Alkenes geometric isomers

Relatively pure alkene geometric isomers are prepared by stereoselective reduction of alkynes. 

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 alkenes, geometric isomers differ from one another by the location of groups on the same or opposite sides of the double bond. Because the double bond of the alkenes is also rigid and there is no free rotation around it, geometric isomers are formed when there are two different groups on each of the carbon atoms attached by the double bond. If both groups are on the same side of the double bond, the molecule is a cis isomer. If the groups are on opposite sides of the double bond, the molecule is a trans isomer. 

In cases where the formation of alkene geometrical isomers was possible the more stable isomer usually predominated. Notably, it was demonstrated that the stereoselectivity was kinetically-controlled no alkene isomerization was observed when single geometrical isomers were resubmitted to the reaction conditions. ... 

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