We are interested in the synthesis of chiral organometallic complexes, with special emphasis on rare earth metals. The complexes are applied as catalysts in stereoselctive and enantioselective small molecule syntheses (e. g. hydroamination, hydrophosphination, hydrosilylation, hydrogenation), as well as catalysts for the stereospecific (co)polymerization of nonpolar and polar monomers.

The hydroamination is a highly atom-economical process in which an amine N-H functionality is added to an unsaturated carbon-carbon bond, either in an intermolecular or intramolecular fashion:

Significant research effort over the last decade has led to the development of numerous catalytic systems based on early and late transition metals. Unfortunately, most of these systems are limited to more special classes of substrates such as activated olefins, for example late transition metal catalysts are restricted to vinyl arenes (e.g. styrene), 1,3-dienes (e.g. cyclohexadiene), strained olefins (e.g. norbornene) and alkynes, using commonly aromatic amines (e.g. aniline).

For a comprehensive review on the hydroamination reaction see: Müller, T. E.; Hultzsch, K. C.; Yus, M.; Foubelo, F.; Tada, M. Chem. Rev. 2008, 108, 3795-3892.

Rare earth metal complexes have been shown to be among the most active and versatile catalysts. They usually react also with non-activated double bonds and simple amines. Most rare earth metal based catalyst systems have utilized cyclopentadienyl ligands as supporting ligand framework and chiral lanthanocene complexes have been applied for enantioselective hydroamination reactions. However, the latter complexes are not configurational stable under the conditions of the hydroamination reaction and readily epimerize via protolytic metal-cyclopentadienyl bond cleavage. This is of course undesirable for efficient asymmetric catalysis. Indeed, achievable enantioselectivities using this chiral lanthanocene catalyst system have been limited below 75% ee.

Giardello, M. A.; Conticello, V. P.; Brard, L.; Gagne, M.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10241

Therefore, non-metallocene catalysts are becoming increasingly important especially in hydroamination catalysis, because the design of the ligands is a crucial element in transition metal catalyzed reactions. Whereas modifications of cyclopentadienyl ligands can be preparatively very tedius, non-metallocene ligands are easily modified, allowing a facile catalyst tuning towards optimal catalytic activity and selectivity.
Our group has prepared various non-metallocene rare earth metal complexes, based on diamidoamine, biphenolate and binaphtholate ligands, which have been applied in diastereoselective and enantioselective hydroamination reactions. For example, cyclization of aminopentenes yield pyrrolidine products. Bicyclization of aminodienes can yield aza-bicyclo[2.2.1]heptanes.

Organometallics 2004, 23, 2601-2612

Tert-butyl substituted biphenolate complexes form homochiral or heterochiral dimeric species. The tert-butyl substituents are not sterically demanding enough to prevent complex aggregation :

Chem. Eur. J. 2003, 9, 4796-4810
Eur. J. Inorg. Chem. 2004, 4091-4101

Binaphtholate complexes with sterically demanding tris(aryl)silyl substituents remain monomeric. They are highly efficient catalysts for intramolecular hydroamination processes. Furthermore, they have been utilized in kinetic resolution of chiral aminoalkenes. One enantiomer of the substrate is cyclized faster than the other, resulting in an enrichment of the faster reacting enantiomer in the pyrrolidine product, while the slower reacting enantiomer remains is enriched in the starting material. Resolution factors of up to 19 have been achieved, allowing the recovery of enantiopure (>98 % ee) substrates in 30-40% yield (at a conversion of 60-70%).

Chem. Commun. 2004, 730-731

J. Am. Chem. Soc. 2006, 128, 3748-3759

Traditionally group 4 metal catalysts have been limited to the hydroamination of alkynes or allenes (with a few recent exceptions). We have developed a simple catalytic system for the intramolecular hydroamination of secondary aminoalkenes based on a readily available cationic zirconocene catalyst:

Angew. Chem. Int. Ed. 2004, 44, 5542-5546; Angew. Chem. 2004, 116, 5659-5663

The first asymmetric base-catalyzed hydroamination using a dimeric chiral diamidobinaphthyl dilithium salt was reported by us recently:

Chem. Commun. 2006, 2221-2223

Finally, we are interested to develop new polymerization catalysts, e.g. for the epoxide carbon dioxide copolymerization:

Dalton Trans. 2005, 1565-1566

J. Organomet. Chem., 2005, 690, 5182-5197

Z. Anorg. Allg. Chem. 2007, 633, 2367-2373.

Opportunities in the Hultzsch group

Graduate students will learn to synthesize and handle highly air and moisture sensitive compounds using Schlenk and glovebox techniques. The work is multidisciplinary, so students will have the opportunity to gain knowledge in very different research areas:

• Ligand design and substrate synthesis
• Organometallic complex synthesis and analysis (variable temperature & multinuclear NMR, X-Ray)
• Asymmetric catalysis (chiral GLC, HPLC)
• Polymer synthesis and analysis (NMR, GPC, DSC)
• Mechanistic and kinetic spectroscopic studies

 

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last modified 09/12/2008 by KCH