Photochemical isomerizations

Oxidopyridazinium betaines isomerize photochemically into pyrimidin-4(3H)-ones (33). Irradiation of 3-oxidopyridazinium betaine or 1-oxidophthalazinium betaine in water affords similarly the corresponding pyridazin-3(2H)-one (35) and phthalazin-l(2H)-one derivative (37). However, photolysis in acetonitrile affords stable diaziridines (34) and (36) which can be converted in the presence of water to the final products (35) and (37) (Scheme 12) (79JCS(P1)1199). 

Perfluoroolefins isomerize photochemically to yield less substituted olefins [272] Photolysis of polyfluorotriarylmtrones leads to polyfluorotriaryloxaziridines [173] (equation 43)... 

Isomerization of ( /Z) isomers is another important transformation. Isomerization of ( ) and (Z-) conjugated amides is effected photochemically " (photo-isomerization " ). There is a rather high energy barrier for the excited state required for (E/Z) isomerization. Isomerization of the C=C units in dienes is also induced photochemically. " Isomerization of cyclic alkenes is more difficult but cyclooctene is isomerized photochemically. " Conjugated aldehydes have been isomerized... 

Photochemical d.s jraw.v-confonnational interconversion is also known to occur in larger polyenes. Brouwer and Jacobs have reported the results of irradiation of E- and Z-2,5-dimethyl-l,3,5-hexatriene (14) in argon matrices at 10 K108. Irradiation of the -isomer gives rise to various retainers, while irradiation of the Z-isomer results only in ,Z-isomerization. Photochemical translcis conformer interconversion has also been observed for , -1,3,5,7-octatricnc in matrices at temperatures below 10 K32. 

Oxidopyridiniums (26 Z = NR, Y = O) are converted photochemically into the bicycle (27) corresponding 3-oxidopyryliums and especially 4-oxidoisochromyliums isomerize more easily (cfi 44 — 45). 5-Oxidopyridazinium betaines are isomerized photochemically into corresponding pyrimidin-4-ones by a similar path (Scheme 6). 

Irradiation of pyran-2-one gives the ketene (46) reversibly. Similar reactions are known for aza and diaza analogues. Thus, l,3-oxazin-6-ones isomerize photochemically to ketene imines (47), and flash vacuum pyrolysis converts the oxadiazinone (48) reversibly into (49). 

Dihydropyridines are isomerized photochemically to 2-azabicyclohexenes (520). 2/7-Thiins behave as dienes in Diels-Alder reactions (Scheme 59). 

The incorporation of a cationic azobenzene derivative, p-( a> -dimethyl-ethanolammonioethoxyj-azobenzene bromide, into nanoporous silica films and the photochemical reactions of the adsorbed dye were investigated. The nanoporous silica films were prepared from tetramethoxysilane and octadecyltrimethyl-ammonium chloride by the rapid solvent evaporation method which we have reported previously. The adsorption of the cationic azo dye was conducted by casting an ethanol solution of the dye onto the nanoporous silica films. Upon UV light irradiation, trans-azobenzene isomerized photochemically to the c/s-form and photochemically formed c/ s-form turned back to the frans-form upon visible light irradiation. The nanoporous silica films were proved to be an excellent reaction media to immobilize organic photocromic species. 

Thus both chemical and physical methods indicate that benzene isomerizes photochemically in the general spectral region 2400-2700 A. Information about wavelength dependence and precise quantum yields is lacking and is badly needed. However, since some or all of the isomers disappear both thermally and photochemically the apparent quantum yield would have to be extrapolated to zero time to give a true value for the quantum yield of the isomerization process. Such data are difficult to obtain but there is reason to believe that they will be available in the near future. 

Pyridazinium-5-olate betaines are isomerized photochemically to corresponding pyrimidin-4-ones by a similar path (Scheme 12). 

When thermochromic anils are incorporated into host crystals of deoxycholic acid or its derivatives, the incorporated anils have been found to exhibit photochromic behaviour as well (Koyama et al., 1994 Kawato et al., 1994b). The incorporated anils are evidently allowed to isomerize photochemically as in the case of derivatives of anils carrying bulky substituents. 

Valence Isomerism.—Photochemical energy storage via the photosensitized valence isomerization of norbornadiene to quadricyclene has continued to attract attention. The effects of excitation wavelength and suppression of side reactions have been studied and the efficiency of various photosensitizers has been compared. It has been concluded that such systems are technically feasible but economically non-competitive at present. Similar valence... 

Products. Photolysis of 1,3-cyclohexadiene gives rise to 1,3,5-hexatriene of unknown stereochemistry. During irradiation in the condensed phase initial product is the m-triene which subsequently isomerizes photochemically. The other primary processes which seem to originate directly in 1,3-cydohexadiene are dehydrogenation ... 

Synthetically, compound 152 was prepared by the Friedel-Crafts procedure from furan-3,4-dicarbonyl chloride and a tetralin derivative (85JCR (S)338). 1,4-Dihydro-l,4-epoxy-5,8-naphthoquinone, when treated with 3,6-di(2-pyridyl)-l,2,4,5-tetrazine, yielded the quinone 153 (75JCS(P1)1339) derivatives of 153 are also known (74CC1034). The parent compound (153) is a stable derivative of the unstable isobenzofuran. Finally, 3-acetyl-2-furyl-1,4-benzo- (or -naphtho-) quinones are isomerized photochemically in aprotic solvents into the quionones 154 (66HCA1806). 

Andersson, J. A., Petterson, R., and Tegner, L (1982). Flash photolysis experiments in the vapour phase at elevated temperatures. I Spectra of azobenzene and the kinetics of its thermal cis-trans isomerization./. Photochem. 20, 17-32. 

However, in this case, the product is a 1 1 mixture of diastereomers Z-13 and E-13, although the base that was used here contains no lithium. After THP deprotection, the mixture can be separated by chromatography to yield both diastereomers stereochemically pure. The undesired diastereomer E-13 can be isomerized photochemically Irradiation with light (k = 280 nm) gives a 3 2 mixture of Z-13 and E-13 in 90 % yield. This process allows the recycling of the undesired diastereomer. ... 

Carbocyanine cations are known to isomerize photochemically in their poly-methinic chain. 3,3 -Diethyloxacarbocyanine iodide (DOCI) and 3,3 -diethyloxadi-carbocyanine iodide (DODCI) are among the most extensively investigated compounds and the cations are believed to exist in solution in the all trans extended structure. After irradiation one of the carbon-carbon double bonds undergoes Z/ -type isomerization and unstable photoisomeric cations are formed as illustrated in Schemes 3.3 and 3.4 [45]. 

Cycloaddition of 1,3-dipolarophiles to alkynes for the synthesis of diazo compounds can also be applied to reaction of diazoalkanes with alkynes (2-91). 2-Diazopropane and 1,2-diarylethynes readily form 3//-pyrazoles (2.229). These pyrazoles isomerize photochemically to the 4-diazo-2-methyl-3,4-diarylbutenes (2.230), i.e., to a vinyldiazo compound (Pincock et al., 1973 Arnold et al., 1976 Leigh and Arnold, 1979). Some cyclopropene (2.231) is formed in a consecutive dediazoniation, i. e., by cyclization of the carbene formed. The method is not useful for unsymmetrically substituted alkynes because these cycloadditions are not regiospecific. It is, however, applicable to the synthesis of diazoalkenes with alicyclic... 

During the past three years, we have studied the reactions of thermally generated silicon atoms with low molecular weight reaetants. Reaction products were isolated in argon matrices and identified by means of IR spectroscopy, aided by calculated vibrational spectra. The method turned out to be very versatile and successful [1-4]. When the reactions of silicon atoms with unsaturated substrates are compared, a general sequence of products can be outlined as follows if a heteroatom is participating in the multiple bond, the silicon atom is bound to a lone pair in the primary product. This primary product can be isomerized photochemically to a cyclic silylene, the formal addition product of the Si atom to the Jt bond, which is the first product in case of substrates with C-C multiple bonds. Consecutive photoisomerizations lead to the formal insertion product of the Si atom into a C-H bond and, finally, to the product(s) of (successive) migration of H atoms to the Si atom. 

An observation of potential relevance is that the 1,3-diphenylallyl carbanion can be isomerized photochemically from the predominant (95%) transjrans conformation to predominantly the cis,trans under appropriate conditions. 

Perfluoropentaethylmethylbenzene isomerizes photochemically to a mixture of Dewar-benzene isomers. Further irradiation converts these into the prismanes. These valence isomers of benzene are remarkably stable, and appear to owe their stability, in part, to destabilization of the parent benzene by non-bonded interactions. 

The stilbenes S, and isomerize photochemically in basic emimCl/AlClj, but not by the electron transfer mechanism. S, and do interconvert but, in addition, yield small amounts of unidentified products. As a result, it is not possible to calculate the PS exactly. An estimate of 55% S, and 45% at the PS seems reasonable. The low value of S, at the PS clearly rules out the electron transfer mechanism even though electron transfer from both S, and S/ to emim+ is calculated by the Rehm-Weller equation to be exothermic. The PS is unusual for the isomerization occurring by the singlet mechanism, however. With >300 mm, which was used here, the PS should have been rich in S unless there is something unusual about the quantum yields of interconversion, and... 

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