Amino acid-functionalized cyclopentadienyl molybdenum tricarbonyl complex and its use in catalytic olefin epoxidation

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 P 2 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 -β-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.