Conformations and barriers to inversion of some cyclic seven-membered α-diketones. A study by dynamic nuclear magnetic resonance spectroscopy and molecular mechanics calculations
Abstract
The dynamic stereochemistry of some seven-membered α-diketones have been studied by 1H n.m.r. spectroscopy and by molecular mechanics calculations. A force-field for α-dicarbonyl compounds has been developed. The cyclic α-diketones studied preferably adopt a distorted chair conformation with C1 symmetry or a C2 symmetric twist-boat conformation with a carbonyl–carbonyl dihedral angle of 87–102°. The inversion of the seven-membered ring system was interpreted in terms of a passage of the two carbonyl units in a planar s-cis conformation. The inversion barriers were found to be 8.6–11.3 kcal mol–1 with negligible entropy of activation. Molecular mechanics calculations indicate that the electrostatic contribution to the inversion barrier is 42–52% of the total barrier heights.