Pyramidal inversion mechanism of simple chiral and achiral sulfoxides: A theoretical study

The pyramidal inversion mechanism of simple sulfoxides was studied, employing ab initio and DFT methods. The convergence of the geometrical and energetic parameters of H 2 SO and DMSO with respect to the Hamiltonian and basis set was analyzed in order to determine a computational level suitable for methyl phenyl sulfoxide ( 3 ), methyl 4‐cyanophenyl sulfoxide ( 4 ), diphenyl sulfoxide ( 5 ), 4,4′‐dicyanodiphenyl sulfoxide ( 6 ), benzyl methyl sulfoxide ( 7 ) and benzyl phenyl sulfoxide ( 8 ). The DFT B3LYP/6‐311G(d,p) level was chosen for further calculations of larger sulfoxides. The barriers Δ E ‡ calculated for the pyramidal inversion mechanism of sulfoxides 3 – 8 are in the range of 38.7–47.1 kcal/mol. These values are in good agreement with the experimental barriers for racemization via the pyramidal inversion mechanism. A resonance effect of a phenyl ring selectively stabilizes the transition state conformations, decreasing the energy barrier for pyramidal inversion by about 3 kcal/mol, compared to a similar molecule without a phenyl substituent. Introducing electron withdrawing groups (cyano) at the para positions of the phenyl ring(s) causes a further decrease of the energy barrier. Chirality, 2007. © 2007 Wiley‐Liss, Inc.