2,5-Bisvinyl-1,4-benzoquinones 1 and 2-vinyl-1,4-benzoquinones 2 substituted with aryl or ester end groups have been synthesized. The 2,5-bisstyryl-1,4-benzoquinones 1 (R=phenyl, p-tolyl, o-tolyl) crystallize with a 7 Angstrom-stacking axis. But only for the o-tolyl derivative the contacts between the vinyl groups are close enough to allow a four-center type photopolymerization. The ester derivative 1 (R=COOEt) has a layer structure and can be photooligomerized in the crystal. The generated cyclobutane subgroups have twofold symmetry. The vinylquinones 2 (R=aryl) may be dimerized photochemically in the crystal at the vinyl groups to centrosymmetric cyclobutanes. Crystals with 4 Angstrom stacking axis are also photoreactive.
2-Styryl-1,4-benzoquinone (1) and compounds 2 and 3 containing 1 as a substructure all proved to be highly reactive towards thermal or photochemical [4pi + 2pi] cyclodimerization reactions. Chemo-, regio- and stereoselective processes lead to dimers (compounds 1-10), which can undergo secondary reactions consisting of the addition of nucleophiles combined with a twofold keto-enol tautomerism (10 --> 12). An alternative process is dehydrogenation/oxidation followed by an intramolecular [4pi + 2pi] cycloaddition (10 --> 11). The same selective [4pi + 2pi] cyclodimerization can be observed in solution upon irradiation (e.g., 1a --> 10a), in contrast to irradiation in the crystalline state which yields a [2pi + 2pi] dimer (e.g., 1a --> 13). If more than one styrylbenzoquinone moiety is present in the same molecule the oligomers 2a (n = 1-3) and 3a (n = 1-4) are obtained. ((C) Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002).
Synthesis, Structures and Topochemistry of 2-Monovinyl-Substituted 1,4-Benzoquinones
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