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

Abstract

The migratory aptitudes of alkyl groups in the gas phase 1,2-Wittig rearrangement have been determined experimentally as follows. An anion Ph–^–C(OR¹)(OR²), on collisional activation, competitively rearranges to the two 1,2-Wittig ions PhC(R¹)(OR²)(O^–) and PhC(R²)(OR¹)(O^–) [R¹ and R² = alkyl and R¹ < R²]. These two ions respectively eliminate R²OH and R¹OH. The smaller alkanol is eliminated preferentially, indicating that R² (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(OMe)₂ shows this loss decreases markedly as R becomes more electron withdrawing, an observation not consistent with a radical reaction. Ab initio calculations [at the CISD/6-311++G**//RHF (and UHF)/6-311++G** levels of theory] have been used to construct potential surface maps for the model 1,2-Wittig systems ^–CH₂OMe→ EtO^–, and ^–CH₂OEt→PrO^–. Each of these exothermic reactions involves migration of an alkyl anion. There are no discrete intermediates in the reaction pathways. There is no indication of a radical pathway for either rearrangement. It is proposed that the gas phase 1,2-Wittig rearrangement involves an anionic migration, and that it is not the barrier to the early saddle point but the Arrhenius A factor (or the frequency factor of the QET), which controls the rate of the rearrangement. Weak H-bonding between the alkyl anion and the oxygen of the neutral carbonyl species acts as a pivot in holding the molecular complex together during the migration process. This electrostatic interaction increases with an increase in the number of hydrogens able to H-bond to oxygen and with the number of equivalent ways this H-bonding can occur. The relative migratory aptitude of alkyl anions bound within these molecular complexes is tert-Bu^– > iso-Pr^– > Et^–  Me^–, an order quite different from the migratory aptitudes 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 controlled reaction in the gas phase.