Carbonyl singlet state

The mechanism of the Patemo-Biichi reaction is not well understood, and while a general pathway has been proposed and widely aceepted, it is apparent that it does not represent the full scope of reactions. Biichi originally proposed that the reaction occurred by light catalyzed stimulation of the carbonyl moiety 1 into an excited singlet state 4. Inter-system crossing then led to a triplet state diradical 5 which could be quenched by olefinic radical acceptors. Intermediate diradical 6 has been quenched or trapped by other radical acceptors and is generally felt to be on the reaction path of the large majority of Patemo-Biichi reactions. Diradical 6 then recombines to form product oxetane 3. 

It is evident from the exceptions noted that the mechanism proposed above does not fully capture the pathways open to the Patemo-Biichi reaction. A great deal of effort has been devoted to deconvoluting all of the possible variants of the reaction. Reactions via singlet state carbonyls, charge-transfer paths, pre-singlet exciplexes, and full electron transfer paths have all been proposed. Unfortunately, their influence on product... 

Thus we end up with an oxygen atom which is somewhat electron deficient and a carbon atom which is electron rich. The oxygen then would be expected to behave as an electrophilic reagent and the carbon (or rather the regions bounded by the w orbital) should behave as a nucleophilic reagent. The amphoteric nature of the carbonyl n- n singlet state is mirrored in its reactivity toward electron-rich and electron-deficient olefins. 

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. 

As is the case with aryl carbonyls previously studied, aliphatic carbonyls add to olefins to form oxetanes. The picture in this case is far more complicated, however, primarily due to the increased importance of singlet state carbonyl addition to the olefin. In Chapter 1 we saw that in an n->v ... 

The capability of 2-hydroxybenzophenone derivatives to dissipate light energy has been ascribed to rapid deactivation of the excited singlet state by intramolecular interaction between the carbonyl and hydroxyl groups, possibly involving reversible H-transfer. These proposals are outlined in Scheme I, where P and PP represent the polymer and photoproduct, respectively. 

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 ... 

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. 

Three types of photoextrusion reactions have been identified in the irradiation of aryl-substituted 1,3,2-dioxathiolane 2-oxides [5 1-1-2 4-2] cycloelimination to produce a carbonyl compound, a carbene and sulfur dioxide extrusion of sulfur dioxide accompanied by a pinacol-like rearrangement to yield an aldehyde or ketone and extrusion of sulfur trioxide to give an alkene <72JOC2589>. Sensitization and quenching experiments indicate that a singlet state is responsible for the cycloelimination reaction, whereas the rearrangement and sulfur trioxide extrusion reactions arise from a triplet state <82JCR(S)175>. 

The rate of intersystem crossing is just as important as its efficiency. Obviously, if the rate of intersystem crossing is faster than that of diffusion in solution (usually on the order of 1010 sec"1), bimolecular reactions of the excited singlet are precluded. Unfortunately, the intersystem crossing rates are available for only a few carbonyl compounds.11,12 It is known that the rate of intersystem crossing for aliphatic carbonyl compounds (e.g., acetone) is slow (4-20 x 107 sec-1)30 in comparison to that for aromatic carbonyl compounds. Thus, aliphatic (and perhaps some aromatic) carbonyl compounds have an opportunity to react in the excited singlet state. 

IR bands for the both states. For example, singlet and triplet states of 2-naphthyl-(methoxycarbonyl)carbenes (NMCs, 17) show very different IR spectra (Scheme 9.4) (1590, 1625, 1640 cm for NMC and 1660 cm for NMC). Both states are observable in this case. In the singlet state, the carbomethoxy group assumes a conformation perpendicular to the naphthylcarbene plane to avoid destabilization of the empty carbenic 2p atomic orbital by the electron-withdrawing carbonyl group, while in the triplet, the methoxycarbonyl group is in the same plane to delocalize an unpaired electron. For this reason, there is a barrier between the two states and hence both of them are observable under these conditions. 

Quenching of Singlets of Carbonyl-Containing Compounds, a. Acetone. Recent reports have shown that singlet complications are not restricted to hydrocarbons for example, the photodecomposition of 1,4-dichlorobutane (52) to free radicals is sensitized by the (n, n) singlet state of acetone.216 Besides the observations that 52 quenches acetone fluorescence and that... 

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