Photodimerization acenaphthylene

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

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 photodimerization of acenaphthylene has been extensively investi-gated<40 46) ... 

As mentioned briefly in Chapter 5, the photodimerization of acenaphthylene is subject to a very interesting heavy-atom solvent effect. The results of the photolysis of acenaphthylene in some heavy-atom solvents are given in Table 10.6.<4a) The data in Table 10.6 show that the heavy-atom solvents n-propyl bromide and ethyl iodide yield product ratios similar to that obtained in the sensitized photolysis, indicating a greater role of the triplet state in... 

Table 10.6. The Photodimerization of Acenaphthylene in the Presence of Some Heavy Atoms...

The photodimerization of various acenaphthylene derivatives has also been reported ... 

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... 

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). 

Indene does not photodimerize unless sensitized,259 whereas acenaphthylene, 16, does so when irradiated directly.280-261 Although it is believed that this reaction involves triplets, the mechanism(s) must be somewhat unusual since the stereochemistry is controlled by the reaction solvent. 

Figure 8.16 [2+2] Photodimerization of acenaphthylene (18) and 1-methylacenaphthylene within cage 2.

The photodimerization of aromatic hydrocarbons is also enhanced in micellar solutions over homogeneous solutions. Acenaphthylene (65), irradiated at the same... 

The photocycloaddition of acrylonitrile to acenaphthylene is found to be competitive with photodimerization in the non-ionic detergent Nippol PBC-34, while... 

The useful ability of micelles to produce high local concentrations of bound organic reactants at low bulk concentrations has been used in photodimerization reactions see references [712, 713] for reviews. The increase in reactant concentration per unit volume of the micelle promotes the probability of encounters between two molecules resulting in up to 1000-fold rate enhancements. The photodimerization of acenaphthylene illustrates this effect nicely cf. Eq. (5-161) [720, 721],... 

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... 

Similar results have been observed in the photodimerization reaction of acenaphthylene [732, 733] cf. Eq. (5-161) in Section 5.5.8. A considerable increase in the production of the traro-adduct was reported in cholesteric liquid-crystalline media compared to the isotropic solvent benzene, in which the cu-adduct is the dominant product... 

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. 

The 1,4-dicyanonaphthalene-sensitized photocyclodimerization of the vinyl ethers (192), yielding (193) has been described. 9,10-Dicyanoanthracene-sensitized dimerization of cyclohexa-1,3-diene affords the two [4+2] adducts (194) and (195) in a total yield of 60% and in a ratio of 4 1. This is to be contrasted with the previous report of cyclohexadiene dimerization where [2 + 2] dimers were also obtained." A detailed study of the photodimerization of some acenaphthylenes (196) has been reported. " The structures and configurations of the products have been elucidated by spectroscopic methods." ... 

In a similar study, Nerbonne and Weiss (25) found that photodimerization of acenaphthylene proceeds 10 times more efficiently in the cholesteric than in the isotropic phase of a 1/1 (w/w) mixture of 5o-cholestan-3j8-yl nonanoate/Sa-cholestan-... 

Rate data have been obtained from a detailed analysis of the previously reported 1420 maximum and fall off in the photodimerization of acenaphthylene with increasing concentrations of dissolved ethyl iodide.1420 The heavy-atom solvent dibromomethane facilitates the cross cycloaddition of acenaphthylene to trans- and cw-penta-1,3-diene by inducing intersystem crossing to the triplet state of the aromatic hydrocarbon. The intermediacy of a biradical is proposed and the stereospecificity can be understood in terms of the maintenance of the stereochemical integrity of the allylic radical units.143... 

Photodimerization of acenaphthylene in cation-exchanged bentonite clays has shown that the cis dimer (135) is formed preferentially with smaller cations. Heavier ions favour the formation of (136). ... 

The cross-photodimerization reaction represents a great challenge because it requires D to selectively recognize two different olefins in a pair-wise fashion prior to irradiation, whereas the cross-coupling reaction under standard conditions shows no preference over the homo-coupling. The cross-photodimerization of acenaphthylene with substituted naphthoquinones within D resulted in exclusive formation of the cross syn-dimer only... 

Scheme 6.5 a) Photodimerization of acenaphthylenes 10a or 10b within nanoreactor D. b) Cross-photodimerization between acenaphthylene and substituted 1,4-naphthoquinone within nanoreactor D (product distribution)... 

Another interesting example of this kind is the photodimerization of acenaphthylene previously discussed in many papers (see [83] and references therein). It is not so surprising that [2 + 2] photodimerization takes place at the ethylenic moiety, but free valence indices, unlike the frontier orbital analysis [83], favor the opposite positions in the naphthalene moiety. In correspondence with the experiment the excitation distributions in Fig. 14.6 unambiguously indicate the double bond sites as most photoactive. Moreover, one can expect more efficient dimerization from the triplet state rather than from the singlet state, and this agrees with [84]. Such examples can be multiplied easily. 

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