Photoisomerization Reactions

Phenomenologically, we can distinguish the following photochemical reactions photoisomerization, photocyclization (photoaddition), photocleavage, hydrogen abstraction, photo-concerted reaction, etc. For a photochemical reaction to occur, efficient absorption of ultraviolet or visible light is necessary, thus the photoreactive molecules should contain in their structure one of the bond types listed in Table 1.13. The characteristics of the typical photochemical reactions of C = C and C = O groups are summarized as follows. 

Photoisomerization Reactions. Photoisomerizations normally require large-size molecular distortions which are greatly influenced by the steric hindrance to molecular mobility afforded by the CD. 

Photochemical reactions can produce highly reactive intermediates for subsequent groimd state reactions. Photoisomerization of the cycloheptenone moiety in 151 led to a highly strained trans enol (152) that readily underwent a Diels-Alder reaction with isoprene to produce a mixture of 153 and 154, both of which were carried forward to a structure related to a naturally occurring tricyclic diterpene, vibsanin E (equation 12.94). Irradiation of the aldehyde... 

Cnppoletti, A., Dirmocenzo, J. R, Goodman, J. L., Gould, I. R. Bond-coupled electron transfer reactions Photoisomerization of norbornadiene to quadticy-clane. /. Phys. Chem. A, 103,11253-11256 (1999). 

Onganer, Y, Yin, M., Bessire, D.R., and Quitevis, E.L., Dynamical solvation effects on the cis-trans isomerization reaction photoisomerization of merocyanine 540 in polar solvents, /. Phys. 

According to Kramers model, for flat barrier tops associated with predominantly small barriers, the transition from the low- to the high-damping regime is expected to occur in low-density fluids. This expectation is home out by an extensively studied model reaction, the photoisomerization of tran.s-stilbene and similar compounds [70, 71] involving a small energy barrier in the first excited singlet state whose decay after photoexcitation is directly related to the rate coefficient of tran.s-c/.s-photoisomerization and can be conveniently measured by ultrafast laser spectroscopic teclmiques. 

Syage J A, Felker P M and Zewail A H 1984 Picosecond dynamics and photoisomerization of stiibene in supersonic beams, ii. Reaction rates and potentiai energy surface J. Chem. Phys. 81 4706-23... 

THE cvcLOBUTADENE-TETRAHEDRANE SYSTEM. A related reaction is the photoisomerization of cyclobutadiene (CBD). It was found that unsubstituted CBD does not react in an argon matrix upon irradiation, while the tri-butyl substituted derivative forms the corresponding tetrahedrane [86,87]. These results may be understood on the basis of a conical intersection enclosed by the loop shown in Figure 37. The analogy with the butadiene loop (Fig. 13) is obvious. The two CBDs and the biradical shown in the figure are the three anchors in this system. With small substituents, the two lobes containing the lone electrons can be far... 

A kinetic scheme and a potential energy curve picture ia the ground state and the first excited state have been developed to explain photochemical trans—cis isomerization (80). Further iavestigations have concluded that the activation energy of photoisomerization amounts to about 20 kj / mol (4.8 kcal/mol) or less, and the potential barrier of the reaction back to the most stable trans-isomer is about 50—60 kJ/mol (3). 

In addition to photoisomerization, there are reversible photochemical reactions of special types for asymmetrical polymethines, produciag sphopyranes (84—86) as ia equation 6, where X = NR, S, or C(CH2)2-... 

Photochromism Based on Geometric Isomerism. The simplest examples of a photochromic reaction involving reversible cis-trans isomerization is the photoisomerization of azobenzene [103-33-3] C22H2QN2 (16). 

Diazirine, fluoromethoxy-nitrogen extrusion, 7, 224 Diazirine, methylvinyl-rearrangement, 7, 221 Diazirines addition reactions to Grignard compounds, 7, 2 0 as carbene precursors, 7, 236 IR spectra, 7, 203 microwave spectrum, 7, 199 molecular spectra, 7, 202-204 nitrogen extrusion, 7, 223 NMR, 7, 202 photoconversion to diazoalkanes, 7, 234 photoisomerization, 7, 221 photolysis, 7, 225-227 quantum chemical investigations, 7, 197 reactions... 

Beckmann rearrangement, 4, 292 pyrolysis, 4, 202 synthesis, 4, 223 Wittig reaction, 4, 294 Wolff-Kishner reduction, 4, 291 Indole, 1-acyl-2,3-disubstituted photoisomerization, 4, 204 photo-Fries rearrangement, 4, 204 photoisomerization, 4, 42 synthesis, 4, 82 Indole, 2-acyl acidity, 4, 297 synthesis, 4, 337, 360 Indole, 3-acyl-acidity, 4, 297 cleavage, 4, 289 reduction, 4, 289 synthesis, 4, 360 Indole, 7-acyl-synthesis, 4, 246... 

Methane, bis(2-acetyl-l-pyrrolyl)-photoisomerization, 4, 203 Methane, bis( 1 -aryltetrazol-5-yl)dichloro-rearrangement, 5, 824 Methane, bispyrrolyl-synthesis, 4, 274 Methane, bis(2-pyrrolyl)-oxidation, 4, 271 reactions... 

In the case of cycloheptenone and larger rings, the main initial photoproducts are the -cycloalkenones produced by photoisomerization. In the case of the seven- and eight-membered rings, the double bonds are sufficiently strained that rapid reactions follow. In nonnucleophilic solvents dimerization occurs, whereas in nucleophilic solvents addition occurs. ... 

The intervention of mesoionic intermediates is suggested by the facile transformation of steroidal dienones, and by a number of acid-catalyzed nonphotolytic reactions which either parallel the photoisomerizations or correlate photoproducts from reactions in protic and aprotic solvents. The isomerization (175) -> (176) -l- (177) has also beeen achieved in the dark by acetic and formic acid catalysis and clearly involves the conjugate acid of the proposed mesoionic intermediate (199) in the dark reaction. Further,... 

The irradiation of 2- and 3-cyanothiophene gave interesting results in agreement with the scheme described above (Scheme 19). The photoisomerization reaction involved only the excited singlet state and Dewar thiophenes were isolated when the reactions were carried out at -68°C and shown to be intermediates in the isomerization reactions [79JCS(CC)881 79JCS(CC)966]. 

Ring-opening products were observed in the photoisomerization of l-methyl-4-phenylpyrazole (73) (95JOC8138 97JOC8325) and in the reaction of 1-phenylpyrazole (74) (Scheme 27) (93JA7645). 

In this chapter I have attempted to collect all the data available on the photoisomerization reactions of pentaatomic aromatic heterocycles. At the end of this work, I can note that all the results fit the hypothesis I reported in the introduction of this chapter. 

The first fully unsaturated 1,4-oxazepines 3 were prepared by the photoisomerization of 3-oxa-6-azatricyclo[3.2.0.02,4]hept-6-enes l.29,30 The reaction proceeds via the bicyclic intermediates 2 which undergo valence isomerization. 

The most widely applicable method for the synthesis of 1-substituted J//-l,2-diazepines 8 is photoisomerization of pyridinium 1-imides 6. The reaction is presumed to proceed by way of the bicydic intermediates 7, which, however, have not been detected (see Houben-Weyl, Vol. 4/ 5a, p584ff) selected examples are given.75-77... 

A variety of heteroannulated 1,2-diazepines 12 has been obtained by photoisomerization of the bicyclic /V-inunes 11, where R1, R2 represents part of a five-or six-mem bered heterocycle.143 The precursors of the imines, the AT ami no mesitylenesulfonates 10 (Ar = Mes), are prepared from the parent heterocycles by reaction with O-mesitylenesulfonylhydroxylamine.144... 

Investigation of the photochemistry of protonated durene offers conclusive evidence that the mechanism for isomerization of alkyl-benzenium ions to their bicyclic counterparts is, indeed, a symmetry-allowed disrotatory closure of the pentadienyl cation, rather than a [a2a -f 7r2a] cycloaddition reaction, which has been postulated to account for many of the photoreactions of cyclohexadienones and cyclohexenones (Woodward and Hoffmann, 1970). When the tetramethyl benzenium ion (26) is irradiated in FHSO3 at — 90°, the bicyclo[3,l,0]hexenyl cation (27) is formed exclusively (Childs and Farrington, 1970). If photoisomerization had occurred via a [(r2a-t-772 ] cycloaddition, the expected... 

It has been found that cyclohexa-2,4- and -2,5-dienones undergo a light-induced valence isomerization reaction in strong acid analogous to the alkylbenzenes, to yield 2-hydroxy-bicyclo[3,l,0]hexenyl cations. The hydroxybenzenium ion (34), for example, underwent a clean photoisomerization to 35 at temperatures below — 60° (Paxrington and Childs, 1970). Cation 35 was also produced upon similar irradiation of 36. 

Category 4. Photoisomerization. The most common reaction in this category is photochemical cis-trans isomerization." For example, cw-stilbene can be... 

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