Reactions photonucleophilic

The photolytic degradation of urea herbicides also proceeds in several steps. In addition to hydrolysis reactions, degradation products hydroxylated in the ring are formed in photonucleophilic reactions. In the case of monuron and linuron, hydrolysis is even preceded by the exchange of the chlorine at< ms of the ring for hydroxyl groups (Rosen et al., 1969 Crosby and Tang, 1969). 

Pentachlorophenol (PCP, XIV) also could be considered to enter into the same photonucleophilic reaction in aqueous solution to give, among other products, tetrachlororesorcinol and chlorinated quinones (46). Further oxidation and condensation reactions then result in several unusual diphenyl ethers and aryloxyquinones (Figure 6). In alkaline solution, photonucleophilic attack of pentachlorophenoxide ion on an ortho chlorine of PCP could produce a chlorinated diphenyl ether which then goes on to form 1,2,3,4,6,7,8,9-octachlorodibenzo-p-dioxin (XV) (40). 

Consequently, the substitution of H for Cl in substances such as the herbicidal benzoic acids could be construed as another example of a photonucleophilic reaction. 

Direct photoreactions of halogenated aromatic compounds in water have received considerably more attention. The products of direct photoreactions include photonucleophilic replacement of halogen by OH, reductive dehalo-genation (i.e., replacement of halogen by hydrogen), photooxidation, and photoisomerization. Several such reactions are exemplified by the photoreactions of DDE in natural waters (eq 12) (38). 

Photonucleophilic substitution of fluoro- and chloro-anisoles has been the subject of three reports within the year. Cornelisse and co-workers have studied the photocyanation and photohydrolysis of 4-fluoro- and chloro-anisoles by laser spectroscopy and report that the initial step of the reaction involves formation of a triplet state transient complex composed of a ground state and an excited state aromatic molecule. Only in the presence of water does the complex yield radical ions and it is this process which determines the product quantum yield. The radical cation then reacts with the nucleophile to give a neutral radical which yields the substituted arene in a single step. Liu and Weiss report on anomalous effects during photonucleophilic aromatic substitution of 2- and 4-fluoroanisoles and also on the photo-... 

The photonucleophilic substitution of five-membered heteroaromatic nitrocompounds has been investigated with the nucleophiles CN-, CNO, OMe-, and water.108 Both 2-nitrofuran and the thiophen undergo smooth substitution of the nitro-group, but 2-nitropyrrole is stable to light. Such substitution reactions may be of synthetic importance as the 2-nitro-compounds are readily available. Light-induced nitro-group replacement has also been observed for 8-nitro-quinoline, which in the presence of a hydrogen donor yields 8-hydroxyquinoline.109 In carbon tetrachloride solution the keto-oxime (52) is reported to be formed. 

Sunlight decomposes many pesticides in water. Water provides a transparent and sometimes reactive medium in which air oxidations for example, are effectively energized by ultraviolet light. In the presence of hydroxide ions or other nucleophiles, photonucleophilic displacement reactions result in hydrolysis and the conversion of aromatic halides to phenols and eventually to insoluble polymers. Photoreduction also may often be interpreted as a hydride ion transfer. Extensive degradation to small fragments and the combination of reactive intermediates with natural substrates may help explain the difficulties encountered in extending laboratory observations to field situations. 

Pentachlorophenol undergoes exchange of its Cl substituents by OH groups in water, presumably also by photonucleophilic aromatic substitution (Wong and Crosby, 1979 Figure 6.18). Dark reactions probably lead to the quinones and ring-opened products observed on prolonged irradiation. 

Peitra, F. "Mechanisms for Nucleophilic and Photonucleophilic Aromatic Substitution Reactions." Quart. Rev., 23,504 (1969). 

Photonucleophilic aromatic substitution reactions of phenyl selenide and telluride with haloarenes have also been proven to involve the S jlAr mechanism, with the formation of anion radical intermediates. Another photonucleophihc substitution, cyanomethylation, proves the presence of radical cations in the reaction mechanism. Liu and Weiss have reported that hydroxy and cyano substitution competes with photo substitution of fluorinated anisoles in aqueous solutions, where cation and anion radical intermediates have been shown to be the key factors for the nucleophilic substitution type. Rossi et al. have proposed the S j lAr mechanism for photonucleophihc substitution of carbanions and naphthox-ides to halo anisoles and l-iodonaphthalene. > An anion radical intermediate photonucleophilic substitution mechanism has been shown for the reactions of triphenyl(methyl)stannyl anion with halo arenes in liquid ammonia. Trimethylstannyl anion has been found to be more reactive than triphenylstannyl anion in the photostimulated electron- transfer initiation step. 

The photo-NOCAS reaction was first described by McMahon and Arnold and is a photonucleophilic Sfj2Ar aromatic substitution between dicyanobenzene and an olefin in the presence of electron donor photosensitizers (phenanthrene or biphenyl) in acetonitrile-methanol solutions. This reaction system has been researched extensively in recent times. As shown in Scheme 6, the single electron transfer from olefin to photo-excited electron-deficient dicyanobenzene forms the cation radical of the olefin, which initiates a quenching reaction with nucleophile solvent methanol molecules and forms the methoxyalkyl radical. Addition of an electron transfer photosensitizer (phenanthrene or biphenyl) to the reaction mixture increases the efficiency of the reaction simply by absorbing more Hght. The excited state of the photosensitizer donates an electron to dicyanobenzene to give the photosensitizer radical cation and dicyanobenzene radical anion. The photosensitizer radical cation then oxidizes the olefin. 

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