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Oxidation of Hydroaromatic Systems. IV. The Chromic Acid Oxidation of Cycloheptatriene’ Paul Muller and Jan Ro;ek* Contribution from the Department of Chemistry, University of Illinois at Chicago Circle, Chicago, Illinois 60680. Received August 10, 1973 Abstract: In the chromic acid oxidation of cycloheptatriene, benzaldehyde in 19-25z yields is formed as the only isolable primary product; benzoic acid is obtained at higher chromic acid :cycloheptatrieneratios. The benzaldehyde isolated from the oxidation of 1,2,3,4,5,6-hexadeuterio~ycloheptatriene contains 6/7 of a proton per molecule evenly distributed over all available positions and must therefore be formed through a symmetrical intermediate; however, no evidence for the formation of a tropenium ion intermediate could be obtained. No significant isotope effect is observed in the oxidation of octadeuteriocycloheptatriene with chromium(V1). A mechanism for the formation of benzaldehyde consisting in a chromium(1V) oxidation of cycioheptatriene to a tropenyl radical and its further oxidation by chromium(V1)directly to benzaldehyde is proposed. It is argued that the results have general validity and that the allylic oxidation products commonly formed in the oxidation of olefins with chromic acid are attributable solely to chromium(1V) and not to chromium(V1).
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he ability of chromium(V1) to oxidize carbonhydrogen bonds has been clearly demonstrated for alcohols, aldehydes, and aliphatic and alkylaromatic hydrocarbons.2 The latter example is of particular importance, as it indicates that the carbon-hydrogen bond is more susceptible to oxidation than the unsaturated system of the aromatic ring. Olefins react with chromic acid to form oxidation products derived from the oxidation of both the double bond and allylic carbon-hydrogen bonds. Available evidence suggests that in monoolefins chromium(VI) attacks the .rr-electron system. 3 c , 4 The purpose of this work was to examine whether chromium(V1) is capable of reacting with an activated u bond in preference to a nonaromatic 7r bond if sufficiently favorable conditions can be provided. Cycloheptatriene (CHT) appeared particularly suitable for this purpose. Hydride abstraction from the methylene group would yield the tropenium cation, hydrogen atom abstraction the tropenyl radical. The cation is well known for its quite unusual stability due to its aromatic character,jv7 and even the free-radical species has been shown to be stabilized to an unexpected degree.8 Moreover, even if hydrogen was abstracted as an atom, the transition state would be expected to assume a more polar character due to the polar and probably cationic 2s
( I ) (a) Part 111: F. Stoos and J. RoEek, J . Amer. Chem. SOC.,94,2719 (1972). (b) Support of this work by the U. S. Army Research Office (Durham) is gratefully acknowledged. (2) For reviews cf. (a) K. B. Wiberg in “Oxidations in Organic Chemistry,” Part A, I
