Chiral Au Research Articles

Homogeneous versus Supported ONN Pincer‐Type Gold and Palladium Complexes: Catalytic Activity

The ONN-tridentate unsymmetrical pincer-type (S)-1-((6-(2-hydroxyphenyl)pyridin-2-yl)methyl)-N-methyl-N-(3-(triethoxysilyl)pro-pyl)pyrrolidine-2-carboxamide ligand has been synthesized by an easy method, in high purity and good yield. Its respective palladium(II) and gold(III) complexes have been prepared as air-stable solids. After deprotonation of the -OH group the coordination of the metal ion is completely stereospecific and gives rise to only one diastereoisomer. The complexes, immobilized onto ordered mesoporous silica (MCM-41), are shown to be very active catalysts in the hydrogenation of prochiral olefins (98% ee was achieved with the heterogenized chiral Au(MCM-41) complex), hydrosilylation, and Suzuki and Heck C-C coupling reactions, under mild conditions. The reactions were studied with the soluble catalysts as well as their heterogenized counterparts. The high accessibility introduced by the structure of the supports allows the preparation of highly efficient immobilized catalysts. The repeated use of the immobilized catalyst in four cycles demonstrates "homogeneous" catalysis with "heterogeneous" catalysts, thus reducing solvent waste and loss of precious metal and/or ligand.

Asymmetric Au(i)-catalyzed synthesis of bicyclo[4.1.0]heptene derivatives via a cycloisomerization process of 1,6-enynes

The enantioselective asymmetric gold-catalyzed cycloisomerization reactions of heteroatom tethered 1,6-enynes are conducted in the presence of a chiral cationic Au(i) catalyst ((R)-4-MeO-3,5-(t-Bu)(2)-MeOBIPHEP-(AuCl)(2)/AgOTf system) in toluene under mild conditions and lead to functionalized bicyclo[4.1.0]heptene derivatives in excellent enantiomeric excesses ranging from 90-98%.

Chirality transfer in nematic liquid crystals doped with (S)-naproxen-functionalized gold nanoclusters: an induced circular dichroism study

In a recent study, we found that nematic liquid crystals (N-LCs) doped with chiral (S)-naproxen-functionalized dodecane thiolate protected gold nanoclusters (Au2, Au3) or non-chiral alkyl thiolate protected Au clusters (Au1) produce thin film textures with characteristic uniform stripe patterns separated by areas of homeotropic alignment. While these textures closely resemble textures commonly observed for chiral nematic (N*-)phases with large helical pitch, so-called cholesteric finger textures, they originate from local concentration differences of the nanoclusters in the N-LC solvent. While areas with higher particle content form linear particle aggregates (stripe domains) due to the surface anchoring of the N-LC molecules to the cluster surface, areas of lower particle concentration give homeotropic alignment as a result of particles residing at the glass–N-LC interfaces. To elucidate and confirm a chirality transfer from the chirally modified gold clusters to the non-chiral N-LC, despite the complex thin film textures, we here present detailed induced circular dichroism (ICD) studies of thin films of 5CB doped with the three different Au clusters. These experiments revealed that the chiral Au nanoclusters (Au2, Au3) successfully transfer chirality to the N-LC host producing a chiral nematic phase (N*) with the opposite helical sense in comparison to the pure, organic chiral dopant dispersed in the same N-LC host. Thus, these results provide the first experimental proof for the usefulness of gold nanoclusters as chiral dopants for N-LCs. In contrast, for the non-chiral Au cluster, at a macroscopic level, no relationship between the cholesteric finger-like textures and chirality was found.

Preparation and characterization of two chiral Au(CN)2-based coordination polymers containing (1R,2R)-N,N′-dimethylcyclohexanediamine

Two chiral heterometallic coordination polymers have been prepared using enantiopure 1R,2R-(N,N′)-bismethylcyclohexanediamine (Me2cydm) as an ancillary ligand along with copper(II) cations and KAu(CN)2. The resultant polymers, Cu(Me2cydm)[Au(CN)2]2 (1) and Cu(Me2cydm)Au(CN)2(ClO4) (2), crystallize in the chiral space groups P21 and C2221, respectively. Polymer 1 consists of 1D zig-zag chains of copper atoms bridged by [Au(CN)2]− groups with pendant [Au(CN)2]− units holding the 1D chains through Au–Au interactions of 3.2799(13) A, generating an overall 2D array. Polymer 2 has both [Au(CN)2]− and ClO4− anions bridging Cu(II) centres in a 2D (4,4) corrugated net motif. Both polymers exhibit weak antiferromagnetic interactions down to 1.8 K with no ordering behaviour. Despite the weak forces holding the 2D sheets in place, the polymers did not exhibit any porosity or vapochromic behaviour, indicating that the structural networks are surprisingly rigid.

Optical Recognition of Surface Chirality at Au(hkl) Single Crystalline Surfaces by Second Harmonic Generation Rotational Anisotropy

Two-dimensional chirality at naturally chiral gold single crystalline surfaces was detected and characterized using optical second harmonic generation (SHG) measurements. SHG rotational anisotropy (SH-RA) patterns at Au(643)S and Au(643)R surfaces were mirror symmetric to each other. Systematic SH-RA measurements at chiral Au(hkl) surfaces with the same step and kink structures but different (111) terrace widths showed a linear correlation between surface step density and SH-RA fitting parameters arising from defects. These results indicate that SH-RA measurements provide information not only on surface chirality but also on density of surface defects.

Asymmetric Intramolecular Radical Vinylation Using Enantiopure Sulfoxides as Temporary Chiral Auxiliaries

A very diastereoselective addition−elimination sequence affords cyclopentane derivatives in high enantiomeric purities. The enantiopure sulfoxide unit serves as a very efficient temporary chiral au...

Synthesis and structure of stable chiral Au I complexes with N, N′-diaryl-α-naphthamidinium- N′-[2,3,4,5-tetra(methoxycarbonyl)cyclopentadien-1-y l] ylides

Novel N-amidine-functionalized cyclopentadiene gold(i) complexes [C5(CO2Me)4{ArNC(α-C110H7)NAr}]Au(PPh3) have been prepared, and their stable chiral structure with the Au-N bond [2.091(6) A] and the additional coordination Au‥π-system of the cyclopentadiene ring has been proved by X-ray crystallography and NMR spectroscopy.