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.