Substituted photoisomerization

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... 

Isothiazole behaves as a typical stable aromatic molecule. Thermolysis of substituted isothiazoles at 590 °C leads to the formation of thioketenes (80MI41700) and phenyl-isothiazoles undergo photoisomerism (Section 4.17.6.2) (73BSF1743, 81T3627). 1,2-Benzisothiazole boils at 220 °C without appreciable decomposition, and the 2,1-isomer... 

Methoxy- and 5-(methylsulfanyl)-l,4-benzoxazepines undergo ring contraction on heating to give 1-substituted isoquinolin-4-ols 2, while photoisomerization affords 2a,7b-dihydrobenzo-furo[3,2-b]azetes 3.32... 

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... 

The photochemical extrusion of S02 from a-phenylsulfonyl-substituted enone systems, to give the analogous /J-phenylenones in modest yield, has also been reported60. Where Z, -photoisomerization is possible, however, for example in compounds such as 28 or 29, photoequilibration of the two isomeric sulfones is the dominant process observed61,62. 

The photochemistry of ally 1-substituted Group 14 metal compounds can be divided into two sections, namely photoisomerization reactions, and those involving some kind of intermolecular reaction. 

We entered this field when we studied the photoisomerization of simply substituted nitrile oxides 138 (R = Cl, Br, CN).188 Matrix irradiation yields the corresponding isocyanates 144. In the case of chloronitrile oxide besides the absorptions of chloroisocyanate189 the bands of chloronitrene190 also appear. It can be assumed that the nitrile oxide-isocyanate rearrangement starts with the ring closure 138 - 140.191192... 

The second example is an intermolecular crystal-state reaction. Cross-conjugated 1,5-disubstituted 1,4-dien-3-ones in solution undergo both cis-trans photoisomerization and photodimerization, yielding complex mixtures of products, including die all-trans-substituted cyclobutane 2 in the case of 1,5-diphenyl-1,4-pentadien-3-one. In contrast, dienones such as 3a in whose crystals adjacent molecules lie parallel and strongly overlapped react in the solid to give 3b as the sole photoproduct. This isomerically pure tricyclic diketone results, formally, from an eight-center dimerization. It is not formed in the reaction in solution, and could be prepared by other methods only with considerable difficulty (4). 

Full details have now appeared concerning the photoisomerization of triquinacene its bridgehead substitution via photochlorination and the S l solvolytic reactivity of these halides The three deuterated, optically active 2,3-dihydrotri-quinacenes 463 of known absolute configuration have been prepared from +)-462 (Scheme XXXVII) The dextrorotatory monodeuterated triquinacene 464 was... 

The addition of O2 to acyclic dienes proved to be strongly dependent on terminal substitution and the substituents at other positions of the conjugated system, and, furthermore, it must be accompanied by photoisomerization of ( ,Z)-dienes to ( , )-dienes because singlet oxygen adds exclusively to ( , )-dienes to yield endoperoxides (cf Tables 6(a), 6(b), and 7, and references therein). 

The excited-state behavior of 1,1,2,2-tetraphenylethene (TPE) has been studied by means of picosecond fluorescence, absorption, and Raman spectroscopies and picosecond optical calorimetry. It has been shown that, like stilbene, TPE derivatives substituted with minimally perturbing stereochemical labels such as methyl groups undergo efficient photoisomerization. However, unlike stilbene, strong spectroscopic evidence exists for the direct detection of the twisted excited singlet state, 5ip herein but traditionally designated as of TPE. 

Compound 122 represents the product of a transposition of the substituents on the benzene ring. Such products are quite characteristic for this photodecarboxylation reaction. Their origin could be explained a priori by the well-known photoisomerization of substituted benzenes320 of type 121 and 126, which are formed primarily. Alternatively, it has been suggested60 that the phototransposition reaction occurs during the course of the decarboxylation. This supposition is based on two facts. First, no photo-Fries... 

Valence isomer formation is a feature also of the photochemistry of naphthalenes (3.33) and anthracenes for naphthalenes, as for benzenes, the extent ol steric crowding helps to determine which type of valence isomer predominates, since there is more severe interaction in the bicyclohexadiene products than in the benzvalene products. Amongst five-membered heteroaromalic compounds there are many known ring photoisomerizations that involve conversion of a 2-substituled to a 3-substituted system (e.g. 3.34). In some cases non-aromatic products can be isolated, such as bicyclo[2.1.0]pentene analogues from thiophenes 13.35). or acylcycfopropenes from furans (3.36) related species may be... 

Photoisomerization of pyran-4-one and substituted derivatives to pyran-2-ones involves a zwitterionic intermediate of similar type (Scheme 7). 

Substituted benzenes have been shown to undergo various valence photoisomerizations to yield bicyclo[2.2 Ojhexadienes, benzvalenes, and prismanes.223 The photochemistry of benzene itself is, at present, rather ill defined. Recently, it has been shown that benzene excited by 2537-A irradiation adds to olefins224-226 and to dienes227 to form 1,2-, 1-3-, and 1,4-bridged products. 

The direct photoisomerization of substituted stilbenes has also received attention. Several 4,4 -disubstituted stilbenes in which one substituent is electron withdrawing and the other electron donating, such as 14, have quantum yields for cis -> trans isomerization similar to that of m-stilbene, but exhibit very low quantum yields for trans -> cis isomerization in hydrocarbon solvents and zero quantum yields in ethanol.250 Likewise, certain salts of 4 -amino-2-styrylpyridine, such as 15, do not undergo direct trans -> cis photoisomerization.251 The strong interactions between the ring systems in the ground states of 14 and 15 are probably increased in the excited states. Consequently the planar... 

A remarkable series of papers has appeared over the last few years reporting the photoisomerization of substituted benzenes, largely in the condensed phase. One of the earliest observations of photoisomerization in benzenes was that of Van Tamelen and Pappas38 who irradiated an ether solution of 1,2,4-tri-I-butyl benzene (1) with a Hanovia Type L lamp and identified a product as the Dewar analog of tributyl benzene (2). This compound reverted thermally to the starting material... 

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