5 years ago

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

The Need for an Alternative to Radicals as the Cause of Fragmentation of a Thiamin-Derived Breslow Intermediate
Michael Bielecki, Ronald Kluger
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 non-radical intermediates that readily form 4, 5, and 6. Perhaps not so radical: A Breslow intermediate surprisingly gives a mixture of products (blue). It has been proposed that these form via radicals (red), but it is now suggested that a non-radical process prevails (green).

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/anie.201702240

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