13C nuclear magnetic resonance spectroscopy of 6- and 7-substituted coumarins. Correlation with Hammett constants

Abstract

The ¹³C n.m.r. spectra of coumarin and 19 6- and 7-substituted derivatives thereof have been obtained and fully assigned. The chemical shifts of the αβ-unsaturated lactone system have been correlated with Hammett constants: C-3 was found to fit very well with σ⁺, and carbons 2 and 4 with σ. A satisfactory fit was found for the two ring-junction carbons with σ_p⁺, but not with σ_m or σ_m⁺. The electronic implications of these relationships are discussed.