Sebastian Torker

Gas-phase thermochemistry of ruthenium carbene metathesis catalysts.

Quantitative energy-resolved collision-induced dissociation cross-sections by tandem ESI-MS provide absolute thermochemical data for phosphine binding energies in first- and second-generation ruthenium metathesis catalysts of 33.4 and 36.9 kcal/mol, respectively. Furthermore a study of the ring-closing metathesis in the second-generation system to liberate norbornene by forming the 14-electron reactive intermediate from the intramolecular pi-complex gives an estimate of the olefin binding energy to the 14-electron complex of around 18 kcal/mol, assuming a loose transition state. The results reported here are in remarkably good agreement with the latest DFT calculations using the M06-L functional.

Readily accessible and easily modifiable Ru-based catalysts for efficient and Z-selective ring-opening metathesis polymerization and ring-opening/cross-metathesis.

Rationally designed Ru-based catalysts for efficient Z-selective olefin metathesis are featured. The new complexes contain a dithiolate ligand and can be accessed in a single step from commercially available precursors in 68-82% yield. High efficiency and exceptional Z selectivity (93:7 to >98:2 Z:E) were achieved in ring-opening metathesis polymerization (ROMP) and ring-opening/cross-metathesis (ROCM) processes; the transformations typically proceed at 22 °C and are operationally simple to perform. Complete conversion was observed with catalyst loadings as low as 0.002 mol %, and turnover numbers of up to 43,000 were achieved without rigorous substrate purification or deoxygenation protocols. X-ray data and density functional theory computations provide support for key design features and shed light on mechanistic attributes.

Simple organic molecules as catalysts for enantioselective synthesis of amines and alcohols

The discovery of catalysts that can be used to synthesize complex organic compounds by enantioselective transformations is central to advances in the life sciences; for this reason, many chemists aim to discover catalysts that allow for preparation of chiral molecules as predominantly one mirror-image isomer. The ideal catalyst should not contain precious elements and should bring reactions to completion in a few hours through operationally simple procedures. Here we introduce a set of small organic molecules that can catalyse reactions of unsaturated organoboron reagents with imines and carbonyls; the products of the reactions are enantiomerically pure amines and alcohols, which might serve as intermediates in the preparation of biologically active molecules. A distinguishing feature of this catalyst class is the presence of a ‘key’ proton embedded within their structure. Catalysts are derived from the abundant amino acid valine and are prepared in large quantities in four steps with inexpensive reagents. Reactions are scalable, do not demand stringent conditions, and can be performed with as little as 0.25 mole per cent catalyst in less than six hours at room temperature to generate products in more than 85 per cent yield and ≥97:3 enantiomeric ratio. The efficiency, selectivity and operational simplicity of the transformations and the range of boron-based reagents are expected to render this advance important for future progress in syntheses of amines and alcohols, which are useful in chemistry, biology and medicine.

Broadly Applicable Z‐ and Diastereoselective Ring‐Opening/Cross‐Metathesis Catalyzed by a Dithiolate Ru Complex

A broadly applicable Ru-catalyzed protocol for Z-selective ring-opening/cross-metathesis (ROCM) is disclosed. In addition to reactions relating to terminal alkenes of different sizes, the first examples of Z-selective ROCM processes involving heteroaryl olefins, 1,3-dienes, and O- and S-substituted alkenes as well as allylic and homoallylic alcohols are reported. Z-Selective transformations with an α-substituted allylic alcohol are shown to afford congested Z alkenes with high diastereoselectivity. Transformations are performed in the presence of 2.0-5.0 mol % of a recently disclosed Ru-based dithiolate complex that can be easily prepared in a single step from commercially available starting materials. Typically, transformations proceed at ambient temperature and are complete within eight hours; products are obtained in up to 97 % yield, >98:2 Z/E, and >98:2 diastereomeric ratio. The present investigations reveal a mechanistically significant attribute of the Ru-based dithiolates that arises from electrostatic interactions with anionic S-based ligands.

Z- and Enantioselective Ring-Opening/Cross-Metathesis with Enol Ethers Catalyzed by Stereogenic-at-Ru Carbenes: Reactivity, Selectivity, and Curtin–Hammett Kinetics

The first instances of Z- and enantioselective Ru-catalyzed olefin metathesis are presented. Ring-opening/cross-metathesis (ROCM) reactions of oxabicyclic alkenes and enol ethers and a phenyl vinyl sulfide are promoted by 0.5-5.0 mol % of enantiomerically pure stereogenic-at-Ru complexes with an aryloxy chelate tethered to the N-heterocyclic carbene. Products are formed efficiently and with exceptional enantioselectivity (>98:2 enantiomer ratio). Surprisingly, the enantioselective ROCM reactions proceed with high Z selectivity (up to 98% Z). Moreover, reactions proceed with the opposite sense of enantioselectivity versus aryl olefins, which afford E isomers exclusively. Preliminary DFT calculations in support of Curtin-Hammett kinetics as well as initial models that account for the stereoselectivity levels and trends are provided.

Reactivity and selectivity differences between catecholate and catechothiolate Ru complexes. Implications regarding design of stereoselective olefin metathesis catalysts.

The origins of the unexpected finding that Ru catechothiolate complexes, in contrast to catecholate derivatives, promote exceptional Z-selective olefin metathesis reactions are elucidated. We show that species containing a catechothiolate ligand, unlike catecholates, preserve their structural integrity under commonly used reaction conditions. DFT calculations indicate that, whereas alkene coordination is the stereochemistry-determining step with catecholate complexes, it is through the metallacyclobutane formation that the identity of the major isomer is determined with catechothiolate systems. The present findings suggest that previous models for Z selectivity, largely based on steric differences, should be altered to incorporate electronic factors as well.

Catalytic enantioselective addition of organoboron reagents to fluoroketones controlled by electrostatic interactions

Organofluorine compounds are central to modern chemistry, and broadly applicable transformations that generate them efficiently and enantioselectively are in much demand. Here we introduce efficient catalytic methods for the addition of allyl and allenyl organoboron reagents to fluorine-substituted ketones. These reactions are facilitated by readily and inexpensively available catalysts and deliver versatile and otherwise difficult-to-access tertiary homoallylic alcohols in up to 98% yield and >99:1 enantiomeric ratio. Utility is highlighted by a concise enantioselective approach to the synthesis of the antiparasitic drug fluralaner (Bravecto, presently sold as the racemate). Different forms of ammonium–organofluorine interactions play a key role in the control of enantioselectivity. The greater understanding of various non-bonding interactions afforded by these studies should facilitate the future development of transformations that involve fluoroorganic entities. Organofluorine compounds are important to medical, agricultural and materials research. Now, small organic molecules may be used to catalyse reactions of fluorinated ketones with organoboron reagents, yielding tertiary homoallylic alcohols in high enantiomeric purity. Electrostatic interactions between a catalysts ammonium group and fluoro-organic substrate are key to control of stereoselectivity.

Preparation of Macrocyclic Z-Enoates and (E,Z)- or (Z,E)-Dienoates through Catalytic Stereoselective Ring-Closing Metathesis

The first examples of catalyst-controlled stereoselective macrocyclic ring-closing metathesis reactions that generate Z-enoates as well as (E,Z)- or (Z,E)-dienoates are disclosed. Reactions promoted by 3.0–10 mol % of a Mo-based monoaryloxide pyrrolide complex proceed to completion within 2–6 h at room temperature. The desired macrocycles are formed in 79:21 to >98:2 Z/E selectivity; stereoisomerically pure products can be obtained in 43–75% yield after chromatography. Utility is demonstrated by application to a concise formal synthesis of the natural product (+)-aspicilin.

Synthesis, isolation, characterization, and reactivity of high-energy stereogenic-at-Ru carbenes: stereochemical inversion through olefin metathesis and other pathways.

The synthesis, isolation, purification (routine silica gel chromatography), and spectroscopic characterization of high-energy endo stereogenic-at-Ru complex isomers, generated by ring-opening/cross-metathesis (ROCM) reaction of the corresponding exo carbenes, are disclosed. We provide experimental evidence showing that an endo isomer can undergo thermal or Brønsted acid-catalyzed polytopal rearrangement, causing conversion to the energetically favored exo carbene.

Catalytic enantioselective 1,6-conjugate additions of propargyl and allyl groups

Conjugate (or 1,4-) additions of carbanionic species to α,β-unsaturated carbonyl compounds are vital to research in organic and medicinal chemistry, and there are several chiral catalysts that facilitate the catalytic enantioselective additions of nucleophiles to enoates. Nonetheless, catalytic enantioselective 1,6-conjugate additions are uncommon, and ones that incorporate readily functionalizable moieties, such as propargyl or allyl groups, into acyclic α,β,γ,δ-doubly unsaturated acceptors are unknown. Chemical transformations that could generate a new bond at the C6 position of a dienoate are particularly desirable because the resulting products could then be subjected to further modifications. However, such reactions, especially when dienoates contain two equally substituted olefins, are scarce and are confined to reactions promoted by a phosphine–copper catalyst (with an alkyl Grignard reagent, dialkylzinc or trialkylaluminium compounds), a diene–iridium catalyst (with arylboroxines), or a bisphosphine–cobalt catalyst (with monosilyl-acetylenes). 1,6-Conjugate additions are otherwise limited to substrates where there is full substitution at the C4 position. It is unclear why certain catalysts favour bond formation at C6, and—although there are a small number of catalytic enantioselective conjugate allyl additions—related 1,6-additions and processes involving a propargyl unit are non-existent. Here we show that an easily accessible organocopper catalyst can promote 1,6-conjugate additions of propargyl and 2-boryl-substituted allyl groups to acyclic dienoates with high selectivity. A commercially available allenyl–boron compound or a monosubstituted allene may be used. Products can be obtained in up to 83 per cent yield, >98:2 diastereomeric ratio (for allyl additions) and 99:1 enantiomeric ratio. We elucidate the mechanistic details, including the origins of high site selectivity (1,6- versus 1,4-) and enantioselectivity as a function of the catalyst structure and reaction type, by means of density functional theory calculations. The utility of the approach is highlighted by an application towards enantioselective synthesis of the anti-HIV agent (−)-equisetin.