Journal Archive

Platinum Metals Rev., 1983, 27, (3), 111

Ruthenium Catalysed Oxidations of Organic Compounds

  • By Ernest S. Gore
  • Johnson Matthey Inc., West Chester, Pennsylvania

Article Synopsis

Ruthenium and its complexes can be used to catalyse the oxidation, both homogeneous and heterogeneous, of a wide range of organic substrates. These include olefins, alkynes, arenes, alcohols, aldehydes, ketones, ethers, sulphides, amines, and phosphines. A wide variety of oxidants can be used under mild conditions; conversions and selectivities are usually high, and the catalyst can be easily recovered. Ruthenium can also be used to catalyse the oxidative destruction of pollutants in both gas and liquid phases.

The synthetic use of ruthenium tetroxide as an oxidant for organic compounds was first reported in 1953 by Djerassi and Engle (1). The scope of ruthenium oxidations was greatly expanded by Berkowitz and Rylander in 1958 (2). All of these early workers used ruthenium tetroxide as a stoichiometric oxidant; however, ruthenium tetroxide is rather inconvenient to use in this way. It is troublesome to prepare, expensive, and its strong oxidising power tends to make it less selective than other oxidants. Caution—ruthenium tetroxide is an extremely powerful and volatile oxidant; it should only be handled in a well ventilated area and when wearing appropriate protective clothing.

Thus it is not surprising that work was soon initiated on using ruthenium in catalytic quantities in oxidation reactions. The first such use of ruthenium seems to have been in an obscure publication in 1956 (3). A more readily available report appeared in 1959 (4). The advantages of catalytic ruthenium oxidations over stoichiometric ruthenium tetroxide have proved to be so convincing that today virtually all ruthenium mediated oxidations are performed catalytically.

While several reviews (5–9) have been written on ruthenium mediated oxidations, the last one available in the West appeared ten years ago and dealt equally with ruthenium catalysed reactions and stoichiometric ruthenium tetroxide reactions (9). With the emphasis shifting to ruthenium catalysed reactions, new reactions have been discovered and conditions have been found which have improved the selectivity and yields of these reactions. Thus it is appropriate to survey the subject again to summarise the state of the art.


Experimental Conditions

Many different ruthenium catalysts and oxidants have been used. Of these, the most common catalysts are RuCl2(PPh3)3, RuCl3.xH2O, and RuO2.xH2O, and the most common oxidants are HOOAc, NaIO4, O2, and NaOCl. Some catalyst/oxidant systems are very selective indeed. For example both RuCl2(PPh3)3/Ph(IOAc)2 and RuCl2(PPh)3)3/N-methylmorpholine-N-oxide specifically convert primary alcohols to aldehydes in high yields (10, 11) and RuCl2(PPh3)3/PhCH=CHCOCH3 converts vicinal diols to vicinal diketones (12).

Conditions for ruthenium catalysed oxidations are very mild; usually a few hours or less at room temperature is sufficient. A variety of solvent systems can be used, and depending on the oxidant a wide range of pH’s can be tolerated. Oxidations with oxygen can be carried out at atmospheric pressure.

Many ruthenium catalysed reactions have been performed in the H2O-CCl4 solvent system. But slow or incomplete reactions are occasionally encountered in this system, especially in the presence of carboxylic acids. Recently it has been found that adding CH3CN to the system greatly improves yields and reaction times (13, 14). When (E)-5-decene was oxidised in H2O-CCl4 only 20 per cent conversion occurred in 2 hours; but on adding CH3CN the reaction was complete in the same period.


Oxidation of Olefins


Cleavage of the Double Bond

When the oxidation of an olefin is catalysed by ruthenium in the oxidation state +3 or higher, the usual result is cleavage of the double bond. Ketones are produced if the carbons are fully substituted; otherwise acids, or occasionally, aldehydes are obtained, see Table I. On the other hand, osmium tetroxide when used catalytically converts olefins to aldehydes rather than to acids (21).


Table I

Oxidation of Olefins with Cleavage of the C = C Bond

Substrate Product Catalyst Oxidant Yield per cent Ref.  
A CH3(CH2)7CH = CH(CH2)7COOH CH3(CH2)7COOH + HOOC(CH2)7COOH RuO4 NaOCl 94 (15)  
B CH3(CH2)7CH=CH2 CH3(CH2)7COOH RuCl3 NalO4 89 (13)  
C HOOC(CH2)10COOH RuO2 O2 94 (16)  
D RuCl3 NaOCl 83 (17)  
E RuO2 NaIO4 62 (18)  
F PhCH = CH2 PhCOOH RuO2* O2 92 (16)  
G PhCH=CH2 PhCHO RuO2 NaIO4 82 (19)  
H RuO2 NaIO4 86 (20)  

3 ppm cobalt naphthenate added


Non-Cleavage of the Double Bond

Only a few non-cleavage reactions are known, and these are given in Table II. The catalyst is usually a +2 ruthenium complex and the products are unpredictable.


Table II

Oxidation of Olefins without Cleavage of the C = C Bond

Substrate Product Catalyst Oxidant Yield per cent Ref.
A CH3(CH2)6CH = CH2 RuCl2(PPh3)3 O2 30, 52 (22)
B RuCl2(PPh3)3 t-BuOOH 53 (23)
C [Ru(trpy)(bipy)(H2O)]2+ electricity — (24)
D RuCl2(PPh3)3 O2 27 (conv.) ratio 1 :5:3 (25)
E RuCl3 NalO4 51, 12 (13)

Oxidation of Alkynes

Terminal alkynes are cleaved to give acids while internal alkynes yield diketones with no cleavage, see Table III.


Table III

Oxidation of Alkynes

Substrate Product Catalyst Oxidant Yield per cent Ref.
A PhC ≡ CPh RuO2 NaOCl 83 (26)
B BuC ≡ CBu RuO2 NaOCl 70, 19 (26)
C PhC ≡ CH PhCOOH RuO2 NaOCl 66 (26)
D tBuC≡ CH tBuCOOH RuO2 NaOCl 60 (26)
E HC ≡ CCH(NH2)CH2CH2COOH HOOCCH(NH2)CH2CH2COOH RuO2 NalO4 50 (27)

Oxidation of Arenes

Ruthenium catalysed oxidations of arenes can proceed in three ways, see Table IV:


Table IV

Oxidation of Arenes

Substrate Product Catalyst Oxidant Yield per cent Ref.
A Ru/Al2O3 O2 — (28)
B Ru/Al2O3 O2 — (29)
C RuCl3 NalO4 94 (13)
D RuO2 NaOCl 80 (30)
E RuO2 NaOCl 70 (31)
F RuO2 NaOCl 50, 5 (31)
G RuO2 NaOCl 7,63 (31)

Alkyl side chains on the phenyl ring can be converted to—COOH (IV-A, B).

The phenyl ring can be cleaved from R-Ph to form R-COOH (IV-C, D).

The phenyl ring can be degraded to form a dicarboxylic acid (IV-E, F, G).

In almost all cases where an alkyl side chain is replaced by a carboxyl group, a heterogeneous catalyst was used, for example IV-A, B. This is one of the few cases in which a heterogeneous catalyst is used in ruthenium oxidations. Oxygen is used as the oxidant since at temperatures below 400°C it can only oxidise ruthenium as far as RuO2, which is insoluble. Stronger oxidants such as NaOCl or NaIO4 will oxidise ruthenium to the +7 or +8 oxidation state and these are soluble.

The effect of the electronegativity of the substituent on the products of the oxidation of naphthalenes can be seen in reactions IV-F and IV-G. An electron-donating substituent favours cleavage of the substituted ring, while an electron-withdrawing substituent favours cleavage of the unsubstituted ring.


Oxidation of Alcohols

This is the most common synthetic use of ruthenium catalysed oxidations. Highly selective conditions are readily available; alcohols can be converted to aldehydes rather than acids and vicinal diols can be readily oxidised to either cleaved or non-cleaved products depending on conditions. In alcohols containing another oxidisable group such as a C=C double bond, a C≡C triple bond, an arene, nitrogen or sulphur, the hydroxyl group is oxidised preferentially (Table V). In substrates containing both primary and secondary alcohols the primary alcohol is oxidised preferentially, (see V-H).


Table V

Selective Oxidation of Alcohols in the Presence of Other Functional Groups

Substrate Product Catalyst Oxidant Yield per cent Ref.
A RuCl2(PPh3)3 94 (11)
B RuCl3 K2S2O8 97 (32)
C RuCl2 (PPh3)3 PhlO 66 (10)
D RuCl2 (PPh3)3 73 (11)
E RuCl2 (PPh3)3 O2 47 (33)
F RuCl2(PPh3)2 65 (11)
G RuCl3 NaOCl 79 (34)
H RuCl2 (PPh3)3 70 (conv.) 5:1 (35)

Secondary Alcohols

These are oxidised cleanly and in good yields to ketones, see Table VI. Cyclobutanols can be oxidised to cyclobutanones (38) (VI-F) in yields higher than with CrO3/oxalic acid (39).


Table VI

Oxidation of Secondary Alcohols

Substrate Product Catalyst Oxidant Yield per cent Ref.
A RuCl3 NaOCl 95 (36)
B RuCl2(PPh3)3 PhlO 86 (10)
C RuCl3 K2S2O8 71 (32)
D RuO2 NalO4 80 (37)
E RuCl3 K2S2O8 95 (32)
F RuO2 NalO4 78 (38)

Primary Alcohols

Primary alcohols are oxidised either to aldehydes (Table VII) or to acids (Table VIII). The outcome of the reaction can be highly selective depending on the conditions used. For example: RuCl2(PPh3)3 with N-methylmorpholine-N-oxide (11), O2 (33), PhCH=CHCOCH3 (40) or Ph(IOAc)2 (10) always gives aldehydes. RuCl2(PPh3)3 with excess PhIO (10), or RuCl3 with HOOAc (42) or K2S2O8 (32) always gives acids.


Table VII

Oxidation of Primary Alcohols to Aldehydes

Substrate Product Catalyst Oxidant Yield per cent Ref.
A CH3(CH2)8CH2OH CH3(CH2)8CHO RuCl2(PPh3)3 90 (11)
B RuCl2(PPh3)3 79 (11)
C PhCH2OH PhCHO RuCl2(PPh3)3 PhCH=CHCOCH3 90 (40)
D RuCl2(PPh3)3 O2 100 (33)
E CH3(CH2)6CH2OH CH3(CH2)6CHO RuCl2(PPh3)3 Phl(OAc)2 97 (10)

Table VIII

Oxidation of Primary Alcohols to Acids

Substrate Product Catalyst Oxidant Yield per cent Ref.
A RuO2 NalO4 83 (41)
B CH3(CH2)6CH2OH CH3(CH2)6COOH RuCl3 HOOAc 83 (42)
C CH3(CH2)6CH2OH CH3(CH2)6COOH RuCl2(PPh3)3 PhlO (3 equ.) 88 (10)
D RuCl3 K2S2O8 97 (32)
E RuCl3 K2S2O8 86 (32)

Diols

Oxidation of diols can give either cleaved or non-cleaved products, see Table IX. Stronger oxidants cleave the diol, generally giving acids (IX-A, B), although under carefully controlled conditions aldehydes can be obtained as the major product (IX-C, D). The use of RuCl2(PPh3)3 and PhCH=CHCOCH3 selectively produces non-cleaved diketones from diols (IX-E, F).


Table IX

Oxidation of Diols

Substrate Product Catalyst Oxidant Yield per cent Ref.
A PhC(CH3)HCH(OH)CH2OH PhC(CH3)HCOOH RuCl3 NalO4 92 (13)
B HOOC (CH2)4COOH RuCl3 NaOCl 90 (36)
C CH3CHO RuCl3 H2O2 88 (43)
D RuCl3 H2O2 — (43)
E RuCl2 (PPh3)3 PhCH=CHCOCH3 85 (12)
F RuCl2 (PPh3)3 PhCH=CHCOCH3 70 (12)

Carbohydrates

Ruthenium catalyses the oxidation of hydroxyl groups in carbohydrates, secondary hydroxyl groups being converted into carbonyls and primary groups to acids. For example L-sorbose is converted to a mixture of erythrose and glycolic acid (44) and sugar, I, is converted to its keto sugar, II, in 100 per cent yield (45).

The yields are usually better when ruthenium is used than when CrO3 is used (9).


Oxidation of Aldehydes and Ketones

Aldehydes are readily oxidised to carboxylic acids, see Table X. There has been little published work on the oxidation of ketones. In one paper dealing with kinetics the authors reported that ketones were converted to diketones (46), for example:

However in this case a ten fold excess of substrate was used. Diketones are cleaved to give a mixture of acids (X-D).


Table X

Oxidation of Aldehydes and Ketones

Substrate Product Catalyst Oxidant Yield per cent Ref.
A CH3(CH2)4CHO CH3(CH2)4COOH RuCl2 (PPh3)3 PhlO 88 (10)
B PhCHO PhCOOH RuCl2 (PPh3)3 PhlO 96 (10)
C RuCl3 K2S2O8 99 (32)
D HOOC(CH2)2COOH + HOOC(CH2)3COOH + HOOC(CH2)4COOH RuCl3 NaOCl 3,56,28 (36)

Table XI

Oxidation of Acyclic Ethers

Substrate Product Catalyst Oxidant Yield per cent Ref.
A CH3(CH2)9OCH3 RuCl3 NalO4 83–96 (13), (14)
B CH3(CH2)5OCH3 RuCl3 NalO4 95 (14)
C RuCl3 NalO4 99 (14)
D RuCl3 NalO4 82,9 (14)
E PhCH2OCH3 RuCl3 NalO4 89 (13)
F RuCl3 NalO4 61,8 (14)
G RuCl3 NalO4 36, 57 (14)
H RuCl3 NalO4 52 (14)
        20  

Table XII

Oxidation of Cyclic Ethers

Substrate Product Catalyst Oxidant Yield per cent Ref.
A RuO2 NalO4 40, 5 (47)
B HOOC(CH2)2COOH RuO2 NalO4 79 (47)
C RuO2 NalO4 56, 14 (47)
D RuO2 NalO4 72 (47)
E RuO2 NalO4 82 (47)

Table XIII

Oxidation of Sulphides and Amines

Substrate Product Catalyst Oxidant Yield per cent Ref.
A Bu2S Bu2SO + Bu2SO2 RuCl3 O2 18, 76 (48)
B Bu2S Bu2SO + Bu2SO2 RuCl2 (AsPPh3)3 O2 93, 7 (48)
C Ru/Al2O3 O2 97 (49)
D PhCH2NH2 PhCN + PhCONH RuCl3 O2 86 (total) (50)
E PhCH2NH2 PhCN RuCl3 K2S2O8 66 (32)
F RuO2 NalO4 60 (51)
G RuO2 NalO4 90 (52)
H RuO2 NalO4 33 (52)
I RuO2 NalO4 71 (52)

Oxidation of Ethers


Acyclic Ethers

Primary methyl ethers, RCH2OCH3, are oxidised to methyl esters, RCOOCH3, in excellent yields, (XI-A, B). Secondary methyl ethers, RR1CHOCH3, on the other hand undergo cleavage to give ketones, RCOR1 (XI-C, D). Benzyl ethers, PhCH2OR, undergo oxidation of the benzyl group to give esters, PhCOOR, in fair to good yields (XI-E, F). Oxidation of unsymmetric ethers, ROR1 where one of the substituents is not aromatic, gives unpredictable results with either R or R1 being oxidised in roughly equal proportions.


Cyclic Ethers

Only carbons next to the ether linkage are oxidised. If both carbons are secondary, the products are mainly lactones with some carboxylic acids depending on the sensitivity of the lactone to hydrolysis (XII-A, B). If one carbon is secondary and the other tertiary the secondary carbon is oxidised preferentially giving a lactone (XII-C, D). Some hydrolysis to keto acids can occur. If both carbons are tertiary, cleavage to diketones occurs (XII-E).

Ethers are a class of compounds for which the yields and selectivities differ significantly when they are oxidised catalytically or stoichiometrically with ruthenium tetroxide. For example, tetrahydrofuran oxidised stoichiometrically with ruthenium tetroxide gives only γ -butyrolactone in 65 to 100 per cent yield (2, 47) but when oxidised catalytically with RuO2 and NaIO4 (47) the products are γ -butyrolactone, 40 per cent, and succinic acid, 5 per cent.


Oxidation of Sulphides and Amines

Sulphides are usually oxidised to a mixture of sulphoxides and sulphones (XIII-A, B), but in at least one case a sulphone was obtained exclusively (XIII-C).

Linear primary amines are oxidised to nitriles with some hydrolysis to the amide (XIII-D, E). Cyclic amines are oxidised to either lactams (XIII-F, G) or imides (XIII-H, I). The yields range from poor to good. As with ethers, only the carbon adjacent to the heteroatom is oxidised and secondary carbons are oxidised preferentially to tertiary carbons.


Oxidation of Steroids

Steroids generally undergo the same reactions that have already been discussed—oxidative cleavage of C=C double bonds to acids (4, 53), degradation of aromatic rings (54), and oxidation of secondary alcohols to ketones (10, 11).

It is interesting that cholesterol, which has both a secondary hydroxyl group and a C=C double bond, does not undergo any reaction (10, 11).

An atypical reaction which some steroids undergo is simultaneous oxidation of a tertiary CH group to a tertiary alcohol, and a secondary CH2 to a ketone (54):


Pollution Control via Ruthenium Catalysed Oxidations

Wet scrubbing with KMnO4 is used commercially to control air pollution. However with some pollutants, notably thiophenes, the reactions are too slow to be useful. It has been demonstrated (55) that oxidation of thiophenes with Ru/NaOCl is more than 100 times faster than with KMnO4. This means that residence times are within the range that wet scrubbing of airborne thiophenes and other sulphur containing pollutants is practicable.

Ruthenium catalysed oxidations have been suggested (56) for removing sulphur containing impurities from various petroleum fractions. Thus sulphur (500 ppm) in an n-paraffin fraction was reduced to less than 50 ppm in 4 hours by treatment with Ru/NaOCl at 20°C.

Ruthenium has also been suggested (57, 58) for the removal of ammonia from waste water by treating the waste at elevated temperatures with oxygen and a supported ruthenium catalyst. Chlorophenols and highly toxic polychlorodibenzodioxins were shown to be effectively destroyed by ruthenium catalysed oxidations (59).

Finally ruthenium has been demonstrated to remove pollutants in the gas phase. Thus 5500 ppm vinyl chloride in air was reduced to 2 ppm by passing the gas over a 0.5 per cent ruthenium on alumina catalyst at a temperature of 376°C (60).


Conclusion

Ruthenium and its complexes are extremely versatile oxidation catalysts. They will catalyse the oxidation of virtually any oxidisable organic functional group and, by choosing the appropriate conditions, the oxidations can be made to proceed in high yield and selectivity even in the presence of other oxidisable groups. Thus they offer a useful alternative to the more classical oxidation reagents.

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