Conversion of tungsten(IV) oxo-alkyne complexes to oxo-vinylidene complexes

A series of chiral tungsten(IV) oxo-alkyne complexes of the type Tp'W(O)(I)(RC equivalent to CR') (R = R' = H (1); R = H, R' = Me (2); R = R' = Me (3); R = Me, R' = Ph (4); Tp' = hydridotris-(3,5-dimethylpyrazolyl)borate) has been prepared. These complexes were synthesized by reaction of Tp'W(O)(CO)(I) with Me3NO in the presence of an excess of alkyne. Phenylpropyne complex 4 was isolated as a 2:1 mixture of alkyne rotamers. For the propyne complex, a second route involving deprotonation of acetylene complex 1 followed by addition of Mel also produced complex 2. The oxidation pathway to 2 with Me3NO resulted in a 1:1.5 ratio of alkyne rotamers, while the elaboration of 1 via deprotonation/methylation gave 2 in a 9:1 ratio of alkyne rotamers. Addition of n-BuLi to propyne complex 2 resulted in deprotonation of the terminal acetylene site, and electrophile addition then produced the vinylidene complexes Tp'W(=C=CMe2)(O)(I) (5) and Tp'W[=C-C(Me)(H)](O)(I) (6) upon addition of Mel or HCl, respectively. X-ray structure determinations for alkyne complexes 2 and 4 and for vinylidene complex 5 are reported.