Ethyltrioxorhenium – Catalytic application and decomposition pathways

doi:10.1016/j.jorganchem.2019.02.004

Journal of Organometallic Chemistry

Volume 885, 1 April 2019, Pages 32-38

Dedicated to Professor Armando J. L. Pombeiro on the occasion of his 70th birthday

https://doi.org/10.1016/j.jorganchem.2019.02.004 Get rights and content

Highlights

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ETO converts cyclooctene to the respective epoxide in good yields and selectivity.
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Addition of a N-Donor ligand alters its reactivity significantly.
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ETO forms a peroxo complex as active species upon addition of TBHP.
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ETO decomposes both, radically and in β-H elimination in diluted solutions of THF.

Abstract

Despite its sensitivity, ethyltrioxorhenium (ETO) is applicable as catalyst in the epoxidation of olefins using either tert-butylhydroperoxide (TBHP) or hydrogen peroxide as oxidants. Conversions of approximately 80% with only epoxide being formed and a turnover frequency (TOF) of up to 1200 h⁻¹ can be achieved with TBHP in 1,1,1,3,3,3-hexafluroroisopropanol (HFI). As proven for its more stable congener MTO, the active species is highly likely an alkyl peroxo species, as shown by ¹⁷O-NMR experiments. Experimental and theoretical studies on the decomposition mechanism of ETO in diluted polar solvents reveal that the degradation pathway proceeds equally via β-hydrogen elimination and radical decomposition.

Graphical abstract

Introduction

MTO (Methyltrioxorhenium) was first prepared in 1979 by aerobic oxidation of trimethyldioxorhenium [1]. One decade later a more applicable straightforward synthesis using Sn(CH₃)₄ and Re₂O₇ was described [2]. Later, the synthesis was stepwise further improved, avoiding the loss of rhenium and the use of toxic tin compounds [3]. MTO has proven to be a highly versatile catalyst and is applied in a broad range of catalytic reactions like olefin metathesis [2], aldehyde olefination [4], (deoxy)dehydration [5,6], arene oxidation [7], Baeyer-Villinger oxidation [8] and epoxidation of olefins [[9], [10], [11], [12]]. In order to tailor its stereo and electronic properties, several derivatives are synthesized by substitution of the methyl group by aryl or alkyl moieties, however, most of them are either instable or catalytically inactive [9,[13], [14], [15], [16], [17], [18]]. Therefore, MTO remained a seemingly isolated case of an active, stable and highly versatile catalyst for a long time.

First catalytically active analogues of MTO are derived by introduction of bulky aryl substituents as in compound 1 (Fig. 1) [19]. By avoiding ortho protons at the aryl substituent, the stability of the resulting trioxorhenium compound is significantly increased, allowing the application of 2,6-xylyltrioxorhenium (XTO) in the catalytic epoxidation of alkenes. Using 1 mol% of the catalyst and TBHP as oxidant 77% yield are obtained in the selective epoxidation of cis-cyclooctene with a turnover frequency of 360 h⁻¹. These results were rather surprising, since MTO has been reported to decompose in the presence of TBHP [20]. ¹⁷O-NMR and Raman experiments suggested the formation of a monoperoxo complex [XyReO₂(O₂)] when mixing XTO and TBHP, which is assumed to be the active species in the epoxidation reaction, in analogy to the case of MTO. More recently, fluorinated aryltrioxorhenium compounds are synthesized and successfully applied in catalysis [21]. 4-(trifluoromethyl)phenyltrioxorhenium (Fig. 1, 1a) oxidizes cis-cyclooctene with TBHP to the respective epoxide with a good turnover frequency of 1420 h⁻¹ rendering it the most active aryltrioxorhenium catalyst in the epoxidation reaction. In combination with the cocatalyst Et₂AlCl, this compound is also capable to convert 1-hexene in a self-metathetic reaction and polymerize norbornene in a ring-opening metathesis polymerization extending the scope of this sensitive substance class.

In contrast, for alkyl substituted trioxorhenium compounds only MTO is known to be an active catalyst. In order to fill this gap, the oxidation chemistry and chemical stability of ethyltrioxorhenium (ETO), the closest congener of MTO is re-examined, which has only attracted little attention after its first synthesis more than 20 years ago [13]. In contrast to MTO, ETO is a liquid at room temperature which tends to decompose at elevated temperatures, turning slowly purple or black.

Section snippets

Computational details

All calculations have been performed with Gaussian-09 [22] using the hybrid density functions O3LYP [23] and the split valence triple-ζ (TZ) basis set 6-311 + G** [24,25]. In combination with the here applied split valence triple-ζ (TZ) basis set O3LYP has previously proven its superior performance [26]. Re atoms have been treated with the Stuttgart/Dresden 1997 relativistic effective core potential (ECP) [27]. Optimizations were obtained without using constraint coordinates. All reported

Experimental section

General. All preparations and manipulations were carried out using standard Schlenk techniques or in an MBraun glove box under strictly oxygen- and water-free argon atmosphere. All reaction vessels were protected from light. Bright-yellow Re₂O₇ was synthesized according to literature procedures [9]. ¹⁷O-labeled MTO was prepared by mixing MTO with 10% ¹⁷O-enriched water, stirring overnight and subsequent drying under vacuum. ETO was prepared according to the literature [13]. Tetrahydrofurane,

Synthesis

ETO is synthesized via the “zinc-route” following an already published procedure using diethylzinc and rhenium heptoxide (Scheme 1).

In order to reach yields of above 80% several factors have to be considered. First of all highly pure, bright-yellow Re₂O₇ is necessary. Even traces of water, e. g. from partially hydrolyzed green Re₂O₇, rapidly decrease the yield. Furthermore the concentration of Re₂O₇ has to be diluted below 0.05 mmol/mL in THF. In higher concentrations yields decrease rapidly.

Conclusion

In this study it is reported that ETO – to date regarded as highly sensitive to temperature and all forms of oxidants – is an active catalyst in the epoxidation of cis-cyclooctene with both H₂O₂ and TBHP. With H₂O₂ as the oxidant, conversions of up to 25% are possible by adding the base 4-tert-butylpyridine stabilizing ETO. With the oxidant TBHP on the other hand conversions up to 80% with TOFs of 1200 h⁻¹ are achieved in HFI at room temperature. In this case, however, addition of 4-tert

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

SH and BH are grateful to the TUM Graduate School for financial support and the Leibniz computing center (LRZ) for providing the computing time on their HPC cluster systems.

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View full text

Ethyltrioxorhenium – Catalytic application and decomposition pathways

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Computational details

Experimental section

Synthesis

Conclusion

Conflicts of interest

Acknowledgements

Inorg. Chem.

Angew. Chem. Int. Ed.

Inorg. Chem.

Organometallics

J. Org. Chem.

Inorg. Chem.

Angew. Chem. Int. Ed.

J. Mol. Catal.

J. Mol. Catal.

Chem. Rev.

Acc. Chem. Res.

Dalton Trans.

Angew. Chem.

J. Organomet. Chem.

Organometallics

Chem. Ber.

J. Organomet. Chem.

J. Organomet. Chem.

Catal. Sci. Technol.

J. Organomet. Chem.

Dalton Trans.

Gaussian 09 Rev. E.01