Rapid intramolecular collapse of betaines derived from 5-phenyldibenzophospholes as compared with those derived from acyclic triarylphosphines; mechanism of the Wittig reaction in protic solvents

Abstract

The reaction of 5-phenyldibenzophosphole or its 2,8-dimethoxy-derivative (VII; R = H or OMe) with styrene oxide in ethanol yields solely the normal Wittig products, styrene and the 5-phenyldibenzophosphole 5-oxide (VIII; R = H or OMe). In contrast, the corresponding reactions of the acyclic triarylphosphines, triphenylphosphine and bis-(p-methoxyphenyl)phenylphosphine, predominantly yield rearranged phosphine oxides, the normal Wittig products being formed only to a small extent. These results are accommodated in a scheme in which rapid collapse of the initially formed cyclic, ring-strained, phospholium betaine (XI; R = H or OMe) occurs to form the normal Wittig products via the unstrained spiro-oxaphosphetan (XII; R = H or OMe). In the case of acyclic phosphonium betaines, collapse via the oxaphosphetan does not occur so rapidly, and the alternative route involving protonation and dehydration of the betaine to form the vinylphosphonium salt (IV) is followed. Hydrolysis of the latter predominantly yields the rearranged phosphine oxides (V).The reaction of 5-phenyldibenzophosphole with phenylacetylene in wet diethylene glycol proceeds via the vinylphospholium salt (XIV; R = X = Ph) to form the ring-expanded phosphine oxide (XVI; R = X = Ph) and the ring-opened phosphine oxide (XVII).