ALKYLIDENE TRANSFER FROM PHOSPHORANES TO TUNGSTEN(IV) IMIDO COMPLEXES

The reactions of WCl2(NPh)(PMePh2)3 with a number of triphenylphosphoranes were surveyed, and alkylidene transfer to give W(=CHR)Cl2(NPh)(PMePh2)2 was observed for variously substituted aryl ylides Ph3P=CHAr', where Ar'= C6H5, C6H4-p-Me, C6H4-M-Me, C6H4-o-Me, C6H4-p-OEt, C6H4-o-OMe, C6H4-p-CF3, C6F5, o-Np, and m-Np, for the vinyl ylide Ph3p=CH-CH=CMe2, and also for the bis(ylide) Ph3P=CH-C6H4-C6H4-CH=PPh3. A mechanism is proposed for the transfer reaction. A crystal structure of W(=CHC6H5-p-Me)Cl2(NPh)(PMePh2)2 indicated an octahedral geometry in which the two mutually trans phosphines and the alkylidene are cis to the apical imido ligand and the alkylidene substituent lies syn to the imido ligand. Reduction of WCl2(N-2,6-C6H3-Me2)-[OCMe(CF3)2]2(THF) in the presence of a number of aryl ylides yielded the corresponding benzylidene complexes W(=CHAr')(N-2,6-C6H3-Me2)[OCMe(CF3)2]2(PPh3), which were stable in solution but only isolable upon addition of a stronger donor ligand than PPh3 (e.g., PMe3). Transfer of o-methoxybenzylidene from phosphorus to reduced WCl2(NAr)[OCMe(CF3)2]2(THF) complexes was especially favorable, as chelation by the o-methoxy group aided the transfer reaction, stabilized the resulting product, and, hence, enabled the synthesis of several arylimido derivatives W(=CHC6H4-o-OMe)(NAr)[OCMe(CF3)2]2(THF), where Ar = Ph, 2,6-C6H3-Me2, and 2,6-C6H3-(i-Pr)2. Crystal structures of the o-methoxybenzylidene complexes, with and without THF coordinated to tungsten, were obtained and supported donation by the o-methoxy group to tungsten. The o-methoxybenzylidene complexes rapidly polymerized norbornene but only slowly metathesized acyclic olefins.