Planning an Alkene Synthesis via the Wittig Reaction

Write two resonance structures for (a) (cyanomethylidene)triphenylphosphorane and (b) (acetylmethylidene)triphenylphosphorane. 

Sample Solution Electron delocalization involving the cyano group generates an additional resonance structure for the ylide. 

To identify the carbonyl compound and the ylide required to produce a given alkene, mentally disconnect the double bond so that one of its carbons is derived from a carbonyl group and the other is derived from an ylide. Taking styrene as a representative example, we see that two such disconnections are possible either benzaldehyde or formaldehyde is an appropriate precursor. 

Either route is feasible, and indeed styrene has been prepared from both combinations of reactants. Typically there will be two Wittig routes to an alkene, and any choice between them is made on the basis of availability of the particular starting materials. 

What combinations of carbonyl compound and ylide could you use to prepare each of the following alkenes  

Step 1 The ylide and the aldehyde or ketone combine to form an oxaphosphetane. 

Step 2 The oxaphosphetane dissociates to an alkene and triphenylphosphine oxide. 

---