Thermochemical Stability of α-Amino-α-carbonylmethyl Radicals and Their Resonance As Measured by ESR

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Abstract
ESR spectra of the captodative α-amino-α-carbonylmethyl radicals 8 have been recorded. No coalescence temperature for the rotation of the two NMe groups was found at temperatures below the decomposition temperature of the radicals. From known coalescence temperatures and rotational barriers of substituted methyl radicals the rotational barrier of ≥17 kcal mol-1 was estimated for the •C−N bond in the radicals 8. Enthalpies ΔHdiss and entropies ΔSdiss of the homolytic dissociation of 7a,c,d into 8a,c,d have been obtained from equilibrium measurements by ESR. By correcting for substituent interaction enthalpies in 7 (steric and geminal), a radical stabilization enthalpy RSE = −20.7 ± 1.0 kcal mol-1 was obtained for 8. By addition of the known RSEs of dialkylamino- and carbonyl groups, a RSE = −9.9 kcal mol-1 is predicted for 8. The difference between the experimental and predicted values of 10.8 kcal mol-1 is attributed to a synergistic captodative substituent effect. A linear correlation between the radical stabilization enthalpies of the radicals 8 and of other mono- and disubstituted alkyl radicals and their ESR aHα coupling constants was found. According to this correlation the reduction of aHα by 1 G corresponds to an increase in RSE of 1.57 kcal mol-1. The large resonance of the captodative α-amino-α-carbonylmethyl radicals 3, expressed by their high RSE, their small aHα coupling constant, and their high rotational barrier, can be rationalized by a strong interaction between the α-amino and the α-carbonyl groups similar to that in amides and expressed in the resonance structures 6.

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