Steric and electronic effects of substituents at arsenic on the decomposition of arsonium betaines

In contrast to the behaviour of the analogous phosphonium betaines, neither the presence at arsenic of the electron-withdrawing 2-furyl substituent nor enclosure of the arsenic in the strained dibenzarsolium ring system promote betaine collapse via attack of betaine oxygen at the arsonium centre. Thus both 2-furyl(methyl)diphenylarsonium iodide and 5-methyl-5-phenyldibenzarsolium iodide on treatment with benzaldehyde and ethanolic sodium ethoxide give exclusive formation of olefin epoxide and tertiary arsine.