Ab initio static and metadynamics investigations of the Wittig reaction

To date, the Wittig reaction remains the most commonly used method in organic chemistry. The synthesis approach yields to a possible functionalization of the olefin product through the transformation of the carbonyl function (ketones or aldehydes) with a phosphoniumylide. In the present work, the two approaches are used to describe the mechanism of the Wittig reaction. Static quantum calculations at the DFT level of theory with a B3LYP functional and 6-31 g(d, p) basis set are carried out and correlated to metadynamics calculations, exploring the free energy landscape of the reaction. The free energy barriers are calculated along the trajectory path, and the mechanism is discussed with the main features observed in the MTD calculations when compared to static quantum investigations. The latter do not allow for the identification of all points that may occur along the reaction path. Static quantum calculation converges to limited geometry states, while the metadynamics converges to several metastable and stable geometries and configurations. Moreover, the strong dependence of the reaction dynamics upon the functional and pseudopotential used highlights the importance of the dispersion forces along the reaction path. A complete description of the reaction mechanism from both the free energy standpoint and the structural configurations of the molecular species is discussed in detail. The differences in the free energy profile are discussed in terms of the limited account of the dispersion interactions within the DFT approach and the standard local XC functionals, confirming the strong non-covalent interactions and molecular rearrangement of charged species that take place all along the reaction path.