Phospholene halides, formation

Other reactions between cyclic phosphorochloridites or cyclic esters and dienes are summarized in Scheme here, A and B are O, S or Se, and may be different or identical, and X = Cl, Br, NCS, or OR, and the conversion is brought about when mixtures of reactants are heated together in sealed tubes for extended time periods. As in the many examples known in which the reacting system, as a whole, is sulphur-free, the nature of the intermediate can be in doubt, and probably depends on the nature of the substituents, particularly those directly connected to phosphorus thus some reactants may interact through a covalent, pentacoordinate species 27, whereas others form a pseudoquatemary intermediate 28. It may also be that one intermediate structure is transformed into the second before the ultimate formation of the 3-phospholene (29). Also, in accord with these ideas, thiophosphorus(III) halides have been obseved to react with a,j5-unsaturated ketones to give, ultimately, l,2-oxa-4-phospholenes as their 2-sulphides (30) (Scheme 3)45 8. reactions carried out in acetic acid solution the products are said to be the esters... 

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