Basicity of the carbonyl group. Part 9. Lewis and Brønsted basicities of enamino-ketones

Abstract

pK_BH⁺ Values and enthalpies of complex formation between boron trifluoride and 30 3-aminocyclohex-2-enones have been investigated. A collection of thermodynamic and spectroscopic arguments, in particular derived from a ¹³C dynamic n.m.r. study, showed the carbonyl oxygen to be the basic site for protonation in aqueous or organic solvents as well as the site for BF₃ co-ordination in dichloromethane. Experimental and theoretical (ab initio calculations) observations in favour of an interaction between H⁺ or BF₃ and the nitrogen atom are discussed. Structural effects on the basicities have been examined and linear correlations between pK_BH⁺ or ΔH°_BF3 and nitrogen substituent constants σ* have been established and discussed. A single extra thermodynamic relationship between pK_BH⁺ and ΔH°_BF3 is found for a large series of unsaturated ketones, including enamino-ketones. This relationship shows that Lewis basicity (ΔH°_BF3) may be used as an indirect measure of pK in a homogeneous series of weak bases for which acidity functions are difficult or impossible to find.