Alkylidene and Metalacyclic Complexes of Tungsten that
Contain a Chiral Biphenoxide Ligand. Synthesis, Asymmetric
Ring-Closing Metathesis, and Mechanistic Investigations
posted on 2003-02-05, 00:00authored byW. C. Peter Tsang, Kai C. Hultzsch, John B. Alexander, Peter J. Bonitatebus,, Richard R. Schrock, Amir H. Hoveyda
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.