Benzophenone, photocycloaddition table

It is conceivable that a carbonyl compound with an n,n triplet energy lower than that of benzophenone could yield the photocycloaddition product in some of these cases. A reaction which may illustrate this point is the photocycloaddition of ethyl glyoxylate to styrene and 1,1-diphenylethylene.66 Unfortunately, the triplet energy of ethyl glyoxalate has not been measured however, there is adequate reason to believe it is lower than that of benzophenone (see Table VI). 

Table II must apply to both reactions. This has been verified by quenching the photocycloaddition of benzophenone to 2-methyl-2-butene with ferric dipivaloylmethane25 and that of benzaldehyde to cyclohexene with naphthalene.37...

One of the limitations of the photocycloaddition reaction is that the unsaturated system may itself act as a quencher. Conjugated dienes fall within this category since they quench the n,ir triplet of some carbonyl compounds. For the photoreduction of benzophenone in benzhydrol, the ratio, kqlka, for m-piperylene is 750 (Table II), which indicates that this diene is indeed an efficient quencher for the reaction. 

The phenylsubstituted benzophenones provide another series to illustrate the correspondence of the photoreduction and photocycloaddition reactions and, in addition, afford another example of an anomaly in the correlation of photochemical reactivity with emission spectra. These experiments are summarized in Table V. This series is of... 

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