Alkylidene and Metalacyclic Complexes of Tungsten that Contain a Chiral Biphenoxide Ligand. Synthesis, Asymmetric Ring-Closing Metathesis, and Mechanistic Investigations

Two complexes that contain the racemic or enantiomerically pure (S) form of the 3,3‘-di-tert-butyl-5,5‘,6,6‘-tetramethyl-1,1‘-biphenyl-2,2‘-diolate (Biphen2-) ligand, W(NAr)(CHCMe2Ph)(Biphen) (2a) and
W(NAr‘)(CHCMe2Ph)(Biphen) (2b) (Ar = 2,6-i-Pr2C6H3; Ar‘ = 2,6-Me2C6H3), were prepared and shown to
be viable catalysts for several representative ring-closing reactions to give products in good yields in most
cases and high % ee in asymmetric reactions. Exploration of the reaction between 2a and a stoichiometric
amount of one desymmetrization substrate allowed two intermediate tungstacyclobutane complexes to be
observed, in addition to the final and quite stable tungstacyclobutane complex formed in a reaction between
the ring-closed product and a tungsten methylene complex. Reactions involving 13C labeled ethylene allowed
for the observation of an unsubstituted tungstacyclobutane complex, an ethylene complex, an unsubstituted
tungstacyclopentane complex, and a heterochiral dimeric form of a methylene complex. The tungstacyclopentane complex was found to catalyze the dimerization of ethylene to 1-butene slowly.