Vinylphosphonium Salts and Allenes from Carbonyl Compounds Using Titanium-Substituted Ylides.

(E)-Vinylphosphonium salts are conveniently obtained from the reaction of carbonyl compounds with titanium-substituted ylide species (Me(2)N)(3)P=CHTi(OiPr)Cl(2) or (Me(2)N)(3)P=CHTi(OiPr)(2)Cl. While a wide variety of nonenolizable aldehydes are tolerated, the steric bulk surrounding the ylide carbon limits the process to highly activated or unhindered ketones. The vinylphosphonium salts may be converted to allenes by deprotonation and condensation with a second aldehyde, thus accomplishing a two-step double olefination allene synthesis. This methodology, along with a previously reported one-pot reaction, comprises the most convenient route available for the synthesis of 1,3-disubstituted allenes. The metal-substituted ylide reagent may be generated in situ from commercially-available starting materials. Two representative vinylphosphonium salts were shown to undergo reversible isomerization to the (Z)-form upon irradiation.