Toward Alkane Functionalization Effected with Cp*W(NO)(alkyl)(η³-allyl) Complexes

Cp*W(NO)( n -alkyl)(η 3 -allyl) complexes result from the selective activations of the terminal C−H bonds of alkanes. Consequently, the reactions of prototypical members of this family of complexes with a range of electrophiles and nucleophiles have been explored with a view to developing methods for functionalizing the newly formed alkyl ligands. The two principal complexes investigated in this regard have been Cp*W(NO)(CH 2 SiMe 3 )(η 3 -CH 2 CHCHMe) ( 1 ) and Cp*W(NO)(CH 2 C 6 H 5 )(η 3 -CH 2 CHCHMe) ( 2 ). It has been found that treatment of 1 and 2 with the oxidant I 2 at −60 °C produces Cp*W(NO)I 2 and terminally functionalized ICH 2 SiMe 3 and ICH 2 C 6 H 5, respectively. Oxidation of 1 by H 2 O 2 also results in the loss of the allyl ligand and production of the known oxo peroxo complex Cp*W(O)(η 2 -O 2 )(CH 2 SiMe 3 ). Treatment of 1 and 2 with electrophiles affords the products resulting from addition of the electrophile to the electron-rich terminus of the σ−π distorted allyl ligands in the reactants. Thus, reagents of the type E-X (E = triphenylcarbenium, H, catecholborane; X = Cl, BF 4 ) liberate CH 3 CH═CHCH 2 E and form the organometallic products Cp*W(NO)(X)(CH 2 SiMe 3 ) and Cp*W(NO)(X)(CH 2 C 6 H 5 ), respectively. Exposure of the tungsten alkyl allyl complexes to isocyanide reagents leads to the formation of complexes bearing β,γ-unsaturated η 2 -iminoacyl ligands that apparently arise from the migratory insertion of isocyanide into the tungsten−allyl linkages. For instance, reaction of 1 with 2,6-xylylisocyanide produces Cp*W(NO)(CH 2 SiMe 3 )(η 2 -CH 3 CHCHCH 2 C═NC 6 H 3 Me 2 ) ( 4a ). Interestingly, gentle heating or chromatography of 4a causes isomerization of the olefin and conversion to the conjugated product Cp*W(NO)(CH 2 SiMe 3 )(η 2 -CH 3 CH 2 CHCHC═NC 6 H 3 Me 2 ) ( 4b ). A similar reaction of 2 with 2,6-xylylisocyanide affords both unconjugated and conjugated isocyanide insertion products, while treatment of 2 with n -butylisocyanide produces primarily the conjugated product. Finally, exposure of these tungsten alkyl allyl complexes to 1000 psi of CO gas generally results in the desired migratory insertion of the CO into the metal−alkyl linkages to form acyl compounds. Hence, 1 is first converted into Cp*W(NO)(C(O)CH 2 SiMe 3 )(η 3 -CH 2 CHCHMe), which then subsequently transforms into isolable Cp*W(NO)(C(O)CH 3 )(η 3 -CH 2 CHCHMe) ( 7 ). Interestingly, 2 does not react with CO under these experimental conditions. Nevertheless, the generality of this mode of reactivity is established by the fact that similar treatment of four other Cp*(W)(NO)(CH 2 CMe 3 )(η 3 -allyl) complexes with CO gas at elevated pressures does afford the corresponding acyl products ( 8 − 11 ). All new organometallic complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of several compounds have been established by X-ray crystallographic analyses.

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