Retro-Wittig process

Both of the above teehniques can be reinforeed by erossover experiments (method C) (12-14,20,2Ic,22,23a). An excess of CIC H CHO (ArCHO) is added to the solution below the temperature for oxaphosphetane decomposition. If stereochemical equilibration occurs exclusively by a retro-Wittig process to give the ylide 33, then an excess of the crossover aldehyde must produee the crossover products. Since 33 would be intercepted by excess ArCHO faster than it can recombine with the original aldehyde, the conversion from one oxaphosphetane diastereomer into the other (i.e., from 31 to 32) by way of any retro-Wittig mechanism will be suppressed using method C. However, it is essential to prove that the oxaphosphetane has not already decomposed prior to the addition of the crossover aldehyde. Otherwise, there is the risk of a false-negative crossover result. 

According to quantum-chemical calculations, decompositions of two last types are possible for betaines of type II (see Section 6). Retro-Wittig decomposition (ii) is the process inverse to their formation. The direction (iii) resulting in the formation of elementaolefins is much more interesting (Scheme 20). 

Aromatic and sterically restricting aliphatic aldehydes in marginal cases lead to retro-oxaphosphetane ring opening [53]. Reversibility has also been excluded as a significant process for semi-stable and stable ylides [43,54]. Therefore, the stereochemistry of the salt-free Wittig reaction would be established in virtually all cases by the formation of cis- or rraw5-oxaphosphetanes under kinetic control. 

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