Dominik Jantke

An efficient protocol for the palladium-catalysed Suzuki–Miyaura cross-coupling

The palladacyclic catalyst precursor received by ortho-palladation of ([1,1′-biphenyl]-2-yloxy)diisopropyl-phosphine represents a highly active system for Suzuki–Miyaura cross-coupling reactions when used in neat water. An efficient, broadly applicable and sustainable aqueous protocol was developed using 2.5 eq. of Na2CO3 as base, allowing the reaction to be performed under air and at ambient temperature with Pd loadings of 0.04 mol%. Coupling products are obtained in high yields and excellent purity by simple filtration with no organic solvents needed throughout the whole reaction. A broad variety of functional groups are tolerated and a large number of substrates can be applied with this protocol. The crystal structure of the palladacyclic catalyst precursor is presented as well as investigations targeting the nature of catalyst activation and the active catalytic species.

Metal-Conjugated Affinity Labels: A New Concept to Create Enantioselective Artificial Metalloenzymes

Invited for this month′s cover is the group of Prof. Jorg Eppinger. The cover picture illustrates the concept of using metal-conjugated affinity labels (m-ALs) to convert proteases into well-defined and catalytically active artificial metalloenzymes. For more details, see the Communication on p. 50 ff.

Ruthenium‐Catalyzed Hydrogenation of Oxygen‐Functionalized Aromatic Compounds in Water

The application of a ruthenium complex, bearing a sulfonated bis‐N‐heterocyclic carbene (NHC) ligand as catalyst precursor for the hydrogenation of aromatic compounds is reported. The reaction proceeds under mild conditions in aqueous phase. The treatment of the Ru complex with 40 bar H2 at 60 °C in water (0.1 M KOH solution) leads to the formation of a catalytically active species, which can be stored and used in catalysis experiments. The catalyst is responsible for the hydrogenation of functionalized aromatic substrates exhibiting endo‐ and exo‐CO bonds, which are relevant products in biomass conversion. Acetophenone is quantitatively reduced to 1‐cyclohexyl ethanol with a selectivity of 100 %. Various other oxygen‐functionalized aromatic substrates were also hydrogenated in moderate to quantitative conversions. To elucidate the nature of the catalytically active species, NMR, UV/vis, and ESI‐MS experiments were undertaken, showing the presence of a mononuclear Ru hydride complex. TEM measurements of a sample of the catalyst solution did not indicate the presence of nanoparticles. Mechanistic investigations point towards a homogenous mechanism.

Phenylalanine – a biogenic ligand with flexible η6- and η6:κ1-coordination at ruthenium(II) centres

The reaction of (S)-2,5-dihydrophenylalanine 1 with ruthenium(III) chloride yields the μ-chloro-bridged dimeric η(6)-phenylalanine ethyl ester complex 3, which can be converted into the monomeric analogue, η(6):κ(1)-phenylalanine ethyl ester complex 12, under basic conditions. Studies were carried out to determine the stability and reactivity of complexes bearing η(6)- and η(6):κ(1)-chelating phenylalanine ligands under various conditions. Reaction of 3 with ethylenediamine derivatives N-p-tosylethylenediamine or 1,4-di-N-p-tosylethylenediamine results in the formation of monomeric η(6):κ(1)-phenylalanine ethyl ester complexes 14 and 15, which could be saponified yielding complexes 16 and 17 without changing the inner coordination sphere of the metal centre. The structure of η(6):κ(1)-phenylalanine complex 17 and an N-κ(1)-phenylalanine complex 13 resulting from the reaction of 3 with an excess of pyridine were confirmed by X-ray crystallography.

Cover Picture: Metal-Conjugated Affinity Labels: A New Concept to Create Enantioselective Artificial Metalloenzymes (ChemistryOpen 2/2013)

Cover Picture Thomas Reiner, Dominik Jantke, Alexander N. Marziale, Andreas Raba, and Jörg Eppinger* The cover picture illustrates the concept of using metal-conjugated affinity labels (m-ALs) to convert proteases into well-defined and catalytically active artificial metalloenzymes. The X-ray structure of the papain-bound inhibitor E64c (orange) served as the basis to predict the orientation of the half-sandwich rhodium(III) moiety (yellow sphere) within the binding pocket of the protease. The well-defined position of the affinity label on the protein surface leads to a distinct environment of the metal center, which translates into enantiomeric ratios of up to 82:18 in the aqueous hydrogenation of ketones. For more details, see the Communication by Jörg Eppinger et al., on p. 50 ff.