Aryl-substituted organomolybdenum(ii) complexes as olefin epoxidation catalysts

Abstract

The epoxidation of selected olefins with a benzyl-substituted organomolybdenum complex and its fluorinated counterpart is described. With hexafluorobenzene (HFB) as solvent, turnover frequencies (TOFs) of >15 500 h −1 are achieved in the epoxidation of cyclooctene with tert-butyl hydroperoxide (TBHP) as the oxidant. The fluorinated complex, ijCpMoIJCO) 3 BzF 5 ], proved to be superior to the non-fluorinated derivative in activity and selectivity with a variety of substrates. This can be explained via X-ray crystallography analysis and with the help of density functional theory (DFT) calculations. Besides, both compounds were applied in two-phase catalytic reactions. Recycling for multiple catalytic runs is achieved without a significant loss of activity.

Kinetic studies of fluorinated aryl molybdenum(ii) tricarbonyl precursors in epoxidation catalysis

“…Compound 4 is additionally compared to known pentafluorobenzylated and benzylated molybdenum tricarbonyl complexes. 27 Compound 4 has a similar activity to [CpMoIJCO) 3 BzF 5 ] (TOF: 2.9 h −1 ). This shows that the addition of two CF 3 groups to the benzyl moiety has a similar effect to full fluorination of the benzyl ring ( Fig.…”

“…When compared to literatureknown benzylated and pentafluorobenzylated derivatives, the structures of the four compounds are most similar to a pentafluorobenzylated compound with respect to the observed bond angles. 27 As the infrared spectra of complexes of the type [CpMoIJCO) 3 R] are well understood, this method was also applied here (see Table 1). [28][29][30][31] However, the mean stretching frequencies are very similar for compounds 1-4.…”

“…Recently, we were able to demon-strate that insertion of a CH 2 bridge in arylated fluorinated and non-fluorinated molybdenum tricarbonyl complexes helps ease the decarbonylation process. 27 A further systematic account on the influence of fluorination on the catalytic epoxidation activity of Mo compounds is given below, based on varying the number and position of CF 3 groups on a benzyl moiety.…”

“…The solvent of choice for mechanistic purposes is CHCl 3 as it was recently found that decarbonylation is quite slow in it and is therefore easier to follow and to distinguish from the catalytic epoxidation process. 20,27 Unfortunately, as the oxidized derivatives and the active species could not be isolated aat least at first glance somewhat clumsy pathway to unravel more mechanistic details is necessary. Comparison of the behaviour of 1-3 shows an increasing induction period (which occurs under applied conditions only with a low excess of TBHP) with an increasing distance from the metal centre to the CF 3 moiety (Fig.…”

Molecular Epoxidation Reactions Catalyzed by Rhenium, Molybdenum, and Iron Complexes

“…To avoid the long activation period necessary to transform [CpMo(CO) 3 CF 3 ] to its oxo species, other fluorinated ligands were attached to a CpMo(CO) 3 moiety and compared to the non‐fluorinated forms. The benzyl (Bz) derivative and the pentafluoro benzyl (Bz F ) moieties were targeted first, also to have another comparison to the Re‐based systems . According to DFT calculations, both Mo compounds should be quite active in epoxidation catalysis due to their comparatively pronounced Lewis acidity.…”