Formation of Breslow Intermediates from N‐Heterocyclic Carbenes and Aldehydes Involves Autocatalysis by the Breslow Intermediate, and a Hemiacetal

Breslow intermediates (BIs) are pivotal species in the Umpolung catalysis effected by N-heterocyclic carbenes (NHCs), ranging from vitamin B1 dependent enzymes to highly selective organocatalytic processes. For their formation from aldehydes and NHCs, a long-standing riddle has been the mechanism by which the zwitterionic primary intermediate is converted to the BI, as the C-to-O 1,2-H-shift involved is kinetically highly disfavored. The kinetic study by Albrecht Berkessel et al. (DOI: 10.1002/anie.202117682) shows that under aprotic conditions, the BI itself effects this critical H-shift in an autocatalytic fashion. Cover art: Andrea Bertl