Nucleophilic Substitution Factors. 1. Coplanar vs. Orthogonal Bimolecular Substitution at a Benzylic Carbon. X-ray Structure of 2-Isobutyl-1,3-dihydrobenzothiophenium Perchlorate

Article abstract of DOI:10.1021/ja00297a031

A stereoelectronic effect in bimolecular nucleophilic substitution at a benzylic center has been observed in the rates of reaction of thiourea in dimethyl-d₆ sulfoxide with a series of sulfonium perchlorates with differing degrees of constraint in the orientation of the reacting C-S⁺ bond with respect to the plane of the aromatic ring.The substrates and their relative rates (at 37 deg C, unless otherwise noted) are as follows: (a, highly constrained) 2-ethyl-1,3-dihydrobenzothiophenium perchlorate (4b), 1,00, and 2-isobutyl-1,3-dihydrobenzothiophenium perchlorate (4c), 2.8; (b, partly constrained) 2-ethyl-3,4-dihydro-1H-2-benzothiopyranium perchlorate (7), 45 (at 82 deg C); (c, unconstrained) S,S-dibenzyl-S-ethylsulfonium perchlorate (5b), 8.1*10³.An (8*10³-fold rate difference was also observed between S,S-dibenzyl-S-ethylsulfonium terafluoroborate (5a) and 2-ethyl-1,3-dihydrobenzothiophenium tetrafluoroborate (4a) with potassium thiocyanate-18-crown-6 in acetonitrile-d₃.Analysis of these rate differences, taken with inspection of molecular models which show that the observed rate differences are not adequately accounted for by simple nonbonding or angle strain effects, clearly indicates a dihedral angle dependent factor consistent with ?-overlap between p-orbitals on the aromatic ring and the carbon atom undergoing substitution.The "orbital-overlap factor" is estimated to increase the reactivity of the benzylic center in 5b (4*10³-fold relative to that of the corresponding ethyl group (in 21).A single-crystal X-ray analysis on 4c showed the salt crystallizes in space group P2₁/n, unit cell dimensions a = 17.382 (1) Angstroem, b = 9.1731(6) Angstroem, c = 8.7828 (3) Angstroem, and β = 91.7 (2) deg, with Z = 4.On the basis of 2054 unique data with F² > 2?, full matrix refinement converged at R = 0.044 for 232 variables.There is a dihedral angle of 3.8 deg between the C2,S,C9 plane and the plane of the aromatic ring consistent with the observed low reactivity of the benzylic center and the present analysis.The entropies of activation for benzylic substitution of the cyclic and acyclic substrates (4b, 4c, and 5b) with thiourea in Me2SO-d₆ are very similar (-12 +/- 1 cal mol^-1 K^-1), in contrast to the great ΔS(excit.) difference reported for an analogous α-carbonyl system by Bartlett and Trachtenberg; the origin of this difference is discussed.