Acetylene-allene rearrangement of propargyl systems X—CH2—C≡CH (X = H, Me, NMe2, OMe, F, SMe): an ab initio study

Acetylene—allene rearrangement in propargyl systems XCH2CH≡CH (X = H, Me, NMe2, OMe, F, and SMe) was studied using the ab initio approach. The relative stabilities of the starting and final propyne structures and the corresponding allenes as well as the structure of intermediate carbanions were considered. n-π-Conjugation was shown to dominate in allene stabilization while the inductive effect of heteroatomic substituents makes at least comparable contribution to stabilization (or destabilization) of the propynyl structure. In particular, relative instability of 1-methoxypropyne can be rationalized by high electronegativity of O atom, which leads to dramatic decrease in the total electron density in the region of the neighboring C≡C triple bond. The influence of substituents on the mobility of the migrating proton was considered for the gas phase and with solvation effects included. Calculations involving electron correlation at the MP2 level of theory were shown to be insufficient for correct reproduction of the energy differences between the corresponding propynes and allene structures. The results of MP4 calculations with inclusion of ZPE correction are in good agreement with the available experimental data.