Triplet carbonyls

Wong S K, Hutchinson D A and Wan J K S 1973 Chemically induced dynamic electron polarization. II. A general theory for radicals produced by photochemical reactions of excited triplet carbonyl compounded. Chem. Phys. 58 985-9... 

Most likely singlet oxygen is also responsible for the red chemiluminescence observed in the reaction of pyrogaHol with formaldehyde and hydrogen peroxide in aqueous alkaU (152). It is also involved in chemiluminescence from the decomposition of secondary dialkyl peroxides and hydroperoxides (153), although triplet carbonyl products appear to be the emitting species (132). 

The maximum of emission from excited triplet carbonyls occurs at 460 nm while that from singlet oxygen at 634 nm (dimol emission) and 1,270 nm (monomol emission) [19]. 

Numerous autoxidation reactions of aliphatic and araliphatic hydrocarbons, ketones, and esters have been found to be accompanied by chemiluminescence (for reviews see D, p. 19 14>) generally of low intensity and quantum yield. This weak chemiluminescence can be measured by means of modern equipment, especially when fluorescers are used to transform the electronic excitation energy of the triplet carbonyl compounds formed as primary reaction products. It is therefore possible to use it for analytical purposes 35>, e.g. to measure the efficiency of inhibitors as well as initiators in autoxidation of polymer hydrocarbons 14), and in mechanistic studies of radical chain reactions. 

A special problem is the high yield of triplet carbonyl compounds being formed — neither the concerted nor the diradical mechanism are fully explaining this fact. Further data on the identities and yields of excited products from different dioxetanes are needed. 

In contrast to 2-alkylarylcarbenes, triplet carbonyl carbenes do not abstract H from 5- or e-CH bonds. Photolysis of diazo compounds (7) in methanol gave products due to Wolff rearrangement (8) and 0-H insertion (9). Sensitized photolysis led, in addition, to the H-abstraction product (10). Analysis of the results indicated that a large proportion of the insertion product (9) arises from the excited diazo compound and that spin inversion of the triplet carbene is faster than H-abstraction from the solvent. Intersystem crossing to the singlet state is a major reaction of all triplet carbonyl carbenes that are not rapidly scavenged intramolecularly. 

The synthesis of oxetanes by the photochemical 1,2-cycloaddition of the carbonyl function in aldehydes and ketones to alkenes [Eq. (69)] was first reported by Paterno274 in 1909, and later reinvestigated by Biichi 275 in 1954. This reaction has recently been extensively reviewed.276, 277 The formation of the oxetane is apparently the result of addition of excited n, n triplet carbonyl to an alkene, although for certain aromatic aldehydes and ketones the mechanism is less clear.278... 

We have also confirmed our previous results on the effect of methoxylation of phenolic hydroxyl groups within lignin. On the basis of our model studies, we suggest that the most likely explanation for this is that triplet carbonyl groups are quenched statically by hydroxyl groups within the lignin structure on timescales less than 20 ns thus reducing the amount of transient detected in our laser photolysis experiments. 

The photolyses of conformationally fixed 3,y-unsaturated ketones can also be performed in constrained media. This was described for the ODPM rearrangement of bicyclo[2.2.1]heptenone and bicyclo[2.2.2]octenone in MY zeolites [47]. It is remarkable that direct irradiation of the guest-incorporated thallium-Y (TIY) zeolites at 254 nm resulted in higher yields of ODPM products than the triplet-sensitized solution photolyses (Sch. 19). This indicates that the triplet carbonyl species are generated more efficiently in the supercages of the zeolites presumably because of the presence of heavy... 

Hydrogen abstraction by triplet carbonyl compounds has been the most widely studied excited state reaction in terms of structural variations in reactants. Consequently, the most detailed structure-reactivity relationships in photochemistry have been developed for hydrogen abstraction. These correlations derive from studies of both bimolecular reaction and intramolecular reactions. The effects of C—H bond strength and the inductive and steric effects of substituents have been analyzed. The only really quantitative comparisons between singlets and triplets and between n,n and 71,71 states have been provided by studies of photoinduced hydrogen abstractions. 

Irradiation of cytosine, cytidine, and 2 -deoxycytidine has recently been reported to produce 4,5 -linked dimers (154 R = H, j3-D-ribose, 2-deoxy-/3-D-ribose, respectively) 210 this reaction is accompanied by deamination. Photodimerization (in aqueous solution) of uracil211 and thymine212 affords the 4,6 -linked dimer (155 R = R = H and Me, respectively) the oxetane (156) (probably formed by 1,2-cycloaddition of a triplet carbonyl to an olefinic bond) is proposed as the precursor.212... 

Several years ago it was found that many aromatic carbonyl compounds are photoreduced by aliphatic and aromatic amines (Cohen et al., 1973 Davidson, 1975). It was suggested that these reactions proceed by electron transfer from the amine to the triplet carbonyl compound. Evidence in support of this mechanism was that the ability of amines to quench the phosphorescence of carbonyl compounds, and to react with them, correlated with the ionisation potential of the amines. On the basis of this evidence the reaction scheme in... 

Optical detection of intermediates produced in the reactions of triplet carbonyl compounds with electron donors has some obvious limitations. However, the technique of CIDNP is proving particularly effective at elucidating the reaction pathways in these systems. The outstanding work of Hendriks et al. (1979) illustrates the power of the technique. Not only was the role of radical ions in the reactions of alkyl aryl ketones with aromatic amines defined but the rate constants for many of the processes determined. The technique has been used to show that trifluoracetyl benzene reacts with electron donors such as 1,4-diazabicyclo[2.2.2]octane and 1,4-dimethoxy-benzene by an electron-transfer process (Thomas et al., 1977 Schilling et al., 1977). Chemically induced dynamic electron polarisation (CIDEP) has been... 

In many cases, the reactions of carbonyl compounds are interpreted in terms of the reactivity of the triplet carbonyl compound. However, the work on [123] in which a fluorescent excited charge-transfer complex was detected, and the finding that some amine radical cations react with the radical anions of carbonyl compounds to produce exciplex fluorescence (Zachariasse, 1974) shows that, although intersystem crossing in carbonyl compounds is usually highly efficient, they may participate in excited singlet-state reactions. 

Thiols and sulphides quench triplet carbonyl compounds. Evidence (including that from CIDNP studies) indicates that these reactions occur by a radical rather than an electron-transfer pathway (Cohen et al., 1979 Ver-meesch et al., 1978). It is interesting to note that sulphides will deoxygenate ketones producing sulphoxides, sulphones and presumably carbenes (Fox et al., 1979). Phosphines quench triplet carbonyl compounds (Davidson and Lambeth, 1969). They also deoxygenate carbonyl compounds to produce phosphine oxides and carbenes, and in this case, the reaction was proposed as occurring by an electron-transfer process (Fox, 1979). 

The rate constants for reactions of all-trans retinal and retinol with have been measured in various solvents. Rate constants increase with dielectric constant, thereby suggesting that charge transfer is involved. Electronically excited triplet species generated during peroxide-catalysed aerobic oxidation of substrates gives rise to fluorescence from acceptors in micelles. Micelle-solubilized chlorophyll is an excellent detector of enzyme-generated triplet carbonyl species, and micelles make photochemistry without light possible for... 

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