Valence photoisomerization

A reactivity index which accurately predicts the site selectivity of the photoisomerization of cycloheptatrienes to their bicyclic valence tautomers fails with 1//-azepines.237 For example, for methyl 2-methyl-1//-azepine-l-carboxyIatc (14), the 1-methyl isomer 16 is the predicted major product in practice the reverse is true. 

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. 

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. 

This chapter has to do with different types of photoisomerization reactions. This includes the formation of constitutional, configurational, conformational and valence isomers. 

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... 

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. 

A direct entry to the quadricyclene series has been gained through photoisomerization of bicyclo [2.2.1 ]heptadienes. Bicyclo [2.2.1 ]hepta-dien-2,3-dicarboxylic acid (Formula 194), undergoes light-induced valence tautomerization to Formula 195 (75). The parent quadricyclene 196 has been prepared by direct irradiation of bicyclo [2.2. l]heptadiene... 

Evidence for the formation of 34 (R = Ph) was provided by neutralization reionization mass spectrometry and more directly by the matrix isolation and spectroscopic investigations on 34 (R = Ph) in an argon matrix at 12 K. The UV spectrum of 34 (R = Ph) exhibits characteristic bands at X = 364, 386, 404, 420, 440, 470 and 502 nm, resembling those of the electronic spectrum of anthracene, but with the expected bathochromic shifts. If one irradiates into the maximum at X = 502 nm, all bands shown in the spectrum disappear completely within 5 minutes. The vanishing of these characteristic bands can again be explained by the photoisomerization of silaanthracene 34 (R = Ph) to the corresponding Dewar valence isomer. 

Figure 3.39 The minimal PSB model, cis-C5/75M7)7 for the PSB photoisomerization. The vertical line indicates the carbon-carbon bond about which the excited state isomerization occurs and defines the two sides used in the definition of the valence bond states.

Su has published a high order computational study of the photoisomerization of arsenin to possible Dewar-type valence isomers <2007JPC971>. The reaction has yet to be observed experimentally. 

It has been recently reported that the E-form of bis(ferrocenylethynyl)-ethane, 97a, can undergo photoisomerization to the Z-form, 97b, by excitation of a charge transfer band with visible light [82]. This structural change affords a decrease in the through bond mixed-valence interaction between two ferrocenes. 

Like proton transfer, photoisomerization is a fundamentally important photochemical process. The two most important forms of photoisomerization are valence isomerization and stereoisomerization. The latter is probably the most common photoinduced isomerization in supramolecular chemistry. It may occur in systems in which the photoactive component has unsaturated bonds which can be excited, and this effect may be exploited for optical switching applications. A number of interfacial supramolecular complexes capable of undergoing cis-trans photoisomerization have been studied from this perspective - some examples are outlined in Chapter 5. 

Photoisomerization of a l-oxa-6,6aA4-dithiapentalene leads to the trans structure which reverts rapidly to the original cis compound by a thermal process (equation 3). The rate of the thermal reversal in a polymethacrylate matrix is much smaller than in cyclohexane or ethanol (73JCS(P1)2837,74AG(E)349,77ACS(B)683). This fact rules out the interpretation of the photoisomerization as a valence isomerism as described in equation (4). Effectively,... 

Photoadditions that arise by initial excitation of the aromatic compound are not common. Benzvalenes are readily attacked by hydroxylic compounds, and so irradiation of benzene in aqueous solutions of acetic acid, for example, results in the formation of a bicyclic product (and an isomer derived from it by subsequent photoisomerization) as a result of addition to the initially formed valence isomer (3.38). A different kind of photoaddition occurs when benzenes react photochemicaliy with amines cyclohexa-I,4-dienes are the major products (3.39), accompanied by cyclohexa-1,3-dienes, and unlike many of the photochemical reactions of benzene this does not suffer loss of efficiency in scaling-up. 

The 1,3-cycloadduct 22 which is isolated (25% yield) together with the expected l,2-bis(trifluoromethyl)cyclo-octatetraene (40%) from the photoreaction of benzene and perfluorobut-2-yne is a valence isomer of the normal 1,3-adduct 23 and may arise by a simple isomerization from it, although it is thought more likely that product 22 arises by sensitized photoisomerization of the 1,4-cycloadduct (24). 

An electrocyclic photochemical rearrangement of oxepins to cyclobutene ring-containing valence isomers has also been reported. Thus 2-oxabicyclo-[3.2.0]hepta-3,6-diene has been isolated upon photoisomerization of 1. Similarly, when 1-benzoxepin 100 was irradiated, the valence tautomer3,4-benz-2-oxabicyclo[3.2.0]-hepta-3,6-diene was formed. ... 

Photoisomerization of isothiazole to thiazole was reported (Eq. 20).4 1 The intermediate was proposed to be a valence-shell extended one comparable to that proposed by Wynberg or an azirin. Reverse isomerization of thiazole to isothiazole was not observed. 

A dipolar intermediate with a valence-shell extension was proposed for the photoisomerization of phenyl isothiazoles (Eq. 21 ).47... 

The difference between such valence-shell extended intermediates and those proposed by Wynberg for his reactions is the presence of the 2,4 bonding or the 2,5 bonding. Significantly, the photoisomerization of methyl-isothiazoles depends on the polarity of the solvents, an observation which supports the suggested formation of zwitterionic intermediates (Eq. 22).48... 

Electron-withdrawing substituents on the norbornadiene C-C double bond facilitate the valence isomerization and also stabilize the quadricyclane formed by photoisomerization, as demonstrated by the selective formation of the methoxycarbonyl-substituted quadricyclane 4 (mp 50-52 C) from both 2 and 3 on direct irradiation. ... 

Qualitative photochemical studies concentrate on diene complexes because of their greater thermal inertness and ease of characterization. Formation of olefin complexes is induced in situ either by photochemical or thermal means and their presence determined by spectroscopy. The photocatalyzed hydrogenation and hydrosilation of 1,3-di-enes the photocatalyzed valence isomerization of norbomadiene to quadricyc-lane and the cis trans photoisomerization of coordinated olefins are potentially usefulHowever, these transformations are not photosubstitution reactions and are not discussed here the reader should consult ref. 1 and references cited therein. Photolysis of olefin complexes leads to olefin loss with high quantum efficiency unless the olefin is a chelating di- or polyene where, as with most chelating ligands, other reactions occur. 

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