Competition of mechanism in nucleophilic substitution of vinyl halides. An unequivocal example of the vinylic SRN1 route

In a search for an unambiguous example of the vinylic Sml route, several vinyl bromides and iodides were reacted mostly with -CH2COCMe3, and sometimes with -CH&OPh, -CH(Me)COEt, and (Et0)zPO- ions, under Fe2+- or photostimulation in MezSO. Vinyl halides having vinylic hydrogens, such as P-bromostyrene, gave acetylenic products, e.g., phenylacetylene or a tertiary PhCZC-substituted alcohol, whereas vinyl halides with allylic hydrogens, such as Me&=C(I)CHMez, gave a substituted allene. Reduction products of the halogen, as well as substitution and rearranged substitution products, were also formed. The operation of ionic elimination-addition routes accounts for formation of most of the products, while the reduction products arise from an intermediate vinyl radical. PhzC=C(Br)Ph (20) and MezC=C(Br)Ph (25) gave both substitution and reduction products, but MezC=C(Br)-t-Bu (23) gave only a reduction product. Formation of substitution products from the conjugated 20 and 25 was ascribed to a reaction via a vinylic Sml route, while lack of substitution in 23 is related to its nonconjugated system and to the consequent higher energy that the radical anion of the substitution product would have. The one here reported seems to be the first case of an exclusive genuine vinylic Sml process.