Leiden, Netherlands
Leiden, Netherlands

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Saisaha P.,University of Groningen | Pijper D.,University of Groningen | Van Summeren R.P.,Royal DSM | Hoen R.,University of Groningen | And 6 more authors.
Organic and Biomolecular Chemistry | Year: 2010

A practical method for the multigram scale selective cis-dihydroxylation of electron deficient alkenes such as diethyl fumarate and N-alkyl and N-aryl-maleimides using H2O2 is described. High turnovers (>1000) can be achieved with this efficient manganese based catalyst system, prepared in situ from a manganese salt, pyridine-2-carboxylic acid, a ketone and a base, under ambient conditions. Under optimized conditions, for diethyl fumarate at least 1000 turnovers could be achieved with only 1.5 equiv. of H2O2 with d/l-diethyl tartrate (cis-diol product) as the sole product. For electron rich alkenes, such as cis-cyclooctene, this catalyst provides for efficient epoxidation. © 2010 The Royal Society of Chemistry.

Pijper D.,University of Groningen | Saisaha P.,University of Groningen | De Boer J.W.,Rahu Catalytics BV | Hoen R.,University of Groningen | And 7 more authors.
Dalton Transactions | Year: 2010

A number of manganese-based catalysts employing ligands whose structures incorporate pyridyl groups have been reported previously to achieve both high turnover numbers and selectivity in the oxidation of alkenes and alcohols, using H2O2 as terminal oxidant. Here we report our recent finding that these ligands decompose in situ to pyridine-2-carboxylic acid and its derivatives, in the presence of a manganese source, H2O 2 and a base. Importantly, the decomposition occurs prior to the onset of catalysed oxidation of organic substrates. It is found that the pyridine-2-carboxylic acid formed, together with a manganese source, provides for the observed catalytic activity. The degradation of this series of pyridyl ligands to pyridine-2-carboxylic acid under reaction conditions is demonstrated by 1H NMR spectroscopy. In all cases the activity and selectivity of the manganese/pyridyl containing ligand systems are identical to that observed with the corresponding number of equivalents of pyridine-2-carboxylic acid; except that, when pyridine-2-carboxylic acid is used directly, a lag phase is not observed and the efficiency in terms of the number of equivalents of H 2O2 required decreases from 6-8 equiv. with the pyridin-2-yl based ligands to 1-1.5 equiv. with pyridine-2-carboxylic acid. © 2010 The Royal Society of Chemistry.

Draksharapu A.,University of Groningen | Li Q.,University of Groningen | Logtenberg H.,University of Groningen | Van Den Berg T.A.,University of Groningen | And 9 more authors.
Inorganic Chemistry | Year: 2012

We report the characterization and solution chemistry of a series of Fe II complexes based on the pentadentate ligands N4Py (1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine), MeN4Py (1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)ethanamine), and the tetradentate ligand Bn-N3Py (N-benzyl-1,1-di(pyridin-2-yl)-N-(pyridin-2- ylmethyl)methanamine) ligands, i.e., [Fe(N4Py)(CH 3CN)](ClO 4) 2 (1), [Fe(MeN4Py)(CH 3CN)](ClO 4) 2 (2), and [Fe(Bn-N3Py)(CH 3CN) 2](ClO 4) 2 (3), respectively. Complexes 2 and 3 are characterized by X-ray crystallography, which indicates that they are low-spin Fe II complexes in the solid state. The solution properties of 1-3 are investigated using 1H NMR, UV/vis absorption, and resonance Raman spectroscopies, cyclic voltammetry, and ESI-MS. These data confirm that in acetonitrile the complexes retain their solid-state structure, but in water immediate ligand exchange of the CH 3CN ligand(s) for hydroxide or aqua ligands occurs with full dissociation of the polypyridyl ligand at low (<3) and high (>9) pH. pH jumping experiments confirm that over at least several minutes the ligand dissociation observed is fully reversible for complexes 1 and 2. In the pH range between 5 and 8, complexes 1 and 2 show an equilibrium between two different species. Furthermore, the aquated complexes show a spin equilibrium between low- and high-spin states with the equilibrium favoring the high-spin state for 1 but favoring the low-spin state for 2. Complex 3 forms only one species over the pH range 4-8, outside of which ligand dissociation occurs. The speciation analysis and the observation of an equilibrium between spin states in aqueous solution is proposed to be the origin of the effectiveness of complex 1 in cleaving DNA in water with 3O 2 as terminal oxidant. © 2011 American Chemical Society.

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