From Solid State Photodimers of Ethyl Coumarin-3-carboxylate to their Alcoholysis Derivatives. A Supramolecular Study

Abstract

The solid state photodimerization of ethyl coumarin-3-carboxylate and its 6-Cl and 6-Br (1a–c) derivatives as well as the methyl and ethyl alcoholysis derivatives of ethyl coumarin-3-carboxylate photodimer are reported in the context of crystal engineering. Ethyl coumarin-3-carboxylates photodimerize topochemically to form anti head-to-tail stereoisomers (2a–c). The extent of lactone methanolysis of 2a depends on the boiling temperature of the solvent to produce 2-(2-hydroxyphenyl)-3-oxo-8bH-4-oxa-cyclobuta[a]naphthalene-1,1,2a-tricarboxylic acid 1,2a-diethyl ester 1-methyl ester (3a) and 2,4-bis-[(2-hydroxyphenyl]cyclobutane-1,1,3,3-tetracarboxylic acid diethyl ester dimethyl ester (4a) in 1:2 proportion, whereas the ethanolysis of 2a only yields 2-(2-hydroxyphenyl)-3-oxo-8bH-4-oxa-cyclobuta[a]naphthalene-1,1,2a-tricarboxylic acid triethyl ester (5a). The molecular structure of 2a–c and 3–5a were elucidated by 1H and 13C NMR spectroscopy. Also the molecular and supramolecular structures of 2a,b and 3,4a were studied by X-ray diffraction. Most of the C–H…X (X = O, Ph), π…π and dipolar interactions in the photodimers 2a,b and derivative 3a are preserved from the corresponding original coumarin monomers. Thus the supramolecular structure of ethyl coumarin-3-carboxylate is conserved through this group of compounds as if they would have a “supramolecular memory”.