Aldehydes oxygen-substituted phosphorus

Scheme 12 Olefination of aldehydes with oxygen-substituted phosphorus ylides...

To explain this phenomenon, there are two seperate processes to consider. The first and most important one for this reason is the formation of the oxaphosphetane. The addition of the ylide to the aldehyde can, in principle, produce two isomers with either a Z or E substituted double bond. The following elimination step is stereospecific, with the oxygen and phosphorus departing in a syn-periplanar transition state. With unstabilized ylides the syn diastereomer of the oxaphosphetane similar to 61 is formed preferentially. This step is kinetically controlled and therefore irreversible, and predominantly the Z-alkene 62 that results reflects this. 

A ketone or aldehyde reacts with a phosphorus ylide to yield an alkene in which the oxygen atom of the carbonyl reactant is replaced by the =CR2 of the ylide. Preparation of the phosphorus ylide itself usually involves 8 2 reaction of a primary alkyl halide with triphenylphosphine, so the ylide is typically primary, RCHP(Ph)3. This means that the disubstituted alkene carbon in the product comes from the carbonyl reactant, while the mono-substituted alkene carbon comes from the ylide. 

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