Carbonyl triplet state

With aromatic carbonyls, oxetane formation appears to arise from the carbonyl triplet state, as evidenced by quenching studies. For example, benzaldehyde irradiated in the presence of cyclohexene yields products indicative of hydrogen abstraction reactions and an oxetane ... 

At this point let us briefly consider the relationship between the carbonyl triplet state and another system capable of hydrogen atom abstraction alkoxy radicals. A comparison of the differences and/or similarities between the reactivity of the carbonyl triplet and that of an alkoxy radical should indicate whether the triplet state behaves as a normal ground state radical or if electronic excitation imparts unique properties leading to reactions not characteristic of ground state radicals. 

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 decarbonylation of dibenzyl ketone has been shown to result from the carbonyl triplet state by its ability to be quenched by 1,3-cyclohexadiene or l,3-pentadiene.<66) Using 1,3-cyclohexadiene as quencher, photodimers of the cyclohexadiene were obtained. Since these are formed only by triplet sensitization,<66) the quenching of ketone triplet states, rather than their excited singlets, was assured. Further evidence for a triplet reaction follows from the fact that decarbonylation could be sensitized by acetone under conditions where the sensitizer absorbed 93% of the light. 

The reactions known to occur from carbonyl triplet states are many, and quite often a particular ketone has completely different photochemical fates in different media. 

Guillet has also shown that 50% of the photochemical reaction in the QH —CO copolymer originates from the carbonyl triplet state which has a lifetime of 10-8 sec 55). 

Role of Hydrogen Bonding. Qualitatively, these results can be understood as effects of hydrogen bonding on the triplet-state decay, as explained below. Hydrogen bonding is known to occur at carbonyl triplet states (12), and probably will also occur at the three methoxyl oxygens of I. 

Carbonyl triplet states continue to be significant subjects of photochemical research. Time-resolved ESR studies on excited triplet 2-methylacetylacetophenone and photoenolization are particularly significant since the enol triplets are not phosphorescent. 

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... 

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