Abnormal reactions of the betaines R3PCH2·CH(Ō)·R

Abstract

The betaines Ar₃P⁺·CH₂·CH(O^–)·Ar′, formed either from methylphosphonium salt and aromatic aldehyde in alcohol–alkoxide or from phosphine and epoxide in alcoholic solution, rearrange to the phosphine oxides Ar₂PO·CHAr·CH₂Ar′, that aryl group migrating which is most stable as the anion. In Wittig olefin synthesis, similar rearrangements are not generally competitive with olefin formation. The analogous betaine formed from diphenylphosphine and styrene oxide rearranges in a similar way but also, depending on the conditions, gives styrene and products derived from betaine dissociation.