Carbonyls triplet excited states

Subsequent efforts to inhibit photochemical yellowing of lignin-containing paper were based on what was believed to be the dominant reaction pathways to yellow chromophores the formation of phenoxy free-radicals by direct excitation of phenolic groups and the abstraction of phenolic hydrogen by the aromatic carbonyl triplet excited state, as shown by reactions 1 and 2 in Scheme 1. The approaches used by researchers were modification of lignin by ... 

Figure 2.2. Important resonance contributors for ntt and titi aromatic carbonyl triplet excited states.

P-Peroxylactones undergo thermal decarboxylation to carbonyl compounds by the initial formation of a 1,5-diradical (238). a-Peroxylactones undergo similar decarboxylation, emitting light since the ketone is generated in the triplet excited state (85,239,240) ... 

The irradiation is usually carried out with light of the near UV region, in order to activate only ihc n n transition of the carbonyl function," thus generating excited carbonyl species. Depending on the substrate, it can be a singlet or triplet excited state. With aromatic carbonyl compounds, the reactive species are usually in a Ti-state, while with aliphatic carbonyl compounds the reactive species are in a Si-state. An excited carbonyl species reacts with a ground state alkene molecule to form an exciplex, from which in turn diradical species can be formed—e.g. 4 and 5 in the following example ... 

All of the elements of stereo- and regioselectivity and reactivity that theory must explain are found in the above reactions. The triplet excited states of the aryl carbonyl compounds demonstrate regioselectivity that has been previously explained on the basis of the relative stabilities of the two possible biradical intermediates, 1 and 2. 65>66> The selectivity... 

Diazo compounds are photoreduced to hydrazines in the presence of silanes and stannanes in a process that apparently involves initial H-atom transfer to an excited state of the diazo compound.105 LFP studies of reactions of Bu3SnH with excited states of diazo compounds 40 and 41 found fast reactions. The singlet diazo species reacted with Bu3SnH about as fast as singlet carbonyl compounds, whereas the triplet diazo species reacted with tin hydride somewhat slower than triplet excited states of carbonyl compounds.100,105... 

Thus, the unknown values of i ed and AGf can be determined from the intercept and slope of the linear plots of (AG /e) versus (LGtJey as shown in Fig. 9. The °red values of various metal ion-carbonyl complexes thus obtained are summarized in Table 2 [113,114]. The °rea values of the triplet excited states are obtained by adding the triplet excitation energies and are listed in Table 2. 

Remarkable positive shifts of the °red values of the singlet excited states of the metal ion-carbonyl complexes as compared to those of the triplet excited states of uncomplexed carbonyl compounds (Table 2) result in a significant increase in the redox reactivity of the Lewis acid complexes versus uncomplexed carbonyl compounds in the photoinduced electron-transfer reactions. For example, photoaddition of benzyltrimethylsilane with naphthaldehydes and acetonaphthones proceeds efficiently in the presence of Mg(C104)2 in MeCN, although... 

Benzophenone and other similar carbonyl compounds react in their triplet excited state with 2,5- and 2,3-dimethylthiophenes, forming oxetanes with unsymmetrical ketones, mixtures of geometrical isomers are formed (Scheme 82). Similar oxetanes could not be obtained from other thiophenes (70CC1474, 73JHC967, 81JHC1065). 

Many photochemical reactions have been reported for unsaturated ketones294 but, despite the ease of intersystem crossing shown by most carbonyl compounds, it is not at all obvious that all the reactions occur from triplet states. We shall mention those that have been demonstrated to involve triplet excited states. 

These photochemical processes originate in most cases from the lowest triplet excited state of the carbonyl reactant, so that they are seldom stereo-selective. However, they show some regio-selectivity when the partner ethylene is substituted with different groups on its C atoms the regio-selectivity can be explained by the relative stabilities of the biradicals which can be formed in the primary photochemical step. 

Orbital Symmetry Conservation in Bimolecular Cycloadditions. The cycloaddition reactions of carbonyl compounds to form oxetanes with ethylenes, as well as those of enones and their derivatives to form cyclobutanes, are examples of reactions which originate from triplet excited states and lead in the first step to biradical intermediates. Such reactions are of course not concerted, and they show little or no stereo-specificity. 

In general an exciplex is formed between the reactant molecules (although it may be non-fluorescent when it is formed from the triplet excited state of the carbonyl compound). The primary photoproducts are radical ions rather than neutral radicals, and these may either recombine to restore the original (ground state) molecules or may undergo further reactions leading to the final reduction products (Figure 4.59). 

Although carbonyl compounds are generally nonfluorescent because of fast intersystem crossing to generate the phosphorescent lowest n,n triplet excited state [187], irradiation of the absorption band due to Mg2+ complex of 1-naphthaldehyde (1-NA) or 2-naphthaldehyde (2-NA) formed in the presence of Mg(C104)2 causes strong fluorescence at 430-440 nm as shown in a general manner (Scheme 22) [188]. 

The corresponding photocyclization of S-arylvinyl sulfides to dihydro-thiophens has been shown to proceed via short-lived thiocarbonyl ylide intermediates, formed in turn by cyclization of triplet excited states. 1-Phenylthio-3,4-dihydronaphthalene (49), for example, affords the colored thiocarbonyl ylide (50)44 the isolated products are derived by processes involving hydrogen abstraction and migration. The greater efficiency of the photocyclization of 2-thioaryloxyenones (51) to dihydrothiophens (52) has been attributed to the additional stabilization afforded to the ylide (53) by the carbonyl group.45 Other similar photocyclizations have been reported.46... 

The biradical corresponding to 16a produces indanol 18 quantitatively, whereas the biradical from 16b undergoes disproportionation to form mainly 19. The quantum efficiency for the formation of 18 was 0.03 in hydrocarbon solvent, and 1.0 in methanol. In contrast, the total quantum yield for 16b was rather low (0.02-0.05) in both hydrocarbon and methanol solvents. These differences were ascribed to a considerably smaller dihedral angle between the carbonyl and t-butyl benzene in the triplet excited state of 16a in comparison to 16b, and a differing rotational freedom in the biradical 17 intermediate species. 

Singlet as well as triplet excited states of oxiranes undergo C—C bond scission to produce carbonyl ylides which, upon cydoaddition with dipolarophiles, give tetra-hydrofuran (THF) derivatives. For example, trans- or ds-stilbene oxide on direct photolysis using 254 nm light in the presence of methyl acrylate gave diastereomeric... 

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