Conversion to alkene

Several methods are available for reductive removal of carbonyl groups from organic compounds. Reduction to methylene groups or conversion to alkenes can be achieved. 

Although not all Wittig reactions proceed by exactly the same mechanism, an oxa-phosphetane intermediate is common to all of the mechanisms (Scheme 2). TTie overall rates of the reactions depend upon the rates of formation of the oxaphosphetane and of its conversion to alkene and phosphine oxide. The stereochemistry of alkene formation depends on the rates of formation and conversion to product of the stereoiso-meric oxaphosphetanes, and upon equilibration of the oxaphosphetane with starting materials and with zwitterionic intermediates in some cases f3-5k... 

It is important to note that the alkyl groups do not simply fly off and become reconnected since then random reattachment would be expected and the substituents are not randomly attached. Second, a reasonable pathway in which there is reversible metal catalyzed formation of substituted cyclobutanes (put together one way and taken apart another) is probably wrong because, when substituted cyclobutanes are subjected to the conditions of the reaction, it is reported that they do rwt undergo conversion to alkenes ... 

---