Arno Döhring

Asymmetric Catalysis of the Carbonyl-Amine Condensation: Kinetic Resolution of Primary Amines

A Brønsted acid catalyzed kinetic resolution of primary amines is described that is based on the condensation between an amine and a carbonyl compound. 1,3-Diketones react with racemic α-branched amines to furnish the corresponding enantioenriched enaminone and recovered starting material. Good to excellent enantioselectivity was observed with both aromatic and aliphatic primary amines. This process represents the first small-molecule catalyzed kinetic resolution of aliphatic amines.

Steric control of the chromium‐catalyzed oligomerization of ethylene1)

Catalysts for the oligomerization and polymerization of ethylene have been obtained by activating phosphinoalkyl-substituted cyclopentadienyl chromium complexes, e.g. (η 1 ,η 3 -R 2 PC 2 H 4 C 5 H 4 )CrCl 2 , with methylalumoxane (MAO). DFT-calculations indicate that the observed dependence of the degree of oligomerization upon the size of the substituents (R) on the P-donor atom is a result of a steric influence on the stability of the transition state for the chain termination step.

Approaching sub-ppm-level asymmetric organocatalysis of a highly challenging and scalable carbon–carbon bond forming reaction

The chemical synthesis of organic molecules involves, at its very essence, the creation of carbon–carbon bonds. In this context, the aldol reaction is among the most important synthetic methods, and a wide variety of catalytic and stereoselective versions have been reported. However, aldolizations yielding tertiary aldols, which result from the reaction of an enolate with a ketone, are challenging and only a few catalytic asymmetric Mukaiyama aldol reactions with ketones as electrophiles have been described. These methods typically require relatively high catalyst loadings, deliver substandard enantioselectivity or need special reagents or additives. We now report extremely potent catalysts that readily enable the reaction of silyl ketene acetals with a diverse set of ketones to furnish the corresponding tertiary aldol products in excellent yields and enantioselectivities. Parts per million (ppm) levels of catalyst loadings can be routinely used and provide fast and quantitative product formation in high enantiopurity. In situ spectroscopic studies and acidity measurements suggest a silylium ion based, asymmetric counteranion-directed Lewis acid catalysis mechanism.Chiral tertiary aldols are encountered in a variety of biologically relevant molecules. Making these valuable compounds directly from unbiased ketones has proven to be extremely challenging. Now it has been shown that sub-ppm levels of in situ generated silylium-based organic Lewis acid catalysts can give quantitative product formation in very high enantiopurity through a Mukaiyama aldol reaction.

Phosphinoalkyl-Substituted Cyclopentadienyl Chromium Catalysts for the Oligomerization of Ethylene

Heterogeneous chromium catalysts are used to prepare much of the Polyethylene manufactured industrially. Related homogeneous catalysts have received less attention and have never found commercial use. Reported here is a new family of phosphinoalkyl-substituted cyclopentadienyl chromium compounds which, after activation with methylalumoxane, give highly active homogeneous catalysts for the oligomerization and polymerization of ethylene whereby the degree of oligomerization is controlled by the size of the substituents on the P-atom. By varying the substitutent the catalysis can be directed to give mainly dimers and trimers, oligomers or polymer. Density functional calculations indicate that this effect is the consequence of an unusually selective steric destabilization of the transition state for termination which proceeds by s-hydrogen transfer to the incoming ethylene in a reaction involving spin inversion.