Cyclopentanone triplet state

Most photodecarbonylation reactions of cyclic ketones, especially in the vapor phase, have been postulated to proceed from various vibrational levels of excited singlet states.321 However, the elimination reaction leading to unsaturated aldehydes has now been shown to occur largely via excited triplet states. In solution, where the lowest vibrational levels of the excited states are rapidly reached, to-alkenals are the major products observed in both photolysis and radiolysis of cyclopentanone and cyclohexanone. The reaction is quenched by oxygen and dienes,322-324 as well as by the alkenal produced in the reaction.325 The reaction is also sensitized by benzene triplets.322,323 With cyclopentanone, quenching by 1M piperylene occurs some 20 times as fast... 

Attempts have been made to look for a triplet state in the photochemistry of cyclopentanone. Experiments intended to detect any light emitted by the excited molecule gave negative results with the vapor and the liquid. Weak light emission has been observed in the glassy state at 77 °K. but such emission could well have come from an impurity present in trace amounts. 

From the photolysis of cyclopentanone in the presence of O218 an upper limit of 0.03 has been estimated for the quantum yield for the formation of cyclopentanone-018 (33). This may be compared with the quantum yield of 0.45 for acetone-O18 formation under identical conditions (39). It has been suggested (but not established) that the reaction between acetone and 021S takes place with the triplet state of the molecule. On this basis, the low efficiency of the corresponding reaction in cyclopentanone has been interpreted to favor the idea that a triplet excited state is not important in the photochemistry of cyclopentanone. 

Kinetic evidence for the importance of triplet states has also been obtained. 4-Pentenal is an important product both in the photolysis111 and radiolysis112 of liquid cyclopentanone. The yield is not affected by conventional radical scavengers such as DPPH or FeCl3 but is decreased by oxygen or 1,3-pentadiene. Phosphorescence is observed in the photolysis of cyclopentanone113 indicating the formation of a triplet state. The effect of 1,3-pentadiene on the yield of 4-pentenal is quantitatively consistent with the scheme... 

Very recently, the fluorescence spectrum77 of cyclopentanone has been studied and found to be unaffected by time, temperature, oxygen, or inert gases. A triplet state is not required to account for the results of either the fluorescence or the photolysis. 

The main conclusion to be drawn from the application of the benzene photosensitization method to the decomposition of cyclobutanone is that the Cj-hydrocarbons originate from the low-lying triplet state of the ketone. However, use of this method in the investigation of cyclopentanone decomposition indicated that reactions I, II and III (if it is a separate primary process) occur from the first excited state of the ketone. This conclusion was based on the quantitative agreement found between the pressure dependence of the decarbonylation-product formation and the fluorescence quenching by cyclopentanone. 

Cyclopentanone emits both fluorescence and phosphorescence in the liquid phase ° The results of Dunion and Trumbore show that the quantum yield of 4-pentenal formation, in the photolysis of cyclopentanone, is decreased by O2 or piperylene . Since both substances are known to be triplet state quenchers, it may be suggested that the triplet state participates in the photolytic decomposition of cyclic ketones in the liquid phase and in solution. 

This reaction is a diastereoselective de Mayo reaction. Its stereochemistry is controlled by the configuration of substrate 7. The P-face of cyclopentanone 7 is effectively shielded by the dimethylphenylsilyl moiety. Therefore, the triplet state of 7 attacks the cyclobutene 6 via the a-face, forming the fourth annulated cyclobutane ring of the ladderane core. Typically for cyclopentanone systems, the annulation proceeds syn-selectively and the product is obtained as a 7 1 mixture of diastereomers 8 and 20, which is separated by preparative TLC yielding pure 8. 

However, the lowest energy triplet state of cyclic enones is a 3jt,jt state, not a 3n,Jt state,761 and it is now believed that the regioselectivity in adduct formation rather reflects differences in the efficiencies of cyclization to products and the efficiencies of regeneration of the starting material via fragmentation of the singlet 1,4-biradicals (Scheme 6.63). For example, [2 + 2] photocycloaddition of cyclopentanone... 

The Norrish Type I reaction usually leads to decarbonylation. This is the case with dicyclopropyl ketone on irradiation at 193 nm. Decarbonylation, however, is a second step and this is preceded by ring opening of the cyclopropyl moieties to diallyl ketone. Calculations have shown that decarbonylation of cyclobutanone occurs from the nji triplet state. The resultant triplet trimethylene biradical undergoes ISC to the ground state before formation of cyclopropane. On the other hand, the cycloelimination reaction to yield ketene and ethene arises from the singlet excited state.Irradiation of cyclopentanone in aqueous and frozen aqueous solutions has been examined and the influence of applied magnetic fields assessed. Photodecarbonylation in the crystalline phase of the ketone (3) at 310 nm takes place stereospecifically with the formation of the cyclopentane derivative (4). The latter can be readily transformed into racemic herbertenolide (5). ... 

The photochemistry of cyclopentanone (29) is characterized in terms of disproportionation versus decarbonylation and/or cyclization versus cleavage reactions. The proposed mechanism involves electronic excitation of the ketone 29 followed by intersystem crossing (ISC) and an a-cleavage reaction in the triplet state... 

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