2,6-dimethyl cyclohexanone, photolysis

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 vapour phase photolysis of epimeric 2, 6- dimethyl cyclohexanone gives stereospecific reaction. In each case, product formation is faster than photochemical interconversion of the epimers and each epimer gives the stereospecific mixture of products. 

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]... 

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