Coupling reactions photochemical

Although silyl compounds have been thus employed in photochemical reactions as all l radical donors, sufficient levels of yields are attained only in quite limited cases. As the caibon-tin bond of cation radicals of stannyl compounds cleaves quite smoothly compared with the corresponding silyl derivatives in oxidation reactions with metallic reagents (see earlier section a-All lthioalkyl Cations from a-Stannyl Sulfides ), we examined the photochemical coupling reaction using organostannyl compounds. 

In aspect of its toxicity, any pathway leading to abatement of chromate(VI) pollution arouse a vivid interest. One of such pathways seems to be created by cooperations between the iron and chromium photocatalytic cycles, which were reported as effectively converting chromate(Vl) into Cr(III) species. Photochemical coupling reactions between polycarboxylate Fe(III) complexes and chromate(Vl) were studied and strong collaboration between both photocatalysts was demonstrated, which was significantly affected by the oxygen concentration (16,17,95,261). On the other hand, chromium(Vl) reduction pho-toinduced by iron(lll) nitrilotriacetate accompanied by nta degradation was found to be independent of the O2 concentration, whereas the oxidation state of the chromium product depended on the pH (257). 

A photochemical coupling reaction of 2-halothiophenes recently published was used for the preparation of a number of naturally occurring bithiophenes. It involves the photolysis of a 2-iodothiophene with another thiophene molecule 87, 88, 89, 90). This method was used... 

TABLE 56.13 Photochemical Coupling Reaction of Cycloalkanone at Room Temperature... 

Reaction that can be carried out by the oxidative coupling of radicals may also be initiated by irradiation with UV light. This procedure is especially useful if the educt contains oleflnic double bonds since they are vulnerable to the oxidants used in the usual phenol coupling reactions. Photochemically excited benzene derivatives may even attack ester carbon atoms which is generally not observed with phenol radicals (I. Ninoraiya, 1973 N.C. Yang, 1966). 

Autoxidation of secondary acetonitriles under phase-transfer catalytic conditions [2] avoids the use of hazardous and/or expensive materials required for the classical conversion of the nitriles into ketones. In the course of C-alkylation of secondary acetonitriles (see Chapter 6), it had been noted that oxidative cleavage of the nitrile group frequently occurred (Scheme 10.7) [3]. In both cases, oxidation of the anionic intermediate presumably proceeds via the peroxy derivative with the extrusion of the cyanate ion [2], Advantage of the direct oxidation reaction has been made in the synthesis of aryl ketones [3], particularly of benzoylheteroarenes. The cyanomethylheteroarenes, obtained by a photochemically induced reaction of halo-heteroarenes with phenylacetonitrile, are oxidized by air under the basic conditions. Oxidative coupling of bromoacetonitriles under basic catalytic conditions has been also observed (see Chapter 6). 

For instance, Kochi and co-workers [89,90] reported the photochemical coupling of various stilbenes and chloranil by specific charge-transfer activation of the precursor donor-acceptor complex (EDA) to form rrans-oxetanes selectively. The primary reaction intermediate is the singlet radical ion pair as revealed by time-resolved spectroscopy and thus establishing the electron-transfer pathway for this typical Paterno-Biichi reaction. This radical ion pair either collapses to a 1,4-biradical species or yields the original EDA complex after back-electron transfer. Because the alternative cycloaddition via specific activation of the carbonyl compound yields the same oxetane regioisomers in identical molar ratios, it can be concluded that a common electron-transfer mechanism is applicable (Scheme 53) [89,90]. 

Thus, we must conclude that it is likely that more success will be achieved if attention is given to photochemical processes eitploying two coupled reactions so that two photons are used for every electron transferred in the ultimate reaction (16). max values for two photosystems are given in column 6 of Table 1. 

Such reactions may be of considerable significance. This is because, if two pendant p-coumarate linkages (or related molecules) are attached to two adjacent polysaccharide chains, an effective means of cross-linking via photochemical coupling could be achieved. However, there is no evidence at present to indicate that these dimers function as either intermolecular or intramolecular cross-linking reagents. 

Unactivated aryl iodides undergo the conversion Arl — ArCHj when treated with tris(diethylamino)sulfonium difluorotrimethylsilicate and a palladium catalyst.131 A number of methods, all catalyzed by palladium complexes, have been used to prepare unsymmetrical biaryls (see also 3-16). In these methods, aryl bromides or iodides are coupled with aryl Grignard reagents,152 with arylboronic acids ArB(OH)2,153 with aryltin compounds Ar-SnR3,154 and with arylmercury compounds.155 Unsymmetrical binaphthyls were synthesized by photochemically stimulated reaction of naphthyl iodides with naphthoxide ions in an SrnI reaction.156 Grignard reagents also couple with aryl halides without a palladium catalyst, by the benzyne mechanism.157 OS VI, 916 65, 108 66, 67. 

The photochemical coupling between methyl 5-iodothiophene-2-car-boxylate (119) and 2-chlorothiophene gave the corresponding bithiophene 140. Also, in this case the reaction with propyne gave a naturally occurring bithiophene 142 isolated from Arctium lappa (89JPP(A)(47)191). 

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