Ferrocenyl Phosphme Complexes of the Platinum Metals in Non-Chral Catalysis THEIR APPLICATIONS IN CARBON-CARBON AND CARBON-HETEROATOM COUPLING REACTIONS

By Thomas J. Colacot Organometallic Chemicals & Catalysts Development, Johnson Matthey, 2001 Nolte Drive, West Deptford, NJ 08066, U.S.A.

The Czsymmetric ligand, 1,l‘-bis(diphenylphosphino)ferrocene, discovered about three decades ago, has opened up a new area of chiral and non-chiral ligands. This ligand is a relatively air- and moisture-stable solid and has a large phosphorus-metal-phosphorus bite angle in its metal complexes. These types of ligands have demonstrated numemus applications in metal- catalysed organic transforrnutions, such as coupling reactions, asymmetric hydrogenation, hydrogen transfer, hydrosilation and allylic alkylation. In this brief review, some recent applications, particularly using platinum metals, will be discussed.

Ferrocenyl phosphines, such as the C2 symmet- aryl (12) substituted phosphines are known in the ric 1,l’-bis(dipheny1phosphino) ferrocene, dppf, literature, and their applications in difficult cou- have emerged as a new class of ligands, with a wide pling reactions are currently being explored. A few range of unique applications, since their first dis- examples of unsymmetrical ferrocenyl phosphines covery and use in 1965 (1). The literature describq have also been reported (13), although their cat- this includes, for example, the first two chapters of alytic applications have not been investigated in the book, “Ferrocenes”, edited by Togni and detail. Different research groups have utilised the Hayashi. In this book they give a detailed account planar chirality of the ferrocene molecule in of the syntheses and catalytic applications of dppf designing novel chiral phosphines - because of (2) and chiral ferrocenyl phosphines (3) up to the their inherent resistance to racemisation. As a year 1995. Recently, Hartwig has emphasised the result the area of chiral ferrocenyl phosphines has applications of dppf and related ligands in carbon- outgrown that of non-chiral ferrocenyl phosphines heteroatom coupling reactions (4). Hayashi (5), (3, 510). Richards (6), Togni 0,Knochel (8), Kagan (9) and It0 (10) have also reviewed the syntheses of chiral Synthesis of 1,l’-Bis(diphenylphosphin0)- ferrocenyl phosphine ligands which have applica- ferrocene and Related Ligands tions in asymmetric syntheses. The first example of a ferrocene-based phos- Aspects of this work, related to the platinum phine ligand, 1,l’-bis(dipheny1phosphino)ferro- group metals, is updated here. In particular this cene, dppf, was synthesised in 1965 by reacting the review covers some recent applications of non-chi- dilithium salt of ferrocene with PhzPC1 (1). ral ferrocenyl phosphines, coordinated to platinum Subsequent process improvements from academic group metals, in homogeneous catalysis. Emphasis (12) and industrial laboratories (14) led to the com- is placed on Kumada-Hayashi, Suzuki, Heck and mercialisation of dppf in multi kilogram quantities. Buchwald-Hdg coupling reactions. Examples of Cp2Fe(PR2)2, where R is an aliphatic group such as t-Bu or i-Pr are also known (12-14). Types of Ligands Seyferth (1 1) and Cullen (1 3) demonstrated an ele- Ferrocenyl ligands are classified into two types: gant way to make a few examples of unsymmetrical chiral and non-chiral. The best known example of ferrocenyl phosphine hgands, where the dilithium a non-chiral phosphine is dppf which has CZsym- salt of ferrocene is reacted initially with RPClz to metry (2). Monodentate (11) and bidentate akyl or produce a C-P-C bridged species. This bridge is

PhhntmMetdr h.,2001,45, (l),22-30 22 2BuLi r-TMEDA

Fe R*PCI -Fe c

RPCI2

Scheme I General routes for the synthesis of various non-chiral ferrocenyl phosphines later opened with RLi, followed by the subsequent pling reactions. The special advantages of using coupling reaction with R'2PC1 to produce ferrocenyl phosphhes in such couphg reactions (CaH,-PR2)Fe(C&-PR'*). There are also reports are reported here. of the synthesis of monosubstituted ferrocenyl phosphines (1 1). Although monosubstituted elec- Carbon-Carbon Coupling tron-rich ferrocenyl hgands are useful for aryl KnmaakHqubi Co@ing chloride coupling reactions, thek syntheses are Kumada and Hayashi were the first group to tricky, tedious and the products are very often demonstrate the extraordinary ability of contaminated with &substituted byproducts - as PdClzdppf, in difficult C-C couplmg reactions the mono lithium salt is not easy to isolate in pure form (1 5). General routes for the syntheses of vat- ious non-chiral ferrocenyl phosphines are summarised in Scheme I.

'Bite angle Applications of Ferrocenyl Phosphines PdClzdppt Numerous patents and publications have appeared during the past decade describing the involving Gngnard reagents and organic bro- applications of ferrocenyl phosphhes in metal- mides. In 1984 Hayashi, Kumada, +chi and catalysed organic reactions. Recently, increasing their coworkers (16) studied the reaction of organ- numbers of pharmaceutical and chemical compa- ic bromides with alkyl magnesium or zinc reagents nies have become interested in palladium- in the presence of various catalysts, such as catalysed carbon-carbon and carbon-nitrogen cou- PdC12(PPh&, Pd(PPh&, PdCbdppe, PdCLdppp

PkzfinumMetah Rm, 2001,45, (1) 23 Scheme I1 Kumada-Hayashi coupling reaction

I X Isolated yield 86-97’!. p-COMe, p-CO?Me, Scheme 111 />-CH:CH:COCHI Suzuki coupling of aryl mesyls using the Ni(0)dppf complex

and PdCl,dppb, where dppe = 1,2-bis(diphenyl- seems to suggest that the smaller Cl-Pd-Cl angle phosphino)ethane, dppp = 1,3-bis(diphenylphos- more strongly influences the reactivity than does phino)propane and dppb = 1,4bis(diphenylphos- the larger P-Pd-P bite angle, see the Table, (17). phino)butane, respectively. In particular see Further work is needed to correlate the reactivi- Scheme I1 for PdClzdppf. ty/selectivity with the X-P-X bond angles (X = The PdClZdppf catalyst gave excellent selectivi- halogen or leaving group). ty with nearly quantitative yield of the desired, coupled product, see the Table. They attributed the SupG CoKpling remarkable success of this catalyst to its larger bite Another interesting application of dppf in C-C angle and a flexible bite size. Gan and Horr called coupling was demonstrated by Suzuki and cowork- PdC1,dppf a ‘magic catalyst’ as its behaviour was ers on reacting aryl boronic acids with more far superior to that of the classical monodentate challenging aryl and vinyl triflates in the presence PPh3-based catalysts or even to that of the of PdC1,dppf (18). The coupling becomes more conventional diphenylphosphinoalkane-based difficult with inherently less reactive and less bidentate hgands (2). Until recently it was believed expensive aryl mesylates and sulfonates. By chang- that the larger P-Pd-P angle and longer P-Pd bond ing from PdClzdppf to NiClzdppf/Zn, Percec distance promoted the facile reductive elimination obtained 81 per cent yield of Cmethoxybiphenyl (that is, coupling), thereby minimising the p- by the Suzuki coupling reaction of phenyl mesylate hydride elimination. However Hayashi’s recent with 4-methoxyboronic acid (19). Miyaura later work with bis(diphenylphosphino)biphenyl, dpbp, extended this work to other examples of aryl

Comparison of Catalytic Activities in the Room Temperature Reaction of Bromobenzene with sec-BuMgCI Catalyst I Reacti; time, Bite angle, Angle sec-BuPh. n- Bu Ph , I P-Pd-P, I CI-Pd-CI,’ YO % (PPh3)zPdClz 24 n/a 5 6 PdChdppe 48 85.8 94.2 0 0 PdClzdPPP 24 90.6 90.8 43 19 PdClzdppb 8 94.51 89.78 51 25 PdClzdpbp 1 92.24 88.21 93 1 PdCIzdppf 1 99.07 87.8 95 2

I

Phbinwm Met& Rm, 2001,45, (1) 24 [Rh(acac) Ln] -7 --Ln Yield, % RB(OH)z + R’CHO DME-H20 or dioxane-H20 PPhl 0 [TOH PCy, 5 R = aryl, 1 -alkenyl; R’ = alkyl, aryl: DME = dimethyl ether dPPP 82 Scheme IV dppb 17 Rhodium catalysed C-C coupling involving u Suzuki reagent dppf 99

mesylates and even more challenging aryl chlo- gave only modest to moderate conversions. rides, and achieved yields of over 90 per cent by Although the results generaJly correlate with the P- using Ni(0)-dppf catalysts, see Scheme 111, (20). In Rh-P bite angles, dppb stands out as an exception: all these cases dppf as ligand is far superior to giving only 17 per cent conversion despite its rela- other biphosphines, such as dppp, dppe and dppb tively large bite angle. This seems to suggest that or monophosphines, such as PCy, and PPhs. Hayashi’s recent observation has to be taken seri- Interestmgly, it was observed by Pndgen that ously (17, and there may be factors other than the there was no couplug reaction between chiral flu- bite angle which decide the reactivity in the cou- orosulfonates and aryl boronic acids in the phg reactions. presence of NiClzdppf,while PdC12dppf gave over 98 per cent yield of the coupled product, 1,fdiaryl Heck Coupling: indene, without any racemisation (21). These com- Only a small number of reports are available on pounds are precursors to SmithIUine Beecham’s: applications of ferrocenyl phosphines in Heck SB 209670 and SB 217242 (code numbers of couplmg, although the importance of dppf in such future drug candidates). Water soluble Ph,P(m- chemistry is documented with several examples NaOS-CaH4)PdClz and [P(o-tolyl)3],PdCl2 are (23). equally effective, but they are either more expen- Butler (24), one of the early workers in this area sive or not readily available. with Cullen (13), recently demonstrated that reac- A C-C coupling of aldehydes with a ‘Suzuki tions using P(i-P+ substituted ferrocenyl reagent’ (organic boronic acid) in the presence of phosphine provided very high yields of coupled rhodium catalysts was also successfully demon- #run$ products (Scheme V) (lb: R, R’ = i-Pr). strated recently by Miyaura, see Scheme IV, (22). The monosubstituted derivative (2a: R = i-Pi) also In this case, with dppf as hgand, 99 per cent con- gave very htgh yields, whereas dppf (la: R, R’ = version was obtained. The other bidentate llgands Ph) and its alkyl-aiyl mixed derivative,

ArI + [PdCIzL/LZ],NEtj. MeCN CuI- Ar-cooM,

-L- Yield, % la 7

Fe Fe lb 100 2a 96 3a 53

I R=Ph, R’=Ph la R = i-Pr 2a R = i-Pr, R‘ = i-Pr lb Scheme V R=Ph, R’=i-Pr 3a Heck coupling using electron-rich ferrocenyl phosphines

Pbtinwnr Metah Rm, 2001,45, (1) 25 + WZdbP1 lBlNAP R"Hz - I Naaf-Bu), 80'C -R --R' Yield, % p-NC Hex 97 Scheme VI p-Me Hex 86 Buchwald coupling involving a primar)?amine

Fe($C5H4PPh2)q5-CgH4P(i-Pr)2 (3a: R = Ph, R' = was the only one which gave consistently higher &Pi) gave only modest and moderate yields, yields of the Stille coupled products when aryl tri- respectively, indicating the importance of aliphatic flates were coupled with organostannanes (27). phosphine moieties in Heck chemistry. Subsequently Hartwig's high throughput Carbon-Heteroatom Coupling screening studies (25) using 45 structurally differ- C-N Coz@ng Using +pf ent phosphines indicated that P(t-Bu)j and The most prominent reaction in this area is car- di(t-butylphosphino)ferrocene, lc, are the two bon-nitrogen bond formation, commonly referred most active systems to date for the Heck olefina- to as Buchwald-Hartwig coupling (4, 28). tion of unactivated aryl bromides, and that Buchwald's work (29) in this area indicated that the di(t-buty1phosphino)ferrocene is the most efficient ligand BINAP (BINAP = 2,2'-bis(dipheny1phos- hgand for olefination of aryl chlorides. phino)-l ,l'-binaphthyl, existing in racemic and Fu's successful work on palladium-catalysed chiral forms) in combination with Pdzdba, Heck, as well as Suzuki and Stille couplings, using (tris(dibenzylideneacetone)dipalladium(O)) is capa- P(f-Bu)3, needs to be mentioned here (26), as it has ble of catalysing the aryl amination process rekindled the interest in these difficult arylchloride involving a primary amine (Scheme Vl). Use of the coupling reactions. However pharmaceutical com- conventional bidentate ligand, dppe or monoden- panies tend to avoid using P(~-BU)~as it is tate P(o-tolyl)j gave unsatisfactory results. extremely air-sensitive and undergoes oxidation if Concurrently, Hartwig (30) identified palladium proper care is not taken. complexes of dppf (that is PdClzdppf, Pd(OAc)z/dppf or Pd(dba)z/dppf) as second gen- SMe Corzplg eration catalysts for similar couplings. These StiUe also observed that PdClzdppf behaves in a complexes are capable of producing high yields of unique way in C-C coupling reactions. Of several coupled products with aryl halides and various ani- palladium phosphine catalysts tried in the study it line derivatives, see Scheme VII. Electron-rich,

WClzdppf/dppf DX+ HzNeR1 NaO(f-BU).- €6-1Oo~C R X = Br, I Yield 80-94.1. R = o-. ni-. p-OMe, Me, Ph Scheme VII R' = p-CI, Ph, H Hartwig coupling

R = OMe, Me, Ph Yield 80-94-1. ArOTf = 2-naphthyl triflate Scheme VIlI R' = H C-N coupling (Buchwald-Hartwig) involving aTl trij7ate

PIarinUmMetuLr Rm,2001,45, (1) 26 Yield ca. 95.1.

Fe PPh2 Scheme IX Highly eficient PPF-OMe (4)assisied C-N coupling e 4 of a secondary amine with t-Bu-CoHaBr electron-poor, hindered or unhindered aryl bro- product (although 43 per cent product could be mides or iodides all performed well in this obtained by using 5 mol% PdClzdppf and 15 molYo chemistry. Hartwig's attempt to use other biden- of dppf (34)). This limitation was overcome by tate ligands, such as bis(diphenylphosphin0)- Buchwald using another ferrocenyl phosphine hg- propane or bis(diphenylphosphino)bemene, in and, (*)-PPF-OMe, 4, which provided a yield of - these types of couplulg reactions were not success- 95 per cent, see Scheme IX, (28,34-35). ful in terms of producing hgh yields of the desired Couplulgs of aryl chlorides with cyclic and couphg products (31). acyclic secondary amines were also difficult with This chemistry has also been extended to the dppf and BINAP. The use of palladacycle 5 in the amination of aryl aiflates with anilines to give Me excellent yields, see Scheme WI, (32). However, electron-poor aryl triflates were susceptible to cleavage under basic conditions. This problem has been overcome by the slow addition of triflates (32) or by the use of the base CszCOs(33). R = ortho - tolyl The unique nature of dppf or BINAP could be associatedwith their steric bulkiness, which may be couplulg of CF3C&Cl with an acyclic amine pro- responsible for the slow rate of P-hydride elimina- ceeded in reasonable yield (60 per cent). When the tion in comparison with the reductive elimination cyclic amine, piperidine, was used the yield process. This would also prevent the formation of increased to 98 per cent (36). Reddy and Tanaka Pd-diamjne complexes. (37) used the electron-rich phosphine, PCy3, to Although there may be a few exceptions where couple chlorobenzene with a secondary cyclic BINAP behaves better than dppf, Hkdg found amine, N-methyl pipemine. Hartwig also per- that dppf is effective in many couplug reactions (4, formed the same type of chemistry very 30). Buchwald in his recent review states "while successfully with several substrates by using optically active BINAP is expensive, the fin- di(t-butylphosphino)ferrocene, lc, (38), while that (*)-BINAP is usually a suitable surrogate has Buchwald followed a similar approach by using the led to its commercial availability" (34). However, bulky PCy2 substituted biphenyl &and, 6, (39). dppf is less expensive than racemic BINAP and is much easier to synthesise and put+.

Limitations of@pf Neither dppf nor BINAP were effective for the amination of aryl bromides, such as Ct-butylbro- Interestingly, &and 6 has been useful even for mobenzene with di-n-butylamine and have couplulg electron-rich aryl chlorides. Subsequently provided only 8 to 9 per cent yield of the desired 2-di(t-buty1phosphino)biphenyl was found to be a

PhhmMet& Rm.,2001,45, (1) 27 Fe PPh, Fe KY2

Nucleophilic corbene, 9

good hgand by the same group. Hartwig has also About five years ago, a group reported a conve- used the ferrocene-based chelating aliphatic phos- nient and efficient synthesis of BINAP by a C-P phines, 7 and 8 originally prepared by Cullen (12a) coupling involving the ditriflate of binaphthol with and Butler (13), to couple an electron-rich aryl PhzPH or PhzP(0)H in the presence of a phos- chloride, MeC&I.&l, with amines, including a pi- phine complex of NiClz (46).Although dppf is a mary amine, n-BuNHz (38). All these studies good hgand for the same reaction, the authors did suggest that bulky electron-rich llgands in combi- not mention any special advantage of dppf over nation with palladium or nickel are extremely other phosphines. A recent non-catalytic study useful for difficult aryl chloride (Arcl) couplmg. In suggests that PdClzdppf, in the presence of this regaxd, Nolan’s original work with the nude- Pd(dba)z, is capable of forming C-P bonds by ophilic carbene ligand 9 is worth mentioning as he reductive elimination (47). However, this is an demonstrated a very efficient coupling of Arc1 area, which needs further exploration. with a wide variety of aliphatic cyclic and acyclic secondary amines, and aliphatic and aromatic Conclusion primary amines (40). The importance of dppf and related ligands in late transition-metal catalysed organic reactions are C-0, C-S and C-P Couphg fairly significant, especially in coupling reactions. Intramolecular C-0 bond formation lea% to The steric bulkiness, large bite angles (for chelating the formation of heterocycles containing 0 atoms ligands) and relative stability of the aliphatic substi- was reported by Buchwald using Tol-BINAP (To1 tuted ferrocenyl phosphines, in comparison to = tolyl) or dppf in the presence of Pd(0Ac)z (41). P(t-B+, are some of the special advantages of Haitwig reported the first intermolecular C-0 cou- these types of ligands which give fade reductive pling of aryl halides, leading to the formation of elimination without any undesired isomerisation ethers, using dppf in the presence of Pd(dba)z (42). (P-hydrogen elimination) leading to byproducts. For the latter reaction, the yields were greater than Many of the ligands such as dppf or its P(i-P+ or 90 per cent. Ni(COD)z, in the presence of dppf or P(t-Bu)* substituted analogs are commercially avail- Tol-BINAP, gave more or less identical yields (68 able, and their air and moisture stabilities increase to 98 per cent) for the formation of t-butyl, methyl further when they are complexed with palladium. and silyl aryl ethers (43). More details on C-0 and Ni(0) complexes also appear to be useful for C-S coupling using dppf ligands have been certain difficult coupling reactions. However, these reviewed recently (4). complexes are fairly difficult to make in large quan- Hartwig’s group (44) has recently identified the tities, and nickel is not a very environmentally in sitr/ formation of an interesting hgand PhSFcP(t- friendly metal for pharmaceutical processes. BU)~,10, by the palladim-catalysed perarylation of The other C-heteroatom coupling reactions are the cyclopentadienylgroup in PFc(t-Bu)z, 2b, (45). also important in the chemical and pharmaceutical This ligand is very useful for room temperature industries. A fuller review, where chiral applica- aromatic C-0 bond formation with yields of up to tions will be discussed in greater detail, is planned 99 per cent (44). for publication during 2001 (48).

PhfiinUmMefuLrRev., 2001, 45, (1) 28 Acknowlegdements 16 T. Hayashi, M.Konishi, Y.Kobori, M. Kumada, T. Thanks are due to Professor J. F. Hartwig (Yale University), Higuchi and K Hirotsu, J. Am. Cbem. Soc., 1984, Professor S. P. Nolan (University of New Orleans) and W. 106,158 Tamblyn (JohnsonMatthey) for valuable suggestions. Professor 17 M. Ogasawara, K. Yoshida and T. Hayashi, S. L. Buchwald @IT) is acknowledged for providing some of Opomet&J, 2000,19, 1567 his recent references at short notice. R Teichman aohnson 18 (a) T. Oh-e, N. Miyaura and A. Suzuki, Synktt, 1990, Matthey) is thanked for support and encouragement in this 221; @) T. Oh-e, N. Miyaura and A. Suzuki,]. Org. work. Cbem., 1993,58,2201 19 V. Percec, J.-Y. Bae and D. H. Hdl, J. 0%.Cbem., References 1995,60,1065 1 (a) G. P. Sollot, J. P. Snead, S. Portnoy, W. R 20 (a) M.Ueda, A. Saitoh, S. Oh-tani and N. Miyaura, Peterson and H. E. Mertoy, Cbem. Absfr., 1965,63, Tetrahedron, 1998,54,13079; (b) S. Saito, S. 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PhhmMetah Rm, 2001, 45, (1) 29 39 D. W. Old, J. P. Wolfe and S. L. Buchwald, J. h. 46 (a) D. Cai, J. F. Payack, D. R. Bender, D. L. Hughes, Cbem. Sot., 1998, 120, 9722 T. R. Verhoeven and P. J. Reider, J. 0~.Cbem., 1994, 40 J. Huang, G. Grasa and S. P. Nolan, 0%.Lett., 1999, 59, 7180; @) D. Cai, J. F. Payack and T. R. 1,1307 Verhoeven, U.S.Patent 5,399,771; 1995 41 M. Palucki, J. P. Wolfe and S. L. Buchwald,]. Am. 47 M. A. Zhuravel, J. R Moncarz, D. S. Glue&, IC-C. Cbem. Sot., 1996,118, 10333 Lam and A. L. Rheingold, Otgmome&, 2000,19,3447 42 G. Mann and J. F. Hartwig,]. Am. Cbem. Sot., 1996, 48 T. J. Colacot, ‘A Concise Update on the 118,13109 Applications of Ferrocenyl Phosphines in Homogenous Catalysis’, Cbem. Rev., to be published 43 G. Mann and J. F. Hartwig,J. Otg. Cbem., 1997, 62, 5413 The Author 44 Q. Shelby, N. Kataoka, G. Mann and J. F. Hartwig, Thomas Colacot is a Development Scientist at Johnson Matthey in West Deptford, U.S.A. His interests include /. Am. Cbem. SOL,2000,122, (43), 10718 organometallic catalysts for chiral and non-chiral applications 45 M. Miura, S. Pivsa-Art, G. Dyker, J. Heiermann, T. and the evaluation of catalysts for organic transformations Satoh and M. Nomura, Gem. Commun., 1998,1889 using high throughput screening.

The 2000 MacRobert Award for the Platinum CRTm The MacRobert Award is the premier prize in the diesel exhaust into less harmful carbon for engineering innovation in the U.K. and is dioxide and water. awarded anndy by the Royal Academy of While the CRFconcept was patented in Engineering to a team responsible for a world- 1989, it could not be commercialised because the lea- engineering development demonstrating diesel fuel then available had a high sulfur con- technological achievement, successful commer- tent which inhibited the generation of nitrogen cial exploitation and benefit to the community. dioxide. The breakthrough occurred in the early In 2000 the MacRobert Award was won for the 1990s when Sweden introduced low-sulfur fuel second time by Johnson Matthey, this time for in response to environmental concerns. Low sul- the diesel emission control system known as the fur fuel and the CRTW are now sold across Continuously Regenerating Trap (CRT*). This Europe, in the U.S.A. and in Japan. revolutionary system has enabled trucks and The MacRobert Award coincided with the buses to control the pollution emitted by their fifth anniversary of the commercial launch of diesel-powered engines, especially the particulate CRY, first described in this Journal in a paper matter (soot), the health impacts of which are by Pelham Hawker (2). Hawker is a member of causing increasirig concern. the winning team, along with Piir Jones and the The MacRobert Award was first presented in co-inventors of CRF, Barry Cooper and Jim 1969, when it was shared between Rolls-Royce, Thoss. Barry Cooper was also in the team which for the Pegasus engine used in the Harrier jump- won the MacRobert Award in 1980 and so jet, and Freeman, Fox and Partners for the Sevem becomes the first person to win the Award twice. Bridge near Bristol. Johnson Matthey won the To celebrate the Award, Johnson Matthey Award in 1980 with its catalytic converter tech- has produced a special commemorative report, nology for passenger cars (1). Other winners “CRT5: Fifth Anniversary Publication”. Copies have included BP, British Gas and ICI. of this may be requested fiom: Ms Vanessa The modular CRFcomprises a filter down- Bystry, Johnson Matthey CSD, Orchard Road, stream of a specially-formulated platinum catalyst. Royston, Herts SG8 5HE, U.K; Fax: +44 1763 The CRY udlises the discovery that nitrogen 253 475 or E-mail: [email protected]. dioxide bums soot at temperatures typical of R. J. D. EVANS diesel exhaust, 2 275°C. The innovative design References of separating catalyst and filter sees the platinum 1 Phtjmm Metub Rev., 1981,25, (l),22 catalyst convert nitric oxide normally present in 2 P. N. Hawker, Plarnum Metuh Rev., 1995, 39, (l), 2 diesel exhaust into nitrogen dioxide. This is then used to bum the diesel soot trapped on the filter. The Author Robert Evans is the Marketing Manager for the Johnson Matthey The platinum catalyst also converts over 85 per Catalytic Systems Division. His interests lie in vehicle emissions cent of the hydrocarbons and carbon monoxide control systems, particularly retrofit applications.

Pbtinum Metub h.,2001, 45, (1) 30