Theoretical and experimental studies of cycloconjugation involving second-row elements

Article abstract of DOI:10.1021/ja00037a040

In a method proposed to investigate conjugative interactions between second-row elements (S, P) amd a π-electron system, barriers to amide rotation have been measured using NMR line-shape methods, for N-acetyl-4-X(hetero)-1,4-dihydropyridines (X = SO₂,7; P(O)OCH₃,13; C(CH₃)₂, 6 (reference)) as well as the dihydro (8a) and tetrahydro (9) derivatives, respectively, of 7. It is suggested that such interactions would stabilize the transition states for amide rotation by cycloconjugation but not the planar equilibrium states, compressing the barriers relative to reference 6. In fact, amide barriers in 7,13, and N-acetylpyrrole are substantially (>6 kcal) less than in 6, 8ab, and 9, all ca. 17 kcal, in all of which saturated centers interrupt possible cycloconjugation. These conclusions are supported by the results of ab initio calculations at the Hartree-Fock level with the 3-21G(*) basis set.