No abstract

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.

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 (Biphen(2-)) ligand, W(NAr)(CHCMe(2)Ph)(Biphen) (2a) and W(NAr')(CHCMe(2)Ph)(Biphen) (2b) (Ar = 2,6-i-Pr(2)C(6)H(3); Ar' = 2,6-Me(2)C(6)H(3)), 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 (13)C 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.