Molybdenum complexes of type (L -L)MoO (O,), (L -L) = chelate ligand), with seven-fold coordination, are employed as catalysts for the epoxidation of olefins.r21 Whether these complexes function as the actual catalytic species in this commercially important reaction or as catalyst precursors is one of the central problems of the still controversial reaction mechan i~m .[~] Associated with this are questions as to the origin of the transferred oxygen atom, the regeneration of the catalyst, and the influence of the solvent and oxidizing agent on the rate of the reaction.For some time our studies have focused on the design of ligands for the adaptation of known catalysts for new solvent systems. For the peroxomolybdenum complexes, . which are only sparingly soluble in most common solvents, we have succeeded, through substitution of 2-(pyrazol-3-yl)pyridine (1) with long-chain alkyl groups, in raising their solubility dramatically such that we may epoxidize olefins in hydrocarbon Moreover, spectroscopic investigations into the reaction mechanism are now made possible with these soluble complexes. Indications that seven-coordinate peroxomolybdenum complexes should not epoxidize olefins in stoichiometric reactions[s1 led us to the synthesis of olefin-substituted derivatives of 1.The synthesis of the ligands 3 was accomplished through nucleophilic substitution of bromoolefins 2 with 1. The conversions of ally1 bromide (2 a) and 4-bromo-but-I-ene (2 b) afforded 2-( 1 -allyl-pyrazol-3-yl)pyridine (3 a) and 2-( 1 -but-3-enyl-pyrazol-3-yl)pyridine Owing to their short-chain substituents, 4 a and 4 b are, however, only poorly soluble in organic solvents and are thus unsuitable for detailed spectroscopic investigations. We nevertheless succeeded in obtaining single crystals of 4a (see Fig. 1). To in-[*] Dr.

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.

Condensation of aromatic acyl compounds with N,Ndimethylformamide diethyl acetal in a pressure tube under microwave heating gives 1-aryl-3-dimethylaminoprop-2-enones in almost quantitative yields. In the presence of hydrazine, these intermediates are transferred to the corresponding 3-arylpyrazoles.

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Substituted N,N-Chelate Ligands -Applications in Molybdenum-Catalyzed Epoxidation of Olefins* Oxodiperoxomolybdenum complexes 4 of substituted 2-[3(5)-pyrazolyl]pyridines (2a-9) were synthesized in order to control the solubility of these complexes in organic solvents. Alkyl side chains (butyl, octyl, octadecyl) increase the solubility of the complexes and enable spectroscopic investigations in solution. Due to the symmetry of the ligands the peroxo complexes 4 appear in two isomeric forms, with the terminal 0x0 ligand in the trans position either to pyridine or to pyrazole. The latter isomer of (C5H,NC3H2N2CH2COOEt)MoO(02)2 (4f) was characterized by an X-ray structure analysis. The alkyl-substituted peroxo complexes are active catalysts for the epoxidation of olefins with tert-butyl hydroperoxide.