COMPETITIVE REACTIVITY OF W-C, W-N, AND W-O BONDS AT THE CP-ASTERISK-W(NO) FRAGMENT - INSERTION REACTIONS OF TERT-BUTYL ISOCYANIDE, P-TOLYL ISOCYANATE, AND CARBON-DISULFIDE

{The three related complexes, Cp*W(NO)(CH2CMe3)(NHCMe3) (1), Cp*W(NO)(OCMe3)-(NHCMe3) (2), and Cp*W(NO)(OCMe3)(CH2CMe3) (3), which differ only in the nature of the pair of groups alpha to the metal center, have been treated with tert-butyl isocyanide, p-tolyl isocyanate, and carbon disulfide in order to establish the relative tendencies of the W-C, W-N, and W-O linkages to undergo insertions of these reagents. The only site of insertion for tert-butyl isocyanide is the W-C bond, the W-N and W-O linkages being unreactive toward this substrate. In contrast, the preferential site of insertion for the two heterocumulenes, p-tolyl isocyanate, and carbon disulfide, was found to be W-N > W-O > W-C and W-N congruent-to W-C > W-O, respectively. These observations can be rationalized in terms of two different mechanisms being operative, one involving initial adduct formation at the metal center and the other involving direct attack at the tungsten-element bond undergoing the insertion. The tert-butyl isocyanide reactions proceed predominantly via the former mechanism, the p-tolyl isocyanate reactions proceed predominantly via the latter mechanism, and the carbon disulfide reactions proceed via both mechanistic pathways simultaneously. Consistently, a kinetic investigation of the reaction between complex 2 and p-tolyl isocyanate reveals that the reaction is second-order overall and first-order in 2 in the presence of an excess of isocyanate, findings indicative of a simple bimolecular reaction between 2 and the isocyanate. The solid-state molecular structure of Cp*W(NO)(eta2-C{NCMe3}CH2CMe3)(OCMe3) (5), formed by the reaction of 3 with tert-butyl isocyanide, has been established by a single-crystal X-ray diffraction analysis. Crystals of 5 are orthorhombic, space group Pbca, with a = 17.459(2) angstrom