Dependence of the Wittig reaction mechanism on the environment and on the substituents at the aldehyde group and at the phosphonium ylide

The B3P86/6-31G* mechanistic results for the Wittig reaction of acetaldehyde, CH3CHO, in vacuo and in tetrahydrofuran (THF) solution in the IEF-PCM framework, with an unsubstituted trimethylphosphonium or triphenylphosphonium ylide, that is Me3PCH2 or Ph3PCH2, have been compared to those recently obtained at the same levels for the reaction of Ph3PCH2 with a bulky chiral aldehyde, (2S,3R)-2,4-dimethyl-3-pyrrol-1-yl-pentanal [TCA (submitted)], here named sys for short. The two model systems show distinct, but similar, behaviors that however differ from the large system one. In particular, betaine-type intermediates are not located in vacuo when Me3PCH2 is used, while only a gauche betaine is obtained using Ph3PCH2; the relevant barriers are anyway smaller than those found for sys. Conversely, in THF, the concerted and stepwise mechanisms are both represented and show TS1 / TSb barriers, that are negligibly small for sys. Thus, in contrast to assessed literature, models with methyl groups in place of phenyl rings and branched aldehyde chains show a different behavior from realistic systems and prevent inferring general rules from their use, suggesting to resort to Ph3PCH2 whose results in vacuo and in THF are closer to sys.