Highly Regioselective Solid-State Photodimerization of Naphthoquinolizinium Salts

Abstract

The photodimerization of benzo-annellated acridizinium derivatives, namely naphtho[1,2-b]quinolizinium and naphtho[2,1-b]quinolizinium bromide, has been investigated in solution and in the solid state. Irradiation of the naphthoquinolizinium salts in solution gave all possible regioisomers in an unselective [4+4] photocycloaddition reaction. In contrast, the irradiation of crystalline samples resulted in regioselective dimerization to give the anti-head-to-tail photodimers. The reactivity in the crystalline phase depended significantly on the solvent used for crystallization. The results are discussed on the basis of X-ray diffraction analyses that showed that the selectivity of the photodimerization in the solid state was the result of anti-head-to-tail preorganization of pairs of molecules in the crystalline state. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)