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-2ylidene complexes (in [D 8 ]THF at room temperature unless otherwise specified). Yields are of isolated products unless otherwise specified. LDA = lithium diisopropylamide.[*] Dr.

Synthesis of Complexes Containing an Anionic NHC Ligand with an Unsubstituted Ring‐Nitrogen Atom

“…Compared with the C carbene resonance found for the related platinum complex of type trans‐ II (Scheme ) containing the NMe,NH‐NHC ligand ( δ =166.9 ppm in CDCl 3 /[D 6 ]DMSO;11a δ =157.8 ppm in CD 2 Cl 2 , see the Supporting Information), the C carbene resonance for 1 in CD 2 Cl 2 is shifted highfield by Δ δ =8.4 ppm compared with the protonated complex, an effect obviously caused by the anionic character of the benzimidazol‐2‐yl ligand of 1 . A similar highfield shift for the C carbene resonance was observed upon N ‐deprotonation of NR,NH‐NHC ligands coordinated to Ru II (NR,NH‐NHC δ =167.7, NR,N‐NHC 156.4 ppm)12e or Ir I (NR,NH‐NHC δ =140.3, NR,N‐NHC 131.3 ppm) 12b. The C carbene resonance in 2 was detected as a doublet of doublets at δ =157.9 ppm (dd, 2 J CP =8.8 Hz, 3 J CP =2.1 Hz), and this value is in accord with the chemical shift recorded for the C carbene atom in the dinuclear platinum complex III ( δ =159.8 ppm, dd, 2 J CP =8.8 Hz, Scheme ) 11a.…”

“…Complexes of type D are extremely rare and have so far only been obtained by deprotonation of coordinated NR,NH‐NHCs 12b. e The direct access to complexes featuring NR,N‐NHC ligands by the oxidative addition of the C2Cl bond of a 2‐halogenato‐benzimidazole is much more facile and allows a rich follow‐up chemistry, such as the N ‐protonation or N ‐alkylation of the NR,N‐NHC ligand. Particularly complexes containing “protic” NHC ligands can be obtained easily this way and these have recently attracted interest as bifunctional catalysts 14…”

The NH Functional Group in Organometallic Catalysis

“…Very recently, Grotjahn found that Ru complex 116 , ligated by a protic N‐heterocyclic carbene–phosphine ligand, was active for transfer hydrogenation of acetophenone (Scheme ) 72. Although the NH moiety does not coordinate directly with the ruthenium center as in the Ru(phosphine) 2 –diamine system, a similar bifunctional mechanism was proposed for the transfer hydrogenation of ketones catalyzed by 116 .…”