Photoreactions of Compounds Containing 6 Heteroatoms Other than Oxygen

O-methyl oxime ether of 2-acetonaphthone has also been reported to undergo facile syn - anti photoisomerization.6 The composition of the photostationary state appears to be concentration-dependent with the sy/i-isomer predominating at low concentrations. Evidence supporting the involvement of the singlet state was obtained from fluorescence-quenching studies and from photosensitization experiments. 

Quantum yields for the direct and 9-fluorenone-sensitized syn anti photoisomerization of 2-naphthaldehyde phenylhydrazone have been determined, and the photoisomerization of a series of 2-benzothiazoylhydrazones has been described.7 

Photocyclization of the azocine (6) in methanol to give the cyclobuta[c]-pyridine (7) has been reported, and is presumably the result of a concerted dis-rotatory process.8 Attempts to reverse this reaction thermally were unsuccessful. 

Analogous cyclizations have been described for a number of 2,3-dihydro-1,2(l )-diazepines (8) and for 3,4-dihydro-l, 2(2tf)-diazepine.e The n.m.r. spectrum of the photoproduct obtained from iV-ethoxycarbonyl-2,3-homoazepine 

Shudo and . Okamoto, Chem. and Pharm. Bull. (Japan), 1974, 22, 1204. 

Rearrangements.—Various examples of Z,E-photoisomeri7ation about the carbon-nitrogen double bond have been reported. The E-esters (1) have been prepared by irradiation of the Z-isomers (2) in pentane-benzene, and quantum yields for the photochemically induced interconversions of the four stereoisomers of l-(methoxyimino)-3-phenylprop-2-ene have been determined. Particular interest has been shown in the isomerization of azines the E — Z photoisomerization of benzophenone-9-anthraldehyde azine is quenched by dyes with the appropriate singlet energy, with the quantum yield for this transformation being influenced by solvent polarity. The composition of the photostationary state has been determined for a number of arylazines  

Sakuragj, and K. Tokumaru, Chem. Lett., 1984, 289. 

Studies of photoisomerization in azoalkanes and azobenzene derivatives have also been described. The E-a,a-dimethylallylazoalkane (4) is converted on irradiation with a nitrogen laser at 25 °C into the thermally labile Z-isomer (5). The triplet-sensitized decomposition of azobis(isobutyronitrile), however, has clearly been shown not to involve an analogous photoisomerization to a thermally labile Z-isomer. Photoisomerization of an azo-group is also implicated in the observed photochromism of benzazoylformazans.  

Photochemically induced Z,E-isomerization has been observed in 4-diethyl-aminoazobenzene and 4-diethylamino-4 -methoxyazobenzene and in the mesoionic azo-compounds (6). The photoisomerization of aromatic azo-compounds on hydrated Si02 and AI2O3 has made available in the adsorbed state Z-isomers which were previously inaccessible. Z-2,3,4,5,6-Pentafluoro-phenylazobenzene, obtained by irradiation of the corresponding E-isomer, is unusually stable in the dark.  

Photoisomerization of an azobenzene function located in a complex molecule is often accompanied by conformational changes. This approach has again been employed in the construction of photoresponsive crown ethers, a topic which has been the subject of a recent review. Cylindrical ionophores in which two diaza-crown ethers are linked by two photoresponsive azobenzene groups change their ability to bind polymethylene-diammonium salts on irradi- 

Useful reviews on the photochemistry of imides, the photoreactions of alkaloids, and selected aspects of photochemically induced preparative heterocyclic chemistry have been published. 

Both direct and triplet-sensitized photoisomerization of pyridylhydrazones has been reported, and a yn-ontZ-photoisomerization of the carbon-nitrogen double bond has been shown to be responsible for the photochromism observed 

The photoisomerization of azo-compounds continues to attract attention. Evidence that photoisomerization of certain steroidal-substituted azo-compounds proceeds by way of an inversion mechanism has been described. Full details have now been reported of the photorearrangement of l,2-diaza-(Z)-cyclo-oct-l-ene (14) to the -isomer (15), and analogous transformations have been observed 

Goemer, H. Grucn, and D. Schulte-Frohlinde, J. Phys. Chem., 1980, 84, 3031. 

Topics reviewed during the year include the photochemistry of indoles, sulfoxides, pyrazoles and isothiazoles, (S-hetero)cyclic unsaturated carbonyl compounds, photoinduced single electron transfer (SET) reactions of amines and of azo compounds, SET reactions of organosilanes and organostannanes with Qo and ketones, photochromic polypeptides and di(hetero)arylethenes, processes in chromophore sequences on a-helical polypeptides,aryl-aryl coupling in furans, thiophenes and pyrroles, [3+2]cycloaddition of aromatic nitriles (and esters) with alkenes, and reactions of benzylsilane derivatives.  

Topics of relevance to the content of this chapter which have been reviewed during the year include photoactive [2]rotaxanes and [2]catenanes, photochemical synthesis of macrocycles, phototransformations of phthalimido amino acids, photoaddition reactions of amines with aryl alkenes and arenes, photoreactions between arenenitriles and benzylic donors, photostability of drugs, polycyclic heterocycles from aryl- and heteroaryl-2-propenoic acids, photoreactions of pyrroles, photoamination reactions in heterocyclic synthesis, switching of chirality by light, photochromic diarylethenes for molecular photoionics and solid state bimolecular photoreactions. 

Reports of single electron transfer (SET) processes were again prominent, as were examples of the use of photochemical approaches to chiral synthesis. 

Several areas dealing with the photochemical processes in silicon containing compounds have also received renewed attention. Thus the photochemical reactivity of oligosilanes, polysilanes and silylenes has been the subject of a detailed review. Aspects of the mechanisms for the processes were highlighted. Other publications have been concerned with the photoreactions of silanes (cyclic and acyclic) disilanes, silenes, etc and the photoinduced electron-transfer reactions involving organosilicon compounds.  

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Organic Aspects of Photochemistry Photoreactions of Compounds Containing Heteroatoms Other Than Oxygen , A. Gilbert, in Photochemistry , ed. D. Bryce-Smith, Royal Society of Chemistry, London, 1970, vol. 1, pp. 351-391. 

Photoreactions of Compounds containing Heteroatoms other than Oxygen... 

IIH6 Photoreactions of Compounds containing Heteroatoms other than Oxygen (CH2) tCH,),... 

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