Ruthenium Catalysed Oxidations of Organic Compounds by Ernest S

Ruthenium Catalysed Oxidations of Organic Compounds By Ernest S. Gore Johnson Matthey Inc., West Chester, Pennsylvania

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 ruthenium tetroxide reactions (9). With the an oxidant for organic compounds was first emphasis shifting to ruthenium catalysed reac- reported in I 95 3 by Djerassi and Engle (I).The tions, new reactions have been discovered and scope of ruthenium oxidations was greatly conditions have been found which have expanded by Berkowitz and Rylander in 1958 improved the selectivity and yields of these (2). All of these early workers used ruthenium reactions. Thus it is appropriate to survey the tetroxide as a stoichiometric oxidant; however, subject again to summarise the state of the art. ruthenium tetroxide is rather inconvenient to use in this way. It is troublesome to prepare, Experimental Conditions expensive, and its strong oxidising power tends Many different ruthenium catalysts and to make it less selective than other oxidants. oxidants have been used. Of these, the most Caution-ruthenium tetroxide is an extremely common catalysts are RuCldPPhJ, powerful and volatile oxidant; it should only be RuCl,.xH,O, and RuO,.xH,O, and the most handled in a well ventilated area and when common oxidants are HOOAc, NaIO,, O,, and wearing appropriate protective clothing. NaOCl. Some catalysdoxidant systems are Thus it is not surprising that work was soon very selective indeed. For example both initiated on using ruthenium in catalytic RuCl,(PPh,)JPh(IOAc), and RuCl,(PPh),)@- quantities in oxidation reactions. The first such methylmorpholine-N-oxide specifically convert use of ruthenium seems to have been in an primary alcohols to aldehydes in high yields obscure publication in 1956 (3). A more readily (10, I I) and RuCl#Ph,)JPhCH=CHCOCH, available report appeared in 1959 (4). The converts vicinal diols to vicinal diketones (I2). advantages of catalytic ruthenium oxidations Conditions for ruthenium catalysed oxida- over stoichiometric ruthenium tetroxide have tions are very mild; usually a few hours or less proved to be so convincing that today virtually at room temperature is sufficient. A variety of all ruthenium mediated oxidations are perfor- solvent systems can be used, and depending on med catalytically. the oxidant a wide range of pH’s can be While several reviews (5-9) have been tolerated. Oxidations with oxygen can be written on ruthenium mediated oxidations, the carried out at atmospheric pressure. last one available in the West appeared ten Many ruthenium catalysed reactions have years ago and dealt equally with ruthenium been performed in the H,O-CCl, solvent system. catalysed reactions and stoichiometric But slow or incomplete reactions are

platinum Melds Rev., 1983,27,(3), 111-125 111 occasionally encountered in this system, ruthenium as far as RuO,, which is insoluble. especially in the presence of carboxylic acids. Stronger oxidants such as NaOCl or NaIO, will Recently it has been found that adding CH3CN oxidise ruthenium to the +7 or +8 oxidation to the system greatly improves yields and reac- state and these are soluble. tion times (13, 14). When (Eh~decenewas The effect of the electronegativity of the sub- oxidised in HzO-CC1, only 20 per cent conver- stituent on the products of the oxidation of sion occurred in 2 hours; but on adding CHFN naphthalenes can be seen in reactions IV-F and the reaction was complete in the same period. IV-G. An electrondonating substituent favours cleavage of the substituted ring, while an Oxidation of Olefins electron-withdrawing substituent favours Cleavage of the Double Bond cleavage of the unsubstituted ring. When the oxidation of an olefin is catalysed Oxidation of Alcohols by ruthenium in the oxidation state +3 or higher, the usual result is cleavage of the double This is the most common synthetic use of bond. Ketones are produced if the carbons are ruthenium catalysed oxidations. Highly selec- fully substituted; otherwise acids, or tive conditions are readily available; alcohols occasionally, aldehydes are obtained, see Table can be converted to aldehydes rather than acids I. On the other hand, osmium tetroxide when and vicinal diols can be readily oxidised to used catalytically converts olefins to aldehydes either cleaved or non-cleaved products depend- rather than to acids (21). ing on conditions. In alcohols containing another oxidisable group such as a C=C double Non-Cleavage of the Double Bond bond, a CGC triple bond, an arene, nitrogen or Only a few non-cleavage reactions are sulphur, the hydroxyl group is oxidised pre- known, and these are given in Table 11. The ferentially (Table V). In substrates containing both primary and secondary alcohols the catalyst is usually a +2 ruthenium complex and the products are unpredictable. primary alcohol is oxidised preferentially, (see V-H). Oxidation of Alkynes Secondary Alcohols Terminal alkynes are cleaved to give acids These are oxidised cleanly and in good yields while internal alkynes yield diketones with no to ketones, see Table VI. Cyclobutanols can be cleavage, see Table 111. oxidised to cyclobutanones (38) (VI-F) in yields Oxidation of Arenes higher than with CrOJoxalic acid (39). Ruthenium catalysed oxidations of arenes Primary Alcohols can proceed in three ways, see Table IV: Primary alcohols are oxidised either to Alkyl side chains on the phenyl ring can be con- aldehydes (Table VII) or to acids (Table VIII). verted to-COOH (IV-A, B). The outcome of the reaction can be highly The phenyl ring can be cleaved from R-Ph to selective depending on the conditions used. For form R-COOH (IV-C, D). example: RuClXPPh,), with N- The phenyl ring can be degraded to form a methylmorpholine-N-xide (I I), O, (33), dicarboxylic acid (IV-E, F, G). PhCH=CHCOCH, (40) or Ph(IOAc), (10) In almost all cases where an alkyl side chain is always gives aldehydes. RuCIXPPhJ, with replaced by a carboxyl group, a heterogeneous excess PhIO (to), or RuC1, with HOOAc (42)or catalyst was used, for example IV-A, B. This is K&OR (32) always gives acids. one of the few cases in which a heterogeneous catalyst is used in ruthenium oxidations. Diols Oxygen is used as the oxidant since at Oxidation of diols can give either cleaved or temperatures below 4ooOC it can only oxidise non-cleaved products, see Table IX. Stronger

Platinum Metals Rev., 1983,27, (3) 112 73 i? Table I P. f Oxidation of Olefins with Cleavage of the C = C Bond t Yield 2 Substrate Product Catalyst Oxidant Ref. c per cent 5 F CH3(CH,),CH = CH(CH,l,COOH CH,(CH,),COOH+ HOOC(CH,l,COOH RuO, NaOCl 94 c W m CH,(CH,),CH = CH, CH3(CH,),COOH RuCI, NalO, 89 W

h) 21 h W v HOOC(CH,),,COOH RuO, 02 94

c c RuCI, 83 W NaOCl

Ph' ph3Frh RuO, NalO, 62

PhCH = CH, PhCOOH RuO," 0, 92

PhCH = CH, PhCHO RuO, NalO, 82

f CHO Br Br RuO, NalO, 86 + Br Br "3 ppm cobalt naphthenate added Platinum Table II Oxidation of Olefins without Cleavage o the C = C Bond ield per cent Meto Substrate Product Catalyst Oxidant

A CH,(CH,),CH = CH, Ru CI ,( PPh 3) , 0, 30,52 5 F c. RuCI,(PPh,) t-BuOOH 53 W B QcH = w, , m W h) 21 [Ru(trpv)(bipv) h C W (H,011 electricity - v 0 ’+ n

27 (conv.) RuCI, (PPh,), D 0, atio 1 :5:3

E RuCI, NalO, 51,12 +0AC OAc 40py+

Table I I I Oxidation of Alkvnes

Substrate Product Catalyst Oxidant 00 NaOCl I1 II A PhC CPh PhCCPh 88 NaOCl I 70, 19 1 (26) B BuC CBu BuCCBu + BuCCOH Ruoz I C PhC E CH PhCOOH RuO, NaOCl 66 (26) D ‘Buck CH ‘BUCOOH I NaOCl I 60 I (26) E HCE CCH(NH,)CH,CH,COOH HOOCCH(NH,)CH,CH,COOH RuO, NalO, 50 (27) 5 P -2 Q Q

m m * 0 0 (9 cn W l- sr: r--

u u % P z

I I 0 0 Arenes IV of '6

Table u z

& I+ I I+ I 00 Oxidation 00 V 0 88 88 0 I

0 0 I bI b1 J- 0

U m 0 n

Yatinum Metals Rev., 1983, 27, (3) 115 I > C d I- (D I- In Q) 8' 0) Q) (D d (0 Ic -In 0 I- Groups

n r n n -n N s K Functional s K Other of

n 1n I V

V +- 2, 2, I V Tabie the Presence the in

0/ .'t Alcohols Q 0I of ON I 0 5I Oxidation

Selective +- 0 I I =N 0, 0 I I

v)L >c 0I n d I0%i

U m u 0 w LL (1 I

Platinum Metals Rev., 1983,27, (3) 116 - hl I- hl zi W m -m - s 1m s

Lo W 7 0 Lo OD m OD b OD 0) I-

P 9 4 4 0m nc Y Y z

n 2 Qa K Qa Alcohols

I VI 0 Secondary Table of 0d' Oxidation

a m u n w LL

Platinum Metals Rev., 1983,27, (3) 117 Table VII Platinum Oxidation of Primarv Alcohols to Aldehydes Substrate Product Catalyst Oxidant field per cent A CH,(CH,),CH,OH CH,(CH,),CHO RuCI,( PPh,), Metals nPH3 90 3 wN\o

5 0 RuCI,(PPh,), 79 F %CHO 3 c. W m WNLo W C PhCH,OH PhCHO RuCI,(PPh,), hCH=CHCOCH, 90 h) 21

h W v

D Ru CI ( PPh,) , 100

c L oo E CH,(CH,l,CH,OH CH,(CH,),CHO RuCI,(PPh,), I Phl (OAc), 97

Table Vlll Oxidation of Primary Alcohols to Acids Substrate Product Catalyst Oxidant Yield per cent Ref.

CH,OH RuO, NalO, 83 (41I A

B CH,(CH ,)&H ,OH RuCI, HOOAc 83 (42)

C CH,(CH,),CH ,OH RuCI,(PPh3), PhlO (3 equ.) 88 (10)

RuCI, K,S,O, 97 (32)

RuCI, 86 0COOH KZSZO8 (32) Platinum Metals Table IX I Oxidation of Diols - Platinum t Substrate Product Catalvst Oxidant field Der ceni Ref. PhC(CH,)HCH(OH)CH,OH PhC(CH ,) H COOH RuCI, NalO, 92 Metals

OH HOOC (CH,),COOH RuCI, NaOCl 90 Platinum 6

Y OH OH

Metals I I CH3CH CHCH, CH,CHO RuCI, H,Oz 88

00 - do" RuCI, H2Oz

c L dOH \o RuCI, (PPh,), 'hCH=CHCOCH3 85

OH OH 00 II 1111 CH3CH CHCH, CH,CCCH, RuCI, (PPh,), 'hCH=CHCOCH, 70

Table X Oxidation of Aldehydes and Ketones I Substrate Product Catalyst Oxidant field per cent a CH,(CH ,),CHO CH ,(CH ,),COO H RuCI, (PPh,), 88

PhCHO PhCOOH RuCI, (PPh,), 96 CI CI

C @-COOH RuCI, 99

HOOC(CH,),COOH + HOOC(CH,),COOH D RuCI, NaOCl 3,56,28 + HOOC(CH,),COOH Platinum Metals -I

221-

NO LDN

c C P ff P P 0 f t m 0 00 0 00 0 0 n m mm m m 6 z zz z ii z i

c -r o m c s J a

0 0 I u

Ethers 0

XI o

Acvclic I u 0

of 0 Table 3= u, A N t+ +I I V Oxidation

c) 11 I I V 0

I u $

a m u n YI LL

Platinum Metals Rev., 1983,27, (3) 120 rial a

P 9 P 4 P 0 0 0 0m 0m i z z z z

Q Q QLT Qa Qa a a Ethers I 0 0 XI1 Cyclic u, I N 0 of I 0 Tabla 0 I u N 0 I V 0 -u 0

Oxidation 70 I 0

4 m u D W

Platinum Metals Rev., 1983,27, (3) 121 0 m

Ba Ba and Amines and Xlll

N 0 Table Sulphides %

of m LAooQoacI I I + I I 0=3 o=vI 0 O= V, 0 o=v I I& cnN V v1, V Om I I m V V Oxidation

Q LJI I IIz I 0 I D o=v, o=v 0" o=v o=v WCI LL I V Om I V I V V

am o nu U (1 I -

Platinum Metals Rev., 1983,27, (3) 122 oxidants cleave the diol, generally giving acids PhCOOR, in fair to good yields (XI-E, F). (IX-A, B), although under carefully controlled Oxidation of unsymmetric ethers, RORI where conditions aldehydes can be obtained as the one of the substituents is not aromatic, gives major product (IX-C, D). The use of unpredictable results with either R or R1 being RuClXPPhJ, and PhCH=CHCOCH, oxidised in roughly equal proportions. selectively produces non-cleaved diketones from diols (IX-E, F). Cyclic Ethers Only carbons next to the ether linkage are Carbohydrates oxidised. If both carbons are secondary, the Ruthenium catalyses the oxidation of products are mainly lactones with some car- hydroxyl groups in carbohydrates, secondary boxylic acids depending on the sensitivity of the hydroxyl groups being converted into carbonyls lactone to hydrolysis (XII-A, B). If one carbon and primary groups to acids. For example L- is secondary and the other tertiary the sorbose is converted to a mixture of erythrose secondary carbon is oxidised preferentially and glycolic acid (44) and sugar, I, is converted giving a lactone (XII-C, D). Some hydrolysis to to its keto sugar, 11, in 100 per cent yield (45). 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 I I1 stoichiometrically with ruthenium tetroxide. For example, tetrahydrofuran oxidised The yields are usually better when ruthenium is stoichiometrically with ruthenium tetroxide used than when CrO, is used (9). gives only y-butyrolactone in 65 to IOOper cent Oxidation of Aldehydes yield (2,47) but when oxidised catalytically with RuO, and NaIO, (47) the products are y- and Ketones butyrolactone, 40 per cent, and succinic acid, 5 Aldehydes are readily oxidised to carboxylic per cent. acids, see Table X. There has been little published work on the oxidation of ketones. In Oxidation of Sulphides one paper dealing with kinetics the authors and Amines reported that ketones were converted to Sulphides are usually oxidised to a mixture of diketones (46), for example: sulphoxides and sulphones (XIII-A, B), but in CH,COCH&H, + CH,COCOCH, at least one case a sulphone was obtained exclusively (XIII-C). However in this case a ten fold excess of subs- Linear primary amines are oxidised to trate was used. Diketones are cleaved to give a nitriles with some hydrolysis to the amide (XIII- mixture of acids (X-D). D, E). Cyclic amines are oxidised to either Oxidation of Ethers lactams (XIII-F, G)or imides (XIII-H, I). The yields range from poor to good. As with ethers, Acyclic Ethers only the carbon adjacent to the heteroatom is Primary methyl ethers, RCH,OCH,, are oxidised and secondary carbons are oxidised oxidised to methyl esters, RCOOCH,, in preferentially to tertiary carbons. excellent yields, (XI-A, B). Secondary methyl ethers, RR'CHOCH,, on the other hand Oxidation of Steroids undergo cleavage to give ketones, RCORI (XI- Steroids generally undergo the same reac- C, D). Benzyl ethers, PhCH,OR, undergo tions that have already been discussed- oxidation of the benzyl group to give esters, oxidative cleavage of C=C double bonds to

Platinum Metals Rev., 1983,27, (3) 123 acids (4, 53), degradation of aromatic rings (54), suggested (56) for removing sulphur containing and oxidation of secondary alcohols to ketones impurities from various petroleum fractions. (10,II). Thus sulphur (500 ppm) in an n-paraffin frac- It is interesting that cholesterol, which has tion was reduced to less than 50 ppm in 4 hours both a secondary hydroxyl group and a C=C by treatment with RdNaOCl at 2oOC. double bond, does not undergo any reaction Ruthenium has also been suggested (57,58) (10, XI). for the removal of ammonia from waste water An atypical reaction which some steroids by treating the waste at elevated temperatures undergo is simultaneous oxidation of a tertiary with oxygen and a supported ruthenium CH group to a tertiary alcohol, and a secondary catalyst. Chlorophenols and highly toxic

CH,~ to a ketone (54): ~ polychlorodibenzodioxinseffectively destroyedwere shown by ruthen- to be effectively destroyed by ruthenium catalysed oxidations (59). Finally ruthenium has been demonstrated to remove pollu- - tants in the gas phase. Thus Ac 0 AcO - 5500 ppm vinyl chloride in air 0 was reduced to 2 ppm by pass- Pollution Control via Ruthenium ing the gas over a 0.5 per cent ruthenium on Catalysed Oxidations alumina catalyst at a temperature of 376OC (60). Wet scrubbing with KMnO, is used com- mercially to control air pollution. However with Conclusion some pollutants, notably thiophenes, the reac- Ruthenium and its complexes are extremely tions are too slow to be useful. It has been versatile oxidation catalysts. They will catalyse demonstrated (5 5) that oxidation of thiophenes the oxidation of virtually any oxidisable organic with RdNaOCl is more than IOO times faster functional group and, by choosing the than with KMnO,. This means that residence appropriate conditions, the oxidations can be times are within the range that wet scrubbing made to proceed in high yield and selectivity of airborne thiophenes and other sulphur con- even in the presence of other oxidisable groups. taining pollutants is practicable. Thus they offer a useful alternative to the more Ruthenium catalysed oxidations have been classical oxidation reagents.

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Platinum Group Metals in Organic Synthesis Modern Synthetic Methods, Volume 3,1983-Transition Metals in Organic Synthesis EDITED BY R. SCHEFFOLD, Salle and Sauerlander, and John Wiley & Sons, 440 pages. To provide chemists with an easy access to The platinum group metals are featured in important and rapidly developing areas of syn- two of the five sections, these being “Principles thetic organic chemistry triennial seminars on of Transition Metals Chemistry” by Professor J. modern synthetic methods are held at K. Stille and “Group VIII Metals in Organic Interlaken, sponsored and organised by The Synthesis” by Professor L. S. Hegedus, both of Association of Swiss Chemists. The May 1983 Colorado State University. The contributions conference was devoted to the transition metals, are well supported by references and this most and the above named volume of contributions is useful book will undoubtedly fulfil its main being co-produced by Salle and Sauerlander, purpose of serving as a guide for chemists with distribution rights in Austria, Germany interested in the application of transition metal and Switzerland (sfr./DM 48), and by John chemistry to organic synthesis, in addition to Wiley & Sons for the rest of the world. aiding participants at the May conference.

Platinum Metals Rev., 1983,27, (3) 125