Hydrocarbons photoisomerization

Sundstrdm V and Gillbro T 1985 Dynamics of trans-cis photoisomerization of stiibene in hydrocarbon solutions Ber. Bunsenges Phys. Chem. 89 222-6... 

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... 

It is now well established that the cation radicals of unsaturated and strained hydrocarbons undergo a variety of isomerization (e.g., Scheme 18) and cycloaddition reactions with much faster rates than those of the corresponding neutral molecules [162-165]. A cation radical chain mechanism analogous to Scheme 17 was reported for one-way photoisomerization of cis-stilbene (c-S) to truws-stilbene (f-S) via photoinduced electron transfer, as shown in Scheme 18 [166], Once c-S + is formed, it is known to isomerize to t-S + [167,168]. The free energy change of electron transfer... 

In hydrocarbon solvents, /3-diketones are predominantly (> 90%) enolized and these solutions have been subjected to flash photolysis, which causes photoisomerization to the diketo form9). Reversion of the diketone to the more stable cis enol was then followed by UV spectroscopy and pseudo first order rate constants at room temperature were in the range 14-68 x 10-3 s-1 (e.g. AA = 23 x 10-3 s 1) with half-lives of several hours. The same research also reveals an alternative transformation on irradiation. In this the cis enol form is converted by rotation about a C-C or C=C bond into one of the possible trans enol isomers. These may then go on to the diketo form but mainly they revert very rapidly to the cis enol with rate constants of 0.1 to 70 s 1 (e.g. AA = 0.27 s-l)9). 

Compounds I and III at 313 and 238-265 nm photodecompose efficiently to CO and hydrocarbon products (i.e., co = 0.77 and 0.87, respectively). In contrast, compounds II and IV photode-composed much less efficiently under identical conditions. However, compound II photoisomerized efficiently to 3-methyl-2-cyclopentenone. This rearrangement is comparable to the photoisomerization of methylcyclopropyl ketone to methylpropenyl ketone (190). 

No gas-phase photolyses have yet been reported with these diazirines. Pyrolysis of the cycloheptyldiazirine yields a mixture of hydrocarbons, and the relative yields of these products are very close to the values obtained by the pyrolysis of diazocycloheptane. Photolysis of these diazirines in the liquid phase yields a hydrocarbon mixture similar to that obtained by pyrolysis, but with small increases in some of the minor products. The photoisomerization reaction leading to the normal diazo compound is readily observed for these two diazirines since the diazo compounds formed are relatively stable. In both cases, during photolysis of the (colorless) diazirines, a deep orange solution develops, and ultraviolet and infrared absorption spectra indicate the formation of the diazo compounds. Photolysis of cycloheptyldiazirine in the presence of acetic acid yields 47% of cycloheptylacetate, which indicates that the photoisomerization reaction under these conditions is a major primary reaction. Under the same conditions cyclooctyldiazirine yields 28% of the corresponding acetate. 

Direct photolysis of caryophyllene (182) and isocaryophyllene (183) gives rise to an extremely complex mixture of isomeric hydrocarbons. The majority of these have now been identified and rationalizations for their formation have been presented.In addition to the photoisomerization between caryophyllene and isocaryophyllene, caryophyllene produces (184)—(188), whereas isocaryophyllene yields (187)—(192). 

Photoisomerization of five-coordinate compounds has also been observed in low-temperature media. Photolysis of Mo(CO)5 (PCX3), (Cx = cyclohexyl) in a hydrocarbon glass at 77°K produces two isomers of Mo(CO) (PCx3) (145) ... 

Both the intermolecular [3 -I- 2] photocycloaddition of arenes to alkenes (see Section 3.2) as well as its intramolecular counterpart (one example is given in Houben-Weyl, Vol. 4/5 a, p 495) afford polycyclic compounds containing a cyclopropane subunit. In recent years it has been shown that by the appropriate choice of substituents on either the alkenyl side chain or on the aromatic ring, the regio- and stereochemical outcome of such 5-phenylpent-l-ene to tet-racycloundecene photoisomerizations can be controlled. Thus the parent hydrocarbon, and... 

E.Z -photoisomerization of the fluorinated l-phenyl-4-carboalkoxybutadienes 21 proceeds regioselectively about the phenyl-substituted double bond, consistent with the Dauben intermediate . Saltiel and coworkers have reported a particularly interesting example of solvent- and substituent-dependent regioselectivity in the E.Z-photoisomerization of , , -l,6-diaryl-l,3,5-hexatrienes (22) the quantum yields for isomerization about the central double bond in the parent molecule (22 X = H) are insensitive to solvent polarity, while those for isomerization about a terminal bond are enhanced seven-fold in acetonitrile compared with hydrocarbon solvents". Isomerization of the substituted derivatives proceeds regioselectively to yield the 1-Z and 3-Z isomers, and in both cases the quantum yield for terminal bond isomerization is enhanced dramatically in polar solvents. The results were explained in terms of competing torsional relaxation about the central and terminal bonds via biradicaloid and zwitterionic twisted intermediates, respectively. For the parent and cyano derivatives, the latter is polarized in the same sense proposed by Dauben and Ritscher. Interestingly, the polarization appears to be reversed in the case of the methoxy derivative. 

Interest is shown still in the isomerization of stilbene and its derivatives. A study of the pathways for the cis -trans isomerization of 4-nitro, 4,4-dinitro, and 4-nitro-4 -methoxy stilbenes has been reported. The dynamics of the photoisomerization of stilbenes in hydrocarbon solution has been studied and a comparison of the trans -cis isomerization of stilbene in low viscosity liquid alkanes and in the gas phase has been carried out. The photoisomerization of stilbene in straight chain alcohols has provided evidence for the existence of rotational relaxation in the excited and the ground states. An analysis of the isomerization rates of photoexcited stilbene has been made and the photobehaviour of poly deuteriated stilbene has been studied. ... 

Arylalkenes, such as stilbene derivatives, are important model compounds for the study of the E Z photoisomerization.529 559 560 The compounds absorb significantly over 250 nm, therefore direct irradiation is technically simple. Photolysis of unsubstituted stilbene in aliphatic hydrocarbons at 313 nm affords a photostationary state (PSS) consisting of 93% of (Z)-stilbene and 7% of ( )-stilbene (Scheme 6.4).112 In addition, a photoinduced 67t-electrocyclic (Section 6.1.2) formation of dihydrophenanthrene with a quantum yield of < > 0.10 competes with (Z)-stilbene isomerization ([Pg.232]

Most of the papers in this section are concerned with photo-oxidation reactions, but a few papers refer to photochemical reactions in polymer matrices. Thus the kinetics of the photo-oxidation of anthracene and naphthacene in solid polystyrene (PS),170 the photoionization of aromatic hydrocarbons dissolved in PMMA and PS,171 the photoreactions of naphthalene in cellulose triacetate,172 the cis-trans isomerization of stilbene residues in the side-chains of polymers,173 intrachain photodimerization in polymers,174 photoisomerization of 1,2-diphenyl-cyclopropane by peptides containing naphthalene in a side-chain,176 and photochemical transformations of poly(vinyl p-azidobenzoate)176 have been reported. 

The direct or sensitized cis-trans photoisomerization has been described by many authors (58). The Hg( P,) photoisomerization was explained on the basis of vibrationally excited triplet molecules. In the case of the Cd( P,), the photosensitization of unsaturated hydrocarbons mainly results in the cis-trans process. The main difference between mercury and cadmium sensitization resides in the lower energy content delivered in cadmium experiments 366.4 in comparison with 469.4 kJ/mol (59). A very recent study of the zinc photosensitization of 2-butene concludes with the cis-trans isomerization process as the main process (60). 

A considerable amount of investigation has been carried out on the unsensitized photoisomerization of mono- and disubstituted stilbenes. The use of the heavy atom effect on intersystem crossing as an elegant tool for gaining mechanistic information has been very nicely demonstrated in this system (Dyck and McClure, 1962 Saltiel, 1964). It was found that, although irans-4-bromostilbene, in contrast to the parent hydrocarbon. 

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