Coupling photochemical

S Pass, B Amit, A Parchomik. Racemization-free photochemical coupling of peptide segments. (Fmoc-amino-acid chlorides) J Am Chem Soc 103, 7674, 1981. 

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]. 

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. 

The photocatalyst used for these conversions can be generated in situ, by photolysis of a zinc dithiolene salt by preformed catalysts , or by particles supported within surfactant vesicles The idea of employing semiconductor surfaces as environments for the enhanced coupling of radicals had previous support in the photochemical coupling of cyclopentadienyl radicals formed by excitation of the corresponding anions at single crystal electrodes... 

Photochemical coupling of halogeno-substituted derivatives with thiophene... 

Bromo-2-thienyl-thiophene-2-carbaldehyde (128), obtained from the photochemical coupling of 5-iodothiophene-2-carbaldehyde and 2-bro-mothiophene, can be reduced with NaBH4 129 was treated with propyne, copper iodide, and Pd(PPh3)4 under phase-transfer conditions to give a natural bithiophene 130 isolated from Arctium lappa (87JOC5243). 

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). 

Recently, several syntheses of phenanthridines have been described in which the biphenyl bond is formed by photochemical coupling. The chief advantage of this method is that relatively simple intermediates are required on the other hand, since dilute solutions must be employed to minimize dimerization of the reactant, the technique is usually convenient only for the preparation of relatively small amounts of material. 

Oxidative photochemical coupling of the anil (116) gives 3,4 7,8-dibenzophenanthridine (117),158 the structure of which was confirmed (as shown) by an independent synthesis from l,2 7,8-dibenzofluoren-one oxime (118).158... 

Pass, S., Amit, B. and Patchornik, A. (1981) Racemization - free photochemical coupling of peptide segments. Journal of the American Chemical Society, 103, 7674-7675. 

Bithienyl derivatives can be prepared by irradiating 2-acetyl-5-iodothiophene or 5-iodothiophene-2-carboxaldehyde in the presence of various thiophene derivatives509,510. Thiazolylthiophene derivatives can be formed by photochemical coupling between iodoth-iophene and thiazole derivatives511. 

Photoreaction of 2-, 3- and 4-iodoquinolines with five-membered heterocycles (pyrrole, iV-methylpyrrole, furan and thiophene) affords the corresponding -(2-heteroaryl)quino-lines (n = 2, 3,4) in appreciable yields526. 3-Halo-1-methylquinolin-2-ones can be converted into 3-aryl-l-methylquinolin-2-ones by photochemical coupling with various aromatic or heteroaromatic compounds527,528. 

The photochemical couplings of 4,5-diphenyl- and 4-phenyl-l,3-diselenole-2-thione to give tetraphenyl- and diphenyl-tetraselenafulvalene, respectively, and elemental sulfur have been reported (79ZC192). In this same report the irradiation of 4,5-diphenyl-l,3-thiaselenol-2-one (52) in the presence of nickel tetracarbonyl gave the nickel thiaselenane (53). 

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. 

Kerrigan, S. Brooks, D. E. Optimization and immunological characterization of a photochemically coupled lysergic acid diethylamide (LSD) immunogen, Bioconjugate Chem. 1998, 9, 596-603. 

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). 

The A-methoxyamino alcohol could then be reduced to the amino alcohol. A photochemical coupling has also been reported. A variation of this reaction treats an imine with Yb in THF/HMPA and then an aldehyde to give a l,2-bis(imine). OS I, 459 II, 71 X, 312 81, 26. 

Figure 86. Quantum Yields O for the Photochemical Coupling of Bromofurfural with Benzene and various Benzene Derivatives in Acetonitrile [78].

By analogy with the quantum yields for the photochemical coupling of bromofurfural with benzene and various benzene derivatives in acetonitrile as shown in Figure 86 [78], the quantum yield of Burger s difurfural process must be expected to lie far below unity. 

A deviant photochemical coupling of the Mannich condensation products 13 gave 2-mor-pholino-l-phenylcyclopropan-l-ols 14 in good yield. 

A full paper expanding on the previously reported photochemical coupling of vindoline (180) with catharanthene (181) to give vinblastine (182) and vincristine (183) has been published. Photolysis of (180) and (181) in aqueous solution at pH 3.5 gives... 

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