Changsheng Cao

Synthesis and characterization of novel chiral NHC-palladium complexes and their application in copper-free Sonogashira reactions.

A new series of chiral N-heterocyclic carbene (NHC) palladium complexes were synthesized from a relatively inexpensive amino acid, l-phenylalanine. All these compounds were fully characterized by (1)H-NMR, (13)C-NMR and elemental analysis. The X-ray molecular structures of two of the complexes were reported. The catalytic activity of the four palladium complexes was successfully tested in the Sonogashira reaction under copper free conditions in air. The palladium complex 3a provided good activity in the Sonogashira coupling reaction.

Efficient synthesis of quinazoline-2,4(1H,3H)-diones from CO2 catalyzed by N-heterocyclic carbene at atmospheric pressure

Under atmospheric pressure, quinazoline-2,4(1H,3H)-diones were obtained from the reaction of 2-aminobenzonitriles with carbon dioxide (0.1 MPa) with a catalytic amount of N-heterocyclic carbene in DMSO. It was found that various electron-donating and electron-withdrawing groups such as –OMe, –F, –Cl, –Br, –CH3, –CF3 and –CN were well tolerated to give the products in almost quantitative yields.

Simple, efficient and reusable Pd–NHC catalysts for hydroamination

A series of chelating NHC–palladium complexes with different alkane-bridges of the type Pd[NHC–(CH2)n–NHC]X2 (X = Br or Cl, n = 2–4) were synthesized, where NHC is a triazolyl-N-heterocyclic carbene donor ligand. The bromide complexes with n = 2 and 3 were characterized by X-ray crystallography. The effects of the length of the bridge and halide ligand on the catalytic reactivity in the hydroamination reaction were investigated. The best catalytic performance was observed with the propylene-bridged Pd–NHC bromide complex 2. The hydroamination of phenylacetylene or 4-methylphenylacetylene with various substituted anilines catalyzed by 2 yielded only Markovnikov addition products in good yield. Interestingly, this homogenous catalyst can be reused three times without significant loss in activity.

Effective, transition metal free and selective C–F activation under mild conditions

A simple and effective aromatic nucleophilic monosubstitution reaction for the synthesis of aromatic amines via selective C–F bond cleavage of various fluoroarenes (mono-, di-, tri-, tetra-, penta- and perfluorobenzene) with primary and secondary aromatic amines under transition metal free conditions is demonstrated. The reaction conditions were investigated thoroughly for a wide range of fluoroarene substrates bearing different numbers of fluorine atoms, and the results showed that the solvent and reaction temperature are very crucial for the successful substitution reactions. The established methods enabled the formation of nonfluorinated and partially fluorinated aromatic amines in good to excellent yields. Several functional groups were tolerated under the optimized conditions.

Cheap Cu(I)/Hexamethylenetetramine (HMTA) Catalytic System for C-N Coupling Reactions

Abstract An efficient C-N coupling reaction of aryl chlorides or bromides with imidazole was successfully performed with cheap and commercially available catalytic system Cu(I)/HMTA. Both aryl bromides and aryl chlorides have been effectively catalyzed by Cu/HMTA, giving products in moderate to good yields. Moreover, aryl halides bearing either electron-withdrawing groups or electron-donating groups could be used in the reaction. GRAPHICAL ABSTRACT

Palladium complexes with picolyl functionalized N-heterocyclic carbene ligands and their application in the Mizoroki–Heck reaction

A series of Pd complexes of picoline-functionalized N-heterocyclic carbenes with different substituents were synthesized. The molecular structures of selected complexes were determined by X-ray diffraction studies, showing a pseudo-square-planar structure with a Pd center surrounded by carbene, pyridine, and two chloride ligands. The influence of the different substituents on the structure and reactivity of the complexes has been studied. The catalytic properties of the complexes were investigated in the Mizoroki–Heck reaction for cross-coupling of bromobenzene with a variety of aryl halides. Their performances varied in these reactions but showed good product regioselectivities.

Synthesis of fluorinated carbazoles via C–H arylation catalyzed by Pd/Cu bimetal system and their antibacterial activities

An effective intramolecular C-H arylation reaction catalyzed by a bimetallic catalytic system Pd(OAc)2/CuI for the synthesis of fluorine-substituted carbazoles from corresponding N-phenyl-2-haloaniline derivatives under ligand free conditions is demonstrated. The established method is effective for both N-phenyl-2-bromoaniline and N-phenyl-2-chloroaniline, and requires the low loading of Pd(OAc)2 (0.5 mol%). A series of new fluorinated carbazoles were synthesized in excellent yields using the protocol (>83%, 19 examples) and were fully characterized by (1)H, (13)C and (19)F NMR spectral data, HRMS and elemental analysis. All compounds were evaluated for their antibacterial activities against four bacteria (Bacillus subtilis, Staphylococcus aureus, Escherichia coli and methicillin-resistant S. aureus with resistance to gentamicin) by serial dilution technique. All tested compounds showed antibacterial activity against three test strains (S. aureus, B. subtilis and MRSA), and most of these compounds displayed pronounced antimicrobial activities against these three strains with low MIC values ranging from 0.39 to 6.25 μg/mL. Among them, compounds 7 and 14 exhibited potent inhibitory activity better than reference drugs meropenem and streptomycin. Three compounds (2, 4 and 5) showed antibacterial activity against E. coli. with MIC values from 12.5 to 25 μg/mL.

Synthesis of novel chiral unsymmetrical imidazolinium bromides

N-heterocyclic carbenes have been successfully used as ligands in organometallic catalysis, and also as organocatalysts in different reactions. A series of novel chiral unsymmetrical imidazolinium salts which are precursors of N-heterocylic carbenes were synthesised effectively from the relatively inexpensive amino acid L-phenylalanine in six steps. These chiral unsymmetrical imidazolinium bromides were fully characterised by NMR and elemental analysis, and their specific rotations were obtained.

The regioselective Larock indole synthesis catalyzed by NHC–palladium complexes

The first practical and regioselective process for the synthesis of 2,3-disubstituted indoles from the reaction of o-iodoanilines or o-bromoanilines and their derivatives with symmetrical and unsymmetrical internal alkynes catalyzed by a ferrocene-functionalized N-heterocyclic carbene (NHC)–palladium complex has been developed, and the indoles were isolated in good yields with high regioselectivity.

Green Synthesis of Alkane Bridged Bisimidazolium Salts Under Solvent-Free Conditions

Abstract A series of N-aryl or N-methyl substituted bisimidazolium dichloride or dibromide salts with different alkyl bridge lengths (n = 1–4) have been synthesized starting from N-arylimidazoles or N-methylimidazoles with dialkyl bromides or the less expensive but less reactive dialkyl chlorides under solvent-free conditions. This green protocol provided most bisimidazolium chloride and bromide salts in good yields, and the reaction time was shortened to a few hours. In two examples, we showed either bisimidazolium dichloride (1a) or dibromide salt (3c) can be easily converted to the corresponding dihexafluorophosphate salt (1a′ or 3c′) in near quantitative yield. GRAPHICAL ABSTRACT