Catalytic applications of transition metals in organic synthesis GRAHAM J. DAWSON and JONATHAN M. J. WILLIAMS Department of Chemistry, Loughborough University of Technology, Loughborough, Leicestershire, LEI I 3TU, UK Reviewing the literature published between 1 July 1992 and 31 August 1993 1 2 2.1 2.2 2.3 2.4 2.5 3 3.1 3.2 3.3 3.4 4 4.1 4.2 4.3 4.4 5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6 6.1 6.2 7 7.1 7.2 7.3 7.4 7.5 7.6 8 9 Introduction Oxidation Oxidation of C-H bonds Dihydroxylation Epoxidation Aziridination 0 t her oxidations Hydrogenation and related processes Hydrogenation Hydroboration Hydrosilylation H y drof ormy lation Lewis acids Diels-Alder and related processes Aldol reactions Hydrocyanation and silylcyanation Other nucleophilic additions Coupling reactions Heck reactions Suzuki-type coupling Stille-type coupling Coupling reactions of other nucleophiles Carbometallation Reactions involving alkynes Hydroxycarbonylation and alkoxycarbonylation Allylic substitution Tandem and cascade processes Reactions involving metal carbenoids C y clo propanation Insertion reactions Miscellaneous Acetalization Thioether formation Conjugate addition Ring fusion and expansion Metathesis Isomerizations Conclusion References 1 Introduction This review highlights significant advances in transition metal catalysis during the period 1 July 1992 to 31 August 1993.The growing body of information in the area enables predictions to be made about the chemoselectivity and stereoselectivity of many homogeneous transition metal catalysts for particular substrates.There has been such a huge volume of publications concerned with transition metal catalysts that it is not possible to provide a comprehensive account. However, we have endeavoured to summarize current areas of interest and to provide commentary on the important advances. Only homogeneous applications have been considered for this review. 2 Oxidation 2.1 Oxidation of C-H bonds The direct oxidation of alkanes by catalysis continues to attract attention. Hirobe and co-workers have reported the oxidation of methylcyclohexane 1 into 1 -methylcyclohexanol3 using 2,6-dichloropyridine N-oxide 2 and a ruthenium porphyrin catalyst.' 1 -0 2 (0.5 mot%) nzene, r.t.. 6h 94% 3 Similarly, Murahashi and co-workers have detailed the ruthenium-catalysed a-methoxylation of tertiary amines2 and also the direct oxidation of alkanes to alcohols and ketones3 Thus, treatment of dimethylaniline 4 with 30% hydrogen peroxide solution and a ruthenium catalyst in methanol afforded the methoxymethylamine 5 .Murahashi and co-workers have also reported the oxidation of alkanes using molecular oxygen in the presence of catalytic copper salts.4 Da wson and Williams: Catalytic applications of transition metals in organic synthesis 77Me Me (DHQD),PHAL ligand. However, in the presence of I 2eq H202 a 4 the achiral catalyst quinuclidine, tetra-substituted alkenes are the most reactive category of alkene. o N \ M e RuCb.nH20 MeOH. r.t., (5 ml%T 2h u k H 2 O M e 67Yo R<R/== R- - 4 5 2.2 Dihydroxylation The asymmetric osmium-catalysed dihydroxylation of alkenes has recently been reviewed.' Crystal structures and synthetic details of the cinchona alkaloids used as ligands for this reaction have appeared? Sharpless and co-workers have demonstrated many synthetic applications of the dihydroxylation process using the commercially available AD-mix-a and AD-mix-/? reagents.[These mixtures contain K,Fe( CN),, K,CO,, K,OsO,(OH),, and either (DHQ),PHAL 6 (for AD-mix-a) or (DHQD),-PHAL 7 (for AD-&-/?).'] For example, the dihydroxylation of enol ether 8 affords a-hydroxy ketone 9,7 and the dihydroxylation of /?, y-unsaturated ester 10 affords the hydroxy lactone 1 1 .8,y 9 (97% e.e.) AD-Mk6 n 10 11 (98% e.e.) There have been numerous reports of other synthetic applications both by the group of Sharpless and by others.Fine-tuning of the ligand has been achieved to accommodate the enantioselective dihydroxylation of cis-disubstituted alkenes using the indolinyl ligand 1 2.1° Similarly, the pyrimidine ligand 13 has been used for improved enantioselectivity in asymmetric dihydroxylations of terminal alkenes.' Dh I ox 12 6 (X=DHQ) 7 (X=DHQD) Ph 13 Dihydroxylation of non-symmetrical dienes'* and rate studies on different categories of olefin have revealed a reactivity hierarchy, as illustrated in Scheme 1 .13 The anomalous position of the tetra-substituted olefin is a consequence of the steric demands of the Scheme 1 The kinetic resolution of alkenes which already contain a chirality centre has been reported using the asymmetric dihydroxylation pro~ess.'~J 2.3 Epoxidation There has been a continued interest in the development of novel catalysts for epoxidation of alkenes. Catalysts which can operate with cheap oxidants and those which can provide asymmetric induction are particularly useful.Mukaiyama and co-workers have reported the oxidation of alkenes into epoxides with molecular oxygen, catalysed by a Co" complex, using propionaldehyde diethyl acetal as the reductant,', in an extension of previous reports employing aldehydes as the reductant.17 An asymmetric variant of this process has been reported using enantiomerically pure manganese complexes to achieve up to 84% e.e. in the epoxidation of alkenes.' Enantiomerically pure ruthenium catalysts have been employed to give 50-80% e.e. in the epoxidation of styrene with iodosylbenzene, but with low yields (12-38'/0).'~ Collman and co-workers have prepared threitol-strapped manganese porphyrins which are capable of catalysing epoxidation with up to 88% e.e.,O Deng and Jacobsen have used the ( salen)MnlI1 complex 14 as a catalyst for the epoxidation of (2)-ethyl cinnamate 15 to give the product 16 with very high enantioselectivity.Treatment with ammonia afforded the ring-opened amide 17 which was readily converted into 18, the side chain of taxol.,' NaOCI, 14 (6 mI%) 4-phenylpyridine-N -oxide ph Ph=CO,Et 16 I 15 (0.25eq) 95-879h.e. 56% 18 17 H Q H 'But 14 But' 78 Contemporary Organic SynthesisAdam and Nestler have developed an interesting titanium-catalysed epoxy-hydroxylation of allylic a l ~ o h o l s . ~ ~ , ~ ~ Photo-oxygenation of the ally1 alcohol 19 affords a 90: 10 mixture of the diastereomeric hydroperoxy homoallylic alcohols 20 and 2 1.19 However, upon treatment with catalytic amounts of titanium tetraisopropoxide, the major diastereomer 20 rearranges more quickly to the epoxy diol products 22 and 23 (95 : 5) than the minor diastereomer 2 1 does to the alternative epoxy diols 24 and 25. 20 22 955 23 ?" 21 24 95:5 25 2.4 Aziridination Transition metal catalysed aziridination of alkenes has been known for several years. However, the recent demonstration of the ability of Cu' or Cu" salts to catalyse aziridination of alkenes with [ N-( p-toluenesulfonyl)imino] phenyliodinane, PhI = N T s , ~ ~ has led to reports of asymmetric variants of this reaction by the groups of Evans and Jacobsen.Evans and co-workers report the use of the bis(oxazo1ine) ligands 26 in the presence of a copper catalyst and the cinnamate 27 with PhI = NTs, to afford the aziridine 28 with excellent enantioselectivity (97% e.e.).25 Jacobsen and co-workers report that whilst salen ligands were found to afford poor results, the diimine ligand 29 was effective for providing asymmetric aziridination under conditions similar to those used by Evans. ph&co2Ph 6 mot% CuOTf, PhI=NTS. Ts I * ph&co2Ph 6 mot% CuOTf, 27 benzene,' 21 oc 28 (97%e.e. 64%) For the chromene 30, asymmetric induction in the product 3 1 was very high ( > 98°h).26 However, not all of the substrates examined by these groups gave such high levels of enantioselectivity. Furthermore, Katsuki and co-workers have reported that the use of enantiomerically pure (salen)manganese( 111) complexes for asymmetric aziridination gives moderate levels of enantioselectivity, but poor yields.27 NC 10 mol% CuOTf, PhI=NTS, CH&I2, r.t., 75% cyy NTs 30 31 (>98% e.e.) 2.5 Other oxidations Larsson and Akermark have reported a catalytic system for allylic acetoxylation. Cyclohexene 32 is oxidized to cyclohexenyl acetate 33 in the presence of catalytic palladium acetate and ferric nitrate in acetic acid under an atmosphere of oxygen.28 Pd(0AC)z (5 mop/.) L Fe(N0&9H20 (5 mot%) AcOH, 0 2 32 OAC 33 (92%) Uemura and co-workers have shown that sulfides 34 may be catalytically converted into sulfoxides 35 using titanium tetraisopropoxide in the presence of (R)-BINOL { (R)-l,l'-bi-2-naphth01].~~ In an alternative procedure, Jacobsen and co-workers have used (sa1en)manganese complexes 14 to catalyse the asymmetric oxidation of sulfides (up to 68% e.e.).30 Ti(OPh4 (10 mot%) 0 s (R)-BINOL (2OmoW) I I 5 Ar0''Me Ar' 'Me 34 H20 2 eq., Bu'OOH 2eq.toluene. r.t., 1 h 35 (up to 73% e.e.) 3 Hydrogenation and related processes This section includes recent advances in catalytic hydrogenation, hydroboration, and hydrosilylation. Additionally, hydroformylation and silylformylation are described here. 3.1 Hydrogenation Faller and Parr have demonstrated the use of chiral poisoning as a novel strategy for asymmetric ~ynthesis.~ Asymmetric rhodium-catalysed hydrogenation was chosen as the example. A mixture of [( S, S)-chiraphosRh] + and [( R, R)-chiraphosRh]+ was employed as the catalyst, and would be expected to afford racemic products in the hydrogenation of dimethylitaconate 36.However, in the presence of a chiral 'poison' 37, one enantiomer of catalyst is deactivated preferentially, and thus asymmetric induction is now observed in the product 38. Da wson and Williams: Catalytic applications of transition metals in organic synthesis 79RaOemiC LZ [(chiraphOsRh)d2*,H2 Z H NMe2 Z 38 (4me.e.) &OPPh2 MeS 36 37 Z = C02Me Phosphine-borane complexes provide a method for stabilizing phosphines to oxidation. A recent report describes the direct use of phosphine-borane complexes in asymmetric synthesis. The ( - )-DIOP-borane complex 39, upon treatment with [( COD)RhCl], and DABCO affords a catalyst which is competent for asymmetric hydrogenation reactions?* -6H3 39 Wang and Backvall have reported the ruthenium-catalysed hydrogenation of imines.Thus, the reaction between isopropanol and the imine 40 in the presence of ruthenium catalyst and base affords the amine 4 1 as the product.,, 41 (93%) 40 Whilst highly enantioselective hydrogenations of ketones and alkenes have been known for a long time, it is only recently that reports of highly enantioselective reductions of imines have been reported. Burk and Feaster have demonstrated the ability of rhodium DuPHOS complexes to catalyse the hydrogenation of N-aroylhydrazones 42 with the formation of product 43 in 72-97% e.eP4 Willoughby and Buchwald have employed enantiomerically pure titanocene complexes for the hydrogenation of imines 44 with the formation of product 45 in 53-98% e.e.35 H N/Ny I Cat; Rh(Et-DUPHOS)* b Et Ll Et‘ enantiomerically pure titanocene catalyst c P h 4 N H2 77%yield 44 H I 43 (88%e.e.) H 45 (9Phe.e.) The highly successful ruthenium-BINAP reagents introduced by Noyori and co-workers are now routinely employed for the asymmetric reduction of ketones and alkene~.,~ A recent development is the in situ formation of the catalyst.A methanolic solution of Ru( acac), and (S)-BINAP were treated with hydrogen at 1000 psi, followed by the introduction of the alkene 46. The product, ibuprofen 47 was formed with 88% e.e.37 46 47 (88%e.e.) Reagents: cat. Ru(aca& cat. (S)-BINAP, MeOH, lo00 p.s.i. H2 3.2 Hydroboration Investigations into the rhodium- and iridium-catalysed hydroboration of alkenes (Scheme 2) have been further documented, in terms of synthetic utility3* and mechani~m.~”~’ These papers effectively summarize and review the catalytic hydroboration process, and the reader is directed to these reports for further information.cat. Rho or Ir(1) kR Scheme 2 3.3 Hydrosilylation Asymmetric hydrosilylation of norbornenes4* and dihydr~furans~~ using an enantiomerically pure palladium catalyst has been reported to give hydrosilylation products of up to 96% e.e. Treatment of norbornene 48 with trichlorosilane in the presence of a palladium catalyst and the enantiomerically pure ligand 49 affords the product 50 with very high enantiocontrol. Lb 48 49 50 (up to 9 6 % ~ ) Ito and co-workers have employed an intramolecular palladium-catalysed bis-silylation of alkenes as a route to polyol ~ynthesis.4~ The reaction of the alkyne 5 1 with catalytic palladium acetate and isocyanide 52 affords the bis-silylated adduct 53 in 85% yield.Hydrogenation and oxidation leads to the formation of 1,2,4-triols 54.45 The palladium-catalysed dimerization/double silylation of 1,3-dienes under ambient conditions has been reported. The reaction of disilane 55 with diene 56 in the presence of a palladium catalyst affords the product 57 in a remarkable 85% 80 Contemporary Organic Synthesismsi.Me Ph-CEC Me3si5=u Ph 51 Ph HKoH . ._ 54 Reagents: Pd(OAc), (0.7-2 md%), Me3CCH,C(Me),NC 52(0.1-0.3equiv), toluene,ll 1 OC 2h Me,Si-SiM+ 55 56 5mt% Pd(dbah DMF r.t. 40h SiMe, Me,Si Me 57 (85%) 3.4 Hydroformylation Totland and Alper have investigated the hydroformylation of vinyl sulfones and vinyl sulfoxides catalysed by the zwitterionic rhodium compkx 58.47 The reaction of ethyl vinyl sulfone 59 with carbon monoxide and hydrogen using catalytic 58 and dppb affords exclusively the branched chain product 60 in 98% yield.CHO MeAS02Et 60 (98%) @S02Et c 59 Reagents: COW2 (600 psi), cat. dppb, CH2C12, 75 OC, Takaya and co-workers have reported an enantioselective rhodium-catalysed hydroformylation of olefins.48 Recent reports of silylformylation of aldehydes and e p o ~ i d e s ~ ~ have appeared. Wright and Cochran have shown that treatment of aldehydes 6 1 with dimethylphenylsilane in the presence of catalytic amounts of [( COD)RhCl], affords the corresponding a-silyloxy aldehydes 62 in 60-90% yield.50 OSiPhMe2 c Ar ArACHO 61 62 Reagents: Me,PhSiH, [(COD)RhClh, (0.5 mot%) CO (250psi), 23 OC, 24 h, THF 6040% 4 Lewisacids Transition metal reagents such as titanium tetrachloride have been familiar Lewis acids for a long time. However, there is a growing tendency to exploit the properties of the later transition metals as Lewis acids, since advantages in terms of catalytic turnover and ligand design may be afforded.4.1 Diels-Alder and related processes Kobayashi and co-workers have reported the use of scandium trifluoromethylsulfonate as a reusable catalyst for the Diels-Alder reaction.51 For example, the reaction of isoprene with methyl vinyl ketone afforded the Diels-Alder adduct 63 in 91% yield using scandium trifluormethylsulfonate as a catalyst.0 SC(OTf), (10 ~ o K ) L' CH2CI2 91 ,O"C, 46 13h * Other examples of transition metal catalysed Diels-Alder reactions include reactions mediated by the ruthenium catalyst 64,52 developed by Bosnich and co-workers, and the polymer-bound, iron-based Lewis acid catalyst 65.53 64 65 Evans and co-workers have prepared enantiomerically pure bis( oxazoline) ligands 66 for use in copper( 11) triflate catalysed Diels-Alder reactions.54 High levels of enantioselectivity were reported for the catalysed reaction between cyclopentadiene and oxazolidinone 67 to afford the Diels-Alder adduct 68. 68 (>98% 8.8. 98:2 endo:exo ) Reagents: 5mol% Cu(OTf)z, CH2C12, -78 "C, 18h Further advances in the use of enantiomerically pure titanium catalysts for the carbonyl-ene reaction have been described.55 Mikami and co-workers have demonstrated an asymmetric desymmetrization of 69 into 70, catalysed by a chiral titanium complex previously developed by this obtained with very high enantioselectivity and very high diastereoselectivity. Interestingly, the reaction of the silyl enol ether 7 1 with methyl glyoxylate and the same catalyst affords the ene-type product 72, rather than a Mukaiyama The product was Da wson and Williams: Catalytic applications of transition metals in organic synthesis 81aldol reaction.Furthermore, remarkably high enantioselectivity and diastereoselectivity is obtained in the aldol-like product.57 o'SiR3 (18 (BINOL)TiCI2 (10 MS ml%) 4h. -@m&le n CH&I* 0 "c -\ 0 27% SIR3 099% syn HKCOIMe 70 >99% 9.9.) OSiMe, 0 + HKC02Me 71 (5 ml%) 58% Me3SiO OH +CO2k Me 72 (982syn:anti 94:6 Z:E 99% e.e.) 4.2 Aldol reactions As well as the highly stereoselective example cited above, more conventional Mukaiyama aldol reactions have recently been reported to be catalysed by scandium trifluor~methanesulfonate,~~ ruthenium catalysts 64,59 mercuric iodide,6O the titanium and zirconium catalysts 73 and 7461 and also the unusual binuclear iron complex 75,62 which offers the possibility of bis-coordination to the carbonyl substrate.ChTi(OTf)2 73 CpzZr(0Tf)ZMF oc-,i.: ;B., oc PPh2 Ph2P' \,go u 74 75 4.3 Hydrocyanation and silylcyanation Faller has used the unusual tungsten complex 76 as a Lewis acid precursor. Upon loss of CO, a potent Lewis acid is formed, which catalyses the addition of trimethylsilyl cyanide to aldeh~des.6~ Asymmetric trimethylsilylcyanation of aldehydes has been achieved with the titanium based catalysts 77,h4 78,6s and 79.66 Inoue has exploited titanium complexes of the peptide 80 as catalysts for the asymmetric addition of hydrogen cyanide to aldeh~des.6~ In the case of a, 6-alkenyl aldehydes 81, the product cyanohydrin 82 was acetylated, treated with catalytic bis( acet0nitrile)palladium dichloride, and hydrolysed to afford the rearranged product 83.6* 79 83 09%e.e.Reagents: (i) Ti(OEt), (10 md%); (ii) A%O, C H N; (iii) 10 I ~ ~ % P ~ C I ~ ( C H ~ C N ) ~ THF;'(ii hydrolysis 4.4 Other nucleophilic additions Nuss and Rennels have employed cationic iridium and rhodium catalysts for the addition of allyltributyltin 84 to aldehydes 85 to afford the homoallylic alcohols 86.69 In the presence of enantiomerically pure catalysts, a small but promising degree of asymmetric induction was observed.m S n B u 3 / 84 + cat. Ir( CO) (PPh&J304 L 01 cat. Rh(CO)(PPh&CD4 R CH&IZ 3147% 86 R J L 05 Shibuya and co-workers have devised an enantioselective hydrophosphonylation of aldehydes 87 which uses diethylphosphonate 88 and a titanium catalyst 78 to afford a-hydroxyphosphonates 89.'O Whilst the enantioselectivities reported are modest (up to 53% e.e.), there is potential for the improvement of this process. OH O H, 7b (20mo1%) AH + 0°C 15h Etfl Ar A P(OEt)2 Ar r\ 0 87 88 89 " 82 Contemporary Organic Synthesis5 Coupling reactions This large section contains a multitude of reactions which are often identified as coupling reactions. Many of these catalysed processes involve the formation of new C-C bonds, and are therefore of great synthetic utility.Only representative examples have been chosen for this section due to the enormous amount of research activity in the area. 5.1 Heck reactions Busacca and co-workers have developed an interesting new vinylamine equivalent for use in the Heck rea~tion.~' Heck reaction between aryl iodides and the vinyloxazolone 90 affords the Heck adducts 9 1. Hydrogenation affords the corresponding phenethylamines 92. 0 P N K O pd(oAC)2 (4 k e ~ 1 H0 )=( N~HCO~, BU,NCI ArI DMF Ph 9o Ph Ph Ph /P: EtOH, HOAc H2 50 psi I\r/\/NH2 92 Another application of the Heck reaction is in the arylation of 4H- 1,3-dioxin 93.72 The arylated product 94 can be converted into cinnamaldehydes 95 upon heating ( via a retro Diels-Alder reaction).Cat. Pd(OAc)z, PPh3 0 AgS03,DMF ArI 60°C * Ar 4745% 94 93 toluene reflw 95 An interesting example of a palladium-catalysed reaction between cyclopentenylzific chloride 96 and diiodobiphenyl97 affords the cyclization product 98.73 The reaction is believed to proceed via the coupled intermediate 99, which is able to undergo an intramolecular Heck reaction. 97 McClure and Danishefsky have reported an intramolecular Heck reaction on the highly functionalized substrate 100 to afford the cyclized product 101, showing the remarkable chemoselectivity afforded by the Heck arylation reaction.74 100 cat. Pd(PPh& EtjN, MeCN 80% loh 1 101 (90%) Asymmetric Heck reactions may be achieved for certain substrates.The use of a palladium BINAP complex as a catalyst for the reaction between dihydrofuran and the alkenyl triflate 102 affords the product 103 in 62% yield and with > 96% e.e.75 ,COPE1 0 ''OTf 102 (R-BINAP)zPd (3 mot%) Proton sponge benzene 6246 1 CO2Et 06 103 (~96% e.e.) Hillers and Reiser have demonstrated that the enantioselectivity and regioselectivity of related reactions are pressure dependent.76 Ashimori and Overman have reported the asymmetric Heck cyclization of the aryl iodide 104.77 Using a palladium BINAP catalyst in the presence of a silver salt, they obtain the product 105 with up to 7 1% e.e. However, in the presence of 1 ,2,2,6,6-pentamethy1piperidine7 and the absence of silver salts, the other enantiomer of the cyclization product is obtained with 66% e.e.! 5.2 Suzuki-type coupling The coupling of boron compounds with organic halides is frequently called the Suzuki coupling.The reaction is also effective using organic triflates as one of the coupling partners.78 Soderquist and Rane have reported a synthesis of ( + )-exo-brevicomin 109 which relies upon a Suzuki coupling followed by an osmium-catalysed asymmetric dihydr~xylation.~~ Da wson and Williams: Catalytic applications of transition metals in organic synthesis 830 1 81% t 0 105 (71%e.e.) Reagents: Pd,(dba), (5 md%),R-(+)BINAP (10 mol%), 1-2eq. Ag3P04, MeCONMe2, 80 "C, 26h Coupling of the borane 106 with (E)-1-bromobut-1-ene affords the isomerically pure alkene 107.Asymmetric dihydroxylation affords the diol 108 which is readily cyclized to 109. L L 106 107 (85%) I E&D,BPHAL reductive coupling of acid chlorides 1 14 with (E)-1,2-bis( tri-n-butylstanny1)ethene 115. Thus, in order to produce these diketones, the alkene has been reduced under the reaction 112 I 110 I cat. Pd(0) F A r 111 Nicolaou and co-workers have reported an exciting example of the use of the Stille coupling for the last step in the total synthesis of the macrocycle rapamycin 1 16 from the acyclic precursor 1 17 and (E)-1,2-bis(tri-n-butylstannyl)ethene 1 15.82b This remarkable cyclization process demonstrates the chemoselectivity of reactions of this type. 5.3 Stille-type coupling Mitchell has recently reviewed the palladium-catalysed reactions of organotin compounds.81 The Stille reaction has been widely exploited recently, and only a tiny selection of such applications can be reported here.Eschavarren and co-workers have prepared 1,4-diketones 1 13 by the palladium-catalysed 114 + mMe 108 (96?/0, 95%e.e.) Whiting and co-workers have examined the reaction of the vinylborate ester 110 with aryl iodides in the presence of a palladium catalyst.80 There are two possible outcomes and either the Suzuki product 1 11 or the Heck product 112 could be obtained. The Heck product appears to be kinetically preferred, although minor changes in conditions affected the product ratio. Me., 117 t mMe Reagents: 115 (1.2eq.), Pr',EtN (1.5eq.), Pd(MeCN)&I2 (20 mol%), DMF/THF (1:1), 25 "C, 24h Moriarty and Epa have demonstrated that alkenyl iodonium salts are reactive components for the Stille coupling.83 Thus, the reaction between the iodonium salt 1 18 and tributylvinyltin 119 affords the coupled product 120 in 5 minutes at room temperature using bis( acetonitri1e)palladium dichloride as catalyst.Vedejs and co-workers have demonstrated that tin reagents 12 1 are unusually reactive in the Stille coupling, and can be used for the selective transfer of primary alkyl groups.84 84 Contemporary Organic SynthesisPh A;, Ph P s n B u , 118 Pd(MeCN)&Iz (5 d%) Ph& m + 5min r.t. 78% 120 5.4 Coupling reactions of other nucleophiles Apart from the use of boron and tin reagents for coupling reactions, other reagents can be employed, including Grignard reagents, organozincs, and other organometallic species.Snieckus and co-workers have employed nickel catalysis for the reaction between arylcarbamates or aryl triflates and Grignard are reported, including the nickel-catalysed coupling of the carbamate 122 with Grignard reagent 123 which affords the coupled product 124 in 83% yield. Many examples awmEt2 TMSCHAgCI Ni(acac)z (5 ml%) 7 EtzO, rl., 83% z - 122 Z= Et&JOC 124 Kocienski and co-workers have shown that the nickel-catalysed coupling of Grignard reagents with 5-alkyl-2,3-dihydrofrans is also effective.86 Thus, the reaction of 125 in the presence of catalytic (Ph,P),NiCl, and phenyl magnesium bromide affords homoallylic alcohol 126 as the product with 96% isomeric purity. One recent application of the cross-coupling of organozinc reagents has been reported by Negishi and co-worker~.~~ Treatment of the vinyl iodide 127 with ethylzinc bromide afforded a zinc alkoxide which could then be coupled with another alkylzinc halide using palladium catalysts to generate the product 128.4347% Me*OH R 1 27 128 Larock and co-workers have reported the coupling of aryl and vinyl halides or triflates with vinylic epoxides,88 vinylic oxetanes,89 and vinylic azetidinones.’O 5.5 Carbometallation Hoveyda and co-workers have continued a study of the zirconium-catalysed ethylmagnesation of alkenes. The reaction of ally1 alcohol 129 with four equivalents of ethylmagnesium chloride and 5 mol% of Cp,ZrCl, affords the intermediate 130, which upon treatment with oxygen affords the dioll31 with 90% d.e.” OH EtMgCl(4 eq.) Et@ Et n-nonyl Et 131 (90%d.e.70%) Conversion of the aUylic alcohol into the corresponding methyl ether affords the alternative diastereomer as the major product (78% d.e.). Furthermore, the use of enantiomerically pure zirconocene catalysts leads to highly enantioselective reactions.Y2 Knochel and co-workers have reported a novel palladium-catalysed intramolecular carbozincation of alkene~.~, The reaction of 132 with diethylzinc and 1.5 mol% of PdCl,( dppf) gives the cyclized organozinc species 133. Transmetallation with CuCN-2LiCl and trapping with benzoyl chloride affords the functionalized cyclopentane 134 in 76% yield. Ph 133 132 / (i)CuCN.PLiCI / (ii)PhCOCI Ph . .. mPh 134 (76%) 5.6 Reactions involving alkynes Ogawa, Sonoda, and co-workers have reported the palladium acetate catalysed addition of aromatic thiols 13594 and benzeneselenol 1369s to acetylenes 137 to give the products 138 and 139.In neither case was the catalyst poisoned by the reagent. 138 X = S 139 X=Se The palladium-catalysed thioboration of terminal alkynes has also been reported.Y6 Thus, treatment of pent-1-yne 140 with 9-(phenylthio)-9-BBN 141 affords the intermediate 142, which can be converted in situ, by addition of iodobenzene, into the coupled product 143 in 95% yield with 99% isomeric purity. + PhSBBN 141 /A&;: K3P04, DMF C3H7 PhS *Ph 143 (95%) Da wson and William: Catalytic applications of transition metals in organic synthesis 85Dixneuf and co-workers have investigated the stereoselective addition of carboxylic acids to alkynesg7 They have also reported that in the presence of the binuclear catalyst precursor [Ru(p-O,CH)(CO),(PPh,)],, hexyne 144 and mandelic acid 145 are converted into the 1,3-dioxolan-4-one 146.98 toluene 100 “c 144 Ph’ 145 146 (9O:lO cis.mns 86%) Kita and co-workers have shown that whilst standard catalysts for the addition of carboxylic acids to alkynes are ineffective in the case of alkoxyalkynes, catalytic amounts of [RuCl,( p-cymene)], were effective for the addition of various carboxylic acids 147 to ethoxyacetylene 148, thereby affording ethoxyvinyl ester 149.” 148 toluene 40 “C 15min 6044% 147 - 1 49 Nuss and co-workers have reported the palladium-catalysed reaction between di-iodoenyne 150 and five equivalents of the alkynyl stannane 15 1 to afford the product 152 in 32% yield, in which three new C-C bonds have been formed.I Bu3Sn+CHflTBDMS HO 161 r C H 2 0 m D M S Larock, Cacchi, and co-workers have exploited palladium catalysts for the formation of furans.lOO Reaction of the alkyne 153 with iodobenzene in the presence of a palladium catalyst directly affords the furan 154 in 57% yield. Meh, M e 2mol%Pd(PPb)4 K2CQ PhI (2 eq.) (3 eq.1 ~ Me& Me Ph 0 DMF 60 “C, 6h 153 154 (57%) Trost and Indolese have reported the novel ruthenium-catalysed addition of alkenes to alkynes.lO’ Thus, the reaction of oct-l-ene 155 and oct-l-yne 156 with a ruthenium catalyst afforded a 69% yield of the branched diene 157 and the linear diene 15 8 (157 : 158,5 : 1). Trost and co-workers have further documented the use of the ruthenium-catalysed ‘reconstitutive condensation’ of acetylenes 159 with ally1 alcohols 160 to afford the ketone 16 1.Evidence has been presented to support the proposed mechanism for the catalytic cycle.102 Applications to the functionalization of steroidal side chains have been reported. lo3 155 CpRu(C0O)CI (5 ml%) DMFH20 (3:l) 100°C. 2h, 69% I 157 5:l 158 doH cat. CpRu(PPh&CI NHQF6 100 “c neat R E + 159 160 161 5.7 Hydroxycarbonylation and alkoxycarbonylation An interesting approach to a-amino perfluoroalkanoic acids has been designed by Uneyama and co-workers. lo4 Palladium-catalysed carbonylation of the imidoyl iodide 162 in the presence of benzyl alcohol affords the corresponding a-imino ester 163, with further transformation giving the a-amino acid 164.163 164 Reagents: CO(latm), Pdp(dba)3CHC13, PhCH20H, K2C03, toluene Ali and Alper have reported a highly regioselective catalytic process for the hydrocarboxylation of methylenecy~loalkanes.~~~ The reaction of methylenecyclohexane 165 with two equivalents of formic acid in the presence of catalytic amounts of 1,4-bis( dipheny1phosphino)butane and palladium acetate under 6.8 atmospheres of carbon monoxide afforded the product 166 in 94% yield. The choice of ligand was crucial for high yields of product. Pd(0Ac)z (2 mol%) dppb (4 mol%) HC02H. 6.8 afrn CO DME 150 “c 1 65 166 (94%) 86 Contemporary Organic SynthesisAlkoxycarbonylation can be achieved using chloroformates as the coupling partner. We are reminded of this in the conversion of the stannylfuran 167 into the corresponding ester 168, which occurs upon treatment with chloroformate 169 under palladium catalysis.lo6 Wang and Alper have reported an unusual rearrangement reaction to afford lactam products.lo7 Treatment of the ketone 170 with catalytic amounts of Co,(CO), and Ru,(CO),, affords the rearranged product 17 1 in 72% yield. 170 171 (72%) O'Connor and Ma have described a useful method for the metal-catalysed decarbonylation of aldehydes 172 into alkanes 173 at room temperature.los The method relies upon the presence of stoichiometric amounts of diphenylphosphoryl azide 174, which prevents the rhodium catalyst from forming inactive carbonyl complexes. 0 phoxF, -t- RCH2CHO cat. Rh(PPh3)CI RCH3 THF r.t. 90-9996 173 PhO' N3 172 1 74 (slow addition) 5.8 Allylic substitution The majority of research in the area of catalysed allylic substitutions is currently centred around palladium catalysis, and has recently been reviewed.loY has been used for the selective 1,4-opening of vinyl epoxides.O Trimethylsilyl cyanide successfully delivers cyanide as a nucleophile in palladium-catalysed allylic substitution.' Tamaru and co-workers have employed cyclic carbonates 175 as substrates.' By using isocyanates as the incoming nucleophiles, a highly regioselective amination occurs by pre-coordination of the nucleophile to give the product 176. For other nucleophiles, the regiochemistry would be expected to involve nucleophilic addition to the less sterically encumbered allyl terminus. ' The use of triphenylsilanol as an oxygen nucleophile Pd(PPh&(2fn01%) TsNCO dbxane 'yo f75 r.t.l l h 0 176 (80%) Palladium-catalysed azidation of 1-alkenylcyclopropyl tosylate 177 affords the azide substituted product 178 with the indicated regiochemistry.' l4 Functional group manipulation affords 2,3-methanoamino acid 179. 177 M-F 79% 178 1 79 Genet and co-workers have reported the use of N, 0-bis-t-Boc protected hydroxylamine 180 as a nucleophile for palladium-catalysed allylic substitution.115 Thus the allyl carbonate 181 is converted into the substitution product 182, which is further transformed into ( + )- N6-hydroxylysine 183. HNZBoc OBoc 180 183 The use of palladium acetate or palladium acetylacetonoate in combination with tributyl phosphine has been recommended as a particularly active catalyst,l16 and has been used in the introduction of exocyclic methylene groups.For example, myrtenyl formate 184 selectively affords /3-pinene 185 upon treatment with this catalyst.li7 (OCHO II 184 i as Three groups have reported the use of the ligand 186 for enantioselective palladium-catalysed allylic substitution.' l8-I2O This ligand provides higher enantioselectivities than previous ligands for some reactions. For example, the reaction of allyl acetate 187 with dimethylmalonate affords the allylic substitution product 188 with up to 99% e.e. These, and related ligands appear to rely upon a disparity in the electronic/steric properties of the two donor atoms.12 palladium-catalysed allylic substitution in the synthesis of aminocyclopentitol glycosidase inhibitors.22 They are pursuing asymmetric variants of these syntheses Trost and Van Vranken report the use of Dawson and Williams: Catalytic applications of transition metals in organic synthesis 87188 (up to 99% 8.8. up to 99%) Reagents: [Pd(q3-C3H&q2 (1 mol%). KOAc,BSA (2 mdo/), H2C(C02Me)2. CH2Clp. 23 "C using highly effective ligands exemplified by structures 189 and 190.'23J24 190 Backvall's group has demonstrated the synthetic power of the palladium-catalysed 1,4-0xidation of conjugated dienes, a reaction which also proceeds via n-allylpalladium intermediates.' 25 A recent development has been the use of tethered nucleophiles which are able to add to both ends of the diene. For example, the reaction of the dienamide 191 with catalytic palladium acetate and excess cupric chloride using oxygen as the primary oxidant afforded the pyrrolizidinone 192 in 90% yield.191 192 3 5.9 Tandem and cascade processes There are a growing number of tandem and cascade processes being reported wherein the catalyst triggers a series of reactions on one molecule. The various components of these reactions are familiar catalytic reactions, such as those that we have seen in the preceding sections. Grigg's research group has accessed various polycyclic structures by identlfylng various 'starter components' for catalytic cascade reactions. For example, the reaction of benzylic bromide 193 with norbornene 194 affords the cyclized product 195.12'j Intermediate palladium alkyl species can also be trapped by hydride ion capture127 or cyanide ion capture.28 have both reported palladium-catalysed tandem reactions as an entry into vitamin D derivatives. Thus, Nuss and co-workers have shown that the palladium-catalysed reaction between the vinyl bromide 196 and the vinyl stannane 197 afforded the cyclized and coupled product 198, which is in equilibrium with 199 via a [l, 71 H shift. The research groups of Nuss129 and 195 Z:E 2:l Reagents: Pd(OAc), (10 md%), PPhs(20 moWo), 2eq. K2C03, EtNCI (leq.), CsHs, 80 "C, 18h. 6 Reactions involving metal carbenoids Whilst many transition metals are believed to catalyse reactions which proceed via intermediate carbenes or carbenoids, the majority of work has been centred around rhodium- and copper-catalysed reactions. The behaviour and chemoselectivity of these carbenoids are becoming more well under~tood.'~~ There are increasingly complex cascade reactions which begin with metal carbenoids and lead to polycyclic products.' 32 6.1 Cyclopropanation Katsuki and co-workers have reported the use of bipyridine ligands for copper-catalysed asymmetric cyclopropanation.33, 34 Cyclopropanation of styrene 200 with t-butyl diazoacetate 201 in the presence of catalytic copper triflate and the ligand 202 afforded the trans cyclopropane 203 selectivity with 92% e.e. 203 (86:14 trans:cis 92Vl.0. 75%) N2CHC02But 0 9 SiM% Me&I 201 2Q2 Enantioselective intramolecular cyclopropanations have also been reported with enantiomerically pure copper*35 and rhodium136 catalysts. Enantiomerically pure polyethylene-bound rhodium( 11) complexes have also been developed, and are especially efficient for enantioselective intramolecular cyclopr~panations.~ 37 8 8 Contemporary Organic SynthesisNot all metal-catalysed cyclopropanation reactions proceed via carbenes.Murai and co-workers have reported the palladium-catalysed reaction of ketone a-carbonates 204 with norbornenes 205 to afford cyclopropane products 206.138 The reaction is believed to proceed via an oxa-n-allylpalladium intermediate rather than via a palladium carbene. 0 Me I1 204 0 Pd(PPh,)4(10mol%) + toluene, reflux 144h * 205 74% V 206 6.2 Insertion reactions Clark has shown that Cu( acac), is superior to Rh,(OAc), for the conversion of 207 into 208.13' The mechanism is suggested to occur through a metal bound oxonium ylide 209, since variation in the catalyst results in differing levels of diastereoselectivity in the product. Reactions involving the corresponding ammonium ylide have been reported by West and Naidu.140 Thus cyclization of 2 10 in the presence of catalytic rhodium(I1) acetate affords the intermediate 2 11, which undergoes a Stevens [ 1,2] shift to afford the piperidine 2 12 in 99% yield.Alternative C-H insertion reactions were not observed. P h , , r 4 210 N2 R~AOAC)~ (3 ml%) CHSl2, r.t. I 21 1 Me 212 Padwa and Kinder have reported the rhodium(~r) acetate catalysed formation of substituted fur an^.'^^ Treatment of the diazo precursor 2 13 affords the bicyclized product 2 14 in 76% yield. A range of similar cyclizations were also reported. Padwa's group has continued its investigations into tandem rhodium- catalysed cyclization/ 1,3-dipolar cycloaddition processes.142 In particular, an intramolecular system 2 15 has been reported to proceed via the intermediate 2 16 to afford the tetracyclic product 2 17 in 88% yield.143 213 21 4 - N2 21 5 cat.Rh2(OAc)4 benzene reflux I hgMe __c @Me H 217 (88%) 21 6 Moody and co-workers have reported the benefits of using rhodium( 11) trifluoroacetamide as a catalyst for O-H insertion rea~ti0ns.l~~ The diazobisphosphonate 2 18 afforded none of the desired O-H insertion product 2 19 in the presence of rhodium( 11) acetate. However, the use of rhodium( 11) trifluoroacetamide as the catalyst afforded an 8 1'/0 yield of product 2 19. 0 0 21 9 21 8 7 Miscellaneous This last section contains a pot-pourri of interesting reactions which do not readily fall into one of the major categories.In some cases, the difficulty of categorization is a reflection of the novelty of the reaction concerned. 7.1 Acetalization A diastereoselective palladium-catalysed acetalization of alkenes has been reported.145 The greatest diastereoselectivity was observed with the t-butyl substituted oxazolidinone 220. Treatment of 220 with methanol, palladium chloride catalyst, and stoichiometric cuprous chloride under an atmosphere of oxygen afforded the product 22 1 with 95% d.e. Da wson and Williams: Catalytic applications of transition metals in organic synthesis 89220 221 (89% 95% d-e.) 231 230 " 7.2 Thioether formation Bulman Page and co-workers have reported an unusual reaction involving the platinum-catalysed formation of thioethers from thiols and akyl halides.'46 For example, treatment of butane thiol, iodobutane, and sodium carbonate with a platinum catalyst affords dibutylsulfide 2 2 2 in 80% yield.BuSBu 222 BUSH (dpprn)PtCI, (5ml%) + Bul Na@3 80% - 7.3 Conjugate addition Tanaka and co-workers have developed the use of the ligand 223 for catalytic enantioselective conjugate addition. The reaction of 224 with methyllithium in the presence of 0.36 equivalents of the ligand 223 and 0.33 equivalents of copper iodide and 0.33 equivalents of THF afford the conjugate addition product, muscone 225 with 99% e.e.147 Such high levels of enantioselectivity hold great promise for reactions involving less catalyst. 225 (99% e.e.) 224 Ito and co-workers have employed the trans chelating ligand 226 for a rhodium-catalysed addition of a-cyan0 carboxylates to Michael acceptors.Thus the reaction between 227 and 228 in the presence of catalytic amounts of RhH(CO)(PPh,), and ligand 226 affords the addition product 229 with 87% e.e.148 H cat. RhH(CO)(PPh3)3 227 Nc$ OP+ Me 228 g e - Me' PPh2 226 H q O P 4 Me' CN 229 (88% 87% e.e.) 7.4 Ring fusion and expansion Huffman and Liebeskind have designed a novel intramolecular carbocyclic ring-fusion process.149 Treatment of 4-cyclopropyl-2-cyclobutenone 230 with 5 mol% RhCl(PPh,), affords the cycloheptadienone 231 in 84% yield. Iwasawa and Matsuo have reported the ring expansion of 1 -alkynylcyclopropanols 232 into cyclopentenones 233 on treatment with 10 molo/o Co,(CO), followed by 20 mol% P(OPh),.lS0 HOf' 232 R b 233 7.5 Metathesis The catalytic ring-closing of dienes has been pursued by Fu and Grubbs.lS1 For example, the reaction of the diene 234 in the presence of the catalyst 235 affords an 85% yield of pyrroline 236.235 benzene, 20 "C. 3h Me Me 234 236 7.6 Isomerizations It is well known that transition metals are able to isomerize alkenes. Particularly useful are the isomerizations of allyl a l ~ o h o l s ~ ~ ~ J ~ ~ and allyl amines, as these next examples illustrate. Mothenvell and Sandham have demonstrated the nickel-catalysed transformation of allylic alkoxides 237 into enolates, and their use in an aldol reaction with benzaldehyde to afford the aldol products 238 and 239.154 Ph 237 (i) cat. [Rh(dppe)lClO, or cat. 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