Nucleophilic Iron Complexes in Proton-Transfer Catalysis: An Iron-Catalyzed Dimroth Cyclocondensation.

The nucleophilic iron complex Bu4 N[Fe(CO)3 (NO)] (TBA[Fe]) is an active catalyst in C-H-amination but also in proton-transfer catalysis. Herein, we describe the successful use of this complex as a proton-transfer catalyst in the cyclocondensation reaction between azides and ketones to the corresponding 1,2,3-triazoles. Cross-experiments indicate that the proton-transfer catalysis is significantly faster than the nitrene-transfer catalysis, which would lead to the C-H amination product. An example of a successful sequential Dimroth triazole-indoline synthesis to the corresponding triazole-substituted indolines is presented.

Gold-catalyzed tandem reactions of methylenecyclopropanes and vinylidenecyclopropanes.

Gold catalysis is often the key step in the synthesis of natural products, and is a powerful tool for tandem or domino reaction processes. Both gold salts and complexes are among the most powerful soft Lewis acids for electrophilic activation of carbon-carbon multiple bonds toward a variety of nucleophiles. The core of these reactions relies on the interaction between gold catalysts and π-bonds of alkenes, alkynes, and allenes. Activation of functional groups by gold complexes provides a useful and important method for facilitating many different organic transformations with high atom efficiency. Although they are highly strained, methylenecyclopropanes (MCPs) and vinylidenecyclopropanes (VDCPs) are readily accessible molecules that have served as useful building blocks in organic synthesis. Because of their unique structural and electronic properties, significant developments have been made in the presence of transition metal catalysts such as nickel, rhodium, palladium, and ruthenium during the past decades. However, less attention has been paid to the gold-catalyzed chemistry of MCPs and VDCPs. In this Account, we describe gold-catalyzed chemical transformations of MCPs and VDCPs developed both in our laboratory and by other researchers. Chemists have demonstrated that MCPs and VDCPs have amphiphilic properties. When MCPs or VDCPs are activated by a gold catalyst, subsequent nucleophilic attack by other reagents or ring-opening (ring-expansion) of the cyclopropane moiety will occur. However, the C-C double bonds of MCPs and VDCPs can also serve as nucleophilic reagents while more electrophilic reagents are present and activated by gold catalyst, and then further cascade reactions take place as triggered by the release of ring strain of cyclopropane. Based on this strategy, both our group and others have found some interesting gold-catalyzed transformations in recent years. These transformations of MCPs and VDCPs can produce a variety of polycyclic and heterocyclic structures, containing different sized skeletons. Moreover, we have carried out some isotopic labeling experiments and computational studies for mechanistic investigation. These reactions always give the desired products with high level control of chemo-, regio-, and diastereoselectivities, making them highly valuable for the synthesis of natural products and to the pharmaceutical industry and medicine in general.

Rhodium(I)-catalyzed cycloisomerization of nitrogen-tethered indoles and alkylidenecyclopropanes: convenient access to polycyclic indole derivatives.

At the end of its tether: A new synthetic protocol for the preparation of polycyclic indole derivatives has been developed from a rhodium(I)-catalyzed cycloisomerization of a nitrogen-tethered indole and alkylidenecyclopropane, affording the corresponding tetrahydro-ß-carboline derivatives in moderate to good yields. Further transformations give a direct and rapid route to tetracyclic compounds through transition-metal catalysis.

Transition metal-catalyzed carbocyclization of nitrogen and oxygen-tethered 1,n-enynes and diynes: synthesis of five or six-membered heterocyclic compounds.

Cycloisomerization of 1,n-enynes and diynes is a powerful method in organic synthesis to access heterocyclic compounds and has drawn increasing attention from organic chemists. In this paper, we attempted to summarize our recent results on the transition metal-catalyzed cycloisomerization to synthesize five or six-membered heterocyclic compounds using 1,n-enynes and diynes having a propargylic ester moiety. First, we will describe the synthesis of 2,3-disubstituted 3-pyrrolines via gold catalyzed cycloisomerization of 1,6-diynes. In addition, we will also disclose a novel silver catalyzed tandem 1,3-acyloxy migration/Mannich-type addition/elimination of the sulfonyl group of N-sulfonylhydrazone-propargylic esters to 5,6-dihydropyridazin-4-one derivatives. Furthermore, we will introduce three interesting examples of the synthesis of bicyclic compounds via titanium or rhodium catalyzed carbocyclization of enynes. In this context, we have presented that 1,n-enynes and diynes containing propargylic esters are highly reactive and useful starting materials for the cycloisomerization catalyzed by a transition metal catalyst.

Gold(I)-catalyzed intramolecular hydroamination and ring-opening of sulfonamide-substituted 2-(arylmethylene)cyclopropylcarbinols.

An interesting gold(I)-catalyzed intramolecular hydroamination and ring-opening of sulfonamide-substituted 2-(arylmethylene)cyclopropylcarbinols has been described in this context. A variety of 4-substituted isoxazolidine derivatives were obtained in good to high yields through a highly regioselective cleavage of a carbon-carbon single bond in the cyclopropane.

[Characteristics of polycyclic aromatic hydrocarbons (PAHS) on airborne particulates in Beijing].

Total suspended particulates (TSP) samples were collected from Sep., 2003 to Jul., 2004 in Beijing, and 15 kinds of PAHs, ranging from 3 to 7 rings were analyzed. The maximum concentrations sigma PAHs and BaP were 705 ng/m3 and 52 ng/m3 respectively. Average sigma PAHs concentrations in four seasons were 46 ng/m3, 16 ng/m3, 52 ng/m3 and 268 ng/m3 respectively; and the average BaP concentrations in four seasons were 2.8 ng/m3, 0.23 ng/m3, 3.3 ng/m3, 16 ng/m3 respectively. Regarding to the meteorological parameters, precipitation distinctly lowered the concentration; in heating period, PAHs concentrations fall with the temperature goes up, but there is no obvious relation between concentration and temperature in non-heating period; the increase of wind speed level causes the decrease of PAH concentrations in the heating period, but relation between PAH concentrations and wind speed varied with aromatic rings of PAHs and levels of wind speed in the non-heating period.

Polycyclic aromatic hydrocarbons (PAHs) in the aerosol in Beijing, China, measured by aminopropylsilane chemically-bonded stationary-phase column chromatography and HPLC/fluorescence detection.

We developed a useful analytical method for the determination of polycyclic aromatic hydrocarbons (PAH) concentrations in the aerosol of China. We used an accelerated solvent extraction (ASE) method for the extraction of PAHs from the aerosol samples, in order to reduce the extraction time and the solvent volume used. The optimum purification method was developed, with aminopropylsilane chemically-bonded stationary-phase column chromatography, in order to remove many co-extractives which cannot be removed by conventional purification methods using silica-gel column chromatography. HPLC/fluorescence detection (FLD) was adopted as the analytical method, because it has very high sensitivity to PAH and it is easy to install, operate, and maintain as compared with GC/MS. With the analytical method developed in this study, the recovery and precision (RSD) for most of the PAHs ranged from 75% to 129% and from 2.8% to 22.7%, respectively. The concentrations of PAHs in the aerosol samples collected from October 2003 to April 2005 in Beijing, China were determined using the newly developed method. SigmaPAHs, which is the sum of the concentrations of all detected PAHs, was 177.8 +/- 239.9 ng m(-3) (n = 64). The SigmaPAHs concentration in the heating season (305.1 +/- 279.0 ng m(-3), n = 33) was 7.2 times higher than that in the non-heating season (42.3 +/- 32.0 ng m(-3), n = 31). These strong seasonal variations in atmospheric PAH concentration are possibly due to coal combustion for residential heating in winter.