Photochemistry photoreduction

Photochemistry Photoreduction Energy transfer Pinacol rearrangement... 

In Chapter 3 we discussed two photochemical reactions characteristic of simple carbonyl compounds, namely type II cleavage and photoreduction. We saw that photoreduction appears to arise only from carbonyl triplet states, whereas type II cleavage often arises from both the excited singlet and triplet states. Each process was found to occur from discrete biradical intermediates. In this chapter we will discuss two other reactions observed in the photochemistry of carbonyls, type I cleavage and oxetane formation. 

Brennan, J.F., and Beutel, J. (1969) Quinone photochemistry. II. The mechanism of photoreduction of 9,10-phenanthrenequinone and 2-tert-butyl-9,10-anthraquinone in ethanol. J. Phys. Chem. 73, 3245-3249. 

An interesting alternative mechanism of activation is the photochemical reduction of Pt(IV) to Pt(II) (Fig. 3). In addition to photoreduction, photosubstitution and photoisomerization can also occur, making the photochemistry of Pt complexes difficult to predict and a careful analysis of the photoproducts imperative (21). We have been involved particularly in the development of photochemotherapeutic agents based on Pt(IV) and the study of their photodecomposition and (subsequent) interactions with... 

A quick survey of the photochemistry of the different complexes described above shows that the mechanism of photoactivation and the subsequent nature of the observed photoproducts varies from complex to complex and from one geometric isomer to another. Photochemical pathways often involve a combination of photosubstitution, photoisomerization, and photoreduction steps. In general, photolysis is rather slow in water and many different products are obtained if the complex is irradiated alone. The presence of nucleophilic biomolecules, on the other hand, can have a major influence, as photoreduction is usually rapid and accompanied by simpler reaction pathways. NMR methods... 

Earlier reviews on the photochemistry of unsatured ketones and amines are available39,40. The photoreactions of a,/i-unsaturated carbonyl compounds in the presence of amines have been reported to yield 1 1 amine adducts32,33 as well as photoreduction... 

Details of nitrobenzene photochemistry reported by Testa are consistent with the proposal that the lowest triplet excited state is the reactive species. Photoreduction, as measured by disappearance quantum yields of nitrobenzene in 2-propanol is not very efficient = (1.14 0.08) 10 2 iD. On the other hand, the triplet yield of nitro benzene in benzene, as determined by the triplet-counting method of Lamola and Hammond 28) is 0.67 0.10 2). This raises the question of the cause of inefficiency in photoreduction. Whereas Lewis and Kasha 29) report the observation of nitrobenzene phosphorescence, no long-lived emission from carefully purified nitrobenzene could be detected by other authors i4,3o). Unfortunately, the hterature value of Et for nitrobenzene (60 kcal mole i) is thus based on an impurity emission and at best a value between 60 and 66 kcal mole can be envisaged from energy-transfer experiments... 

Other authors 82.83) also found a delicate concentration and viscosity-dependent balance between a nitro-nitrite-isomerisation 8,8i) route and photoreduction governing the photochemistry of the unsubstituted pjnidine-N-oxide la). 

The photochemistry of phthalimide systems was thoroughly investigated by many groups over the last two decades. This chromophore shows a broad spectrum of reactivity leading mainly to cycloaddition and photoreduction products by either intermolecular or intramolecular processes. In the presence of electron donors, the electronically excited phthalimide could also undergo electron transfer and act as an electron acceptor. 

Platinum(ni).—Unlike the photochemistry of [PtXg] " (X = Br or I) in which only photoaquation is observed,flash photolysis of [PtClsV" in its Cl Pt charge-transfer band has been shown to result also in photoreduction. The unusual Pt" complex [PtC ]" is produced via the mechanism... 

Photoexcited nitrobenzene may be used for benzylic hydroxylation (at C-9) of 17/3-acetoxy-3-methoxyoestra-l,3,5(10)-triene. The photochemistry of the 17/3-nitro-steroid (217) is markedly solvent dependent, the major products being in ether the 17-desnitro-compound (218), in propan-2-ol the hydroxylamine (219), and in EtOH-NaOEt the hydroxamic acid (220) and the cyclopropane (221). The hydroxamic acid (220) is probably formed through the oxaziridine (Scheme 7). Although there are analogies to this in the photochemistry of nitrones and oximes, the photoreduction of a nitroalkane in propan-2-ol to an alkyl-hydroxylamine appears to have no precedent. Further studies of photochemistry of conjugated... 

Recent work on the photochemistry of ketimines has shown that they do not undergo reduction unless ketones are present. Thus chemical sensitization is entirely responsible for the photoreduction of benzophenone methylimine (22)105 while intramolecular chemical sensitization has been suggested as the mechanism for reduction of the acylketimine (23).119 In related work,... 

Photoreduction and Photoaddition Reactions of Heterocyclic Compounds , D. G. Whitten, in Photochemistry of Heterocyclic Compounds , ed. O. Buchardt, vol. 4 of General Heterocyclic Chemistry Series , Wiley, New York, 1976, pp. 524-573. 

The photochemistry of Eosin under both reductive and oxidative conditions has been studied by several groups [145-151], Photoreduction by amines such as tribenzylamine (R = CH2, R" = ) produces two leuco analogues, the dihydro derivative, and the cross-coupled product formed from the amine radical and the dye radical anion (2) [152], In addition, debromination of Eosin is reported during photobleaching with amines and phenols. The reader however is referred to the extensive studies of Rose Bengal dehalogenation by Paczkowski and Neckers [153]. Radiolysis of Eosin in methanol shows that debromination is a consequence of the photochemical decomposition of semireduced Eosin [154],... 

Recently Fouassier and Chesneau [219] studied the photochemistry of the system Eosin-PDO-MDEA in aqueous acetonitrile using steady-state irradiation and laser flash photolysis. The photopolymerization of methyl methacrylate (MMA) sensitized by the photoreduction of Eosin is investigated in acetronitrile to understand the mechanism of initiation and the enhancement in the rate of polymerization caused by the presence of PDO, 3. Rates, quantum yields of photopolymerization, and number average molecular weights of the polymer are determined with MMA (7 M), Eosin (3 x 10 5 M), and MDEA (0.1 M) in the presence and in the absence of 2 x 10-3 M PDO. 

This review has been concerned with the photochemistry of heterocyclic systems photoreactions which are more correctly designated as reactions of functional groups have, in general, been omitted or not seriously considered. Thus, the formation of pinacols by the photoreduction of such heterocyclic ketones as 3-acetylpyridine 453 and the keto sulfone (422)454 has not been included, nor has cis-trans isomerization been reviewed. 

Photooxidation of coordinated oxalate has been known since the earliest studies of transition metal photochemistry (42). In these reactions oxalate ligand is photooxidized to CO2, and up to two metal centers are reduced by one electron (e.g. ferrioxalate). We wondered whether the oxalate ligand could be a two-electron photoreductant, by simultaneous or rapid sequential electron transfer, with metals prone to 2e redox processes. Application of this concept to l6e square planar d complexes, Equation 15, was attractive because it should produce solvated I4e metal complexes that are inorganic analogues of... 

A well studied example of a photoreduction which is accompanied by decomposition is the CT state photochemistry of Co(III)-ammine complexes.2b) In recent years substantial effort182,183,2b has been directed towards understanding the photoreductions like reaction (65). Reactions of this type almost certainly origi-... 

No attention is given to the mechanistic importance of a reaction rather, an attempt has been made to concentrate on reactions that have an actual (potential) synthetic role. This is not always an obvious selection, because photochemistry has not been sufficiently used in such syntheses, and mechanistic studies are not necessarily a reliable guide towards this aim. As an example, hydrogen abstraction by ketones (Scheme 1.3), which probably is the most thoroughly studied photochemical reaction, is not mechanistically discussed. Neither is presented the resultant photoreduction of ketones (Scheme 1.3, path a), because this will hardly ever become a sensible synthetic alternative for the reduction. However, other reactions arising from the same primary photoprocess, namely bimolecular reduction (path b) and... 

In these studies it appeared that low-temperature photoreduction of the bound iron-sulfur centers is not greatly decreased (about 2x) by the removal of phylloquinone (Setif et al, 1987). Low temperature photochemistry was measured by three methods the total amount of iron-sulfur centers A and B reduced by continuous illumination at 77K, the extent of reduction of these centers by saturating laser flashes at 77K, and the flash-induced formation of triplet P-700. These methods show that all the reaction centers are still able to oxidize P-700 and to reduce the iron-sulfur centers. This observation raises serious questions concerning the role of phylloquinone or the significance of low-temperature photochemistry. For the moment we consider that room temperature data are more reliable in indicating that A] is a phylloquinone. 

Among transition metal complexes the iron compounds are those for which LMCT photochemistry is of crucial environmental relevance [51, 61, 62], Their photoreductions yield the Fe11 species and ligand radicals (L ), eg ... 

Cu11 complexes undergo photoreduction to the Cu1 species and may be the photocatalyst in photo-oxidation cycles of organic environmental matter, quite similar to the Fem species [20, 81] (see Figure 9.11). Dissolved copper compounds are important to transformation reactions, because they react with hydroperoxyl (H02) and superoxide (02 ) radicals much faster than other species present in the solution. Oxidation of Cu1 and Fe11 by H202 is a source of the OH radicals in oceans comparable with nitrite photolysis, whereas photochemistry of Cu11 chlorocom-plexes provides Cl radicals [81] ... 

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