Study on the reactive transient α‐λ3‐iodanyl‐acetophenone complex in the iodine(III)/PhI(I) catalytic cycle of iodobenzene‐catalyzed α‐acetoxylation reaction of acetophenone by electrospray ionization tandem mass spectrometry

RATIONALE

Hypervalent iodine compounds are important and widely used oxidants in organic chemistry. In 2005, Ochiai reported the PhI‐catalyzed α‐acetoxylation reaction of acetophenone by the oxidation of PhI with m‐chloroperbenzoic acid (m‐CPBA) in acetic acid. However, until now, the most critical reactive α‐λ³‐iodine alkyl acetophenone intermediate (3) had not been isolated or directly detected.

Electrospray ionization tandem mass spectrometry (ESI‐MS/MS) was used to intercept and characterize the transient reactive α‐λ³‐iodine alkyl acetophenone intermediate in the reaction solution.

The trivalent iodine species was detected when PhI and m‐CPBA in acetic acid were mixed, which indicated the facile oxidation of a catalytic amount of PhI(I) to the iodine(III) species by m‐CPBA. Most importantly, 3·H⁺ was observed at m/z 383 from the reaction solution and this ion gave the protonated α‐acetoxylation product 4·H⁺ at m/z 179 in MS/MS by an intramolecular reductive elimination of PhI.

These ESI‐MS/MS studies showed the existence of the reactive α‐λ³‐iodine alkyl acetophenone intermediate 3 in the catalytic cycle. Moreover, the gas‐phase reactivity of 3·H⁺ was consistent with the proposed solution‐phase reactivity of the α‐λ³‐iodine alkyl acetophenone intermediate 3, thus confirming the reaction mechanism proposed by Ochiai. Copyright © 2012 John Wiley & Sons, Ltd.