Anion‐Exchange‐Triggered 1,3‐Shift of an NH Proton to Iridium in Protic N‐Heterocyclic Carbenes: Hydrogen‐Bonding and Ion‐Pairing Effects

Abstract

The chemistry of N-heterocyclic carbenes (NHCs) has become one of the most active and exciting topics in synthesis and catalysis.[1] NHCs bearing N-alkyl or -aryl wingtips are predominant, and there are only limited, although increasing, literature reports on NHC systems with hydrogen wingtips ligated on transition metals such as iridium, [2] rhodium, [3] osmium, [4] ruthenium, [4a-c, 5] rhenium, [6] manganese, [7] chromium, [8] and platinum ( Figure 1). [9] In general, these NHprotic NHC complexes can be synthesized by 1) protonation of 2-or 4-pyridyl or 2-imidazolyl complexes; [6,10] 2) cleavage of N À C [2c, 5a] or N À Si [9] bonds within NHC units; 3) intramolecular attack of a ligated isonitrile by a pendent NH 2 or OH group generated in situ; [5c, 8] and 4) metal-mediated tautomerization of N-heterocycles, a process involving CÀH activation.[2-4, 5a,b, 11] Among these methods, metal-mediated tautomerization is most intriguing in that N-heterocycle to NHC tautomerization is important not only in biological process [12] but also in C À C bond formation, where rhodium [3] and ruthenium [5b] protic NHC complexes are established as active catalysts.However, almost all the literature reports in this area are limited to the synthetic aspects, reactivity, catalytic applications of such complexes, and the energetics of N-bound and Cbound tautomers.[13] Mechanistic studies on this tautomerization process are rare and only two reports are known. [3a, 4e] Furthermore, few experimental studies have been carried out on the the most important N À H bond-formation step that generates NHCs (or the microscopic-reverse process). Bergman, Ellman, and co-workers reported the first studies on the tautomerization of 3-methyl-3,4-dihydroquinazoline via a rhodium(III) hydride intermediate, which, on the basis of theoretical studies, undergoes b-hydride insertion to give the NHC product.[3a] We feel that controllable interconversions between the protic NHC and the metal hydride precursor can greatly alter the electronic effect of metal center, an important feature in catalysis and molecular recognition. Thus it is highly desirable that the thermodynamics between protic NHC complexes and the metal hydride precursors can be readily tuned. We now report the synthesis of a series of rare 18-electron iridium(I) protic NHC complexes, which upon anion exchange can undergo 1,3-shift of the NH proton to iridium to give iridium(III) hydrides. Significantly, the thermodynamics of this process can be readily tuned by the counteranion, the solvent, and the phosphine coligand.

A Labile and Catalytically Active Imidazol‐2‐yl Fragment System

“…[3] In these systems, with commonly used ancilliary NHC ligands that are substituted at nitrogen atom(s) by alkyl, aryl, or other groups, [2,3] all catalytic transformations take place at the metal center, which is stabilized and/or activated by the NHC ligand. However, transformations that may possibly involve both the metal center and at one ring nitrogen of the NHC ligand are much less common, [4, 5c,e-g] and are limited to protic NHC complexes [4][5][6] or their conjugated bases. Thus, the NÀH function of a protic NHC complex (A or D; Scheme 1) could behave as Brønsted acid, whereas the basic nitrogen atom in the imidazol-2-yl complex (B or C) may behave as a Brønsted base.…”

An exploration of the mechanisms for the formation of 3,4-bis(4-aminofurazan-3-yl)-furoxan by density functional theory

“…Since barriers of the reactions that involve hydrogen transfer can be considerably lowered by the appearance of a water molecule [35][36][37][38][39][40][41], the assistance of an explicit catalyzed water molecule for this electrophilic substitution was also considered. For the water-assisted hydrogen transfer (TS1'), the water molecule hydrogen-bonding with the chlorine gradually transfers a hydrogen atom to the chlorine through the hydrogen bond [40], with one of the amino hydrogen atoms gradually transferring to the water oxygen. It is seen from Fig.…”

Theoretical studies of iridium-mediated tautomerization of substituted pyridines

“…Mechanistic studies on this tautomerization process, however, are rare [32,38,39]. Bergman, Ellman, and coworkers reported the first studies on the tautomerization of 3-methyl-3,4-dihydroquinazoline, and a rhodium(III) hydride intermediate was established as an intermediate, which, on the basis of theoretical studies, undergoes bÀhydride insertion into the proximal heteroaryl group to give the NHC product (Scheme 1) [32].…”

“…Bergman, Ellman, and coworkers reported the first studies on the tautomerization of 3-methyl-3,4-dihydroquinazoline, and a rhodium(III) hydride intermediate was established as an intermediate, which, on the basis of theoretical studies, undergoes bÀhydride insertion into the proximal heteroaryl group to give the NHC product (Scheme 1) [32]. We recently reported the microscopic reverse process, and 1,3-shift of the NH proton to the iridium(I) center was observed upon anion exchange, leading to iridium(III) hydrides (Scheme 2) [39]. The sharp contrast of the kinetics of this proton shift process measured at different water concentrations in CD 2 Cl 2 (20 ppm versus 0.14 M of water), together with DFT studies, all point to a water-assisted 1,3-proton shift mechanism via a six-member transition state.…”

“…It should be noted that even 20 ppm of water in "anhydrous" CD 2 Cl 2 would assist this process. We now focus on a deeper mechanistic understanding of our recently published work on protic NHCs by examining several 2,3 0 -bipyridyls and related systems using the density functional theory (DFT) [39,40]. Water-assisted proton transfer mechanism is consistently followed.…”