THE MECHANISM OF DOUBLE OLEFINATION USING TITANIUM-SUBSTITUTED YLIDES

The adduct 3, derived from TiCl3(OiPr) and (Me2N)(3)P=CH2, engages in a complicated set of interactions with NaN(SiMe3)(2) and aldehydes, re sulting in the requirement to use excess amounts of both reagents for the one-pot synthesis of allenes. When TiCl2(OiPr)(2) is used instead, ligand substitution reactions with NaN(SiMe3)(2) are diminished and s o stepwise transformations call be accomplished without excess amounts of each reagent. The selective production of vinylphosphonium salts a nd byproduct titanium oxides from Ti-substituted ylides and aldehydes is proposed to arise hom the presence of a chloride leaving group on t he metal. isolated vinylphosphonium compounds may be deprotonated with phenyllithium to give thermally sensitive allenic phosphoranes, which have been characterized by low temperature multinuclear NMR. The reac tion of allenic phosphoranes with aldehydes affords oxaphosphetane and betaine intermediates which appear to interconvert upon warming to pr oduce allene and phosphine oxide. Dimethylamino-substituted phosphorus components are required for high yields in both steps of the allene-f orming process, presumably to boost the reactivity of the hindered Ti- substituted ylide reagents and to stabilize the allenic phosphorane un it so that it may be trapped by aldehyde, The placement of chiral grou ps on the phosphorus methylide or aldehyde components results in low l evels of enantiomeric and diastereomeric induction, respectively, duri ng allene formation. In two cases, the diastereomeric ratios of initia lly-formed oxaphosphetanes have been found to differ from the diastere omeric composition of their product allenes, offering examples of the phenomenon known as ''stereochemical drift''. However, oxaphosphetane/ betaine formation from allenic phosphorane and aldehyde has been found to be irreversible, suggesting that an intramolecular betaine olefin isomerization is responsible for the loss of stereochemical integrity during the Wittig step.