Free energy landscapes in THF for the Wittig reaction of acetaldehyde and triphenylphosphonium ylide

The B3P86/6-31G* mechanistic results for the Wittig reaction of triphenyl-phosphonium ylide and acetaldehyde, in vacuo and in tetrahydrofuran (THF) in the IEF-PCM framework, led to distinct behaviors. In particular, only a gauche betaine (Bg) was obtained in the gas phase and the relevant barriers were smaller than those found for a realistic system (a bulky chiral aldehyde [Alagona & Ghio, Theor Chem Acc (2009) 123, 337]). Conversely, in THF, both mechanisms (via oxaphosphetanes only and via betaines plus oxaphosphetanes) were represented with significant TS1 / TSb barriers. Furthermore, an additional gauche betaine, named B’g, was located [Alagona & Ghio, Int J Quantum Chem (2010) 110, 765]. A number of open questions however called for an answer. The main issues addressed in this study are: (i) Can Bg and B’g proceed to products along different reaction paths on the same hypersurface in THF? (ii) If it is not so, can they interconvert into each other? (iii) Might the reaction proceed via an anti betaine-type intermediate? (iv) Is it possible to find a pathway leading directly to a transition state along the path to products without passing through an oxaphosphetane intermediate, as suggested on the basis of quantum molecular dynamics simulations on trimethylphosphonium ylide in dimethylsulfoxide [Ziegler et al., J Phys Chem A (2005) 109, 5136]? The answers to these questions depend on the fact that the phenyl groups at the phosphonium ylide are of paramount importance for obtaining a correct reaction behavior.