Issue in Honor of Prof. James Bull ARKIVOC 2002 (ix) 55-61 ISSN 1424-6376 Page 55 © ARKAT USA, Inc Unusual expressions of Michael reactivity of 1,3-bis(phenylsulfonyl)allene James R. Bull, Nicholas S. Desmond-Smith, Steven J. Heggie, Roger Hunter,* and Sophie C. M. Rees-Jones Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa E-mail: roger@science.uct.ac.za Dedicated to Professor James Ronald Bull on occasion of his retirement as Mally Professor of Organic Chemistry at the University of Cape Town Abstract: An improved procedure for producing the potent cycloaddition dienophile 1,3- bis(phenylsulfonyl)allene 1 is described based on ensuring removal of excess triethylamine in the work-up of the final dehydroiodination step. Triethylamine, as well as other tertiary amines, promote trimerisation of 1 to 3 (X-ray shown), and a possible mechanism involving potentiation via a Michael addition to 1 is proposed. A further expression of the potent Michael acceptor character of 1 is provided by insertion into the α-position of THF to form 5. Mechanistically, this is suggested as proceeding via an oxonium-ion ylide, also generated via initial Michael addition of THF to 1. Keywords: Michael addition, electrophilic allene, oxonium-ion ylide, α-insertion Introduction We have recently reported the synthesis of 1,3-bis(phenylsulfonyl)allene 1 and its application as a useful dienophile in Diels-Alder reactions. 1 During development of its chemistry, several unusual facets of reactivity were noted based on its ability to act as a potent Michael acceptor. This paper reports on two unusual expressions of Michael-based reactivity of 1, as well as an improved procedure for preparing it. Results and Discussion The first observation was that the final key dehydroiodination step of the synthesis (details in experimental) invariably gave quantities of a by-product 3, the amount depending on work-up and isolation procedure, Scheme 1.