Photooxidation of oxime ethers in the presence of nucleophiles was shown to result in nucleophilic on the oxime ether nitrogen. In this work we present the results of our studies on the oxidative cyclization reactions of 2’-arylbenzaldehyde oxime ethers with the aryl ring acting as the nucleophile. Reaction of the biaryl oxime ether in the presence of a catalytic amount of 9,10-dicyanoanthracene (DCA) as the photosensitizer resulted in the formation of the corresponding phenanthridine. The reaction is regiospecific for meta-substituted oxime ethers, favoring cyclization to form only the corresponding 2’-phenanthridine, which is believed to be due to a steric effect. Measured oxidation potentials (cyclic voltammetry) and calculated ionization potentials show a good correlation with the Hammett coefficients, providing support for an initial electron transfer step. A plot of the relative product yield against s+ shows a concave downward trend for strong electron-donating substituents, which suggests a change in the rate-limiting step. Step 1 (electron transfer) is the rate-limiting step for compounds with electron-withdrawing or weak electron-donating substituents (s+ &gt; –0.3), whereas step 2 (nucleophilic attack) is rate-limiting for compounds with strong electron-donating substituents (s+ &lt; –0.3). On the basis of these mechanistic studies we conclude that this oxidative cyclization reaction is initiated by an electron transfer step followed by a nucleophilic attack of the aryl ring on the nitrogen of the oxime ether radical cation intermediate (Scheme 1).

Mechanistic studies on the catalytic oxidative cyclization reactions of 2’-arylbenzaldehyde oxime ethers

249th National Meeting (2015)

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