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

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 C i symmetry were formed whereas the dimers 9k and 9t have C s 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.