Photochemical isomerization compounds

The photochemical isomerization of pentaatomic heterocyclic compounds has been the object of several reviews, starting with that of Lablache-Combier in 1971 (71BSF679). After this work, reviews of the whole field (76MI123 77CRV473 ... 

Recently, we reported our first attempts to reach a unified description of the photochemical isomerization of pentaatomic heterocyclic compounds [99H(50)1115 99MI233]. If the first excited state of a molecule is populated, this molecule can... 

The irradiation of imidazole derivatives such as 81 gave isomeric compounds 82 (Scheme 30) (67TL5315 69T3287). Dewar isomers are invoked to justify the observed photochemical behavior. 

Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-cis photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [29-31]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CgAzoCioN+ Br induced by photoirradiation [32]. 

Photochemical isomerization of trans- -ionol (128) in benzene gave the 1-cis-isomer in high yield. The photochemical behaviour of y,5- and S,e-unsaturated carbonyl compounds of the dihydroionone series has been studied in detail, and... 

Pericyclic reactions are unimolecular, concerted, uncatalyzed transformations. They take place in a highly stereoselective manner governed by symmetry proper-ties of interacting orbitals. - Characteristic of all these rearrangements is that they are reversible and may be effected thermally or photochemically. The compounds in equilibrium are usually interconverted through a cyclic transition state,224 although biradical mechanisms may also be operative. A few characteristic examples of pericyclic rearrangements relevant to hydrocarbon isomerizations are presented here. 

More detailed consideration of light absorption and consequent chemical changes is left to Chapter 13, but it is appropriate here to summarize briefly the types of compounds that are convenient photochemical radical sources. Many of the substances we have been discussing as thermal radical sources absorb light in the visible or ultraviolet and can be decomposed photochemically. The azoalkanes are particularly versatile they absorb around 350 nm and decompose cleanly to nitrogen and two radicals just as in the thermal reaction. As we have already noted, a preliminary photochemical isomerization to the cis isomer precedes the homolysis, which is actually a thermal decomposition of this unstable form.78 CIDNP observations confirm a stepwise decomposition pathway, and clarify the various reactions of the radicals produced.79... 

Many ortho photocycloadducts are thermally and photochemically labile compounds. When a mixture of isomeric adducts is formed from a particular combination of arene and alkene, its composition is often time dependent. Ratios of... 

Compounds 63 and 64 coexist as a 1 1 equilibrium mixture upon irradiation of one or the other in C6D6 (mercury high-pressure lamp Phillips, HPK 125W, Duran-50 filter). On heating of each of the two isomers separately at 150 °C, compound 65 (yield 100% or 48%, respectively) is formed by a skeletal rearrangement. Finally, compound 67 can undergo complete photochemical isomerization to furnish an isomeric mixture of two compounds 65 and 66 (ratio 80 20) (Scheme 6) <1997AGE1337>. 

However, not all the six prepared compounds (Scheme 3b) underwent photochemical isomerization the photoisomerization reactivity of these dithienylethene-bridged diporphyrins strongly depended on the distance between the porphyrin and dithienylethene. The three compounds, in which porphyrin and dithienylethene were directly linked (1), linked through a 1,4-phenylene (2), and linked through meso-ethynylene (5), did not exhibit any photochromism. The authors attributed it to the efficient quenching of the excited dithienylethene by the attached porphyrin moiety through intramolecular energy transfer. 

The present procedure is that of Schafer8 9 and is the first method available for large-scale preparation of a Dewar benzene. Other syntheses of compounds containing the Dewar benzene skeleton have generally involved photochemical isomerization of the corresponding benzene iso-... 

The preparation of 3,1-benzoxazepines by photochemical isomerization of quinoline A-oxides constitutes a rather general entry into this class of seven-membered heterocycles. Since the structure of the photoisomer of 2-phenylquinoline A-oxide was first recognized as 2-pheny 1-3,1-benzoxazepine by Buchardt et al.,3 the scope of this method for oxidative ring expansion of six-membered heterocyclic A-oxides to 1,3-oxazepines has been extensively explored.4 For example, irradiation of 2-cyano-, 2-phenyl-, and 2-methoxyquinoline A-oxides affords the corresponding 2-substituted 3,1-benzoxazepines in 70-90% yield.5 However, isolation of the moisture-sensitive parent compound was only recently accomplished in the submitters laboratories.6... 

Demonstration of cis addition with acetylene is unequivocal, in that the stereochemistry of the resulting bis(dihaloboryl)ethylene has been shown directly by spectroscopic evidence (90) and by conversion of the initially produced cis isomer to the trans compound by photochemical isomerization (27). Stereochemical inferences in the olefin addition reactions are based on the assumption that hydrolysis and oxidation of the bis(boryl) compounds to the corresponding diols occurs with retention of configuration, as is the case in closely related systems 19). The NMR spectrum of the addition product of B2CI4 with 1,3-cyclohexadiene is also consistent with cis addition (120). 

A quantum chemical study of the mechanism of the cis-trans photochemical isomerization in retinal like protonated Schiff bases uses the model compound 1-imino-2.3-pentadiene23. The lowest excited state is B like but the second A like state is particularly labile photochemically. The photoisomerization of all trans-retinal has been studied in a variety of organic solvents and organized media232 The structure of the isomers involved in the photoisomerization of retinoic acid and photoprotective effects in model physiological solutions have been determined233. picosecond time-resolved absorption study of... 

Stiver and Yates have studied the photochemical reactions of some hydroxy-keto steroids (28, 29). Irradiation of the isomeric compounds (28a, 29a) showed that the products obtained, (30) and (31) respectively, had retained the configuration of the carbon to which the hydroxy group is attached. The use of deuteriated derivatives (28b, 29b) has identified the hydrogen abstraction processes involved in the conversion of these ketones into the lactones (30b) and (31b) respectively. The authors " propose that there are two major factors which control the stereospecificity of the reactions. These are the shape of the hydroxy-bearing C-atom and the hydrogen transfer within the biradical formed on Norrish Type I fission. The stability of the biradical intermediate clearly plays an important part in determining the outcome of the reactions. 

The synthesis of unsubstituted Dewar benzene was accomplished by van Tamelen (3) 4). Thus, dihydrophthalic anhydride was photochemically isomerized to a [2.2.0]-ring system which was decarboxylated oxidatively to Dewar benzene. This compound has a higher stability than expected from the high strain of its ring system (t1/2 = 37.2 h at 24.3°). This stability was later explained by the rule of the conservation of the orbital symmetry by Woodward and Hoffmann. 

Mesoionic five-membered heterocycles are also an important group of heterocyclic compounds. These compounds are very useful intermediates in organic synthesis because they react as dipolar components in 1,3-dipolar cycloaddition reactions. They are photochemically isomerized to other dipolar compounds via Dewar-type intermediates. These dipolar compounds are converted to products different from those formed in the thermal reaction (118) U8). (Concerning this type of reaction, see ref.102).)... 

Irradiation (quartz filtered) of the oxazolone derivative (199) in acetonitrile results in decarbonylation and the formation of the imine (200). When this imine is formed in the presence of allyl alcohols, trapping (a thermal reaction) results in the formation of the ethers which are also photochemically reactive and are transformed by a Norrish type II hydrogen abstraction process into the isomeric compounds (201). Oxy-Cope rearrangement of (201) yields the second product (202) isolated from the initial irradiation. 

Photolysis of a-diazo esters in the presence of benzene or benzene derivatives often results in [2-1-1] cycloaddition of the intermediate acylcarbene to the aromatic ring, thus providing access to the norcaradiene (bicyclo[4.1.0]hepta-2,5-diene)/cyclohepta-l,3,5-triene valence equilibrium. The diverse effects that influence this equilibrium have been discussed (see Houben-Weyl, Vol. 4/3, p509). To summarize, the 7-monosubstituted systems obtained from a-diazoacetic esters exist completely in the cycloheptatriene form, whereas a number of 7,7-disubstituted compounds maintain a rapid valence equilibrium in solution. On the other hand, several stable 7-cyanonor-caradienes are known which have a second 7t-acceptor substituent at C7 (see Section 1.2.1.2.4.3). Subsequent photochemical isomerization reactions of the cycloheptatriene form may destroy the norcaradiene/cycloheptatriene valence equilibrium. Cyclopropanation of the aromatic ring often must compete with other reactions of the acylcarbene, such as insertion into an aromatic C H bond or in the benzylic C H bond of alkylbenzenes (Table 7). 

The light-induced isomerization of bicyclo[2.2.0]hexa-2,5-dienes to prismanes (see Houben-Weyl, Vol. 4/5 a, pp232 and 472) has, until recently, been of little preparative use as the starting compounds were usually obtained by irradiation of a suitable benzene precursor, and only photoequilibria were observed between the different benzene valence isomers. Recent examples of successful light-induced syntheses of prismanes involved fast photochemical isomerization of a sterically crowded Dewar benzene 1 to less sterically crowded 2, which then underwent a (slower) ring closure to the corresponding prismane 3. ... 

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