The Chemistry of Ruthenium Oxidation Complexes

Abstract

This chapter introduces the topic and scope of the book and principally concerns the basic preparation, physical and chemical properties of Ru-based ­oxidation catalysts, then summarising the catalytic oxidations which they accomplish. More detail on these is given in the succeeding four chapters. The major oxidants RuO₄ (1.2.1), perruthenate [RuO₄]⁻ (1.3.1) – mainly TPAP, (ⁿPr₄N)[RuO₄], ruthenate [RuO₄]²⁻ (1.4.1), trans-Ru(O)₂(TMP) (1.4.2.5), RuCl₂(PPh₃)₃ (1.9.3) and cis-RuCl₂(dmso)₄ (1.9.4) are covered in some detail, but many other catalysts are also discussed. In some cases brief comments are made on the mechanisms involved when data on these are given in the cited papers. There is also an Appendix (1.11) which gives brief details on the preparation of four ruthenium oxidation catalysts and selected model oxidations using them.

Appendix: Brief Resumé of Preparations of Ru Oxidants and Oxidation Reactions

In this short section brief experimental details are given for the in situ preparations of RuO₄, [RuO₄]⁻ and [RuO₄]²⁻, the isolation of solid TPAP and of trans-Ru(O)₂(bpy){IO₃(OH)₃}1.5H₂O. There are also brief notes on specific oxidation procedures using these oxidants with which the author has had some experience. There are several good, practically explicit references in the literature to procedures with Ru oxidants, e.g. that by Haines [51, 202], Courtney [60] and by Lee and van der Engh [203].

All preparations, particularly those involving RuO₄, should be carried out in a fume cupboard. It must be remembered that there is always a potential hazard involved in the handling of high oxidation state ruthenium compounds and of many of their co-oxidants.

1.1.1 Preparations of RuO₄ In Situ for Oxidations

1.1.1.1 The Classic Sharpless Procedure for Oxidations with RuO₄

Although this method is quite old it has stood the test of time, and is the basis of many modern oxidations with RuO₄. Sharpless used it for oxidations of alcohols, ethers, aromatic rings and for alkene cleavage, so clearly it has a high range of applicability.

A representative experiment involved RuCl₃ (0.005 g, 0.022 mmol) added to a solution of Na(IO₄) (0.88 g, 4.1 mmol) in water (3 cm³), CCl₄ (2 cm³) and CH₃CN (2 cm³) with the substrate (1 mmol of E-decene in the example given); the mixture was stirred for 2 h [260]. This author recommends pre-dissolution of the RuCl₃ in water (ca. 3 cm³) for some 12 h beforehand, and since Na(IO₄) is not very soluble suggests the use of more water, e.g. 10 cm³.

1.1.1.2 Cleavage of Alkenes with RuO₄

Cyclohexane is more environmentally acceptable than the more commonly-used CCl₄. A recent cleavage of a mono-fluorinated alkene to a ketone was thus effected. The alkene (70 mg, 0.28 mmol) was dissolved in a mixture of acetonitrile (0.5 cm³) and cyclohexane (0.5 cm³) and treated with RuCl₃.nH₂O (0.05 g, 0.2 mmol) and Na(IO₄) (0.24 g, 10 mmol) in water (1 cm³). The mixture was stirred for 1.5 h, the product extracted with diethylether and dried over MgSO₄ [330]. Other examples, using RuCl₃/aq. IO(OH)₅/C₆H₁₂-CH₃CN have been given [216].

1.1.2 Preparations and Use of [RuO₄]⁻

1.1.2.1 Preparation of TPAP

The reagent is available commercially from a number of suppliers (this is not the case, of course, for RuO₄), but the solid reagent may be simply and cheaply prepared in the laboratory. A pre-made aqueous solution of RuCl₃ (0.13 g, 0.5 mmol) in water (5 cm³) is added to a solution of Na(BrO₃) (6.0 g, 50 mmol) and anhydrous Na₂(CO₃) (1.58 g, 15 mmol) in water (20 cm³). These solutions are stirred together at room temperature until the typical green colour of [RuO₄]⁻ appears. An aqueous solution of (ⁿPr₄N)OH (0.1 g, 0.5 mmol) is then added with stirring; the deep green precipitate extracted into alcohol-free CH₂Cl₂ (50 cm³), the solution dried over anhydrous Na₂(CO₃), concentrated in vacuo and recrystallised from high-grade ethanol-free CCl₄. This gives a yield of some 200 mg (ca. 0.5 mmol), sufficient for several catalytic oxidations. The preparation may be scaled up if required [213].

The salt is said to be prone to explosion at temperatures above 60°C although the author has not experienced this. It should not occur since TPAP is normally used at room temperatures, but it should be stored in a refrigerated desiccator.

1.1.2.2 Oxidations with TPAP

The recently published total synthesis of rapamycin involves several steps in which TPAP was used under a variety of conditions [173].

Primary alcohol to aldehyde. The alcohol (2 g, 7.9 mmol) was added to NMO (1.4 g, 11.0 mmol.) with oven-dried 4 Å powdered molecular sieves (PMS) in 1:1 CH₂Cl₂:CH₃CN (40 cm³) and stirrecd for an hour before addition of TPAP (0.28 g, 0.79 mmol) and the green solution stirred for an hour before Celite filtration, in vacuo concentration and purification [173].

Secondary alcohol to ketone. As a step in the synthesis of avermectin B1a the alcohol (0.024 g, 0.04 mmol) in CH₂Cl₂ with TPAP (0.015 g, 0.004 mmol) and PMS (0.25 g) was mixed with NMO (0.005 g, 0.045 mmol) in CH₂Cl₂ (1 cm³) and stirred for 1 h under argon, filtered through a Florisil pad and extracted wioth EtOAc [95].

Diol to lactone. To the diol (0.09 g, 0.31 mmol) in CH₂Cl₂ (2.4 cm³) was added NMO (0.1 g, 0.92 mmol) and 4 Å PMS (0.11 g) and the mixture stirred for 10 min.; solid TPAP (0.01 g, 31.0 μmol) was added in portions, and after 1 h the reaction mixture was purified by flash chromatography [173].

1.1.2.3 Production of [RuO₄]⁻ in Aqueous Base: the [RuO₄]⁻ – (BrO₃)⁻ Reagent

This can be used for oxidations in aqueous base of primary alcohols, aldehydes, activated alkyl halides, cis-diols and nitroalkanes to carboxylic acids, and of secondary alcohols and secondary halides to ketones.

A solution of RuCl₃ (0.003 g, 0.011 mmol) in water (1 cm³), prepared some 12 h in advance, is added with stirring to a solution of Na(BrO₃) (1.2 g, 8 mmol) and anhydrous Na₂(CO₃) (0.8 g, 7.5 mmol) in water (10 cm³). This gives a yellow-green solution of [RuO₄]⁻ buffered to about pH 11; the electronic spectrum should have bands at 378 and 310 nm [213].

1.1.3 Preparations and Use of [RuO₄]²⁻

This may be used for oxidations in aqueous base of primary alcohols, aldehydes, activated alkyl halides, cis-diols and nitroalakanes to carboxylic acids, and of ­secondary alcohols and secondary halides to ketones.

To a solution of RuCl₃ (0.008 g, 0.03 mmol) in water (3 cm³), prepared some 12 h in advance, is added an aqueous solution of K₂(S₂O₈) (0.7 g, 2.6 mmol) or the more soluble Na₂(S₂O₈) in M KOH or NaOH (50 cm³) with stirring. This gives a deep orange-red solution of [RuO₄]²⁻ at pH 14; the electronic spectrum should have bands at 466 and 386 nm [213].

1.1.3.1 Oxidation of Amines with Aqueous [RuO₄]²⁻

A useful example is the oxidative dehydrogenation of primary amines to nitriles. The amine (2 mmol) is added dropwise or in small portions to a vigorously stirred solutuion of [RuO₄]²⁻ prepared as above (100 cm³); the reaction is complete when the dark orange colour of [RuO₄]²⁻ reappears. The solution is extracted with diethylether (3x25 cm³), dried over MgSO₄ and the ether removed [549].

1.1.4 Preparation of trans-Ru(O)₂(bpy){IO₃(OH)₃}1.5H₂O [568]

To a saturated aqueous solution of Na(IO₄) (1.89 g, 9 mmol) is added RuO₂.nH₂O (0.5 g, 4 mmol) suspended in water (5 cm³). The resulting orange solution ­containing RuO₄ is added to (bpy) (0.31 g, 2 cm³) in 1:1 acetone-water (20 cm³) and stirred for 5 min. The orange product is filtered off.

1.1.4.1 Epoxidation of Alkenes with trans-Ru(O)₂(bpy){IO₃(OH)₃}1.5H₂O

The complex (0.012 g, 0.02 mmol) is stirred in water (20 cm³) at 2°C for 15 min and the alkene (2 mmol) in CH₂Cl₂ (30 cm³) with Na(IO₄) (3.0 g, 14 mmol) added, and the mixture stirred for 15 h at 2°C. By addition of aqueous M NaOH solution the pH is adjusted to 12 and the mixture extracted with CH₂Cl₂ (4 × 20 cm³), the extracts combined and dried over anhydrous MgSO₄ and the sovent removed [568].