Oximes generally have a high barrier to inversion, and accordingly this
reaction is envisioned to proceed by protonation of the oxime hydroxyl, followed
by migration of the alkyl substituent "trans" to nitrogen. The N-O bond
is simultaneously cleaved with the expulsion of water, so that formation of a
free nitrene is avoided.
An efficient Beckmann rearrangement at room temperature involves an eosin Y
catalyzed, visible-light-mediated in situ formation of the Vilsmeier-Haack
reagent from CBr4 and a catalytic amount of DMF. This operationally
simple method for the activation of ketoximes avoids the need for any corrosive,
water-sensitive reagents and elevated temperatures.
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S. Chandrasekhar, K. Gopalaiah,
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