Chiral Palladium(II) Complexes Possessing a Tridentate N‐Heterocyclic Carbene Amidate Alkoxide Ligand: Access to Oxygen‐Bridging Dimer Structures

Abstract

N-Heterocyclic carbenes (NHCs) and their palladium complexes have been developed to facilitate the formation of carbon-carbon and carbon-heteroatom bonds.[1] NHC complexes exhibit unique chemical properties such as strong Pd-NHC s bonding, which enhances the stabilities of active organometallic compounds relative to conventional phosphane complexes.[2] Moreover, chiral NHC ligands have been synthesized to promote asymmetric catalysis. [3] Most of the chiral NHC ligands that have been utilized for asymmetric Pd II catalysis are monodentate, as Lee and Hartwig demonstrated moderate enantioselectivities (71-76 % ee) in a-arylation.[4] However, monodentate ligands caused practical difficulties including concomitant formation of inactive palladium-ligand complexes, such as those with a trans conformation. Bidentate NHC ligands exhibited better stabilities and selectivities: Douthwaite reported better enantioselectivities (up to 92 % ee) for asymmetric allylic alkylation [5a, b] than reactions employing the corresponding monodentate ligand.[5c]We envisioned tridentate NHCs would enhance the stabilities of Pd II complexes and enantioselectivities of various asymmetric reactions. As depicted in Figure 1, we sought a "chiral {Pd(OAc) 2 } complex" and designed and synthesized novel chiral tridentate NHC-Pd II complexes (II). Notably, ligand systems with NHCs, amidates, and oxygen functionalities (a C,N,O triad) could exert high electron densities and strong coordination on the Pd II complexes to increase stabilities even in nucleophilic solvents such as water and alcohols. Therefore, labile ligands such as water, alcohols, and acetonitrile are likely to be removed easily and thus enhance the reactivities and efficiencies of NHC-Pd catalysts.We report herein the synthesis of chiral tridentate NHC-Pd II complexes and their applications in an asymmetric oxidative Heck-type reaction as a proof of concept.The preparation of chiral ligands 4 is shown in Scheme 1. Hydroxyamide compounds 2 were prepared by reduction of amino acids 1 and subsequent N-alkylation with bromoacetyl bromide. Treatment of 2 with benzimidazole in the presence of KOH in DMF provided compounds 3, which were subjected to methylation to yield the amido alcohol substituted benzimidazolium salts 4. The structure was confirmed by 1 H NMR spectroscopic analysis; new peaks assigned to the NCH 3 appeared at d = 4.18 (4 a) and 4.15 ppm (4 b). Also, the imidazole H resonances shifted significantly as expected for iodine salts, appearing at d = 9.55 (4 a) and 9.50 ppm (4 b).Because direct coordination of ligands 4 to palladium was not efficient under numerous conditions, the ligands were transferred to palladium via silver NHC complexes.[6] As described in Scheme 2, compounds 4 a and 4 b were treated with Ag 2 O in CH 2 Cl 2 to give silver NHC complexes.

Tridentate, anionic tethered N-heterocyclic carbene of Pd(II) complexes

“…The reaction mixture was stirred under refluxing for 15 h. After cooling the solution at room temperature, a white solid, which is the desired product, was filtrated and then washed with THF. These compounds were reported previously [10]. 6, 137.6, 123.6, 122.9, 61.0, 56.4, 50.6, 35.8, 28.2, 19.5, 18.1.…”

“…The filtrate was concentrated under reduced pressure, which was purified by column chromatography on silica gel using AcOEt followed by MeOH as an eluent to afford 2 or 5 as a white solid. These compounds were reported previously [10]. (d, J = 6.6 Hz, 3H), 0.90 (d, J = 6.6 Hz, 3H); 13 C NMR: d 169.3,139.4,128.9,121.7,62.9,58.3,50.3,30.0,19.9,18.7.…”

“…Recently, we developed a novel chiral tridentate NHC-ligand and their Pd(II) complexes (Scheme 1) [10]. Importantly, the Pd(II) complex derived from chiral b-amino alcohol catalyzes an asymmetric oxidative Heck-type reaction with excellent enantioselectivity (up to 98% ee).…”

Synthesis of pincer-type N-heterocyclic carbene palladium complexes with a hemilabile ligand and their application in cross-coupling catalysis

“…Alternatively, the presence of HOAc as the product from chelation of the carboxylate when starting with complex 5 may induce nonproductive side reactions such as hydro-dehalogenation. Attempts to improve catalysis by abstracting the chloride ligand in complex 7 with AgBF 4 only gave catalytically inactive species and no product formation was observed even after 24 h. This result may be due to the formation of a bimetallic system [27]. General reaction conditions: styrene (560 µmol), NaOAc (440 µmol), bromoanisole (400 µmol) and palladium complex (1 mol%) in DMA (3.0 mL) at 50°C, 2 h; NMR spectroscopic yield.…”

Synthesis, Characterisation and Antimicrobial Activity of Dimeric and Monomeric Copper(II)-Schiff base Complexes

“…The literature provide with information on dimeric and monomeric square planar palladium(II) derivatives of N-heterocyclic carbene amidate alkoxide having both N-Pd-O chelating stretching modes. The Xray structural analysis of these palladium compounds showed that the dimer has shortened metal oxygenbridging bond length in comparison to the oxygen-terminal bond in the monomer [19]. The structural determination of dimeric square pyramidal copper(II) compounds derivative of N-(salicylidene)-N'-(imidazol-2-ylmethylene)-1,3-propanediamine) also showed elongated metal oxygen-bridging bond length in addition to shortened ones which are associated with the axial or equatorial positon of the oxygen atoms within the structure.…”