The Need for an Alternative to Radicals as the Cause of Fragmentation of a Thiamin‐Derived Breslow Intermediate

Abstract

Mandelylthiamin (1)i saconjugate of benzoylformate and thiamin that loses CO 2 to form the classic Breslow intermediate (2), whose expected fate is formation of the thiamin conjugate of benzaldehyde (3). Surprisingly,i tw as observed that 2 decomposes to 4 and 5 and rearranges to 6 in competition with the expected protonation to give 3.R ecent reports propose that the alternatives to protonation arise from homolysis followed by radical-centered processes.I ti sn ow found, instead, that the spectroscopic observations cited in support of the proposed radical pathways are likely to be the result of other events.A na lternative explanation is that ionization of the enolic hydroxy group of 2 and resultant electronic reorganization leads to C À Cbond cleavage and nonradical intermediates that readily form 4, 5,and 6. Scheme 4. An anionic mechanism for the fragmentation and rearrangement reactions of the Breslow intermediate. Angewandte ChemieCommunications 6323