu, Adamantyl). Catalysts for Enantioselective Olefin Metathesis Reactions

Two 3,3‘-dialkyl-5,5‘,6,6‘-tetramethyl-1,1‘-biphenyl-2,2‘-diols (where alkyl = t-Bu, adamantyl) were prepared in two steps and resolved as the menthol phosphate derivative. Addition of the dipotassium salt of each biphenolate to various Mo(N-Aryl)(CHR)(OTf)2(DME) complexes produced racemic and enantiopure compounds of the type Mo(N-aryl)(CHR)(biphenolate). X-ray crystallographic studies of syn-Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)[(S)-Biphen] and syn-Mo(N-2-CF3C6H4)(CHCMe3)[(S)-Biad](pyridine) proved the absolute stereochemistry of the biphenolate ligands. Neophylidene and neopentylidene complexes were found to have predominantly the syn conformation in solution. The [syn]/[anti] equilibrium constant for Mo(N-Aryl)(CHR)[Biphen] complexes increased in magnitude with decreasing size of the arylimido ligand, and decreased upon reducing the steric bulk of the alkylidene substituent. The rates of exchange of syn and anti isomers, as determined by single-parameter line shape analysis and by spin saturation transfer, were found to be on the order of ∼1 s-1 at 22 °C.