Dimerization, acenaphthylene

Figure 10.2. Ratio of cis-to-trans acenaphthylene dimers found in (1) benzene and (2) cyclohexane as a function of acenaphthylene concentration.

Figure 10.3. Stem-Volmer plot of the quenching of acenaphthylene dimer formation by ferrocene. (Reproduced with permission from Ref. 41.)...

Photochemical cycloaddition of cyclohex-2-ene-l,4-dione (31) and acenaphthylene (32) in dichloromethane gives the iM-,un//,c -adduct 33 in 15% yield, in addition to large amounts of acenaphthylene dimers, which are insoluble and precipitate during the photolysis.100... 

Table 7. Product Yields and Ratios for Acenaphthylene Dimerization...

Photocycloreversions have been reported in 1,2,3,4-tetraphenylcyclo-butane and in the endoperoxide of 1,4,9,10-tetraphenylanthracene, and fluorescence spectroscopic studies have shown that cycloreversion of cis- and trtms-acenaphthylene dimers (178) to acenaphthylene proceeds by way of the excited singlet state. 

Figure 15.16 Syn/anti ratio of acenaphthylene dimer formed in alkylamine- or perfluoroalk-ylamine-intercalated graphite oxide as a function of acenaphthylene content The broken lines are guides for eyes. (Reprinted with permission from [85] Copyright (1999) Royal Society of Chemistry.)...

The quenching of the trans dimer with oxygen and ferrocene indicates that this product is formed almost entirely from the triplet state. It is possible to calculate the amount of triplet-derived product in benzene by subtracting the amount of product obtained in the presence of oxygen from the amount of product obtained in the absence of oxygen. Such a calculation indicates that acenaphthylene triplets in benzene give both trans and cis dimers in the ratio of 74 26. The triplet state accounts for almost all of the trans product and about 10% of the cis product. The break in the slope of the Stem-Volmer plot for the trans dimer (Figure 10.3) may be attributed to the presence of two excited species which are quenched at different rates. These two species could be (a) two different monomeric acenaphthylene triplet states 7 and T2 or (b) a monomeric acenaphthylene triplet state 7 and a triplet excimer. This second triplet species is of relatively minor importance in the overall reaction since less than 5% of the total product in an unquenched reaction is due to this species. 

The photocycloaddition of maleic anhydride to acenaphthylene has been studied by Hartmann and Heine.(107a> Irradiation of acenaphthylene in the presence of maleic anhydride in light-atom solvents (dioxane, acetone, or acetonitrile) yields only dimers or copolymers of acenaphthylene. In heavy-atom solvents (dichloromethane, dibromomethane, or iodomethane), however, dimerization is suppressed and cycloaddition with maleic anhydride predominates ... 

Cowan and Drisko have studied the photodimerization of acenaphthy-lene(130,l36) in detail and have concluded that the cis dimer is derived primarily from a singlet excimer state of acenaphthylene while the trans dimer is derived entirely from the acenaphthylene triplet state ... 

The amount of trans dimer formed upon the photolysis of acenaphthylene in heavy-atom solvents could be correlated with the square of the spin-orbit coupling constant for the heaviest atom in the solvent (see Table 5.11) ... 

The ratio of cis (27) to trans (26) dimers produced in this reaction has been found to be solvent and concentration dependent, as can be seen in Table 10.3. The data in Table 10.3 show that the cis dimer predominates in all three solvents. It is also seen that as the acenaphthylene concentration is increased, proportionally more cis dimer is formed. This effect is slightly more pronounced in cyclohexane than in benzene (see Figure 10.2) Increases in the yield of dimer with increasing concentration would of course be expected... 

The symbols A, (A - A)s, and Aa refer to acenaphthylene monomer, its singlet excimer, and its stable dimer(s), respectively. Experimental support for the singlet excimer mechanism [(10.27b)-(10.27d)] has been provided by Chu and Kearns,<47) who have shown that the cis (but not the trans) dimer in EPA (300-195°K) gives a broad fluorescence emission due to a singlet excimer. This excited state was also demonstrated to lead to dimer dissociation. 

A2 is the dimer, HA is ethyl iodide, and Q is ferrocene. This mechanism is essentially the same as that proposed for the dimerization of acenaphthylene. The steps involving possible singlet excimer formation and singlet dimerization have been excluded since they are at best relatively unimportant processes in heavy-atom solvents. Using the steady-state approximation, we can... 

The first photochemical reactions to be correlated with PMO theory were the dimerizations of anthracene, tetracene, pentacene, and acenaphthylene. 36> More detailed energy surfaces for the photodimerization reactions of butadiene have also been calculated. 30> In the category of simplified calculations lie studies of the regiospecificity of Diels-Alder reactions 37>, and reactivity in oxetane-forming reactions. 38,39) jn these... 

Photolytic. Based on data for structurally similar compounds, acenaphthylene may undergo photolysis to yield quinones (U.S. EPA, 1985). In a toluene solution, irradiation of acenaphthylene at various temperatures and concentrations all resulted in the formation of dimers. In water, ozonation products included 1,8-naphthalene dialdehyde, 1,8-naphthalene anhydride, 1,2-epoxyacenaphthylene, and 1-naphthoic acid. In methanol, ozonation products included 1,8-naphthalene dialdehyde, 1,8-naphthalene anhydride, methyl 8-formyl-1-naphthoate, and dimethoxyacetal 1,8-naphthalene dialdehyde (Chen et al., 1979). Acenaphthylene reacts with photochemically produced OH radicals and ozone in the atmosphere. The rate constants and corresponding half-life for the vapor-phase reaction of acenaphthylene with OH radicals (500,000/cm ) at 25 °C are 8.44 x lO " cmVmolecule-sec and 5 h, respectively. The rate constants and corresponding half-life for the vapor-phase reaction of acenaphthylene with ozone at 25 °C are... 

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). 

The yellow, nonfluorescent, hydrocarbon acenaphthylene in solution dimerizes on irradiation to form a product with a cyclobutane structure ... 

The evidence from photosensitization derives from the Rose Bengal (RB) sensitized dimerization of acenaphthylene.1 15 It is observed that one RB molecule can yield, by repeated excitation, dimeric molecules with a high conversion. This requires that dimers move away from the RB molecules and that monomers move to occupy these vacant places. Evidence has been presented1 that the RB molecules are firmly fixed to the surface and thus the acenaphthylene monomers and dimers must be undergoing extensive motion on the surface during... 

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. 

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