Cyclopentanone photolysis

QUANTUM YIELDS OF PRODUCT FORMATION FROM CYCLOPENTANONE PHOTOLYSIS... 

Srinivasan (1964) has reviewed the early studies of cyclopentanone photolysis. From the data available at that time, he favored the direct formation of these products... 

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... 

Exercise 28-10 Write a mechanism for formation of cyclobutane from the photolysis of cyclopentanone, and ketene from the photolysis of cyclobutanone. 

Photolysis of cyclopentanone leads to the formation of carbon monoxide, ethylene, cyclobutane (3), and 4-pentenal (28). An early report of the formation of butenes (27) has not been substantiated by later work. The yield of the gaseous products agrees with eq. 1 at 3130 A. and temperatures up to 125° (33) and at shorter wavelengths up to 300° (3). 

Evidence against 3b as a source of ethylene has been obtained from a study of the photolysis of cyclopentanone-2,2,5,5-d4(I) (28). In this... 

Since the photolysis of simple aliphatic ketones leads to monoradicals, it has been postulated that the photolysis of alicyclic ketones should lead to diradicals. In cyclopentanone, the initially formed diradical may... 

It is essential to note that the mechanism which involves diradical intermediates is based purely on analogy and as such remains to be proved. If tetramethylene diradicals are formed, it is surprising that, unlike monoradicals, they do not recombine with each other or abstract hydrogen atoms from other molecules. It has been claimed that photolysis of cyclopentanone in the presence of ethylene leads to cyclohexane and hexenes as products presumably through the reactions 5 and 6 (17). 

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. 

Fig. 1. Effect of methane on the photolysis of cyclopentanone at 3130 A. (33). Average ketone pressure 11.6 mm. room temperature , cyclobutane A, pentenal , ethylene O, cyclopentanone accounted for in products.

Gas Photolysis of cyclopentanone Thermal decomposition Cyclopropane13 Cyclobutane ... 

The diradical mechanism as applied to this system is as hypothetical as in the cases of cyclopentanone and cyclohexanone. The only data on the use of radical scavengers is a report on the photolysis of cycloheptanone in the presence of 3.2 mm. of oxygen. In this case, both of the C hydrocarbons and 6-heptenal was observed to be formed. It is quite likely that the reactions 30-33 are concerted processes. 

A preliminary study of the photolysis of 2-methyl cyclopentanone (XVII) and 3-methyl cyclopentanone (XVIII) has appeared (18). At... 

In a qualitative study of the photolysis of 2-ethyl cyclopentanone (XIX) (20) it lias been observed that ethyl cyclobutane is a product. 

It has been observed that the formation of the olefin and carbon monoxide, 45, is ten times more important than the formation of the bicyclic hydrocarbon and carbon monoxide, 46, at 80° and 80 mm. pressure even at 3130 A. The formation of the strained bicyclic hydrocarbon is evidently not a favorable reaction although this may not be the only consideration. In the case of camphor it should be interesting to find out if an optically active isomer of the ketone on photolysis will give rise to an optically active trimethyl bicyclo [2.1.1] hexane (XXVI). A concerted reaction, analogous to the formation of cyclobutane from cyclopentanone, may lead to only an optically active product. 

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. 

Using 2,2-dimethylcyclobutanone [26a], as a specific example, initial excitation to produce an excited state species followed by a-cleavage would produce the acyl alkyl biradical [30]. Subsequent decomposition of [30] would then afford ester [27a] (via ketene), cyclopropane [28a], and acetal [29a], the observed photoproducts. The intermediacy of biradical [30] was supported by (a) the nearly exclusive formation of methyl acetate (as opposed to methyl isobutyrate), (b) the exclusive formation of the 5,5-dimethyl substituted acetal [29a] (as opposed to its 3,3-dimethyl substituted isomer), (c) its role as a common intermediate for all products, and (d) analogy to the photochemistry of cyclopentanones and cyclohexanones. Recently, Wasacz and Joullie have reported that photolysis of oxacyclohexanone [32] affords a 3% yield of acetal [29a] (18). It is conceivable that the formation of [29a]... 

The pyrolysis of tetramethylenediazirine in the gas phase is a first-order reaction yielding only cyclopentene and nitrogen. Similarly, the treatment of the tosylhydrazone of cyclopentanone with base under aprotic conditions yields cyclopentene as the only hydrocarbon product. Photolysis of this diazirine yields cyclopentene as the principal hydrocarbon product (99.2%), but very small quantities of bicyclo[2,1,0]-pentane (0.3%) and methylenecyclobutane (0.1%) are also formed. In addition, about 0.5% of another hydrocarbon was detected but not identified. Its early position of the chromatogram indicates that it may be a fragmentation product. 

Reaction I (in a peculiar form), as well as reactions II, III and IV all occur in the photolysis of cyclobutanone, though the quantum yield of step IV is very low. The occurrence of reaction III in the photolysis of cyclopentanone was not reported however, in that of cyclohexanone and cycloheptanone again all four reactions seemed to take place. 

McGee also found the cyclopropane/propene ratio to decrease with increasing pressure in the photolysis of cyclobutanone. However, his data indicated that the change is entirely due to an increase in propene formation, while cyclopropane formation is claimed to be independent of pressure. McGee suggested, on the basis of these results, that propene and cyclopropane were not formed from the same excited state. The deuterium content of the olefin, formed in the photolysis of cyclopentanone-2,2,5,5-rf4, also indicates that the hot cycloalkane is not the only source of the CH2 = CH(CH2) 4CH3 product - . 

A pressure increase, brought about by an increase in the concentration of the ketone or by the addition of an inert gas, enhances the formation of the unsaturated aldehyde as compared to that of CO. The value of awehyde increases at the expense of 0co> thus, the ketone consumption yield is independent of pressure. This seems to be generally valid in the photolysis of the cyclic ketones it was confirmed, for instance, for cyclopentanone °, cyclohexanone 2-methyl cyclohexanone and 2,6-dimethyl cyclohexanone . An increase in wavelength also favours the formation of the aldehyde as compared to decarbonylation in the photolysis of cyclobutanone , cyclopentanone and cyclohexanone . At 3130 A, the decrease in temperature has a similar effect on the product distribution in the photolysis of cyclopentanone , cyclohexanone , 2-methyl cyclohexanone , and 2,6-dimethyl cyclohexanone to that caused by the increase in wavelength or pressure. However, at shorter wavelengths, the quantum yields seem to be independent of temperature . ... 

The reactions taking place in the vapour phase also occur in the condensed phase, and their mechanisms are probably similar. However, as may be expected on the basis of the results obtained for the gas phase photolysis, the formation of olefins, cycloparaffins, and CO is of less importance, while that of the saturated aldehydes is more important in the liquid phase or solution, where energy dissipation by collision is more efficient. The decarbonylation products were shown to be only of minor importance in the photolysis of liquid cyclopentanone and cyclohexanone . The unsaturated aldehyde was found to be the main product in the liquid-phase photolysis of cyclopentanone (methyl cyclohexanone . Unsaturated aldehydes were also identified in the photolysis products of other cyclic ketones in the liquid phase as well as in solution . ... 

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. 

Photolysis of bicyclo[3.2.2]octa-3,6-dien-2-one derivatives, for example the adduct of tropone with benzyne 33 a in water gave the three-membered ring derivative 35 a, while photolysis in acetonitrile gave the cyclopentanone derivative 37 a which must arise from the 1,3-shift product 36a.This different behavior in these two solvents was rationalized in the following way the efficient [3,3] shift of 33 a to cyclopropyl ketene 34 a is thermally reversible, so that in the absence of a nucleophile 35a is not observed. In that case, the less efficient 1,3-shift to form 36a and then 37a is the only detectable reaction. The ketene 34a was detected as the major product of photolysis of adduct 33 a at low temperature methanolysis of 34 a at — 80 "C gave only the enfifo-configurated ester 35 a. 

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