Solvolysis photochemical

Photochemical solvolysis reactions are directly analogous to ordinary solvolysis reactions except that the absorption of light energy is necessary for reaction to occur ... 

Primary vinyl cations are highly labile species. Detailed study on solvolysis of chiral A -iodane (R)-69 in methanol was carried out by Okuyama and Fujita [108]. They concluded that the solvolysis does not involve the generation of the primary vinyl cation 71, but instead the chiral secondary vinyl cation 70, produced via the reductive elimination of iodobenzene with concomitant trans fiC-C migration. Lodder and coworkers, however, found that photochemical solvolysis of 2-phenyl-l-propenyl(phenyl)-A3-iodane produces a primary vinyl cation via an SN1 pathway [109]. 

Direct photolysis of aqueous solutions of the 2-chloro-.v-iriaz.ine herbicides (atrazine, simazine, propazine) proceeds via excitation of the triazine molecule, followed mainly by dechlorination and hydroxylation to form the corresponding hydroxytriazine (Pape and Zabik, 1970 Khan and Schnitzer, 1978 Chan et al, 1992 Lai et al, 1995 Schmitt et al, 1995 Sanlaville et al, 1997 Torrents et al, 1997 Texier et al, 1999b Hequet et al, 2001). This observation - plus the fact that when 2-chloro-v-triazine herbicides are photolyzed in methanol, ethanol, and n-butanol, the respective 2-alkoxy derivatives are formed - indicates a mechanism involving photochemical solvolysis rather than the involvement of hydroxyl radicals. This conclusion is supported by the fact that the rate of oxidation of atrazine was unaffected by the presence of either bicarbonate ion (Beltran et al, 1993) or ferf-butanol (Torrents et al, 1997), both strong hydroxyl radical scavengers. 

This intriguing observation led the present author to investigate whether or not the phenomenon was more general and to search for a quantum mechanical basis for the chemistry. Two studies were carried out 1 10 11). In the first1,10) a photochemical solvolysis of substituted-phenyl trityl ethers was encountered. Here p-nitro-phenyl and p-cyanophenyl trityl ethers were found to solvolyze thermally in the dark faster than the meta-isomers as expected. However, the meta-isomers solvolyzed more readily on irradiation, and the quantum yields were higher for the meta-isomers. Note Equations 1. 

Still another example of the phenomenon was encountered in a study of the photochemical solvolysis of 4-methoxybenzyl, 3-methoxybenzyl, and 3,5-dimethoxy-benzyl acetates and chlorides 1- U). Note Equation 2. 

Irradiation of the iodoketones (99) has shown that the products obtained are formed by competing radical and cationic processes. A previous account of the photochemical solvolysis of the chloroketone (100) reported the formation of two photoproducts (101) and the solvolysis product (102). A reinvestigation of this reaction has shown that the only photoproduct from the reaction is the alcohol (101). The second product (102) is formed by an acid catalysed thermal process. ... 

The situation is quite different for the photochemical solvolysis reactions of vinyl iodonium salts. In all cases studied thus far these reactions involve direct, unassisted heterolytic cleavage of the vinylic C-I bond, yielding primary and endocyclic secondary vinyl cations. For example, photosolvolysis of ( )-styryl(phenyl)iodo-nium tetrafluoroborate (37) very efficiently yields the products resulting from heterolytic cleavage of the vinylic C-I bond, depicted in Scheme (Also... 

The photochemical solvolysis of E) and (Z)-2-phenyl-l-propenyl(phenyl)io-donium tetrafluoroborate (40) in methanol and TFE yields the heterolytic vinylic C-I bond cleavage products shown in Scheme 54 and summarized in Table 16. ... 

Gronheid, R., Lodder, G., Ochiai, M., Sueda, T, and Okuyama, T, Thermal and photochemical solvolysis of (E)- and (Z)-2-phenyl-l-propenyl(phenyl)iodonium tetrafluoroborate benzenium and primary vinyhc cation intermediates, /. Am. Chem. Soc., 123, 8760, 2001. 

Photosolvolysis of some vinyl iodonium substrates can be compared with thermal solvolysis of the same substrates. Photochemical reaction of (E)- and (Z)-2-phenylprop-l-enyl(phenyl)iodonium tetrafluoroborate (31) in methanol and TFE gave products of heterolytic cleavage of vinylic C-I bond as summarized in Table III (for the product structures see eq 14.17 In contrast to... 

Finally, the case of benzene derivatives having electron withdrawing substituents will be discussed. Experimental support for mein electron withdrawal by nitro and cyano groups has been found (3, (>, 20) in the photochemical solvolyses of nitrophenyl and cyanophenyl trityl ethers. For example, it is found that in the dark w-nitrophenyl trityl other (XLII) is essentially unreactive in 90% aqueous dioxanc while the para isomer (XLIII) solvolyzes smoothly this is in accord with ground-state, expectation, for p-nitrophenolate is a better departing anion than -nitropheiiolate. In contrast, on irradiation under the same conditions, m-nitrophcnyl trityl other solvolyzes rapidly to m-nitrophenol and tri-phenylcarbinol, as major products, while the para isomer s (XLIII) solvolysis is scarcely enhanced beyond experimental error. V similar... 

Photochemically-induced addition of bromotrichloromethane to 1-ethoxy-1-trimethyl-silylethene, the ethyl enol ether of acetyl trimethylsilane, generates a 1 1 adduct which provides 3,3-dichloropropenoyl trimethylsilane (20) on solvolysis. Treatment of this material with lithium alkyl cyanocuprates resulted in addition-elimination to give the -isomers of the 3-substituted a,/J-unsaturated acyl silane products (Scheme 50)135. 

This chapter will focus on o- and p-quinone methides and will be divided into two parts. The first will present methods for generating quinone methides in photochemical and solvolysis reactions and will emphasize how the structure and stability of quinone methides dictates the pathways for their formation. The second section will discuss the results of experiments to characterize the reactivity of quinone methides with nucleophilic reagents, and the broader implications of these results. The scope of this presentation will reflect our interests, and will focus on studies carried out at Buffalo. We considered briefly writing a comprehensive chapter on quinone methides, but abandoned this idea when we learned of Steven Rokita s plans to edit a 12-chapter text, which presents an extremely comprehensive coverage of the chemistry and biochemistry of quinone methides.9... 

Cristol and his colleagues have contributed greatly to the understanding of the photochemical transformations of compounds which undergo light-induced solvolysis and photo-Wagner-Meerwein reactions. There are several other studies which are important additions to this body of work which are not covered in this review, and we refer the reader to these references [78-83]. 

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