Synthesis, Structures and Topochemistry of 2-Monovinyl-Substituted 1,4-Benzoquinones

Abstract

In the course of topochemical investigations of substituted quinones the 2-monovinyl-substituted 1,4-benzoquinones 1 were synthesized by electrochemical oxidation of the corresponding 1,4-dimethoxybenzene derivatives 3 to the quinone bisketals 4 and subsequent hydrolysis. In solution, the aryl-substituted vinylquinones 1a−1t underwent unexpected Diels-Alder additions. The stereochemically unique course of the reaction was proved spectroscopically and by X-ray structure analysis of the dimer 8k. As a basic requirement for the topochemical studies, the crystal structures of eight quinones 1 were determined by X-ray diffraction. In the crystals of 1a, 1c, 1e, 1f, 1i and 1n with an α-type packing arrangement, the vinylic double bonds have short contacts (< 4.4 Å) across a centre of symmetry. The quinone 1k crystallizes in a β arrangement with short distances (< 4.0 Å) between molecules related by translation and in a γ arrangement without suitable contacts. The crystals of all quinones 1 were irradiated with UV light to perform [2+2] cycloadditions. The photoreactive quinones dimerized exclusively at the vinylic double bonds, resulting in the corresponding cyclobutanes 9. Topochemically controlled the dimers 9a−9c, 9e, 9l−9n, 9q, 9r and 9u with molecular Ci symmetry were formed whereas the dimers 9k and 9t have Cs symmetry. The structures of the cyclobutanes were determined by spectroscopical investigations and in the case of 9b, 9e and 9r additionally by X-ray analysis. Despite short contacts, crystals of 1f and 1i were photostable. This is probably because of insufficient lattice flexibility indicated by relatively high densities.