On the photodimerization of acenaphthylene

Table 10.3. Solvent and Concentration Effects on the Photodimerization of Acenaphthylene<41)...

Koser, G.F. and Liu, V.-S., Heavy-atom effect on the photodimerization of acenaphthylene substituent analysis on the efficiency of external aromatic perturbers, /. Org. Chem., 43, 478,1978. Muszkat, K.A. and Sharafi-Ozeri, S., Electronic overlap population as reactivity measure. The photodimerization of acenaphthylene, Chem. Phys. Lett., 38, 346, 1976. 

The [2+2]-photodimerizations of coumarin and its derivatives proceed selectively in soHd inclusion complexes with CDs. The yield and distribution of the photodimers are very different from those in the neat coumarin soHds. For the 4,7-dimethyl (70a) and the 4,6-dimethyl (70b) derivatives, for example, photodimers are obtained by irradiation of the CD complexes as shown in Table 73.1, whereas no dimer is formed from neat soHd coumarins. Depending on the substitution pattern of the coumarin molecules and the type of CD employed, complexes whose host/guest ratios are 1 1,1 2, and 2 2 are identified. The stereochemistry of the dimers depends on the cavity size of the CD and the relative orientations of coumarins within a complex. The photodimerization of acenaphthylene 41 is accelerated by the formation of 1 2 inclusion compound between y-CD and 41 to give cis (71a) and trans (71b) dimers (cis trans = 99 1). In contrast, the dimerization is inhibited by P-CD due to the formation of 1 1 inclusion complex. 

At acenaphthylene concentrations as low as 2 10 m, facile photodimerization takes place in benzene in the presence of nonionic or anionic surfactants, whereas in pure benzene at these concentrations no detectable amount of the two dimers is found [720]. Furthermore, the cis/trans product ratio is slightly dependent on the type of surfactant used [720, 721]. The product ratio during the course of photodimerization of 1-substituted acenaphthylenes is influenced by both solvent polarity and the addition of... 

That is, the ordered structure of the cholesteric mesophase affects the formation of the traTO-adduct advantageously. Furthermore, the trans/cis product ratio depends significantly on the initial acenaphthylene concentration. In isotropic solutions, the dimerization of singlet-excited acenaphthylene molecules is known to yield exclusively the czv-adduct, whereas a mixture of cis- and traTO-adducts results from triplet-excited solute molecules. The lowering of cu-adduct production in the mesophase has been attributed to the enhanced efficiency of the triplet reaction in comparison with the singlet reaction, as shown by quantum yield measurements [732]. The increase in triplet reaction efficiencies has been ascribed to the increase in the fraction of acenaphthylene-acenaphthylene collisions which have coplanar or parallel-plane orientations with respect to the surrounding solvent molecules, and not to the increase in the total number of collisions per unit time [732]. See references [713, 732, 733] for a more detailed discussion of this photodimerization reaction. 

It may be suspected that the genuinely topotactic (as secured by the molecular precision of the AFM [18]) photodimerization of 2-benzyl-5-benzyli-denecyclopentanone [118] might be a good candidate for a quantitative preparative photo dimerization to give the head-to-tail anti-[2+2] dimer. Early quantitative solid-state [2-1-2] photodimerizations (most of the published mechanistic interpretations of which can no longer be accepted) are listed in [110]. These deal with the anti dimerization of acenaphthylene-1,2-dicarboxylic anhydride, the head-to-head syn dimerization of acenaphthylene-1-carboxylic acid, the syn dimerization of 5,6-dichloroacenaphthylene, and the thermally reversible head-to-tail anti dimerization of seven ( )-2,6-di-f-butyl-4-(2-aryl-ethenyl)pyrylium-trifluoromethanesulfonates. All of these reactions proceed fully specific. On the other hand, quantitative photoconversions of a 1 1 mixed crystal of ethyl and propyl a-cyano-4-[2-(4-pyridyl)ethenyl]cinnamates gives mixtures of diesters with one (A>410 nm) or two cyclobutane rings (no cutoff filter). 

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