SYNTHESIS AND CHARACTERIZATION OF THE ASYMMETRIC COMPLEXES CP-ASTERISK-W(NO)(R)(R') [R, R' = ALKYL, ARYL] AND REGIOSELECTIVE INSERTIONS OF CO INTO THEIR W-C SIGMA-BONDS

Treatment of Cp*W(NO)(Cl)2 (CP* = eta5-C5Me5) with 0.5 equiv of a R2Mg.X(dioxane) reagent (R = CH2CMe3, CH2SiMe3, Ph, o-tolyl) results in the formation of the alkyl and aryl chloro complexes Cp*W(NO)(R)(CI), which have been isolated and fully characterized. Metathesis of these complexes with 0.5 equiv of another R'2Mg.x (dioxane) reagent (R' = CH2CMe3, CH2SiMe3, Me, Ph, o-tolyl) affords a series of 16-electron, mixed alkyl and aryl complexes,; Cp*W-(NO)(R)(R'), having all ten combinations of R not-equal R'. These complexes have been characterized by conventional spectroscopic methods. The competitive reactivity of the different tungsten-carbon sigma bonds in these species toward CO insertion has been investigated. Each complex reacts with 1 equiv of CO under ambient conditions to produce the corresponding eta2-acyl complex presumably via an initial carbonyl adduct. An X-ray diffraction analysis of Cp*W(NO)(Ph)-eta2-C{O}CH2CMe3) has been performed to confirm the existence of the eta2-acyl linkage in one of these complexes: space group P2(1)/n, a = 11.759(3) angstrom, b = 16.028(3) angstrom, c = 12.022(2) angstrom, beta = 107.95(2)-degrees, Z = 4, V = 2155.5 angstrom3, R(F) = 0.024 for 3249 data (I(o) greater-than-or-equal-to 2.5sigma(I(O))). These 18-electron acyl compounds are isolable except in the cases when the CO insertion occurs into a W-CH2-SiMe3 linkage. All complexes containing a (trimethylsilyl)methyl ligand form enolate species, Cp*W(NO)(R)(OC{=CH2}SiMe3) upon reaction with CO, probably via formation of an intermediate acyl complex and subsequent rearrangement via a 1,2-silyl shift. The relative migratory aptitudes of the alkyl and aryl ligands have been established by conclusive identification of the inserted products. In order of decreasing migratory aptitude, the trend is CH2CMe3 > CH2SiMe3 > o-tolyl > Ph > Me. The migratory aptitude of the various ligands appears to be primarily a function of the steric bulk of the ligand, the most sterically demanding ligands migrating to CO preferentially.