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 CpW(NO)(Cl)2 (CP* = eta5-C5Me5) with 0.5 equiv of a R2Mg .X(dioxane) reagent (R = CH2CMe3, CH2SiMe3, Ph, o-tolyl) results in th e formation of the alkyl and aryl chloro complexes CpW(NO)(R)(CI), wh ich have been isolated and fully characterized. Metathesis of these co mplexes with 0.5 equiv of another R'2Mg.x (dioxane) reagent (R' = CH2C Me3, CH2SiMe3, Me, Ph, o-tolyl) affords a series of 16-electron, mixed alkyl and aryl complexes,; CpW-(NO)(R)(R'), having all ten combinati ons of R not-equal R'. These complexes have been characterized by conv entional spectroscopic methods. The competitive reactivity of the diff erent tungsten-carbon sigma bonds in these species toward CO insertion has been investigated. Each complex reacts with 1 equiv of CO under a mbient conditions to produce the corresponding eta2-acyl complex presu mably via an initial carbonyl adduct. An X-ray diffraction analysis of CpW(NO)(Ph)-eta2-C{O}CH2CMe3) has been performed to confirm the exis tence 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 whe n the CO insertion occurs into a W-CH2-SiMe3 linkage. All complexes co ntaining a (trimethylsilyl)methyl ligand form enolate species, CpW(NO )(R)(OC{=CH2}SiMe3) upon reaction with CO, probably via formation of a n intermediate acyl complex and subsequent rearrangement via a 1,2-sil yl shift. The relative migratory aptitudes of the alkyl and aryl ligan ds have been established by conclusive identification of the inserted products. In order of decreasing migratory aptitude, the trend is CH2C Me3 > CH2SiMe3 > o-tolyl > Ph > Me. The migratory aptitude of the vari ous ligands appears to be primarily a function of the steric bulk of t he ligand, the most sterically demanding ligands migrating to CO prefe rentially.