Bond length and reactivity: the gauche effect. A combined crystallographic and theoretical investigation of the effects of β-substituents on C–OX bond length

Abstract

The ‘variable oxygen probe’ is applied to systems with the general structure Y–C–C–OX (Y = F, H, SiR₃), using both crystal structure correlations (including 25 new structures) and ab initio calculations (SCF[DZP]), for 20 structures, Y–CH₂–CH₂–OX: Y = H, F and SiH₃(gauche and trans); X = CH₃, CHO (E and Z) and NO₂. The calculations reproduce conformational preferences well (all our 2-fluoroethyl derivatives crystallise with F gauche to OX). Both crystal and calculated structures give linear bond-length/pK_HOX correlations. From the crystal structures: definitive correlations are derived for primary and secondary alcohol derivatives; no significant difference is observed between axial and equatorial cyclohexyl systems; β-fluorine has a small bond-shortening effect on the C–OX bond, which is greatest for the best leaving groups OX; and data for two systems with β-silicon are consistent with a bond-lengthening effect. The inductive effect of β-fluorine, observed as its effect on the C–OX bond length, shows no significant dependence on geometry.

Calculated bond lengths give similar results, but show the different sensitivities to varying OX expected from frontier orbital considerations (σ_Y–C–σ*_C–OX for Y = Si > H > F). The length of the C–OX bond in Y–CH₂–CH₂–OX is a linear function of the electronegativity of Y. 1,3-Interaction energies calculated (at the MP2[DZP] level) for the isodesmic reaction Y–CH₂–CH₂–OX + CH₃–CH₃→ CH₃–CH₂–OX + Y–CH₂–CH₃ give a measure of the destablising effect of β-fluorine and the stabilising effect of β-silicon in these systems. In three different systems which react with participation of adjacent σ-bonds, the lengthening of the C–OX bond, observed in the ground state using the variable oxygen probe, is not accompanied by significant involvement of the adjacent orbitals. It is suggested that C–OX bond breaking, and the further structural reorganisation, are not closely coupled in such reactions.