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 ⁴J_{H3exo–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)–H_3exo–C(5)–H_5exoσ–σ overlap. A number of chemical shift and coupling correlations are reported.