Photoreduction by Amines

Cohen SG, Parola A, Parsons GH (1973) Photoreduction by amines. Chem Rev 73 141-161... 

Fluorescence of PDC is also quenched by amines. The ordering of reactivity is tertiary > secondary > primary, which follows inversely the ionization potential (Table 9.13). The results are explained as indicating that PDC undergoes photoreduction by amines, thereby forming triplet charge-transfer intermediates as the primary step in quenching. Therefore, the mechanism of the PDC reaction is not the same as the proposed mode of reaction of PDC, which involves direct formation of an yhde intermediate by electrophilic attack on the lone-pair electrons of the amine (Table 9.13). ... 

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],... 

Cohen has recently reviewed the large amount of research performed on photoreduction by amines 152>. The overall reaction is different for tertiary amines than for primary and secondary, since radicals with the structure R2C—NH—R are able to reduce ground state ketone in the same way that a-hydroxy radicals do. 

Photoreduction by amines differs from photoreduction by alcohols in two respects quantum yields are always lower than maximal and rate constants for amine quenching of triplet ketones are very large. These two facts led Cohen 153> and Davidson 154> to suggest that amines react with excited carbonyl compounds by electron transfer followed by proton transfer from the amine radical cation. 

In contrast, photoreduction by amines obeys a general mechanism (Scheme 3) that involves a charge-transfer process. It should be noted that the azoalkane 3c, which exhibits a low intersystem crossing (,sc ca. 0.1), resists photoreduction. The efficiencies of photoreduction for the azoalkanes 3 by amines... 

The photoreduction of aryl ketones by amines generally occur via a charge transfer interaction between the triplet state of ketone and the amines, as shown in the following scheme. A ketone radical anion and an amine radical cation are formed in the intermediate stage. 

An analogous single electron transfer producing two-electron reduced products was encountered in Whitten s study of the photoreduction of thioindigo by amines, eq. 99 (304) ... 

Another example concerning the reduction of carbonyl compounds also relates to the salt effect theme. Shaefer and Peters (1980), Simon Peters (1981,1982,1983,1984), Rudzki et al. (1985), and Goodman and Peters (1986) described photoreductions of aromatic ketones by amines. In this case, the addition of excess NaC104 results in considerable retardation, even prevention, of final product formation. The two fundamental steps in this photoreduction consist of rapid electron transfer from the amine to the photoactivated ketone (in its triplet state), followed by the slow transfer of proton from the amine cation radical to the carbonyl anion radical ... 

Scheme 2. Ion pair formation in the photoreduction of benzophenone by amines [19,22,23a]...

Cohen and Guttenplan were the first to recognize that the photoreduction of triplet benzophenone by amines occurs via PET followed by proton transfer to... 

Examples of well-known photochemical reactions which involve electron transfer include the primary step in plant and bacterial photosynthesis [2], the photoreduction of ketones by amines [3], a series of sensitized isomerizations of olefins and small ring compounds such as cyclopropanes or of strained polycyclics such as quadricyclane to norbornadiene or Dewar benzenes to benzenes [4], and the reactions of electron-rich substrates in the presence of oxygen which proceed via superoxide [5]. These reactions and others have proved valuable for synthetic applications in addition to their fundamental interest to photochemists. 

As a function of N,N-dimethyl ethanolamine concentration, the pKslymerization rate passed throi h a maximum, as shown in F. 2. Such behaviour parallels that observed for photoreduction of fluorenone by other tertiary amines, e.g. the quantum yield for photoreduction by triethyiamine has been measured (5) as 0.09 in neat amine, 0.9 in... 

Naphthyl ketones, which are generally unreactive toward photoreduction by 2-propanol, are efficiently reduced by amines. Electron transfer (cf. Section 5.4.4) is considerably faster than hydrogen abstraction. Thus, the reaction cannot be quenched using common triplet quenchers although it proceeds from the triplet state. 

The photoreduction of carbonyl compounds or aromatic hydrocarbons by amines was one of the early electron-transfer reactions to be studied. Observation of products from primary electron transfer depends on the facility of a deprotonation of the amine, which must be fast compared to back electron transfer. For amines without a hydrogens, quenching by back electron transfer is observed exclusively (Cohen et al., 1973). The solvent plays a quite important role since it determines the yield of radical ion pairs formed from the exciplex (Hirata and Mataga, 1984). 

Photoreduction of benzophenones and acetophenones by amines has been studied in detail. A mechanism has been derived for the reaction of benzophenone with N.A -dimethylaniline that involves a triplet exciplex in the formation of the radical ion pair, as indicated in Scheme 49. 

In addition to the chloroacetates, as shown by comparisons with the estimates in Table V, the half-lives for both lindane and mirex in the natural water samples would have been considerably longer than those observed, had reaction with solvated electrons in bulk solution been the dominant mechanism for photoreaction. The higher efficiency of these halocarbon reactions may be attributable to sorption of the chloroacetates on the NOM, which permits more facile electron capture. Other possible pathways for reactions of sorbed halocarbons include direct photoreduction by excited states of the NOM, which, like solvated electrons, also are quenched by oxygen. Alternatively, the enhancement may involve other direct electron-transfer mechanisms such as amine-halomethane reactions. These alternative possibilities are examined in the following section. 

Enolization of triplet 5,8-dimethyl-1-tetralone by hydrogen transfer effects" ", spin-lattice relaxation of pentacen-6,13-quinone triplets in a benzoic acid host crystal involves proton tunnelling", and photoreduction of benzophenone and thioxanthen-9-one by amines" are other researches of photochemical significance. Interactions of formate ions with triplet states of benzophenone-4-carboxylate and -4-sulphonate, 1,4-naphthoquinone, and anthraquinone-2-sulphonate have been examined systematically by laser flash photolysis". ... 

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