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-TOLYLISOCYANATE, AND CARBON-DISULFIDE

The three related complexes, CpW(NO)(CH2CMe3)(NHCMe3) (1), Cp*W(NO)(O CMe3)-(NHCMe3) (2), and CpW(NO)(OCMe3)(CH2CMe3) (3), which differ onl y in the nature of the pair of groups alpha to the metal center, have been treated with tert-butyl isocyanide, p-tolyl isocyanate, and carbo n 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 onl y 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-tol yl isocyanate, and carbon disulfide, was found to be W-N > W-O > W-C a nd W-N congruent-to W-C > W-O, respectively. These observations can be rationalized in terms of two different mechanisms being operative, on e involving initial adduct formation at the metal center and the other involving direct attack at the tungsten-element bond undergoing the i nsertion. The tert-butyl isocyanide reactions proceed predominantly vi a the former mechanism, the p-tolyl isocyanate reactions proceed predo minantly 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 is ocyanate reveals that the reaction is second-order overall and first-o rder in 2 in the presence of an excess of isocyanate, findings indicat ive of a simple bimolecular reaction between 2 and the isocyanate. The solid-state molecular structure of CpW(NO)(eta2-C{NCMe3}CH2CMe3)(OCM e3) (5), formed by the reaction of 3 with tert-butyl isocyanide, has b een established by a single-crystal X-ray diffraction analysis. Crysta ls of 5 are orthorhombic, space group Pbca, with a = 17.459(2) angstro m, b = 20.393(3) angstrom, c = 15.075(3) angstrom, and Z = 8; the stru cture was solved by conventional heavy-atom methods and was refined by full-matrix least-squares procedures to R(F) = 0.032 and R(wF) = 0.02 7 for 3856 reflections with I greater-than-or-equal-to 3 sigma(I). Fin ally, the reactivity of 1-3 and the related chloro complexes, CpW(NO) (XCMe3)Cl (X = NH, CH2 and O), with CO2 has also been investigated and indicates that, in general, the W-N linkage is the preferential site Of CO2 insertion.