Molybdenum‐Catalyzed Olefin Epoxidation: Ligand Effects

Abstract

Abstract:We synthesized substituted pyrazolylpyridine ligands to examine their donor properties by spectroscopic (IR, NMR) and computational (AM 1 ) methods. The influcnce of the substitution patterns on spectroscopic and thermodynamic features ofcorrelates with the activities of the complexes in catalytic olefin epoxidation reactions. This further proof for the relation between the Lewis acidity and the catalytic activity of epoxidation catalysts supports a reaction mechanism in which the peroxo complex activates the oxidizing agent (H,O,, ROOH) instead of directly transferring an oxygen atom from a q2-peroxo ligand to the olefin.

Spectroscopic Evidence for Intramolecular MoVI(O2)····HO−C Hydrogen Bonding in Solution

“…However, this geometry is not From investigations on the isomer ratios (A:B) of peroxomolybdenum complexes bearing pyrazolylpyridine ligands in accordance with the assignments of the spectra of these complexes, which we published in previous papers [1] [4] [5] [6] , with electron-withdrawing and -donating substituents [1] , we can estimate the enthalpy of the intramolecular hydrogen-a contradiction which had to be clarified.…”

“…These spectroscopic findings are in agreement with the results of Extended Hückel calculations on used as catalysts or stoichiometric reagents for the oxidation of organic and inorganic substrates [2] . We have inves-the model complex (NH 3 ) 2 MoO(O 2 ) 2 , which proved that the three highest occupied molecular orbitals of such comtigated the role of seven-coordinate oxodiperoxomolybdenum complexes in the catalytic epoxidation of olefins [1] pounds possess mainly π* character at the peroxo ligands and only low σ* and π* character at the oxo ligand. From and have focused on bidentate N-alkylated 2-[3(5)-pyrazolyl]pyridine ligands, first described by H. Brunner et al [3] .…”

“…By means of NMR spectroscopy it was found that these absorptions of peroxomolybdenum complexes bearing pyrazolylpyridine ligands with unfunctionalized alkyl isomers are in equilibrium in solution [14] . In a detailed study, we recently showed that the isomer ratios correlate side chains, like [2-(3-butylpyrazol-1-yl)pyridine]oxodiperoxomolybdenum(VI) [6] , which cannot participate in any with the individual donor properties of the heterocyclic ring systems [1] . intramolecular hydrogen bonding, show only weak solvent dependences [acetone: 962, 957 cm Ϫ1 (2 ϫ ν MoϭO ), 877, 868 Solutions of rac-2 and (1S,2S)-2 show the same behavior, which does not depend on the electronic properties of the cm Ϫ1 (2 ϫ ν OϭO ); CHCl 3 : 963, 956 cm Ϫ1 (2 ϫ ν MoϭO ), 878, 866 cm Ϫ1 (2 ϫ ν OϭO )].…”

“…The latter feature enables a correlation of electronic gies of these bonds should be low. To overcome this problem, we synthesized new N,N-donor ligands of the pyrazoligand properties with epoxidation activities of the catalysts [1] . From spectroscopic and kinetic investigations on lylpyridine type bearing OH functions in the side chain, which should lead to the formation of an intramolecular peroxo complexes bearing olefinic side chains, we postulated a new reaction mechanism for the catalytic olefin hydrogen bond.…”

New Insights into the Mechanism of Hydroperoxide Activation by Investigation of Dynamic Processes in the Coordination Sphere of Seven-Coordinated Molybdenum Peroxo Complexes

“…This special feature gave us insight into the role of the Lewis acidity of the metal centre which can be tuned by the donor properties of the chelate ligands and allowed for the first time to establish a quantitative relationship between the structure of an epoxidation catalyst and its activity. [27] Scheme 2…”