3821
Photooxidation of Aqueous Benzene. 11. Role of Benzvalene in the Formation of Cyclopentadienecarboxaldehyde ls2
Sir : The formation of an acidic aldehyde upon irradiation of aerated aqueous benzene at 253.7 nm has been reported previously, 3, and the intermediacy of an oxygen adduct with benzvalene3 or another labile isomer of benzene4 has been postulated. In the preceding communication2 we have identified this aldehyde as 1,3-cyclopentadiene-l-carboxaldehyde(1) or its anion l a . We now wish to report that formation of the photoaldehyde does not require 02,but only aqueous benzvalene (2). In water free from 02,one molecule of benzvalene is oxidized to the aldehyde while a second is reduced to 1,4-cyclohexadiene (3).
c-01
2
L 1
In the presence of 02,two molecules of benzvalene are likewise required to produce one molecule of the aldehyde: 1,4-cyclohexadiene is not formed in this case, but a peroxide is. In both cases, the reactions following the photoisomerization of benzene5 are purely thermal and appear to be radical chain processes. Aldehyde formation occurs equally well in the presence or absence of O2 and exhibits the same dependence on acidity from pH 4 to 12. Our conclusions are derived both from photochemical studies and from dark experiments with benzene solutions of benzvalene.6 In a particularly enlightening series of experiments, the solutions of preformed benzvalene were distilled in vacuo Torr) into a 1000-fold volume of thoroughly degassed water at pH 10. The carboxaldehyde was formed within minutes in good yield, as determined by the uv absorption2s3 of the anion in the still-evacuated vessel. Since the aldehydic oxygen must have been acquired from water, a sealch was made for a complementary reduction product. No Hz was found above the solution, but benzene recovered by evacuation and analyzed by gc was found to contain 1,4-cyclohexadiene in an amount comparable to that of the aldehyde. The origin of the cyclohexadiene was established by isotopic labeling: cyclohexadiene formed from the benzvalene solution mixed (after preparation) with benzene-& (0.3 mol fraction) contained no deuterium ; that formed from benzvalene-& in benzene-& was exclusively C6HzD6. These results demonstrate that the cyclohexadiene is formed by addition of two H atoms from water to a molecule of benzvalene. In contrast to the rapid formation of aldehyde in these experiments, it was (1) Based on work performed under the auspices of the U. S.Atomic Energy Commission. (2) Paper I: L. Kaplan, L. A. Wendling, and K. E. Wilzbach, J . Amer. Chem. Soc., 93, 3819 (1971). ( 3 ) E. Farenhorst, Tetrahedron Letr., 4835 (1968). (4) M. Luria and G . Stein, Chem. Commun., 1650 (1970). ( 5 ) I
