Mechanistic insights into the triazolylidene-catalysed Stetter and benzoin reactions: role of the N-aryl substituentElectronic supplementary information (ESI) available: All spectroscopic data and procedures for novel compounds. CCDC 895406 (41), 895407 (54) and 895408 (55). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2sc22137c

The in situ observation, isolation and reversible formation of intermediate 3-(hydroxybenzyl)azolium salts derived from NHC addition to a range of substituted benzaldehydes is probed. Equilibrium constants for the formation of these 3-(hydroxybenzyl)azolium salts, as well as rate constants of hydrogen-deuterium exchange ( k ex ) at C(α) of these intermediates for a range of N -aryl triazolinylidenes is reported. These combined studies give insight into the preference of N -pentafluorophenyl NHCs to participate in benzoin and Stetter reaction processes. The in situ observation, isolation and reversible formation of intermediate 3-(hydroxybenzyl)azolium salts derived from NHC addition to a range of substituted benzaldehydes is probed.