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

Abstract

N-heterocyclic carbenes (NHCs) [1] and their complexes [2] are excellent catalysts for a broad array of organic transformations, where the NHC ligands impart useful electronic and steric properties to metal centers. [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. Moreover, reactivity of A with a base could lead to B, a transient species with both a vacant metal coordination site and a basic nitrogen atom, which could bind (C) and activate a substrate (C to D). These reactivity patterns might also be compatible with protic NHC complexes derived from other NHC ligands.Herein, we report a versatile organometallic system in which the combined reactivity of the imidazol-2-ylidene or imidazol-2-yl fragments and the metal center lead to ligand exchange processes that involve A and B, hydrogen activation (B!C!D), and ultimately, catalytic behavior (D + ketone! E!B + alcohol). Key differences between results herein from previous work [4] include: 1) greatly enhanced rates of reaction (for example, ligand exchange within minutes instead of days); 2) the ability to tune ligand exchange rates over several orders of magnitude; and perhaps most importantly 3) catalytic behavior, which was not at all apparent before. Moreover, we show that 15 N chemical shift information on natural-abundance samples gives valuable information on the environment of the imidazol-2-yl or imidazolylidene ligand.Reaction of 1 [4] with [CpRuCl(cod)] (Cp = cyclopentadienyl, cod = 1,5-cyclooctadiene) in THF at 100 8C led to phosphorus coordination and complete tautomerization of the imidazole to carbene 2, which shows a low-field 1 H NMR signal (d = 10.28 ppm, NH) and a doublet in the 13 C{ 1 H} NMR (d = 184.1 ppm, 2 J CP = 22.5 Hz, C2) that are consistent with structure 2 (Scheme 2). Scheme 1. Imidazol-2-yl and imidazol-2-ylidene fragment systems. The square indicates a vacant site.Scheme 2. Synthesis and reactivity of imidazol-2-yl and imidazol-2-ylidene complexes (in [D 8 ]THF at room temperature unless otherwise specified). Yields are of isolated products unless otherwise specified. LDA = lithium diisopropylamide.[*] Dr.

Complexes Bearing Protic N-Heterocyclic Carbenes: Synthesis and Applications

“…33 A 1 H NMR titration study of one of these complexes, with the hydrogen-bond acceptor DMPU, demonstrated the ability of the NH group of the protic NHC ligand to form intermolecular hydrogen bonds. Similar hydrogen bonds between an NH group of a protic NHC ligand and a hydrogen-bond acceptor were observed for complexes [4], 29 [8], 31 [10], 32 and others. 20c,23k Such NH£X hydrogen bonds represent an interesting feature as they may be utilized for substrate recognition and orientation in catalytically active complexes bearing protic NHC ligands.…”

“…31,32 Reaction of 7 with [RuCl(Cp)(cod)] (Cp: cyclopentadienyl, cod: cyclooctadiene) proceeds directly to ruthenium(II) complex [8] as the exclusive reaction product (Scheme 5, top). 31 The oxidative addition of 7 to [IrCl 2 (Cp*)] 2 proceeds stepwise. At ambient temperature, the phosphine complex [9] was isolated.…”

“…The complexes have also been used for the activation of dihydrogen and as catalysts in transfer hydrogenation reactions. 31,32 The phosphine-tethered benzimidazole derivatives 11 33 and 13, 14 34 have also been employed for the preparation of complexes bearing protic NH,NR-NHC ligands (Scheme 6). The diphenylphosphine-tethered benzimidazoles of type 11 (n = 1 and 2) react with [RuCl(Cp*)] 4 to give Ru II complexes of type [12] by the formal tautomerization of the benzimidazole group to an NH,NR-NHC ligand.…”

“…While oxidative addition to Pd 0 gave exclusively the dinuclear complex [30] in 42% yield, 40 the reaction with Pt 0 led to the formation of a mixture of mononuclear complex [31] and dinuclear complex [32] in a ratio of 4:1. 45 The dinuclear complexes [30] and [32] are obtained by the intermolecular attack of the unsubstituted ring-nitrogen atom of intermediate I at the metal atom of a second complex with the loss of a phosphine ligand.…”

“…Some interesting features emerge from the comparison of the structural data for complexes [30] 40 and [31] 45 to those of . 40 Metric parameters in the dinuclear complex [30] do not differ significantly from equivalent parameters found in the NH,NMe-NHC complexes of type trans-[21]BF 4 .…”