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The proximate coupling constant, 5 J (H,CH 3 ), and the torsional mobility of the thiomethyl group in some thioanisole derivatives

1991; NRC Research Press; Volume: 69; Issue: 4 Linguagem: Inglês

10.1139/v91-094

1480-3291

Ted Schaefer, Rudy Sebastian, Salman R. Salman, James Baleja, Glenn H. Penner, David M. McKinnon,

Chemical Reaction Mechanisms

The proximate coupling constants, 5 J, between ortho and methyl protons in thioanisole and 18 of its derivatives are discussed as conformational indicators. On the assumption that 5 J varies as cos 4 θ, for 0° ≤ θ ≤ 90°, θ being the angle by which the methyl group twists out of the aromatic plane, 5 J for θ = 0° follows as −0.43 (2) Hz from the known internal barrier in thioanisole in solution. A measurement of 5 J in meta- or para-substituted thioanisole derivatives then yields an approximate value for the twofold barrier to rotation about the Csp 2 —S bond. For derivatives containing an ortho substituent, 5 J yields an estimate of the torsion angle for the thiomethyl moiety. In some instances these angles are compared with those derived from long-range 1 H, 13 C and 13 C, 13 C coupling constants. The size of the ortho substituent appears to have only a small effect on the magnitude of 5 J. The major determinant of the latter appears to be the manner in which the substituent perturbs the mobile bond order of the Csp 2 —S bond. Key words: spin–spin coupling constants, thioanisole derivatives; 1 H NMR, thioanisole derivatives; conformations, thioanisole derivatives; conformations, torsional motion of SCH 3 group.