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

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posted on 2003-02-05, 00:00 authored by W. 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.