Carbonyl compounds, photoreduction

Hydrogen abstraction (Section 4.9) by excited carbonyl compounds is one of the most fundamental reactions in organic photochemistry. In a classical photoreduction reaction, an excited carbonyl compound, such as a ketone, undergoes hydrogen abstraction from a hydrogen donor [H] to form the ketyl radicals, which subsequently abstract another 

Photoreduction of aliphatic ketones may involve both singlet and triplet excited states, but the quantum yield for product formation via singlet is usually low because other competing processes, such as radical pair recombination, are involved. The rapid intersystem crossing (ISC) in aryl ketones (Section 2.1.6) allows triplet reactivity. Ketones with n,7t lowest triplets, having an unpaired electron localized in an n-orbital on oxygen, are far more reactive than those with k.k lowest triplets.863 

Considerable photoreductive reactivity is also observed for substrates which are poor hydrogen donors but good electron donors (i.e. possessing a low j) (Table 6.7). Such compounds are able to reduce efficiently both n.n and n,7t triplets via partial or complete electron transfer (Section 5.2) followed by proton transfer. Amines are very common electron donors depending on their structure, either an N—H or C—H bond is cleaved via a 

Case Study 6.16 Chemistry in ionic liquids - photoreduction 

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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. 

The photoreductions of a number of carbonyl compounds with either lowest mr or jiji triplet states in the presence of tributyltinhydride are reported24. The carbonyl compounds include cyclohexanone and acetone which possess nrr lowest-energy triplets, and 2-acetonaphthone, 1-naphthaldehyde and 2-naphthaldehyde which possess lowest-energy mr triplets. In the case of the two njr triplets, a simple mechanism is proposed which involves the abstraction of a hydrogen atom from the tributyltinhydride by the triplet state... 

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... 

Little has been reported concerning the mechanism of the photocycloaddition reaction however, much is known about the photoreduction of carbonyl compounds.15,16 It has been shown that both hydrogen abstraction, leading to photoreduction, and most photocycloaddition reactions of carbonyl groups are characteristic of the same type of excited state reagent, that is, the carbonyl n,n state.17 Furthermore, much is known about the emission (phosphorescence and fluorescence) of carbonyl compounds, and all of this knowledge can be brought to bear upon the photocycloaddition reaction. 

One of the limitations of the photocycloaddition reaction is that the unsaturated system may itself act as a quencher. Conjugated dienes fall within this category since they quench the n,ir triplet of some carbonyl compounds. For the photoreduction of benzophenone in benzhydrol, the ratio, kqlka, for m-piperylene is 750 (Table II), which indicates that this diene is indeed an efficient quencher for the reaction. 

Thus, the observations that (a) dienes quench the photoreduction reaction (b) the isomerization and dimerization of dienes is sensitized by the it,n carbonyl triplet and (c) there is a lack of photocycloaddition products with dienes, taken in conjunction with the relative energy levels of carbonyl compounds and dienes, form a consistent picture. 

Table IV summarizes the pertinent characteristics of some of the naphthyl carbonyl compounds. All of these compounds emit from a it,7T triplet very similar to that of naphthalene. Those that have been studied are resistant to photoreduction in isopropyl alcohol and photocycloaddition with 2-methyl-2-butene25 and isobutylene.17 Significant oxetane formation was, however, observed with the aldehydes, albeit with only moderate efficiency (quantum yield approximately one-tenth that of benzaldehyde).25...

Direct evidence for triplet-triplet transfer has been provided by sensitizing both the isomerization of cis- or ira/w-olefins,28 38 79,80 and the dimerization of some cyclic olefins81-83 with carbonyl compounds. Furthermore, the phosphorescence of some carbonyl compounds can be quenched by olefins (for example, acetone with 2-pentene30 and phenyl-cyclopropyl ketone with 2-methyl-2-butene37). On the other hand, the phosphorescence of benzophenone is not quenched by 2-methyl-2-butene37 nor is the photoreduction of benzophenone quenched efficiently by cyclohexene (Table II). 

In solution these esters undergo a variety of transformations which are dependent on the reaction conditions. In benzene, decomposition to carbon monoxide and carbonyl compounds is observed either upon direct irradiation94 or with benzophenone sensitization.33 In cyclohexane a complex product mixture is obtained.95 Addition of solvent to the carbonyl group is observed when the reaction is carried out in cyclohexene.54 At room temperature photoreduction takes place when the reaction is carried out in a secondary alcohol.96-97 However, in the case of the phenylglyoxylates quite a different reaction is observed when the reaction is carried out at elevated temperatures. The ester is reduced to the mandelate ester of the solvent alcohol, and the alcohol moiety of the ester is oxidized to the corresponding carbonyl compound. The pyruvates have not been studied at an elevated temperature. 

The prototype of photoreduction reactions is hydrogen abstraction by carbonyl compounds in presence of suitable H-donors. Such H-atom transfer may be visualized to occur first by transfer of an electron followed by proton transfer. An electron deficient centre is the seat of reaction and the efficiency of the reaction depends on the nucleophilic nature of the donor. 

A variety of H-donors have been used for the photoreduction of carbonyl compounds. They include amines, alcohols, hydrocarbons,. phenols and amides. The presence of compounds with double bonds lead to oxetane formation. 

Figure 4.56 Mechanism of the photoreduction of carbonyl compounds with hydrogen atom donors, ZH. Ketyl radicals and donor radicals are formed in the primary photochemical process...

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 ... 

Imines can be photoreduced by hydrogen donors such as propan-2-ol, but in some cases it is clear that a small amount of carbonyl compound (arising from hydrolysis of the inline) is necessary to initiate the process. For example, N-alkylimines of benzaldehyde give ethane-1,2-diamine derivatives on irradiation in 95% ethanol (5.6), but they are inert when irradiated in a perfectly dry alcohol. The C=N chromophore is capable of abstracting a hydrogen atom, and both the photoreduction of a tetrahydropyridine (5.7) and the photoaddition of p-xylene to an isoxazoline (5.8) occur by such a direct reaction. 

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. 

It is unlikely that the asymmetric photoreduction procedure described here will be able to compete with the available thermal methods for the reduction of carbonyl compounds. Generality of the zeolite-based method even with respect... 

One of the earliest photoreactions to be studied was the photoreduction of benzophenone (Ciamician and Silber, 1900)—that is, the conversion of a carbonyl compound into an alcohol by an intermolecular hydrogen abstraction reaction. Intramolecular hydrogen abstraction by the carbonyl group, usually from the y site, is referred to as a Norrish type II reaction. Hydrogen abstraction by olefins and heterocycles has also been observed. 

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, 395-99 bcnzophenone, 395. 397-98 benzophenones and acetophenones, 467 carbonyl compounds. 466 naphthalene by iriethylamine, 466 Phoioseleciion. 272. 275 Photosensiiizalion. 292. 407. See also Sensitization... 

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