Anna Michrowska

A Highly Efficient Ruthenium Catalyst for Metathesis Reactions

The development of accessible metathesis catalysts that combine high activity with excellent tolerance to a variety of functional groups has been key to the widespread application of olefin metathesis in organic synthesis. In spite of the general superb application profile of the ruthenium carbene 1a, its limited thermal stability and the low activity towards substituted double bonds are major drawbacks.[1] Specifically, the preparation of substituted olefins with electron-withdrawing functionality (such as a,b-unsaturated carbonyl compounds, nitriles, sulfones, etc.) through cross metathesis (CM) with terminal alkenes remains a difficult task. The newly introduced highly active ruthenium alkylidene complexes with sterically demanding N-heterocyclic carbene (NHC)[3] ligands have dramatically alleviated this limitation.[2] Compounds of type 1b and 1c were found to be efficient catalysts in the reactions of previously metathesisinactive substrates, including a,b-unsaturated olefins (Scheme 1).[2,4]

The Proline-Catalyzed Double Mannich Reaction of Acetaldehyde with N-Boc Imines

Double-cross: Proline catalyzes the double Mannich reaction of acetaldehyde with N-Boc imines in excellent yields (up to 99 %; Boc = tert-butoxycarbonyl) and close to perfect diastereo- and enantioselectivities. Depending on the choice of catalysts, both the chiral, pseudo-C(2)-symmetric diastereomer and the corresponding meso compound can be prepared. Cross double Mannich reactions of acetaldehyde with two different imines are also demonstrated.

A green catalyst for green chemistry: Synthesis and application of an olefin metathesis catalyst bearing a quaternary ammonium group

The novel catalyst 8, bearing a polar quaternary ammonium group, is very stable and can be easily prepared from commercially available reagents. Catalyst 8 can be efficiently used for olefin metathesis not only in traditional but also in aqueous media. Various ring closing-, cross- and enyne-metathesis reactions were conducted in water–methanol mixtures in air. The electron withdrawing quaternary ammonium group not only activates the catalyst chemically, but at the same time allows its efficient separation after reaction. Application of 8 leads to organic products of high purity, which exhibit very low ruthenium contamination levels (12–68 ppm) after filtering through a pad of silica gel.

A Highly Active Aqueous Olefin Metathesis Catalyst Bearing a Quaternary Ammonium Group

A polar olefin metathesis catalyst that bears a quaternary ammonium group was prepared from commercially available reagents. The electron-withdrawing quaternary ammonium group not only activates the Ru catalyst electronically but at the same time makes the catalyst more hydrophilic. The catalyst can therefore be efficiently used both in traditional media, such as dichloromethane and toluene, as well as in technical-grade alcohols, alcohol-water mixtures and in neat water. Various metathesis reactions, including ring-closing, cross- and enyne metathesis, were conducted in these solvents in the presence of air. In addition, the Ru catalyst can act as an inisurf (initiator and surfactant) molecule, promoting metathesis under heterogeneous aqueous conditions.

Concise synthesis of ricciocarpin A and discovery of a more potent analogue

Cascade reactions enable the rapid build-up of molecular complexity from relatively simple starting materials. Both rapid construction and the ability to prepare related structures are crucial to the study of biological activities. Here, we report an efficient, highly enantioselective and diastereoselective total synthesis of ricciocarpin A. The key feature of the synthesis is a one-pot, three-step, organocatalytic reductive Michael-Tishchenko cascade. The conciseness and flexibility of this approach not only resulted in the synthesis of the natural product, but also of its antipode and four other structural analogues. A preliminary biological evaluation of these compounds identified an analogue with significantly improved molluscicidal activity.

Cross-metathesis reaction of vinyl sulfones and sulfoxides

Abstract Cross-metathesis reactions of α,β-unsaturated sulfones and sulfoxides in the presence of molybdenum and ruthenium pre-catalysts were tested. A selective metahesis reaction was achieved between functionalized terminal olefins and vinyl sulfones by using the ‘second generation’ ruthenium catalysts 1c–h while the highly active Schrock catalyst 1b was found to be functional group incompatible with vinyl sulfones. The cross-metathesis products were isolated in good yields with an excellent (E)-selectivity. Both the molybdenum and ruthenium-based complexes were, however, incompatible with α,β- and β,γ-unsaturated sulfoxides.

A simple and practical phase-separation approach to the recycling of a homogeneous metathesis catalyst

The air stable asarone-derived Ru carbene 16, a robust olefin metathesis catalyst, can be easily separated after reaction by deposition on silica gel and reused up to nine times. This procedure provides products of excellent purity with low Ru content.