Wanzhi Chen

Direct Synthesis of Iron, Cobalt, Nickel, and Copper Complexes of N-Heterocyclic Carbenes by Using Commercially Available Metal Powders†

The development of transition-metal-catalyzed organic transformations based on the first-row transition metals such as Fe, Co, Ni, and Cu is of importance because of their relatively low cost and toxicity relative to precious metals. The strongly sdonating N-heterocyclic carbenes (NHCs) offer a good opportunity to tune the reactivity and selectivity of transition-metal catalysts. A large number of NHC complexes of precious metals have recently been prepared and characterized, and their catalytic applications in various organic transformations have been widely studied. In contrast, catalytic processes based on NHC complexes of non-noble metals such as manganese, iron, and cobalt have not been well studied. 4] One of the reasons is that NHC complexes of the first-row metals have to be prepared from free carbenes or through reactions of basic metal acetates, alkoxides, or amides with imidazolium salts. The generation of free carbenes often requires special bases, and handling unstable free carbenes requires harsh conditions. The second route is also restricted because metal acetates, alkoxides, and amides are not always available, and such metal salts are moisturesensitive. The most popular approach to noble-metal–NHC complexes is the transmetalation of NHCs from silver complexes that can be synthesized by treating Ag2O with the imidazolium salts. However, the carbene-transfer reactions of silver–NHC complexes with light metal salts such as Mn, Fe, and Co are not simple. Thus, the development of practical and convenient synthetic approaches to NHC complexes of non-noble metals is highly desired. As a continuation of our research on the nickel and palladium chemistry of NHCs, we herein report practical and convenient synthetic procedures of divalent iron, cobalt, nickel, and copper complexes by direct reactions of metal powders with imidazolium salts or silver–NHC complexes. Treatment of [Ag3(L1)2(CH3CN)2](PF6)3 [8f] (L1 = bis(pyridylimidazolylidene)methane) in situ generated from bis(pyridylimidazoliumyl)methane dihexafluorophosphate [H2L1(PF6)2] and Ag2O with an excess of commercially available iron, cobalt, and nickel powders allowed the isolation of [M(L1)2](PF6)2 [M = Fe (1); Co (2); Ni (3)] in 51.7–94.1% yields (Scheme 1). Complexes 1–3 could also be obtained in

Room-temperature Kumada cross-coupling of unactivated aryl chlorides catalyzed by N-heterocylic carbene-based nickel(II) complexes.

The Kumada cross-coupling reaction of a variety of unactivated aryl chlorides, vinyl chlorides, and heteroaryl chlorides catalyzed by nickel(II) complexes containing pyridine-functionalized NHC ligands is described. The catalysts are so active that the reactions proceed at room temperature in excellent yields.

Direct C–N Coupling of Imidazoles and Benzylic Compounds via Iron-Catalyzed Oxidative Activation of C–H Bonds

Iron-catalyzed direct C-N bond formation between imidazoles and benzylic hydrocarbons is described. The reaction utilizes an inexpensive iron catalyst-oxidant system that is suitable for the coupling of a range of benzylic C-H bonds with various imidazoles.

Efficient Negishi coupling reactions of aryl chlorides catalyzed by binuclear and mononuclear nickel-N-heterocyclic carbene complexes.

We describe the first nickel-N-heterocyclic carbene catalyzed Negishi cross-coupling reaction of a variety of unactivated aryl chlorides, heterocyclic chlorides, aryl dichlorides, and vinyl chloride. The mononuclear and binuclear nickel-NHC complexes supported by heteroarene-functionalized NHC ligands are found to be highly efficient for the coupling of unactivated aryl chlorides and organozinc reagents, leading to biaryls and terphenyls in good to excellent yields under mild conditions. For all aryl chlorides, the binuclear nickel catalysts show activities higher than those of mononuclear nickel complexes because of possible bimetallic cooperative effect.

Copper-Catalyzed N-Methylation of Amides and O-Methylation of Carboxylic Acids by Using Peroxides as the Methylating Reagents

The copper-catalyzed N-methylation of amides and O-methylation of carboxylic acids by using peroxides as the methylating reagent are described. Various amides and carboxylic acids were methylated affording N-substituted amides and esters. Tentative mechanistic studies suggest that this reaction is likely to involve a radical process.

Iron-Catalyzed N-Alkylation of Azoles via Cleavage of an sp3 C–H Bond Adjacent to a Nitrogen Atom

Iron-catalyzed direct C-N bond formation between azoles and amides is described. The oxidative coupling reactions of sp(3) C-H bonds adjacent to a nitrogen atom in amides and sulfonamides with the N-H bond in azoles proceeded smoothly in the presence of FeCl(2) and di-tert-butyl peroxide (DTBP).

Ortho-Functionalization of 2-Phenoxypyrimidines via Palladium-Catalyzed C−H Bond Activation

The palladium-catalyzed direct acetoxylation and arylation of 2-aryloxypyrimidine has been described. The aromatic C-H bonds may be functionalized in moderate to excellent yields providing a facile method for the synthesis of phenol derivatives, which show antimycobacterial and herbicidal activities.

Cu-NHC-TEMPO Catalyzed Aerobic Oxidation of Primary Alcohols to Aldehydes

Imidazolium salts bearing TEMPO groups react with commercially available copper powder affording Cu-NHC complexes. The in situ generated Cu-NHC-TEMPO complexes are quite efficient catalysts for aerobic oxidation of primary alcohols into aldehydes. The catalyst is easily available, and various primary alcohols were selectively converted to aldehydes in excellent yields.

Copper-Catalyzed Synthesis of N-Fused Heterocycles through Regioselective 1,2-Aminothiolation of 1,1-Dibromoalkenes

The first aminothiolation of 1,1-dibromoalkene is described using an inexpensive copper/N,N-dimethylethylenediamine catalyst. The method provides a powerful means of using easily available 1,1-dihaloalkenes as precursors to fused heterocycles.

Pd-catalyzed efficient one-pot sequential cross-coupling reactions of aryl dihalides.

The palladium complex containing N-heterocyclic carbene ligands catalyzes one-pot sequential Heck/Suzuki, Heck/Heck, and Heck/Sonogashira coupling reactions of aryl dihalides to afford unsymmetrically substituted arenes in excellent yields.