Cyclopentadienyl molybdenum alkyl ester complexes as catalyst precursors for olefin epoxidation

Abstract

New molybdenum complexes of the type [CpMo(CO) 3 X] containing ligands of the formula X = CHR 2 CO(OR 1 )where R 1 = ethyl (1), menthyl (4), and bornyl (5) and R 2 = H; R 1 = ethyl and R 2 = methyl (2) and phenyl (3) have been synthesized and characterized by NMR and IR spectroscopy and X-ray crystallography.These compounds have been applied as catalyst precursors for achiral and chiral epoxidation of unfunctionalized olefins with tert-butyl hydroperoxide (TBHP) as the oxidant at 22°C (in CH 2 Cl 2 ) and 55°C (in CHCl 3 ). The substrates cis-cyclooctene, 1-octene, cis-and trans-stilbene, and trans-β-methylstyrene were selectively and quantitatively converted into their epoxides using a catalyst :substrate : oxidant ratio of 1 : 100 : 200 within 4 h at room temperature in CH 2 Cl 2 and within 15 min at 55°C in CHCl 3 . Complexes 1-5 are precursors of active epoxidation catalysts and turnover frequencies (TOFs) of ca. 1200 h −1 are obtained with cis-cyclooctene as the substrate. No enantioselectivity is observed with trans-β-methylstyrene as the substrate despite the application of enantiomerically pure precatalysts. In situ monitoring of catalytic epoxidation of cis-cyclooctene with complex 5 by 1 H and 13 C NMR spectroscopy suggests that the chiral alkyl ester side chain is retained during oxidation with TBHP. During epoxidation, the primary catalytic species is the dioxo complex [CpMoO 2 X]. After near complete conversion of cis-cyclooctene to its epoxide, further oxidation of the dioxo complex to oxo-peroxo complex [CpMo(η 2 -O 2 )(O)X] takes place. The oxo-peroxo complex is also an active epoxidation catalyst.

Oxidation of [CpMo(CO)3R] olefin epoxidation precatalysts with tert-butylhydroperoxide

“…It is also found that no precipitate forms during catalytic epoxidation with complexes 1 and 5-8 (Section 3.5 and Ref. [38]). Formation of a precipitate during catalytic epoxidation of alkenes with similar Mo-based complexes has also been reported, but the nature of the precipitate has remained elusive [10,28,[44][45][46][47].…”

“…Complexes 1-5 [38,39], 6 [40,41], 7 and 8 [39,41] were prepared by known methods. All NMR experiments were carried out in CDCl 3 solvent under ambient conditions.…”

“…The persistent Cp signal at 6.45 ppm in 1 H NMR and 112.3 ppm in 13 C NMR is thus also assigned to complex II. The short-lived signals at 6.47 ppm and 111.9 ppm in 1 H and 13 C NMR respectively can be assigned to the Cp ligand of the dioxo complex [CpMo(O) 2 R], I on considering the in situ IR spectroscopic evidence (discussed below), previous kinetic studies and crystallographic evidence [10,19,22,38]. The two transient signals observed in 1 H NMR during oxidation of precatalyst 5 at 2.85 and 6.52 ppm are assigned to the Cp ligand of species designated as A and species B respectively.…”

“…The stretching frequencies for terminal CO observed at 2023 and 1931 cm À1 and for the ester AC@O group at 1669 cm À1 (in C 6 D 6 ) [38] shift to 2034 cm À1 (a) and 1952 cm À1 (b), and 1672 cm À1 (c) respectively on addition of TBHP ( Fig. 4(b)), disappear completely after 4 h. Two new bands are observed for the ester group -1717 cm À1 (c 0 ) and 1620 cm À1 (c 00 ) which suggests that two oxidized CpMo species must be present that retain the R group.…”

“…The signals at 111.9 and 113.5 ppm disappear altogether after 65 min. 95 Mo NMR of the reaction mixture after 24 h shows a broad signal at À628 ppm and is identified as the oxo-peroxo complex II from comparing the 95 Mo chemical shifts for similar complexes [CpMo(O)(O 2 )CH 3 ] (À609 ppm) and [CpMo(O)(O 2 )CF 3 ] (À709 ppm) [22,24,38]. The persistent Cp signal at 6.45 ppm in 1 H NMR and 112.3 ppm in 13 C NMR is thus also assigned to complex II.…”