Oxaphosphetane ratios, kinetic

In nearly all cases, (Z)-alkene selectivity is higher for tertiary than for unbranched aliphatic aldehydes. The eombination of a tertiary aldehyde and bulky phosphorus ligands in the ylide usually results in the highest Z E ratios, except for the cases already mentioned where oxaphosphetane intermediates undergo cis-trans equilibration according to control experiments. The kinetic oxaphosphetane ratios follow the general rule that cis selectivity is higher for tertiary than for unbranched aldehydes. If this rule is not reflected in the empirical alkene ratios, then equilibration of intermediates is a distinct possibility. 

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. 

In the reaction of a phosphonium ylide with an aldehyde or ketone, a mixture of E- and Z-alkenes can result. In general, it is found that a resonance-stabilized ylide gives rise predominantly to the fJ-alkene, whereas a non-stabilized ylide usually gives more of the Z-alkene. The stereochemistry of the alkene product must arise from the stereochemistry of the oxaphosphetane, as the second step (the breakdown of the oxaphosphetane) takes place by way of a concerted syn elimination. Therefore, of the two diastereomeric oxaphosphetanes, the cis isomer leads to the Z-alkene and the trans isomer to the E-alkene (2.73). With a non-stabilized phosphonium ylide, the formation of the oxaphosphetane is thought to be irreversible. Therefore the Zr-E ratio is a reflection of the stereoselectivity in the first, kinetically controlled step. The preference for the formation of the cis oxaphosphetane has been attributed to the minimized steric interactions in the transition state involving orthogonally aligned reactants. 

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