abstract
The complex [(N-benzyloxycarbonylprolyl) cyclopentadienyl]-Mo(CO)(3)Me was prepared by the reaction of the amino acid-functionalized cyclopentadienyl-lithium reagent with Mo(CO)(6), and subsequent methylation with CH(3)I. The complex was characterized by FTIR, (1)H and (13)C NMR, and elemental analysis. A single-crystal X-ray diffraction study showed that the complex is chiral, crystallizing in the orthorhombic P2(1)2(1)2(1) space group with the Flack parameter refining to -0.007(16), which unequivocally confirms the presence of an enantiomerically pure compound. The complex was examined as a catalyst precursor in the liquid-phase epoxidation of trans-beta-methylstyrene at 280-330 K, using either tert-butylhydroperoxide (tert-BuOOH), cumylhydroperoxide or urea-hydrogen peroxide adduct as oxidant, and, optionally, a co-solvent. With chloroform and tert-BuOOH, the catalytic activity surpasses that previously reported for chiral complexes of the type CpMo(CO)(3)X, giving the epoxide isomers ((R, R)-(+) and (S, S)-(-)-1-phenylpropylene oxide) in excellent selectivity, albeit with negligible enantiomeric excess. FTIR spectroscopy showed that the oxidative decarbonylation of the tricarbonyl complex with tert-BuOOH is fast under the reaction conditions used, and that the structural integrity of the amino acid-functionalized cyclopentadienyl group is retained during this process. The use of the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate as a solvent for the catalyst generates a biphasic liquid-liquid reaction system that enables, at the end of a catalytic cycle, the separation and reutilization of the catalyst. (C) 2009 Elsevier B.V. All rights reserved.
keywords
CAMBRIDGE STRUCTURAL DATABASE; TERT-BUTYL HYDROPEROXIDE; ENANTIOSELECTIVE EPOXIDATION; ASYMMETRIC EPOXIDATION; TUNGSTEN COMPLEXES; CRYSTAL-STRUCTURES; INDENYL LIGANDS; DERIVATIVES; LIBRARIES; CHEMISTRY
subject category
Chemistry
authors
Abrantes, M; Paz, FAA; Valente, AA; Pereira, CCL; Gago, S; Rodrigues, AE; Klinowski, J; Pillinger, M; Goncalves, IS
our authors
Anabela Tavares Aguiar Valente
Principal Researcher
Filipe Alexandre Almeida Paz
Principal Researcher
Isabel Maria de Sousa Gonçalves
Associate Professor
Martyn Pillinger
Principal Researcheracknowledgements
The authors are grateful to the Fundacao para a Ciencia e a Tecnologia (FCT) for financial support (including Project PTDC/QUI/65427/2006 and post-doctoral grants to C.C.L.P. and S.G.). We also wish to thank the FCT for specific funding toward the purchase of the single-crystal diffractometer. Filomena Goncalves is acknowledged for the measurement of the elemental analysis data, and Luis Martins is acknowledged for assistance with the FTIR experiments.