Conjugated Peterson alkenation

Unlike the Peterson alkenation, which is in principle similar, the phosphine oxide anion addition can be controlled to produce predominantly the erthyro isomer (206). The threo isomer can be obtained by selective reduction of the a-ketophosphine oxide (210), allowing highly stereoselective alkene fonna-tion. Since a two-step sequence is employed, this reaction does not require a stabilizing functionality to be conjugated to the phosphine oxide in order to produce the alkene. In fact, unlike the phosphonate HWE reagents, the reaction of a ketophosphine oxide (211) with a carbonyl derivative does not occur to produce the unsaturated carbonyl (213 Scheme 30). ° The addition step is presumably too rapidly reversible and the elimination of phosphine oxide too slow. 

How can the Z selectivity in Wittig reactions of unstabilized ylids be explained We have a more complex situation in this reaction than we had for the other eliminations we considered, because we have two separate processes to consider formation of the oxaphosphetane and decomposition of the oxaphosphetane to the alkene. The elimination step is the easier one to explain—it is stereospecific, with the oxygen and phosphorus departing in a syn-periplanar transition state (as in the base-catalysed Peterson reaction). Addition of the ylid to the aldehyde can, in principle, produce two diastere-omers of the intermediate oxaphosphetane. Provided that this step is irreversible, then the stereospecificity of the elimination step means that the ratio of the final alkene geometrical isomers will reflect the stereoselectivity of this addition step. This is almost certainly the case when R is not conjugating or anion-stabilizing the syn diastereoisomer of the oxaphosphetane is formed preferentially, and the predominantly Z-alkene that results reflects this. The Z selective Wittig reaction therefore consists of a kinetically controlled stereoselective first step followed by a stereospecific elimination from this intermediate. 

While these examples point to the possible use of the Wittig and the Peterson reactions as complementary methods for (Z)- and ( )-alkene formation in cases with conjugating fiinctiotuility, it must be emphasized that no systematic predictive rule can be rqrplied to the possible selectivity, and this area remains one of active research. 

The exocycllc alkene (1) has been prepared from the corresponding ketone by Wittig and Peterson oleflnation. When excess of the lithium salt of ethyl trimethylsllylacetate was used,the conjugated... 

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