Cyclopentanone primary process

The details of the photochemical primary process in cyclopentanone are fairly well understood (33). Since the spectrum in the first absorption region appears to be similar to that of the simple aliphatic ketones, it has been assumed that initial excitation is to the upper singlet state and that the zero-zero band is near 3600 A. 

Although the picture of the photochemical primary processes in cyclopentanone which has been presented seems self-consistent, a number of minor points still have to be explained. These are (a) the dependence of the ratio of ethylene to cyclobutane on the geometry of the system (6) the puzzling fact that a constant fraction, between 2/10 and 3/10, of the initially excited molecules seem to return to the ground state without decomposition, by a route that is virtually unaffected by pressure. Before this can be explained it is necessary to confirm the value for the quantum yield for decomposition and (c) the fact that 2.5 kcal./mole of energy affects the reaction path profoundly. In the ground state the enthalpies of 2 and 3 differ by 19 kcal./mole at 25° while 3 and 4 may be estimated to differ by 15 to 20 kcal./mole. This phenomenon may be explained when a clear understanding of the excited state of the molecule is obtained. 

This contrasts sharply with the behavior of cyclopentanone and other alkanones for which the most important primary process from is intersystem crossing. 

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. 

Reaction of 3,3-disubstituted-l,4-pentadiene 92 with a primary amine under cyclohydrocarbonylation conditions yielded cyclopenta[. ]pyrrole 96 as the predominant product accompanied by a small amount of cyclopentanone 95 (Scheme 15). This unique reaction is proposed to proceed through a cascade hydrocarbonylation-carbonylation process. The first hydrocarbonylation of 92 and the subsequent carbocyclization formed cyclopentanoylmethyl-Rh complex 93. If 93 immediately reacts with molecular hydrogen, 2-methylcyclopentanone 95 is formed. However, if CO insertion takes place faster than the hydrogenolysis, cyclopentanoylacetyl-Rh complex 94 is generated, which undergoes the Paal-Knorr condensation with a primary amine to yield cyclopenta[. ]pyrrole 96. ... 

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