Metallosalen complexes are able to catalyze several transformations via a bimetallic, cooperative pathway. Various strategies to induce simultaneous activation by two metal centers with multinuclear catalysts have been reported to date, leading to higher reaction rates and selectivities in existing reactions. In some cases, new reactions could be realized that were previously not possible. The design principles, the most successful approaches and the catalytic applications of multinuclear metallosalen catalysts are discussed in this feature article.

Monodentate phosphoramidites based on BINOL or substituted BINOL are excellent ligands for the rhodium-catalysed asymmetric hydrogenation of olefins. Very high enantioselectivities were obtained with MonoPhos (7a) the simplest member of this class, a ligand that is prepared in a single step from BINOL and HMPT. Turnover numbers up to 6000 have been obtained in the hydrogenation of dehydroamino acid derivatives. Enantioselectivities in the hydrogenation of dehydroamino acids are solvent dependent; in non-protic solvents they range from 95 ± 99%. Itaconic acid and its dimethyl ester could be hydrogenated with 96 and 94% e.e., respectively. Hydrogenation of aromatic enamides gave the corresponding acylated amines in 86 ± 94% e.e. Several analogous phosphoramidite ligands have been prepared. Surprisingly, bidentate ligands gave poorer results, both in terms of rate as well as enantioselectivity. Taddol-based phosphoramidites led to poor e.e. and slow rates. Methyl substituents at the 3,3'-position of BINOL led to a sharply reduced rate and a somewhat lower enantioselectivity. Bromo substituents at the 6,6'-position led to a slightly reduced rate but little effect was seen on enantioselectivity. Use of octahydro-MonoPhos (11) gave results that were very similar to those obtained with 7a. The rate of the reaction is dependent on the hydrogen pressure, however, the enantioselectivity is not affected. The rate of the dehydroamino acid hydrogenation also increases if the ligand to rhodium ratio is reduced from 2.2 to 1.5 or even to 1.0; yet, there is no deleterious effect on the enantioselectivity. Catalytic activity ceases with L/Rh 3 when dehydroamino acid derivatives were used as substrate. The reaction shows a positive non-linear effect, which confirms the presence of Rh-complexes with more than one ligand. Following the hydrogenation of methyl 2-acetamidocinnnamate with Rh(nbd) 2 BF 4 /7a by electrospray mass spectrometry showed the presence of several rhodium species. Notable are the presence of [Rh(7a)] 3 and [Rh(7a)] 4 . There is at present insufficient evidence to conclude if the active catalytic species carries one or two ligands. In view of the low cost of MonoPhos this invention might well lead to a broader application of asymmetric olefin hydrogenation for the production of enantiopure amino acids and amines.

The direct chemo-enzymatic DKR of racemic beta-haloalcohols is reported, yielding the corresponding optically active epoxides in a single step. The mutant haloalcohol dehalogenase HheC Cys153Ser Trp249Phe is used for the asymmetric ring closure, whereas racemization of the remaining enantiomer of the haloalcohol is achieved using the new iridacycle 3, one of the most effective racemization catalysts to date for beta-haloalcohols.

One hand mapping: By means of scanning tunneling microscopy, solvent‐induced homochirality is shown to emerge in self‐assembled monolayers of achiral molecules at the liquid–solid interface (see picture). The chirality of the solvent directs the macroscopic chirality of the monolayer. The dynamics of the monolayer structure as it evolves towards homochirality are probed by time‐dependent measurements.

The self-assembly features of a series of (non)symmetrical Zn(salphen) complexes have been studied in detail by X-ray crystallography, NMR and UV-vis techniques. The combined data demonstrate that the stability of these dimeric assemblies and the relative position of each monomeric unit within the dinuclear structure depend on the location and combination of the aromatic ring substituents.

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We describe a selective and template-controlled synthesis of a series of Zn(8) metal complexes based on a bis-nucleating salen ligand scaffold. Our results, a combination of X-ray analysis and solution studies, show that discrete, shape-persistent metal clusters can be prepared in high yield. Their activity in organic carbonate catalysis is a function of the metal-connecting fragment present in the exterior of the cluster complex. The high stability of the clusters has been confirmed by (1)H, (13)C (DEPTQ) and DOSY NMR, gel permeation chromatography, high-performance liquid chromatography, and mass spectrometry.

ABSTRACT:New catalysts based on Yb(salen) complexes active for the copolymerization of cyclohexene oxide (CHO) and CO 2 to give poly(cyclohexene)carbonate (PCHC) are reported. In combination with co-catalytic, nucleophilic chloride additives these new (binary) catalysts provided good conversion and selectivity for PCHC formation with average turnover frequencies of up to 35 h -1 and narrow molecular weight distributions. The best results were obtained with the binary catalyst system 1 (0.1 mol%)/NBu 4 Cl (0.05 mol%), and at 90 °C a conversion of 57% was reached after 18 h with a TOF of 31 h − 1 , and the polycarbonate had an M n of 10.2 Kg/mol and a PDI of 1.54. Comparative catalysis studies have also been performed with a series of literature systems based on transition metal/lanthanide salen complexes, and the newly presented catalysts show comparatively good activity as well as copolymerization selectivity. MALDI-ToF mass spectrometric analysis revealed that trace water contamination and/or traces of 1,2-cyclohexane-diol were responsible for chain transfer effects limiting to some extent the maximum molecular weights that can be achieved in the current reactor set up.4