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).
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.