BOND-DISSOCIATION ENERGIES OF 3,5,5-TRIMETHYL-2-OXOMORPHOLINES AND 4,5,5-TRIMETHYL-2-OXOMORPHOLINES BY PHOTOACOUSTIC CALORIMETRY - AN ASSESSMENT OF THE ADDITIVITY OF SUBSTITUENT EFFECTS

Carbon-hydrogen bond dissociation energies (BDE) and absolute rate constants for hydrogen atom abstraction by the tert-butoxyl radical have been measured for a number of aliphatic amines, including two captodatively substituted compounds (4,5,5-trimethyl-2-oxomorpholine (1) and 3,5,5-trimethyl-2-oxomorpholine (2)). The bond energies, determined by photoacoustic calorimetry, cover a range of about 15 kcal mol-1 (ca. 90 kcal mol-1 for the aliphatic amines to 75 kcal mol-1 for 4,5,5-trimethyl-2-oxomorpholine). The reactivity of the C-H bonds with the tert-butoxyl radical illustrates the importance of the stereoelectronic effect: in this case, conjugation between the incipient radical center and the developing pi-system. In addition, there is evidence suggesting that relief of ring strain also plays an important role in determining both the BDE and the reactivity of the amine. The estimation of the captodative stabilization energies of the title compounds from BDE data is difficult for a number of reasons. The usual assumption that relative BDE(C-H) values are a measure of relative radical stabilization energies (RSE) is questioned. For cyclic esters evidence suggests that strain effects on the hydrocarbon versus those on the radical may have a significant effect. In addition, we have shown that the addition of RSE's from a methane based scale will overestimate the combined effect of two substituents on a C-H bond. We conclude that the method proposed originally by Benson provides a better estimate of multiple substituent effects on the BDE's. The analysis of the BDE's using this approach leads us to the conclusion that, for 1, the substitutent effects on BDE(C-H) appear to be synergetic by ca. 9 kcal mol-1. This should not be interpreted as a measure of the captodative effect on the radical since it is difficult to estimate the extent to which the relief of ring strain influences the BDE.