Access to a Cationic, Electron-Poor N-Heterocyclic Carbene with a Quinazolinium Core by Postsynthetic Modification of Related Neutral Derivatives

Abstract

Derivatives of cationic six-membered N-heterocyclic carbene G with a quinazolinium scaffold are easily obtained by abstraction of an alcoholate group from related neutral precursors which are easily accessible. On the basis of this route, chalcogene adducts and metal complexes of G, which is a benzo-anellated pyrimidiniumylidene, could be prepared. Evaluation of the ligand properties by IR and 77 Se NMR spectroscopy indicate that cationic carbene G is a much more powerful π-acceptor (δ( 77 Se): 768 ppm) and a significantly weaker overall donor (TEP: 2073 cm −1 ) than related neutral carbene 4 (361 ppm, 2054 cm −1 ). DFT calculations provide a rationale for these experimental observations. AuCl complexes of both carbenes were used as catalysts for the cyclization of a propargylamide to a methyleneoxazoline. The complex containing the cationic carbene featured a much higher activity.

Optimizing Catalyst and Reaction Conditions in Gold(I) Catalysis–Ligand Development

“…192 Ganter and coworkers prepared a gold complex containing a cationic 6-membered ring NHC with a quinazolinium structure (NHC-126) by treatment of the corresponding neutral species, [Au(NHC-129)Cl], with HOTf (Scheme 67). 193 The cationic NHC-126 ligand was found to be significantly less donating and a much better π-acceptor [TEP = 2073 cm −1 , δ( 77 Se) = 768 ppm] than the neutral NHC-129 carbene [TEP = 2054 cm −1 , δ( 77 Se) = 361 ppm]. The gold-catalyzed cyclization of a propargylamide was chosen to evaluate the ligand effect.…”

“…The activity of the cationic species was higher (62 and 99% yield after 1, and 7 h, respectively) than that of the neutral complex (18 and 62% yield after 1 and 7 h, respectively). 193…”

“…The reaction was carried out in acetonitrile, at room temperature, using 5 mol % of the Au-NHC ( NHC-126 , 129 ) complexes and 5 mol % of AgOTf. The activity of the cationic species was higher (62 and 99% yield after 1, and 7 h, respectively) than that of the neutral complex (18 and 62% yield after 1 and 7 h, respectively) …”

N-Cyclopropenio-imidazol-2-ylidene: An N-heterocyclic carbene bearing an N-cationic substituent

“…For a preliminary evaluation, the intramolecular cyclization of propargylamide 8 and hydroarylation of propargyl aryl ether 10 were selected as model reactions, since they had previously served as standard catalytic reactions for gold catalysts bearing cationic NHCs (Scheme 3). 16,25,26 The activity of catalyst [6](OTf) was compared with that of the neutral reference systems AuCl(IMes) and AuCl(IPr). In the cyclization of propargyl amide 8, 16,25 catalyst [6](OTf) performed almost as good as the two neutral catalysts at 0.5 mol% with a 86% NMR yield observed after 24 hours.…”

“…16,25,26 The activity of catalyst [6](OTf) was compared with that of the neutral reference systems AuCl(IMes) and AuCl(IPr). In the cyclization of propargyl amide 8, 16,25 catalyst [6](OTf) performed almost as good as the two neutral catalysts at 0.5 mol% with a 86% NMR yield observed after 24 hours. Decreasing further the catalyst loading to 0.25 mol% had a significant effect on the reaction rates, and the cyclization did not proceed to completion for all catalysts, even after 80 hours.…”