| Abstract
| - The adduct 3, derived fromTiCl3(OiPr) and(Me2N)3PCH2, engages in acomplicated set ofinteractions with NaN(SiMe3)2 andaldehydes, resulting in the requirement to use excessamountsof both reagents for the one-pot synthesis of allenes. WhenTiCl2(OiPr)2 is used instead,ligandsubstitution reactions with NaN(SiMe3)2are diminished and so stepwise transformations can beaccomplished without excess amounts of each reagent. The selectiveproduction of vinylphosphonium salts and byproduct titanium oxides from Ti-substituted ylides andaldehydes is proposed toarise from the presence of a chloride leaving group on the metal.Isolated vinylphosphoniumcompounds may be deprotonated with phenyllithium to give thermallysensitive allenic phosphoranes, which have been characterized by low temperature multinuclearNMR. The reaction ofallenic phosphoranes with aldehydes affords oxaphosphetane and betaineintermediates whichappear to interconvert upon warming to produce allene and phosphineoxide. Dimethylamino-substituted phosphorus components are required for high yields in bothsteps of the allene-formingprocess, presumably to boost the reactivity of the hinderedTi-substituted ylide reagents and tostabilize the allenic phosphorane unit so that it may be trapped byaldehyde. The placement ofchiral groups on the phosphorus methylide or aldehyde componentsresults in low levels ofenantiomeric and diastereomeric induction, respectively, during alleneformation. In two cases,the diastereomeric ratios of initially-formed oxaphosphetanes have beenfound to differ from thediastereomeric composition of their product allenes, offering examplesof the phenomenon knownas “stereochemical drift”. However, oxaphosphetane/betaineformation from allenic phosphoraneand aldehyde has been found to be irreversible, suggesting that anintramolecular betaine olefinisomerization is responsible for the loss of stereochemical integrityduring the Wittig step.
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