SNAr-Derived Decomposition By-products Involving Pentafluorophenyl Triazolium Carbenes

Abstract

Pentafluorophenyl triazolium carbenes, widely used in NHC-catalysis, can decompose by several mechanisms. Under high concentration conditions, the azolium may undergo a pentafluorophenyl exchange by a proposed SNAr mechanism to give an inactive salt. In the presence of appropriate substrates, cyclization on the ortho-position of the arene can occur, also by SNAr. These adducts provide a potential pathway for catalyst decomposition and serve as a caveat to the development of new reactions and catalysts.

An Unexpected Inactivation of N-Heterocyclic Carbene Organic Catalyst by 1-Methylcyclopropylcarbaldehyde and 2,2,2-Trifluoroacetophenone

“…Although the formation of the NHC-bounded side reaction products might be sometimes observed during the investigations of various NHC organocatalytic transformations, to the best of our knowledge, there has been very limited reports on the characterization of those NHC-bounded reaction products ( Berkessel et al, 2013 ; Berden et al, 2021 ). A representative study was from Rovis and co-workers in 2013, when they reported a cascade cyclization reaction between the triazolium NHC catalyst bearing an N -pentafluorophenyl ( N -C 6 F 5 ) group and the isatin-derived α , β -unsaturated ester substrate ( Rovis et al, 2013 ) ( Figure 1B ). The spirocyclic product was characterized via X-ray analysis on the product crystals.…”