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Pdf 593.66 Kb 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).
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