Excited States of Carbonyl Compounds

Aromatic ketones have lowest Si(n,7t ) states and the (n,7t ) and (71,71 ) triplet states are close together in energy. Which of these is the lower in energy depends on the nature of the substituents on the aromatic rings and on external factors such as the nature of the solvent. 

Electron-donating substituents in conjugation with the carbonyl group have the effect of stabilising the (7r,7r ) triplet state but destabilise the (n,7t ) triplet state. On the other hand, the (n,7r ) triplet state is stabilised relative to the (n,n ) triplet state by the presence of electron-accepting substituents. 

Because an increase in solvent polarity results in a shift to longer wavelength of (71,71 ) absorptions and longer wavelength of (n,7t ) absorptions, polar solvents stabilise the (7t,7t ) triplet state relative to the (n,7t ) triplet state. This effect may result in the (7t,7t ) triplet being the lowest triplet state. 

Intersystem crossing in aromatic ketones is very efficient, so that their photochemistry is dominated by the triplet-state processes. The efficient formation of triplet states, the small singlet-triplet energy and the accessible long-wavelength absorption make the aryl ketones excellent triplet sensitisers. 

The principal reaction types for ketone (n,7t ) excited states are  

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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. 

Synthetic application of Paterno-Bilchi reaction of simple dienes with carbonyl compounds is rare. While seemingly an extension of the photocycloaddition of olefins and carbonyl compounds, the reaction between dienes and carbonyls is often complicated by the fact that triplet excited states of carbonyl compounds are quenched by dienes, although the formation of oxetanes can be observed during these reactions. Recall also that the photosensitized dimerization of diene triplet excited states is also a well known reaction vide infra), these two observations would seem to naturally limit the synthetic potential of this process. Kubota and coworkers found that irradiation of propanal in the presence of 1,3-cyclohexadiene produced oxetanes 164a and 164b in a 4 1 ratio (Scheme 37). ... 

The n,n and n,n excited states of carbonyl compounds show very different patterns of chemical reactivity. It is useful to represent the n,n state of carbonyl compounds as shown in Figure 12.42, in which there is appreciable radical character on both the carbon atom and the oxygen atom and in which there is charge donation from oxygen to carbon. ... 

The high reactivity of the excited states of carbonyl compounds is due to half-filled orbital of oxygen. 

Oxetans.—It has been known for many years that the electronically excited states of carbonyl compounds will add to double bonds to form oxetans. The majority of examples of the Paterno-Buchi reaction, as it is known, involve addition of a carbonyl triplet state to a tt-system, most commonly an isolated double bond. It is accepted that an intermediate in this reaction is a 1,4-biradical with a lifetime long enough to destroy the stereochemical integrity of an acyclic double bond. Intramolecular reactions are of synthetic value since in many cases they proceed regiospecifically. For example, 3-ent/o-acylbicyclo[3.3.1]non-6-enes give exclusively 2,4-oxo-bridged protoadamantanes upon excitation... 

Styrene, the most efficient quencher of triplet excited states of carbonyl compounds among a great variety of olefins employed by Ausloos and Rebbert (1964), gave no detectable influence on the ethene and carbon monoxide quantum yields when 20.1 Torr was added to 19.9 Torr of 2,2-dimethyl-3-hexanone (Nicol and Calvert, 1967). However, an almost quantitative removal of radical products of 1-propyl and 2-methyl-2-propyl radicals resulted. Since 4>co was unaffected by styrene addition, the radical... 

When 14C-benzoin (19) or its methyl ether (20) is used as photosensitizer for polymerizations, more of the sensitizer is incorporated in polymer than can possibly be accounted for by the initiation process. The reactions have the characteristics of mono-radical polymerizations and separate experiments with thermal initiators have shown that transfer to the carbonyl compound is of little importance. It appears that photo-excited states of these compounds, but not the ground states, can engage... 

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. 

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... 

Just as it is certain that the initial state in the vapor phase photolysis is the upper singlet, it is equally certain that in the photochemistry brought about by sensitization by triplet states of carbonyl compounds, the initial excited state is the first triplet. In fact, this is the only state that would be accessible to the molecule with the excitation energy that is involved. 

Alcohols (as well as amines, sulfides and many hydrocarbons) may act as overall hydride donors towards excited nonbonding-rr " states of carbonyl compounds and heterocycles. The synthetic as well as the photophysical aspects of these processes have been discussed extensively. These reactions will not be dealt with further here. Singlet oxygen also accepts hydride from alkoxides in the gas phase the mechanisms of such reactions have received considerable study. ... 

Ordinary aldehydes and ketones can add to alkenes, under the influence of UV light, to give oxetanes. Quinones also react to give spirocyclic oxetanes. This reaction, called the Patemo-BUchi reaction,is similar to the photochemical dimerization of alkenes discussed at 15-61.In general, the mechanism consists of the addition of an excited state of the carbonyl compound to the ground state of the alkene. Both singlet (5i) and n,n triplet states have been shown to add to... 

The excited state of the carbonyl compound is the (n, it ) state where one electron is excited from the HOMO to the LUMO. The SOMO is the n-orbital on the carbonyl oxygen atom. The SOMO is the antibonding jt -orbital. 

Lamola AA, Sharp LJ (1966) Environmental effects on the excited states of o-hydroxy aromatic carbonyl compounds. J Phys Chem 70 2634—2638... 

Electronically excited states of organic molecules, acid-base properties of, 12,131 Energetic tritium and carbon atoms, reactions of, with organic compounds, 2, 201 Enolisation of simple carbonyl compounds and related reactions, 18,1 Entropies of activation and mechanisms of reactions in solution, 1,1 Enzymatic catalysis, physical organic model systems and the problem of, 11, 1 Enzyme action, catalysis of micelles, membranes and other aqueous aggregates as models of, 17. 435... 

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... 

The [2+2]-photocycloaddition of carbonyl groups with olefins (Paterno-Buchi reaction) is one of the oldest known photochemical reactions and has become increasingly important for the synthesis of complex molecules. Existing reviews have summarized the mechanistic considerations and defined the scope and limitations of this photocycloaddition73. Although this reaction likely proceeds via initial excitation of the carbonyl compound and not the excited state of the diene, the many examples of this reaction in natural product synthesis justify inclusion in this chapter. 

The first step of the reaction involves the (n, it ) excited state of the carbonyl compound reacting with the ground-state alkene. For aromatic ketones, rapid intersystem crossing from the excited singlet state to the excited triplet state occurs, forming initially a 1,4-biradical and then the oxetane ... 

As far as we are aware, these observations are the first that show that the well-known Norrish Type I reactions of p,7-unsaturated carbonyl compounds can take place by excitation of the alkene moiety rather than the carbonyl group. This unusual reactivity may be due to the fact that the TiC-ir, -ir ) excited states of 53 and 55 possess sufficient energy to promote the homolytic allylic bond fission to form the stabilized pentadienyl radical 57. As a result, photodecarbony-lation competes favorably with the ODPM rearrangement. 

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