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

Mandelylthiamin (1) is a conjugate of benzoylformate and thiamin that loses CO2 to form the classic Breslow intermediate (2), whose expected fate is formation of the thiamin conjugate of benzaldehyde (3). Surprisingly, it was observed that 2 decomposes to 4 and 5 and rearranges to 6 in competition with the expected protonation to give 3. Recent reports propose that the alternatives to protonation arise from homolysis followed by radical-centered processes. It is now found, instead, that the spectroscopic observations cited in support of the proposed radical pathways are likely to be the result of other events. An alternative explanation is that ionization of the enolic hydroxy group of 2 and resultant electronic reorganization leads to C-C bond cleavage and nonradical intermediates that readily form 4, 5, and 6.