Excitation, of carbonyl compounds

A second source of UV-induced radicals to initiate lipid oxidation is excitation of carbonyl compounds (88). The carbonyl n ti transition (340 kJ/mol) occurs when light is absorbed at 350 nm and lower wavelengths (87). 

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... 

The photochemistry of carbonyl compounds has been extensively studied, both in solution and in the gas phase. It is not surprising that there are major differences between the photochemical reactions in the two phases. In the gas phase, the energy transferred by excitation cannot be lost rapidly by collision, whereas in the liquid phase the excess energy is rapidly transferred to the solvent or to other components of the solution. Solution photochemistry will be emphasized here, since both mechanistic study and preparative applications of organic reactions usually involve solution processes. 

Excitation of the ii-tt transition of carbonyl compounds entails transfer of an electron from an orbital essentially localized on oxygen to... 

Photochemical elimination reactions include all those photoinduced reactions resulting in the loss of one or more fragments from the excited molecule. Loss of carbon monoxide from type I or a-cleavage of carbonyl compounds has been previously considered in Chapter 3. Other types of photoeliminations, to be discussed here, include loss of molecular nitrogen from azo, diazo, and azido compounds, loss of nitric oxide from organic nitrites, and loss of sulfur dioxide and other miscellaneous species. 

The photocycloaddition of an aldehyde or ketone with an olefin to yield an oxetane was reported by Paterno and Chieffi in 1909. 58> Contemporary studies on the synthetic utility and mechanistic features were initiated nearly 50 years later by Biichi et al. 59) Two review articles summarizing synthetic aspects of Paterno-Biichi reactions have been published 6.12)) and mechanistic studies have been reviewed several times. 6,38,60-62) The reaction involves the addition to olefin of a photo-excited carbonyl moiety. This circumstance makes it advantageous to review this reaction before a discussion of olefin-olefin additions, because the solution photochemistry of carbonyl compounds is probably better understood than any other aspect of organic photochemistry. Many of the reactions of carbonyl compounds have been elucidated during studies of the important phenomena of energy transfer and photosensitization. 63-65)... 

Predict the products of the reactions of excited-state carbonyl compounds with alkenes (Paterno-Buchi reaction). 

The second principal reaction of the (n,jr) excited state of carbonyl compounds is the abstraction of a hydrogen atom from another molecule ... 

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... 

The excited states of carbonyl compounds are often considered to be similar to alkoxyl radicals because of the unpaired electron on the oxygen atom. In particular, the benzophenone n—tt triplet mostly reacts in the same manner as and at a similar rate to t-BuO radicals. 

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 most important and difficult questions to answer for any photochemical reaction is which excited state is involved. Since these are the reagents, it is obviously important, if generalizations are to be made, to know which state is responsible for a given reaction. The question is difficult to answer because several different types of excited states, both singlet and triplet, are attainable even with the simplest of carbonyl compounds, and their reactivity may, in some cases, be similar. All of the discussion thus far has implied that the photocycloaddition reaction is characteristic of the n,n state. What is the evidence that this state can be involved and what is the character of this state which makes it reactive ... 

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. 

Given this structural representation of the n-ir excited state, one has to determine the extent to which the photochemistry of carbonyl compounds is actually explicable as continuous electron redistribution processes of such a species. In actual fact, it is found (3-6) that n-ir photochemical reactions are amenable to mechanistic discussion in electron detail using this model of the excited state. 

The photochemical reaction of carbonyl compounds and alkenes, which is referred to as the Paterno-Buchi (PB) reaction, was developed in 1909 [13], and is currently one of the most widely used methods for oxetane synthesis (Scheme 7.4). As exemplified in the PB reaction of benzophenone with 2-methylpropene [14], a selective formation of the oxetane is possible even when the photochemical reaction involves highly unstable molecules that is, the excited state of carbonyls. Due to its synthetic importance and mechanistic interest, the PB reaction is the most extensively studied synthetic method for oxetanes. Thus, several extensive reviews describing the PB reaction have been published since 1968, and the reader is directed towards these for further information [15]. In this chapter, methods that allow for the control of the regioselective and stereoselective formation of synthetically important oxetanes will be described. 

One of the most common photochemical reaction pathways of carbonyl compounds is the formation of a diradicaloid excited state which is able to abstract a hydrogen atom at the y (or, more rarely, e) position, followed by either fragmentation or recombination. This process, which is known as the Norrish type II reaction, has a parallel in the photochemistry of nitro groups the intramolecular hydrogen abstraction of excited ortho-nitrotoluene is actually one of the very early synthetic photochemical transformations [9]. It has been exploited in a family of photolabile protecting groups, most prominent among which are derivatives of ortho-nitrobcnzyl alcohol, as introduced in 1966 by Barltrop et al. (Scheme 13.1) [10, 11],... 

Because of their low triplet energies (55-60 kcal/mol), 1,3-dienes are often used as quenchers for excited triplet states of carbonyl compounds. Besides... 

In contrast to photocycloaddition reactions of carbonyl compounds to electron-rich alkenes (which proceed with a low degree of stereoselectivity in the case of triplet excited carbonyls), reactions with electron-deficient alkenes, such as cyanoalkenes, are, although rather inefficient, but highly stereoselective [101]. Kinetic analysis showed that these reactions involve the... 

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