The benzylic anomeric effect. Internal rotational potentials of ArCH2X compounds (X = F, Cl, SH, SCH3, S(O)CH3, SO2CH3)

Abstract

The apparent twofold barriers to rotation about the exocyclic C—C bonds in ArCH2X (X = SH, SMe, S(O)Me, SO2Me) compounds have been determined in CS2 and in benzene-d6, from the magnitudes of long-range proton–proton coupling constants. The work extends earlier studies of the systems X = F, Cl. Except for X = F, all of the title compounds have a low energy conformation in which the C—X bond is perpendicular to the plane of the benzene ring. The introduction of two meta-chlorine substituents leads to a 2 kJ/mol relative destabilization of this structure for X = F, Cl, SH. The trend in the rotational barriers associated with the sulfur substituents is SH, [Formula: see text], SO2Me. Abinitio calculations, with geometry optimization at the STO-3G level for X = F, Cl, SH, reproduce the experimental results, and also the magnitudes of the rotational barriers. Quantitative PMO analyses of these abinitio wavefunctions have been employed to distinguish between "steric" and "other" interpretations, and reveal that the preferred conformation of X = F is determined by a first order perturbational effect, manifested in an electrostatic attraction between the heteroatom and a syn-periplanar ortho-hydrogen. For X = Cl, the PMO analysis suggests that a [Formula: see text] interaction, analogous to the [Formula: see text] interaction postulated as the origin of the anomeric effect in heterocyclohexanes, plays a dominant role. For X = SH, both [Formula: see text] and [Formula: see text] interactions are found; the latter is not possible in the anomeric effect itself. It is concluded that a benzylic anomeric effect exists, and that its magnitude, as a function of X, is S(O)Me, SO2Me > Cl > SH, SMe > F.