Photolysis mechanism indoles

It was noted above that the photolysis of indole and aldehydes in the solid state leads to diindolymethanes by a mechanism postulated to involve an oxetane intermediate (Scheme 14). The reaction also proceeds when aromatic aldehydes and indole or 2-methylindole are irradiated with UV light in acetonitrile solution (Scheme 28) [61]. Under these circumstances, it has been proposed that light-induced electron transfer from the indole to the aldehyde yields a ketyl radical anion and the indole radical cation. Proton transfers, coupling, and elimination of water can then yield an electrophilic alkylidene indolenine 65 which can react with indole thermally to give the observed product. 

Indoles were high (0.02-1.2), evidence that auto-catalysls occurred In distilled water. In aqueous CRM-1, Indoles underwent sensitized photolysis, possibly through a superoxide or singlet-oxygen Intermediate. The same photoproduct of 3-MI, o-(N-formyl)amlnoacetophenone, was found In both distilled water and In aqueous CRH-1. Carbazole underwent direct photolysis In both distilled water and aqueous CRM-1, Indicating that It has a different photolysis mechanism from that of Indoles In the two systems studied. 

In all cases, the Initial rates for the Indoles In aqueous CRM-1 were elevated compared with those In distilled water. The Increases ranged from 1.3-fold for the 2-MI/3-MI pair up to 5.0-fold for 7-MI. Carbazole, by contrast, had a 2.2-fold reduced rate In aqueous CRM-1, which Is Indicative of differing photolysis mechanisms for carbazole and Indoles. 

Photolysis Mechanism. The photophyslcs and photochemistry of the Indole ring have been comprehensively reviewed in Refs. 21-23. The unique properties of the Indole-rlng excited states, either the singlet or the triplet. Include effects by solvents, temperature, and polarity. 

Because of the complex chemical nature of aqueous CRM-1, and the multiple pathways of photolysis of Indoles In solutions, we can only speculate as to the exact mechanism or mechanisms of the sensitization process In this matrix. He found, however, that the photoproduct obtained from 3-MI In distilled water was the same photoproduct Isolated In aqueous CRM-1 solution. This observation Is discussed In the next section. 

A number of interesting photocyclizations have been reported in aromatic nitro compounds. The first authenticated example is probably that of Tanasescu392 who showed that photolysis of 2-nitrocinnamic acid (370) led to the formation of 3-hydroxy-3/7-indole-2-carboxylic acid 1-oxide (371). Although the mechanism of... 

The quenching mechanism has been investigated by a picosecond laser photolysis technique for the lumiflavin (Lf) and riboflavintet-rabutylate (RFTB) as fluorescers, and indole, N-methylindole and phenol as quenchers in various solvents of different polarity, and the electron transfer from the quencher to the fluorescer has been confirmed by transient absorption spectral measurements (5). The reaction scheme of the fluorescence quenching is given in... 

Photolysis of a solution of indole containing the pentafluorophenyl ester of trifluromethylsulfonic acid yields a mixture of 2- and 3-pentafluoro-phenylindole in 45% and 24% yields, respectively [70]. The authors suggest a photochemical electron-transfer mechanism in which the ester radical anion is produced and ionizes to yield a pentafluorophenyl radical. By... 

Shimizu et al report that while [2.2] paracyclophane (55) undergoes two-photon dissociation in low temperature matrices by way of the triplet state, in the gas phase, the efficient two-photon process proceeds via a hot molecule formed by internal conversion from the initially formed singlet excited state. The photocleavage of 2-nitrobenzyl ethers and ester has been widely reported and has now been evaluated as a deprotection methodology for indoles, benzimidazole, and 6-chlorouracil (Voelker et al). The mechanism of the cleavage of such compounds is considered to involve the o-quinonoid intermediate, but previously these had only been deduced from transient electronic spectra produced in flash photolysis experiments. Infrared spectral data from photochemical studies of 2-nitrobenzyl methyl ether in argon and nitrogen matrices have now been published which confirm that the intermediate does indeed have the o-quinonoid structure... 

Carbazole exhibited a photolysis rate comparable to that of the faster Indoles. All test compounds exhibited high correlation coefficients In the first-order regression analyses. The Indoles had a broader range of rates and generally behaved less linearly than carbazole beyond 90 min of exposure. While the photolysis of carbazole was nearly linear through 180 min, that of the Indoles generally decreased. This difference in behavior indicates that photolysis of the two classes of chemicals in this matrix occurs by different mechanisms. 

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