Quantum photocyclization

Photocyclization rate constants are the primary and most direct reactivity measures. However when such data are unavailable, photocyclization quantum yields (0c in Tables 1—9) can serve as reactivity measures provided closely similar or parallel processes are considered. Under such conditions photoequilibrium concentrations (c in Tables 1 -9) or even chemical yields (for the direct photoaromatization) are equally useful. Photocyclization reactivity can be expressed either relative to that of the parent molecule or for a series of parallel processes,... 

Photochemical cyclohexanol formation can proceed efficiently (although quantum yields are low, slowing the reaction), and can also show high diastereoselectivity18. In this case, a methyl substituent a to the carbonyl markedly enhances the yield of photocyclization. 

Hereafter, a and b indicate the closed- and open-ring form isomers, respectively. The photocyclization and cycloreversion quantum yields were determined to be 0.46 and 0.015, respectively.1121 In the absence of oxygen, the coloration/decoloration cycle could be repeated more than 2000 times.[13] The basic performance of diaryl-ethenes is described below. 

Wagner et al. [8] have further reported that the irradiation of several ring-substituted a-(o-tolyl) acetophenones (25) leads to quantitative photocyclization yielding 2-phenyl-2-indanols (26) in high quantum efficiency (0.42-1.0). Based on the large quantum yield and large triplet decay rates (k t = 3.1—31 M-1), it was suggested that the reactive syn conformer is readily accessible and the excited conformer attains equilibrium before the 5-H-abstraction takes place (Scheme 8.6). 

One of the first examples of 8-hydrogen abstraction in acyclic ketones was the photocyclization of P-alkoxy ketones, in particular of P-ethoxypropiophenone (34), to the corresponding furanol derivatives 37 (Scheme 8.10). It was revealed that formation of enol 36 as well as a reversion to the starting ketone occur by 1,4-hydrogen transfer from the 1,5-biradical 35 causing the lower quantum efficiency for cyclization [12]. 

The quantum efficiency of this photocyclization was lower in alcoholic solvents than in hydrocarbons, which was in sharp contrast to the solvent effect observed on the formation quantum efficiencies of products resulting from y-H abstraction. It was also observed that the addition of an alcoholic solvent lowered the overall... 

Table 8.1 Photocyclization quantum yields of o-alkoxyphenyl ketones in benzene.

The photocyclization of o-alkoxy phenyl ketones to yield benzofuranols (57 and 58) represents one of the earliest example of 8-H-abstraction from the lowest n, n triplet Wagner et al. [18] have provided detailed photokinetic data studying the photocyclization of a variety of o-alkoxyphenyl ketones 56, and have revealed that quantum efficiency for cyclization for 56d was the lowest (0.023) and that for 56f the highest (1.00). The diastereoselectivity for cyclization of 56 was found to be higher in benzene and lower in polar solvents. From the estimated kH values (0.6-25 x 106 s 1), it was inferred that the low rate constant for 56e (8 x 106 s ) compared to that for 56g (25 x 106s 1) i s due to the alkyl chain in the alkoxy groups that points away from the o-carbonyl moiety in the most populated equilibrium conformations (Table 8.1). 

Direct irradiation of Z-enaminonitrile (8) and Z-enaminoisocyanide (9) revealed (Scheme 5) that both undergo Z— isomerization and photocyclization to 7, the N-2 -C-3 interchange product. In addition, cyclization of Z-9 to 7 was observed when the former was heated to 80 °C. Although the quantum yields shown in Scheme 3 reveal that the photocyclization of enaminonitrile Z-8 to imidazole 7 is a very inefficient reaction, chemical and quantum yields show that the photocleavage-photocyclization pathway via an isocyanide intermediate is a major route for the N-2-C-3 interchange phototransposition reaction <97JOC8325>. 

The effects of the substituent steric hindrance of the R group in fulgide 37b-37f on the quantum yield for the photoreactions have been reported by Yokoyama et al.42-44 and Kiji et al 45 These authors demonstrated that steric hindrance has an important effect on the quantum yield of the photocyclization (0E c) and the E — Z isomerization ( E, Z). The results are shown in Table 4.7. 

Table 4.21. Quantum Yield of Photocyclization Reaction of Fulgides and Fulgimides in Solution and Polymer Matrix51 91...

Indolyl fulgenate is a new kind of photochromic compound. The EvaZ photoisomerization is also involved, as shown in Scheme 32. The quantum yields of EvaZ photoisomerization are summarized in Table 4.28. In general, the quantum yields of E va Z photoisomerization of fulgide and its derivatives are much lower than those of photocyclization (see Section 4.5.3). 

The photocyclization of 3-nitro-2-pyridyl-DL-leucine (354) to 2-isobutyl-imidazo[4,5-i]pyridine (355) has been studied spectrophotometrically and the quantum yield of the reaction determined as a function of pH (72JCS(P2)2218). 3-iV-Methanesul-fonamidopyridine 1-oxide (356) is acylaminated at C-2 by phenylbenzimidoyl chloride (357) and the intermediate 2-acylaminated product (358) cyclized to 2,3-diphenyl-imidazo[4,5-i]pyridine (359) (74JOC1802). 

Ito Y, Matsuura T. A simple method to estimate the approximate solid state quantum yield for photodimerization of trans-cinnamic acid. J Photochem Photobiol 1989 50 141-145. Ito Y, Matsuura T, Fukuyama K. Efficiency for solid-state photocyclization of 2,4,6-triisopropylbenzophenones. Tetrahedron Lett 1988 29 3087-3090. 

Quantum efficiencies for the conversion of BR to M have been reported to be about 0.25 at room temperature [48,278], and not much higher [48] at -40°C. The apparent yield depends, of course, on the photostationary state between BR and K, as well as on any photoconversions involving the other intermediates of the photocycle. Such effects will be minimized with flashes of low intensity and sufficiently short duration, but thermal decay of the intermediates leading directly to BR, and other branching reactions are difficult to assess. 

Bicyclo[1.1.0]butane is usually a side product of the photocyclization of butadiene to cyclobutene (Srinivasan, 1963) in isooctane, the quantum yield ratio is I 16 (Sonntag and Srinivasan, 1971). It becomes the major product in systems in which the butadiene moiety is constrained near an s-trans conformation and bond formation between the two terminal methylene groups that leads to cyclobutene is disfavored. An example is the substituted diene 88 in Scheme 30, for which the bicyclobutane is the major product a nearly orthogonal conformation should result from the presence of the 2,3-di-r-bu-tyl substituents (Hopf et al., 1994). 

Direct photolysis of 50-53 in 02-saturated acetonitrile solution also leads to the corresponding carbazoles with a quantum yield of ca 0.64 in all cases (equation 13)161. Apparently, substituents have only a little effect on the chemical yield of carbazole produced by steady-state irradiation in aerated acetonitrile. However, an attempt to carry out such a photocyclization reaction by using photoinduced electron-transfer sensitization has failed, presumably due to fast back electron transfer that quenches the net reaction. It is also interesting to note that chemical oxidation and electrochemical oxidation of 50-53 does not result in carbazoles. Instead, benzidine products are formed. These results are consistent with the AMI calculations, which suggest that the cyclization reaction is both kinetically and thermodynamically more favorable from the triplet state than from the cation radical or dication. 

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