Kaikai Wu

Pt-Sn/γ-Al2O3-catalyzed highly efficient direct synthesis of secondary and tertiary amines and imines.

Versatile syntheses of secondary and tertiary amines by highly efficient direct N-alkylation of primary and secondary amines with alcohols or by deaminative self-coupling of primary amines have been successfully realized by means of a heterogeneous bimetallic Pt-Sn/γ-Al(2)O(3) catalyst (0.5 wt % Pt, Pt/Sn molar ratio=1:3) through a borrowing-hydrogen strategy. In the presence of oxygen, imines were also efficiently prepared from the tandem reactions of amines with alcohols or between two primary amines. The proposed mechanism reveals that an alcohol or amine substrate is initially dehydrogenated to an aldehyde/ketone or NH-imine with concomitant formation of a [PtSn] hydride. Condensation of the aldehyde/ketone species or deamination of the NH-imine intermediate with another molecule of amine forms an N-substituted imine which is then reduced to a new amine product by the in-situ generated [PtSn] hydride under a nitrogen atmosphere or remains unchanged as the final product under an oxygen atmosphere. The Pt-Sn/γ-Al(2)O(3) catalyst can be easily recycled without Pt metal leaching and has exhibited very high catalytic activity toward a wide range of amine and alcohol substrates, which suggests potential for application in the direct production of secondary and tertiary amines and N-substituted imines.

Highly Active Ruthenium(II) Complex Catalysts Bearing an Unsymmetrical NNN Ligand in the (Asymmetric) Transfer Hydrogenation of Ketones

N-Heterocylic ligands have recently become more and more attractive in homogeneous catalysis and organic synthesis because their organometallic complexes usually exhibit higher reactivity and better stability than those with phosphine ligands. Tridentate NNN ligands such as Pybox, 2,6-bis ACHTUNGTRENNUNG(imino)pyridines,[3] terpyridines, and other symmetrical NNN ligands have demonstrated their potential applications. However, examples of unsymmetrical tridentate NNN ligands and their transition-metal complexes are scarce. Transition-metal complexes bearing an unsymmetrical polydentate ligand are usually bestowed with good catalytic activity. Thus, unsymmetrical pyridyl-based NNN ligands are strongly desired for the construction of highly active transition-metal complex catalysts. Functional metal complexes were obtained with symmetrical bis(benzimidazol-2-yl)pyridines and bis(N-alkylbenzimidazol-2-yl)pyridines, and the electronic properties of the respective ruthenium complexes have been described. However, only a few of them have been applied as catalysts. We recently synthesized very efficient Ru pyridyl–pyrazolyl-based NNN complex catalysts for transfer hydrogenation of ketones. Ru-catalyzed transfer hydrogenation reactions with 2-propanol as the hydrogen source have been extensively studied by Noyori et al. Baratta’s group recently reported highly active Ru 2-(aminomethyl)pyridinylphosphane complex catalysts for the transfer hydrogenation of ketones. A few Ru complex catalysts featuring no N H functionality have also been documented for the same purpose. Herein, we report the construction of unsymmetrical (chiral) pyridyl– benzimidazolyl-based NNN ligands and their Ru complex catalysts for the (asymmetric) transfer hydrogenation of ketones by following the strategy shown in Scheme 1. The mono-N-alkyl derivative of bis(benzimidazol-2-yl)pyridine (1), that is, 2, and bis(N-propylbenzimidazol-2yl)pyridine (3) were obtained by reacting 1 with 1-bromopropane in the presence of NaH (Scheme 2). Reactions of ligands 1–3 with one equivalent of [Ru ACHTUNGTRENNUNG(PPh)3Cl2] in refluxing methanol afforded Ru complexes 4–6 in 76–91 % yields. Treatment of 5 with K2CO3 base in CH2Cl2 formed

Effect of Alumina Support on Catalytic Performance of Pt-Sn/Al2O3 Catalysts in One-Step Synthesis of N-Phenylbenzylamine from Aniline and Benzyl Alcohol

The effect of alumina on the catalytic performance of Pt‐Sn/Al2O3 catalysts in the green synthesis of secondary amines by N‐alkylation of amines with alcohols based on the borrowing hydrogen strategy was investigated. N‐alkylation of aniline with benzyl alcohol to produce N‐phenylbenzylamine was used as a model reaction. Three different alumina supports were selected, and the corresponding catalysts were prepared by complex impregnation under vacuum. The supports and catalysts were characterized using N2 adsorption‐desorption, mercury intrusion porosimetry, X‐ray diffraction, transmission electron and scanning electron microscopies, CO chemisorption, H2 temperature‐programmed reduction, and NH3 temperature‐programmed desorption. The results show that the catalysts with small Pt particles that were highly dispersed on the alumina supports and interacted weakly with the supports had high catalytic activities. The large pore volumes and pore size distributions of the alumina supports helped diffusion and adsorption of the reactants on the catalyst surface and increased the catalytic activity; they also promoted removal of the products from the catalyst surface and enhanced the catalytic stability. However, strong acidity and acid distribution of the alumina supports decreased the selectivity for secondary amines and reduced the catalyst stability.

Iron-Catalyzed Oxidative C–H Functionalization of Internal Olefins for the Synthesis of Tetrasubstituted Furans

Tetrasubstituted furans were efficiently synthesized from Fe(OAc)2-catalyzed C-H/C-H cross-dehydrogenative-coupling (CDC) reactions of activated carbonyl methylenes with S,S-functionalized internal olefins, that is, α-oxo ketene dithioacetals and analogues, under oxidative conditions. β-Ketoesters, 1,3-dicarbonyls, β-keto nitrile, and amide derivatives were used as the coupling partners. The resultant alkylthio- and carbonyl-functionalized furans could be further transformed to diverse arylated furan derivatives and furan-fused N-heterocycles, respectively. The control experiments have revealed a radical reaction pathway.

Brønsted acid catalyzed PhSe transfer versus radical aryl transfer: linear codimerization of styrenes and internal olefins.

Brønsted acid p-TsOH·H2O-catalyzed hydrovinylation of styrenes with internal olefins α-oxo ketene dithioacetals was efficiently achieved in the presence of N-phenylselenophthalimide (N-PSP), regioselectively affording Markovnikov phenylselenative hydrovinylation products through PhSe transfer of the phenylseleno ketene dithioacetal intermediates. Radical reduction of the resultant PhSe-alkyl-substituted ketene dithioacetals with AIBN/n-Bu3SnH gave the corresponding anti-Markovnikov hydrovinylation products formally from the linear codimerization of styrenes and the internal olefins.

Brønsted Acid‐Mediated Annulation of α‐Oxo Ketene Dithioacetals with Pyrroles: Efficient Synthesis of Structurally Diverse Cyclopenta[b]pyrroles

Brønsted acid-mediated annulation of internal olefins α-oxo ketene dithioacetals to pyrroles was efficiently achieved to afford cyclopenta[b]pyrroles. A pair of Brønsted acids with acid strengths, that is, trifluoroacetic acid, and para-toluenesulfonic acid hydrate, were applied to promote the annulation reactions. The resultant products were readily oxidized to sulfones by meta-chloroperoxybenzoic acid. Subsequent treatment with 1,8-diazabicyclo[5.4.0]undec-7-ene gave desulfurized terminal olefins or [2+2] cycloaddition products from the desulfurized olefin intermediates. The present protocol provides facile access to structurally diverse cyclopenta[b]pyrrole derivatives under mild conditions.

Iron-Promoted Difunctionalization of Alkenes by Phenylselenylation/1,2-Aryl Migration

Iron-promoted difunctionalization of α,α-diaryl and α-aryl-α-alkyl allylic alcohols has been efficiently achieved by means of N-(phenylseleno)phthalimide (N-PSP) under mild conditions. An in situ generated phenylselenium cation (PhSe+) was added to the olefinic C═C bond to initiate the regioselective phenylselenylation with concomitant 1,2-aryl migration, following a migration preference contrary to the well-known radical pathway. Hydrazonation of the resultant alkene difunctionalization products, that is, α-aryl-β-phenylselenyl ketones, and subsequent copper-catalyzed dehydroselenylation efficiently afforded functionalized 2-pyrazoline derivatives.