ABSTRACT
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
The use of olefin metathesis as a construction tool for multimetallic salen-based structures is described. The approach involves mono- and diallyl-functionalized metallosalen complexes that can be directly coupled by metathesis leading to dimetallic species or mixtures of linear and cyclic oligomers. The metathesis of bis-allyl Ni(salen) complexes has been studied in detail. At high concentration it is possible to selectively obtain di-Ni species rather than heavier oligomers while under dilute conditions cyclic rather than linear oligomers are preferentially obtained. A mono-allyl Zn(salphen) complex was efficiently coupled using metathesis to give the di-Zn(salphen) product, which was subsequently transmetalated with a variety of metals to yield dimetallic salens of potential catalytic interest. Finally, a tetranuclear Zn(4) macrocycle was also prepared using buildings blocks obtained by metathesis from commercially available precursors. The methods described herein allow for the facile construction of multi-centered Schiff base complexes of catalytic or supramolecular interest.
ABSTRACT
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.
Subject(s)
Chelating Agents/chemistry , Organometallic Compounds/chemistry , Catalysis , Molecular StructureABSTRACT
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.
ABSTRACT
The construction of novel (macrocyclic) multinuclear Co(III)salen catalysts is reported. Olefin metathesis has been used as a key construction tool for the multimetallic structures starting from versatile allyl-substituted salen scaffolds. The Co(III) complexes were tested in the hydrolytic kinetic resolution of (rac)-1,2-epoxyhexane and epoxide ring opening reactions using methanol as the nucleophile. The preliminary results suggest a cooperative mode of catalysis in the case of bis-Co(III)salen macrocycle 10.
ABSTRACT
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.
Subject(s)
Alcohols/chemistry , Epoxy Compounds/chemistry , Halogens/chemistry , Catalysis , Cyclization , Kinetics , Molecular Structure , StereoisomerismABSTRACT
3-Alkenyl and heteroaryl chloroalcohols have been obtained in excellent enantiomeric excess (>99%) by enzymatic kinetic resolution using the haloalcohol dehalogenase HheC. Yields were close to the theoretical maximum for all substrates employed. Furthermore, the applicability of this methodology on multigram scale has been established.
Subject(s)
Alcohols/chemistry , Chlorine/chemistry , Catalysis , Chromatography, Thin Layer , Hydrolases/chemistry , Hydrolysis , Kinetics , Models, Chemical , Molecular Conformation , Molecular Structure , Stereoisomerism , TemperatureABSTRACT
A conceptually new, simple and practical method for the syn-nucleophilic displacement of aryl and vinyl epoxides and aryl aziridines with (substituted) phenols, using aryl borates as activating nucleophiles under neutral conditions, is reported.
