THE RAPID STEPS IN NUCLEOPHILIC VINYLIC ADDITION ELIMINATION SUBSTITUTION - RECENT DEVELOPMENTS

In an earlier Account we addressed the question of a single-step vs multistep mechanism in nucleophilic vinylic substitution.3 This is formulated in eq 1, where Nu-, LG, and Y and Y' are the nucleophile, the leaving group (nucleofuge), and the electron-withdrawing activating substituents, respectively. The so-called “addition-elimination” route2 commences by nucleophilic attack on Cβ, and the question is whether it gives a discrete carbanion 1 which then expels the LG- (multistep route a) or, on the other hand, it is coupled with nucleofuge expulsion (concerted single-step route b). We suggested that “when the electron-withdrawing abilities of Y and Y' decrease or the leaving ability of X increases, ke1 will increase simultaneously with the decrease of k1 until they merge to a single-step process. “3 In other words, route b may be enforced if the lifetime of the intermediate is shorter than that of a bond vibration. On the basis of kBr /kC1 and kC1/kF element effects, the stereochemistry, and MO calculations we concluded that almost all the results are consistent with route a.3. © 1992, American Chemical Society. All rights reserved.