A bimolecular mechanistic model avoiding the formation of free C2 is here proposed. This involves concerted 1,1-substitution directly on 2 by nucleophilic attack from e.g. the galvinoxyl oxygen atom and with iodobenzene acting as a nucleofuge, to form a b-labelled product if isotopic substitution is present in 2 and a-labelled product for 1,2-substitution (Figure ). The transition state structures for these two alternatives are shown in Figure . Classical nucleophilic substitution at trigonal 23 and digonal carbon is suggested to proceed via a 1,1-