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Volume 145, 2010
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Dynamic path bifurcation for the Beckmann reaction: observation and implication

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The reaction of oximes to amides, known as the Beckmann rearrangement, may undergo fragmentation to form carbocations + nitriles instead of amides when the cations have reasonable stability. The reactions of oxime derivatives of 1-substituted-phenyl-2-propanones and 3-substituted-phenyl-2-butanones in aqueous solvents gave both rearrangement and fragmentation products, the ratio of which was dependent on substituents. Transition state (TS) optimizations and intrinsic reaction coordinate (IRC) calculations for the reaction of 1-phenyl-2-propanone oximes showed that there is a single TS for each substituted compound. The IRC path from the TS either led to a rearrangement product or a fragmentation product depending on the substituent; the IRC path changes from rearrangement to fragmentation when substituent X becomes more electron donating. Ab initio dynamics simulations were found to follow the IRC path for X = p-NH2 and p-MeO giving fragmentation products, and almost so for X = p-NO2 giving the rearrangement products. However, in a borderline case where X is less donating than p-MeO or less withdrawing than p-NO2, the trajectories did not follow the minimum energy path on the potential energy surface, but gave both rearrangement and fragmentation products directly from the single TS. This is a novel example of path bifurcation for a closed shell anionic reaction. It was concluded that a reactivity-selectivity argument based on the traditional TS theory may not always be applicable even to a well-known textbook organic reaction.

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The article was received on 01 Apr 2009, accepted on 15 Apr 2009 and first published on 23 Sep 2009

Article type: Paper
DOI: 10.1039/B906159B
Citation: Faraday Discuss., 2010,145, 327-340
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    Dynamic path bifurcation for the Beckmann reaction: observation and implication

    H. Yamataka, M. Sato, H. Hasegawa and S. C. Ammal, Faraday Discuss., 2010, 145, 327
    DOI: 10.1039/B906159B

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