Irradiation of N-acetyldiphenylmethyleneamine (II) in toluene solution gave N-(1,1,2-triphenylethyl)acetamide and N-(1,1-diphenylmethyl)acetamide in 11% and 4.4% yields, respectively. The former is an addition product of toluene to the C=N bond of imine II, and the latter a hydrogenation product of II. The same kind of addition and hydrogenation products as those in toluene were obtained on irradiation of imine II in ethylbenzene, cumene, o-, m-, and p-xylene, mesitylene, p-methoxytoluene, and p-cyanotoluene, respectively. Two kinds of competitive reaction processes are proposed from the results of the substituent effects for the phenyl ring of toluene, the effect of the additives such as sensitizers and quenchers, and the dependence of the photoreaction yields on the concentration of the additive. The photoreaction proceeds not only by the “chemical sensitization” mechanism derived from the ketyl radical of benzophenone, a hydrolyzed product of II, but also by the mechanism through the excited state of II itself.
The photochemical conversion of N-acetyldiphenylmethyleneamine (I) in toluene to produce an addition product N-(1,1,2-triphenylethyl)acetamide and a hydrogenation product N-(diphenylmethyl)acetamide has been found to increase remarkably by displacement of para-hydrogen of toluene with a halogen (chlorine or bromine). A number of halogenated solvents, especially brominated solvents such as dibromomethane and bromobenzene, also promoted the photoreaction of the imine I with toluene. The extent of the promotion depends on the kind of a halogen atom of the solvent molecule. The reaction rate was accelerated with the number of halogen atoms of the solvent molecule. The conversion rate increased very remarkably when the concentration of bromobenzene increased from 0 to 1 mol/l. The reaction via chemical sensitization by benzophenone, a contaminant in imine I, was shown to be insensitive to the heavy atom solvent. The heavy atom effect can be attributed to an increase in the rate of intersystem crossing from the excited singlet state to the triplet state of the imine I. Imine I in the excited state was confirmed to have a hydrogen abstracting character.
Irradiation of N-(diphenylmethylene)acetamide (N-acetyldiphenylmethyleneamine, 1) in cyclopentene gave N-[1,1-diphenyl-1-(cyclopenta-2′-enyl)niethyl]acetaniide, the addition product of cyclopentene to the C=N bond of imine 1, in 31% yield. The photoreaction of 1 was observed in other cyclic and acyclic olefins. The reactivity of the olefins decreases in the order; cyclopentene>cyclohexene>cyclooctene>2-pentene\simeq2-hexene. The photoreaction is promoted by the presence of the heavy atom solvent or the sensitizer. The mechanism of photoreaction is discussed in terms of the excited triplet state of the imine.
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