Iodinated vinylidene complex

Stoichiometric or catalytic use of W(CO)s-THF leads to successful cycloalkenylation of iu-acetylenic silyl enolates such as 42a in the presence of water (Scheme 23).321 321a The W(CO)s-promoted cyclization of iu-iodoacetylenic silyl enolate 42b affords the iodine-migrated product in good yield.322 This observation is indicative of the presence of the vinylidene complex intermediate 43. 

Iwasawa et al. have demonstrated that the endo-seleclive cycloalkenylation of co-acetylenic SEE is successfully achieved by stoichiometric or catalytic use of W(C0)5 THF (Scheme 10.101) [269]. In the reaction of SEE 99 fhe mode of cyclization endo or exo can be controlled by appropriate choice of the silyl group, the amount of W(CO)5, and the solvent. The W(CO)5-promoted cyclization would proceed by nucleophilic addition of fhe enolate to a W(CO)5-coordinated alkyne and/ or a vinylidene W(CO)5-complex. Quite recently, the cyclization of co-iodoacetylenic SEE such as 100 has been found to afford fhe iodine-migrated products in good yields [270]. This observation indicates the presence of the vinylidene complex intermediate 101. 

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