PHOTOCHEMICAL DIMERIZATION MODES OF 9-METHOXYANTHRACENES AND 9-(2-HYDROXY-2-PROPYL)ANTHRACENE

Irradiation of 9-methoxyanthracene in ether solution at room temperature gives, in a ratio of about 55:45, head-to-tail and head-to-head substituted dianthracenes, which have been isolated by colum chromatography on silica gel. Photochemical dimerization of 9-methoxy-10-methylanthracene afforded the known centrosymmetrical 4pi + 4pi cycloaddition product. By contrast, photoexcitation of 9-methoxy-10-methylanthracene in the presence of 9-methoxyanthracene results mainly in the formation of the two possible heterodimers by 4pi + 4pi cycloaddition. Photochemical head-to-tail dimerization of 9-(2-hydroxy-2-propyl)anthracene gives two diastereomeric dianthracenes which, due to their rotationally blocked 2-hydroxy-2-propyl substituents, are characterized by i- and C2-symmetry, respectively. Deviations from planarity of the aromatic pi-system in crystalline 9-methoxy-, 9-methoxy-10-methyl-, and 9-(2-hydroxy-2-propyl)anthracene, and the effects of substituents on the molecular topology of various dianthracenes, have been evaluated by X-ray diffraction.