Axial and equatorial cyclohexylacyl and tetrahydropyranyl-2-acyl radicals. An experimental and theoretical study.
Axial and equatorial cyclohexylacyl and tetrahydropyranyl-2-acyl radicals gave distinct EPR spectra thanks to surprisingly large beta-hydrogen atom hyperfine splittings that enabled them to be characterized and monitored. DFT computations indicated that the axial species (X = CH(2)) had a higher barrier to rotation about the (O)C(alpha)-C(beta) bond. The computed difference Delta H degrees for the axial and equatorial radicals (R = H, X = CH(2)) was 0.8 kcal mol(-)(1).
Acyl radicals: the relationship between electron spin resonance spectra, structure, and stability in a family of σ-radicals
A series of saturated and unsaturated acyl radicals, RĊO, have been generated in fluid solution, principally by the photolysis of di-t-butyl peroxide in the presence of the corresponding aldehyde. Most of the acyl radicals show well-resolved hyperfine splittings, and the values of a(Hβ) correlate with the n.m.r. coupling constants 3J(CHCHO) in the parent aldehyde. It is concluded that the acyl radicals and the corresponding aldehydes have similar structures.
Conformational effects in compounds with six-membered rings—VI : Conformational equilibria in 5-alkyl-1,3-dioxane
Abstract Conformation equilibria in 5-alkyl-1,3-dioxanes have been studied by determining the equilibria between stereoisomeric pairs of 2,5-disubstituted-1,3-dioxanes in which the 2-substituent (t-butyl or p -nitrophenyl) was large enough to act as a conformational locking group; the results were independent of the 2-substituent. The more fully studied 2-t-butyl series gave enthalpy and entropy differences for the conformations of 5-methyl, 5-ethyl and 5-isopropyl-1,3-dioxanes which were smaller than those for the analogous alkylcyclohexanes, in agreement with the difference in van der Waals radii of oxygen atoms and methylene groups. cis -2,5-Di-t-butyl-1,3-dioxane has an axial 5-t-butyl group. The enthalpy and entropy differences vary with the size of the alkyl group in an irregular manner which is not consistent with the usual assumptions made in the conformational analysis of chair-chair equilibria in 6-membered rings and which shows the inadequacy of considering only free energies.