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

Under aprotic conditions, the stoichiometric reaction of N-heterocyclic carbenes (NHCs) such as imidazolidin-2-ylidenes with aldehydes affords Breslow Intermediates (BIs), involving a formal 1,2-C-to-O proton shift. We herein report kinetic studies (NMR), complemented by DFT calculations, on mechanism this kinetically disfavored H-translocation. Variable time normalization analysis (VTNA) revealed that orders reactants vary for different NHC-to-aldehyde ratios, indicating and ratio-dependent mechanistic regimes. propose high H-shift takes place in primary, zwitterionic NHC-aldehyde adduct. With excess aldehyde, zwitterion is equilibrium hemiacetal, which occurs. In both regimes, critical auto-catalyzed BI. Kinetic isotope effects observed R-CDO are line our proposal. Furthermore, we detected an H-bonded complex BI NHC (NMR).