Basicity of the carbonyl group. Part 13. 4-Substituted camphors as models for transmission of polar effects. A calorimetric, infra-red and 1H NMR study
Abstract
In a series of 4-substituted camphors, in which the substituent, X, varies from ethyl through hydrogen to nitro, we have shown that the enthalpies of complexation, with BF3 in dichloromethane, which cover a range of 14.5 kJ mol–1 correlate well with r > 0.99 with polar substituent constants with the sensitivity ρ*ca. 8.
IR spectroscopy of the free ketones and the more polar BF3 complexes indicates that the latter are ca. 1.44 times more sensitive to substituent variation.
Complexation occurs at both sites when X = OAc and only at the ketone when X = CO2Me.
We rationalise the smaller sensitivity of the protons to substituent-induced 1H shift displacement when in the E-complexed form to the electron-attracting effect of complexation that leads to diminished electronic screening. The 4JH3exo–H5exo coupling is diminished by complexation which enhances hyperconjugation between the C(3)–Hσ and the carbonyl p orbitals to the detriment of back lobe C(3)–H3exo–C(5)–H5exoσ–σ overlap. A number of chemical shift and coupling correlations are reported.