Michael Limbach

The First Catalytic Synthesis of an Acrylate from CO2 and an Alkene—A Rational Approach

For more than three decades the catalytic synthesis of acrylates from the cheap and abundantly available C(1) building block carbon dioxide and alkenes has been an unsolved problem in catalysis research, both in academia and industry. Herein, we describe a homogeneous catalyst based on nickel that permits the catalytic synthesis of the industrially highly relevant acrylate sodium acrylate from CO(2), ethylene, and a base, as demonstrated, at this stage, by a turnover number of greater than 10 with respect to the metal.

Selective alkylation of amines with alcohols by Cp*-iridium(III) half-sandwich complexes.

[Cp*Ir(Pro)Cl] (Pro = prolinato) was identified among a series of Cp*-iridium half-sandwich complexes as a highly reactive and selective catalyst for the alkylation of amines with alcohols. It is active under mild conditions in either toluene or water without the need for base or other additives, tolerates a wide range of alcohols and amines, and gives secondary amines in good to excellent isolated yields.

Alcohol amination with ammonia catalyzed by an acridine-based ruthenium pincer complex: a mechanistic study.

The mechanistic course of the amination of alcohols with ammonia catalyzed by a structurally modified congener of Milsteins well-defined acridine-based PNP-pincer Ru complex has been investigated both experimentally and by DFT calculations. Several key Ru intermediates have been isolated and characterized. The detailed analysis of a series of possible catalytic pathways (e.g., with and without metal-ligand cooperation, inner- and outer-sphere mechanisms) leads us to conclude that the most favorable pathway for this catalyst does not require metal-ligand cooperation.

Nickel‐Catalyzed Direct Carboxylation of Olefins with CO2: One‐Pot Synthesis of α,β‐Unsaturated Carboxylic Acid Salts

The nickel-catalyzed direct carboxylation of alkenes with the cheap and abundantly available C1 building block carbon dioxide (CO2 ) in the presence of a base has been achieved. The one-pot reaction allows for the direct and selective synthesis of a wide range of α,β-unsaturated carboxylates (TON>100, TOF up to 6u2005h(-1) , TON=turnover number, TOF=turnover frequency). Thus, it is possible, in one step, to synthesize sodium acrylate from ethylene, CO2 , and a sodium salt. Acrylates are industrially important products, the synthesis of which has hitherto required multiple steps.

Ruthenium Carbenes Supported on Mesoporous Silicas as Highly Active and Selective Hybrid Catalysts for Olefin Metathesis Reactions under Continuous Flow

In the search for a highly active and selective heterogenized metathesis catalyst, we systematically varied the pore geometry and size of various silica-based mesoporous (i.e., MCM-41, MCM-48, and SBA-15) and microporous (ZSM-5 and MWW) versus macroporous materials (D11-10 and Aerosil 200), besides other process parameters (temperature, dilution, and mean residence time). The activity and, especially, selectivity of such linker-free supports for ruthenium metathesis catalysts were evaluated in the cyclodimerization of cis-cyclooctene to form 1,9-cyclohexadecadiene, a valuable intermediate in the flavor and fragrance industry. The optimized material showed not only exceptionally high selectivity to the valuable product, but also turned out to be a truly heterogeneous catalyst with superior activity relative to the unsupported homogeneous complex.

Linker Free, Silica Bound Olefin Metathesis Catalysts: Applications in Heterogeneous Catalysis

A set of heterogenized olefin-metathesis catalysts, which consisted of Ru complexes with the H(2)ITap ligand (1,3-bis(2,6-dimethyl-4dimethyl aminophenyl)-4,5-dihydroimidazol-2-ylidene) that had been adsorbed onto a silica support, has been prepared. These complexes showed strong binding to the solid support without the need for tethering groups on the complex or functionalized silica. The catalysts were tested in the ring-opening-ring-closing-metathesis (RO-RCM) of cyclooctene (COE) and the self-metathesis of methyl oleate under continuous-flow conditions. The best complexes showed a TON>4000, which surpasses the previously reported materials that were either based on the Grubbs-Hoveyda II complex on silica or on the classical heterogeneous Re(2)O(7)/B(2)O(3) catalyst.

Unsymmetrical Ru-NHC catalysts: a key for the selective tandem ring opening-ring closing alkene metathesis (RO-RCM) of cyclooctene.

Dissymmetry for selectivity: NHC ligand with two different pendant group allows the selective formation of cyclic oligomers in place of polymers opening new strategy to generate macrocycles.

Ring‐Opening Polymerization and Copolymerization of Propylene Oxide Catalyzed by N‐Heterocyclic Carbenes

Herein, we demonstrate the application of adducts of various N‐heterocyclic carbenes (NHCs) with CO2 (i.e., NHC–CO2) as precatalysts in the ring‐opening homopolymerization of propylene oxide (PO) onto diethylene glycol as a chain starter to give well‐defined polyether diols. The influence of various NHCs on the structure of the polymers and the mechanism of this reaction were investigated both experimentally and through DFT calculations. With this methodology, copolymers of PO and the monomers ε‐caprolactone and (S,S)‐lactide are accessible.

A Ru-Vinylvinylidene Complex: Straightforward Synthesis of a Latent Olefin Metathesis Catalyst

The development of ruthenium-based olefin metathesis has had an enormous impact on organic synthesis and polymer chemistry. 2] The versatility of metathesis reaction types and reaction conditions has led to an explosion of tailor-made catalyst designs with fine-tuned properties. However, methodologies to introduce the metathesis-active alkylidene moiety into the Ru complex have still been quite limited. To date, Ru–benzylidene and Ru–indenylidene complexes 1 and 2 a represent the basic motifs for which the majority of synthetic efforts towards the development of future generation catalysts are based upon, mainly owing to the fact that they are also commercially available.

Bulky N-Phosphino-Functionalized N-Heterocyclic Carbene Ligands: Synthesis, Ruthenium Coordination Chemistry, and Ruthenium Alkylidene Complexes for Olefin Metathesis

Ruthenium chemistry and applications in catalytic olefin metathesis based on N-phosphino-functionalized N-heterocyclic carbene ligands (NHCPs) are presented. Alkyl NHCP Ru coordination chemistry is described, and access to several potential synthetic precursors for ruthenium alkylidene complexes is outlined, incorporating both trimethylsilyl and phenyl alkylidenes. The Ru alkylidene complexes are evaluated as potential olefin metathesis catalysts and were shown to behave in a latent fashion. They displayed catalytic activity at elevated temperatures for both ring closing metathesis and ring opening metathesis polymerization.