The gas phase 1,2-Wittig rearrangement is an anion reaction. A joint experimental and theoretical study

The migratory aptitudes of alkyl groups in the gas phase 1,2-Wittig rearran gement have been determined experimentally as follows. An anion Ph-C-(OR1)( OR2), on collisional activation, competitively rearranges to the two 1,2-Wi ttig ions PhC(R-1)(OR2)(O-) and PhC(R-2)(OR1)(O-) [R-1 and R-2 = alkyl and R-1 < R-2]. These two ions respectively eliminate (ROH)-O-2 and (ROH)-O-1. The smaller alkanol is eliminated preferentially, indicating that R-2 (the larger alkyl group) is migrating preferentially (observed tert-Bu > iso-Pr > Et > Me): a trend generally taken to indicate a radical reaction. However , a Hammett investigation of the relative losses of MeOH from R-C6H4- C-(OM e)(2) shows this loss decreases markedly as R becomes more electron withdra wing, an observation not consistent with a radical reaction. Ab initio calc ulations [at the CISD/6-311 + + G**//RHF land UHF)/6-311++G** levels of the ory] have been used to construct potential surface maps for the model 1,2-W ittig systems -CH2OMe--> EtO-, and -CH2OEt-->PrO-. Each of these exothermic reactions involves migration of an alkyl anion. There are no discrete inte rmediates in the reaction pathways. There is no indication of a radical pat hway for either rearrangement. It is proposed that the gas phase 1,2-Wittig rearrangement involves an anionic migration, and that it is not the barrie r to the early saddle point but the Arrhenius A factor (or the frequency fa ctor of the QET), which controls the rate of the rearrangement. Weak H-bond ing between the alkyl anion and the oxygen of the neutral carbonyl species acts as a pivot in holding the molecular complex together during the migrat ion process. This electrostatic interaction increases with an increase in t he number of hydrogens able to H-bond to oxygen and with the number of equi valent ways this H-bonding can occur. The relative migratory aptitude of al kyl anions bound within these molecular complexes is tert-Bu- > iso-Pr- > E t- much greater than Me-, an order quite different from the migratory aptit udes of anions expected from thermodynamic considerations. This conclusion indicates that great care must be exercised in utilising thermodynamically derived migratory aptitudes to explain the course of a kinetically controll ed reaction in the gas phase.