Catalytic applications of transition metals in organic synthesis CHRISTOPHER G. FROST and JONATHAN M. J. WILLIAMS Department of Chemistry, Loughborough University of Technology, Loughborough, Leicestershire, LEI I 3TU, UK Reviewing the literature published between 1 September 1993 and 31 August 1994 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 7 8 9 10 11 12 Introduction Oxidation E poxidat ion Dihydroxylation Oxidation of alcohols Oxidation of hydrocarbons Other oxidations Hydrogenation and related processes Hydrogenation H ydrosilylation Hydroboration and diboration Hydroformylation Lewis acids Friedel-Crafts Ally lation Diels- Alder Carbonyl-ene reactions Coupling reactions Heck reactions Suzuki coupling Stille coupling Coupling of other organometallics Allylic substitution Carbonylation and related reactions C y clizations Cyclizations to form aryl rings Tandem and cascade reactions Reactions involving metal carbenoids Conjugate addition and substitution Catalysed nucleophilic additions Metathesis Miscellaneous Conclusion References 1 Introduction This review highlights the advances in transition metal catalysis made in the period 1 September 1993 to 3 1 August 1994.During this period, there have been many advances in the field of homogeneous transition metal catalysed reactions. One of the most prominent areas of research within the field has been the further development of asymmetric catalysis using enantiomerically pure ligands associated with transition metal catalysts. There are a growing number of catalytic reactions in which the enantiomeric excess of the product is > 90%.There has been such a huge volume of publications concerned with transition metal catalysts that it is not possible to provide a fully comprehensive account. We have endeavoured, however, 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 Two important objectives with oxidation reactions are selectivity and efficiency. There are examples here of transition metal catalysed oxidation reactions which are chemoselective as well as stereoselective. The cheapest oxidant is air, and catalytic systems which employ air as the stoichiometric oxidant are especially appealing.2.1 Epoxidation Further developments within asymmetric epoxidation using enantiomerically pure manganese salen complexes have been reported. Jacobsen and co-workers have described the sterically and electronically optimized (sa1en)Mn complex 1, which was employed in the catalytic oxidation of the diene 2 to the monoepoxide 3' and also of l-phenylcyclohexene (4) to the corresponding epoxide 5.2 H Q H 0.2 eq. P h C N + - O - cat. 1 4"C, €120, NaOCl 2 45% yield 3 (64% e.e.) O'C, CH&12 4 5 (92% e.e.) Frost and Williams: Catalytic applications of transition metals in organic synthesis 65The related catalyst 6 has been prepared by Katsuki and co-workers, and has provided high levels of asymmetric induction for the epoxidation of conjugated cis-alkenes, such as the conversion of 7 into 8.3 PhIO cat.6 8 (94%e.e.) a 8% a ' c yield - 7 A remarkable enantioselective aerobic epoxidation of alkenes catalysed by the manganese complex 9 has been rep~rted.~ The (2)-alkene 10 is converted into the epoxide 11 with 80% e.e.in the presence of oxygen and pivaldehyde. However, the yield and diastereoselectivity of the reaction are less satisfactory. 9 BU'CHO 3 ~ 1 % 9 1 atm. air P P M e r.t., benzene 10 80% 8.8. 63:37 cis :trans 2.2 Dihydroxylation There still remains considerable debate over the precise mechanism of the enantioselective osmium-catalysed dihydroxylation of alkenes in the presence of cinchona-derived catalysts 12 and 13.5-7 However, the synthetic importance of this reaction is evident from the increasing range of substrates which have been successfully employed.The Sharpless group have provided many examples recently, including the conversion of the allyl halide 148 and the allyl sulfide 169 into the corresponding diols 15 and 17 with excellent levels of asymmetric induction. DHQ DHQD buffered AD-ma-B *cl O%, Bu'OH-H~O -- 14 MeSOzNHz OH 75% yield 15 (95%e.e.) OH E Ph!3&Ph buffered A M i - B P h S d Ph O'C, Bu'OH-HpO 16 75% yield 6H 17 (98%e.e.) Hale and co-workers have reported that the conversion of the silyl protected allyl alcohol 18 into the product 19 occurs with good enantioselectivity, but with an opposite sense of asymmetric induction to that predicted based solely on the steric demands of the substituents on the a1kene.I' Me OSiPhz13ut Me OSiPhzBut K O%, AD-ma-B Bu'0H-H20 HC!? (91 Yo e.e.) 70% yield 18 2.3 Oxidation of alcohols Backvall's group has provided two interesting examples of the transition metal catalysed oxidation of secondary alcohols to the corresponding ketones.Treatment of cyclohexanol(20) with manganese dioxide and potassium carbonate and catalytic amounts of both the ruthenium complex 2 1 and the quinone 22, afforded cyclohexanone (23)." -8 20 MlYo 22 0.5 MI% 24 2 MI% 25 0.6-1.5% O2 in N2 W% yield 20 23 But In a related system, it was possible to use air as the stoichiometric oxidant, in conjunction with an additional catalyst 25. The alternative ruthenium 66 Contemporary Organic Synthesiscatalyst 24 was employed, and cyclohexanol was converted into cyclohexanone in 89% yield with this unusual triple catalytic system.' 24 2.4 Oxidation of hydrocarbons Selective oxidation of alkanes is a daunting objective, and whilst there is still some way to go before chemo- and stereo-selective oxidation of alkanes can be reliably achieved with high efficiency, there are a few examples of highly selective reactions.For example, the functionalization of alkanes with sulfuryl chloride, catalysed by the cobalt complexes 26 and 27, has been described.13 These reagents are highly chemoselective, with catalyst 26 converting cyclohexane (28) into the chloride 29, whilst the use of catalyst 27 affords the c hlorosulfonate 30. so*cr I CI I 0- :7 0 : -0 29 72% yield 74% yield 30 20 2.5 Other oxidations Goti and Romani have described a catalytic oxidation of secondary amines into the corresponding imines,14 a reaction which has received much less attention than the related oxidation of alcohols to ketones.The secondary amine 3 1 was converted into the imine 32 by treatment with N-methylmorpholine N-oxide (NMO) and catalytic amounts of tetra-n-propylammonium perruthenate (TPAP). Phn'Nn Ph 32 0.05 eq. TPAP phAyAph 1.5eq.NMO * H r.t.. MeCN 88% yield 31 Murahashi and co-workers have reported the osmium trichloride catalysed oxidation of alkenes with peracetic acid to afford a-ketols. These workers indicate how this is quite different from the osmium tetroxide catalysed oxidation of alkenes to afford di01s.l~ Oct-1-ene (33) was converted into the a-ketol 34 by treatment with peracetic acid in the presence of catalytic amounts of OsC1,.3 Hydrogenation and related processes Transition metal catalysed hydrogenation reactions have been known for a long time, and even asymmetric variations of this reaction are over twenty years old. There is still room for progress, however, as the following examples illustrate. 3.1 Hydrogenation The effective cationic rhodium catalyst 37 for the hydrogenation of aldehydes and ketones 35 under mild conditions into the corresponding alcohols 36 has been reported by Burk and co-workers.16 0 cat. [(COD)Rh(DiPFc)]+OTf(37) OH II b I Di P Fc=l , 1 '&is( d iisapropylphosphino)ferrocene Lemaire and co-workers have demonstrated that the ligand 38 is effective in the rhodium-catalysed transfer hydrogenation of methyl benzoylformate to the product 39 giving over 99% e.e.and 100% conver~ion.'~ Other substrates, however, afforded lower levels of asymmetric induction. OH 39 (99%e.e.) Me Me r.t KOH The range of substrates which are efficiently hydrogenated in the presence of the titanocene catalyst 40 has been extended.I8 The enamine 4 1 is converted into the amine 42 with 92% e.e.," and the alkene 43 is hydrogenated to 44 with 99% e.e.20 (li) 2.5 q. PhSiH3 W 'active catalyst' 40 15 p.8.1. H2 r.t PhACH2 75%yield Ph+Me 42 (92%e.e.) 41 Frost and Williams: Catalytic applications of transition metals in organic synthesis 67L P h 5 ' % 4 0 - * r.t.. 48 h Ph Aph Ph 43 91% yield 44 (99%e.e.) Burk and co-workers have also reported the synthesis of a range of amino acid derivatives via asymmetric hydrogenation with DuPHOS rhodium catalysts.21 The ( E ) - and (Z)-enamides 45 and 46 were both converted into the same enantiomer of product 47 with very high enantioselectivity upon treatment with ligand 48 and a rhodium catalyst under two atmospheres of hydrogen.Faller and Tokunaga have provided a further example of chiral poisoning.22 Treatment of the racemic ruthenium complex 49 with (1 R,2 S)-ephedrine (50) deactivates one of the enantiomers of the catalyst. The unpoisoned enantiomer remains available to effect a kinetic resolution in the hydrogenation of racemic cyclohexenol5 1. At 77% conversion, recovered cyclohexenol5 1 was found to have > 95% e.e. and Jac~bsen,*~ with their respective co-workers have independently described enantiomerically pure bimetallic complexes, 55a and 55b.These complexes are anticipated to afford two-point binding for suitable substrates, and the Kagan group have shown that catalyst 55a is able to catalyse asymmetric hydrogenation reactions. It may be possible to use such bimetallic complexes to provide highly selective reactions, and it seems likely that this design strategy will afford interesting results in the future. r 7 + L 56a Kagan J r l+ L 55b Jacobsen J m i H A C -1 0.1 mol% [(COD$3h+lOff 45 47 (99.6% e.e.) NHAc 0.1 mOl%m -NHAc 2 atm. H2 -CO2Me c ~ H A C I 0.1 mot% [ ( w R h + p T f C02Me 0.1 mOl%48 46 47 (99.4% e.e.) 48 OH Takaya and co-workers have shown that the H,-BINAP ruthenium complex 52 is an effective asymmetric hydrogenation catalyst.23 For example, the alkene 53 was converted into the anti-inflammatory drug (S)-ibuprofen (54) with 97% e.e.In some cases this catalyst proved to be superior to the more normal BINAP derived catalysts. I I 3.2 Hydrosilylation Takeuchi and co-workers have reported a highly selective hydrosilylation of propynylic alcohols with complete control over regiochemistry and alkene geometry.26 Alkyne 56 was converted into the vinyl silane 57 with 92% yield. 2 4 Ho 56 HPsiEt3 0.5 m ~ l % Rh(C00)2BF4 1 mot% PPhi 1.5 eq. EtSiH 50°C. acetone, - 16h -u 02% yield 57 Kobayashi and Nishio have described a one-pot preparation of homoallylic alcohols from 1,3-dienes via a hydrosilylation-aldehyde-coupling sequence.27 Cyclopentadiene (2) is reacted with trichlorosilane and a palladium catalyst, and the intermediate allylsilane 58 is reacted with benzaldehyde to form the alcohol 59 with high yield and excellent syn selectivity.90 'C 2 PhCHO 0%. DMF 01% yield OH I A palladium-catalysed silylstannylation of alkenes has been reported.28 For example, treatment of the silylstannane 60 and norbornene (61) with a palladium catalyst affords the derivative 62 in 89% yield. 6, 130%, toluene, 16h 95% yield 62 68 Contemporary Organic Synthesis3.3 Hydroboration and diboration Brown and co-workers have reported that the rhodium complex 63 is able to effect hydroboration of styrenes 64 with good enantioselectivities, as determined after conversion into the corresponding alcohols 6 5 .29 65 (78-94% e.e.) 64 20 *c H& I HO- -9% yield 63 An unusual platinum( o )-catalysed diboration of alkynes has been described.30 The reaction of alkyne 66 with the diboron species 67 with 3 mol% of Pt(PPh,), affords the addition product 68 in 86% yield.68 3.4 Hydroformylation Doyle and co-workers have reported a highly regioselective hydroformylation of alkenes catalysed by [Rh( COD)( OAC)],.~' Thus styrene is hydroformylated to afford a 96 :4 ratio of the branched to linear aldehydes 69 and 70. Takaya and co-workers have shown that in the same process the phosphinephosphite ligand 7 1 provides good enantioselectivity in the reaction, but with a 90: 10 ratio of the branched to linear aldehydes.32 90: 70 10 4 Lewis acids Ruthenium complexes have continued to be used as Lewis acids. Ma and Venanzi have used the ruthenium catalyst 72 to effect the acetalization of o-salicaldehyde (73) with 1,2-ethanediol(74) to afford the 1,3-dioxolane 75.33 The same workers have also reported the hydrolysis of acetals, including 1,3-dioxolanes by the use of a ruthenium catalyst.34 Nitriles are converted into esters on reaction with an alcohol in the presence of the ruthenium catalyst 76.35 For example, the nitrile 77 and methanol are converted into the methyl ester 78 in 86% yield. The same catalyst in the presence of water converts nitriles into the corresponding primary a m i d e ~ .~ ~ I I azeotropic diaillatbn 7 3 0 90% yield 72 = R~(MeCN)(triphos)(OTf)~ 3 mot% RuH,(PP& 76 C11H&O*Me C11H23CN l.Mq.H@. MeOH * eealedtube 77 18O'C, 24h -yield 78 4.1 Friedel-Crafts The use of scandium triflate as a catalyst has been further developed.This catalyst has been applied to Friedel-Crafts acylation reactions. Thus, treatment of anisole (79) and acetic anhydride with 20 mol% scandium triflate afforded the acetylated product 80 in 89% yield.37 The use of 1 mol% catalyst under otherwise identical conditions afforded a 62% yield of product. The catalyst could be recovered from the aqueous layer by simple extraction. 20 50%. m ~ l % AC20 MeNOa Sc(OTr), 4h * $ -vleld 6 79 Me 80 4.2 Allylation Scandium triflate has also been reported to catalyse the allylation of carbonyl compounds with tetraall~ltin.~~ Thus, treatment of o-salicaldehyde 73 with tetraallyltin in water/tetrahydrofuran ( 1 :9) and 5 mol% scandium triflate affords the adduct 8 1. It would be expected that most other Lewis acids would not tolerate the presence of water or the presence of the phenol.73 81 The related catalyst, scandium perchlorate, has been employed in the C-glycosidation reaction between 82 and 83 to afford the product 84 with excellent a-~electivity.~~ Frost and Williams: Catalytic applications of transition metals in organic synthesis 69OBn I Keck and Geraci have reported a simple procedure for the enantioselective allylation of aldehydes with allyltributylstannane 87 using catalytic amounts of pre-mixed titanium tetraisopropoxide and ( R)-BINOL.40 Using this methodology, benzaldehyde ( 8 5 ) was converted into the product 86 with 96% e.e. b S n B u 3 / 87 H ,OH 0 phAM 10 mi% Ti(OPrJ), Ph'- . .. .. 20 ml% ( R )4 INOL 85 -20%. C H & , 50h 86 (96?he.e.) 98% yield 4.3 Diels-Alder Asymmetric catalysis with scandium reagents has been achieved in the Diels-Alder reaction.41 A catalyst 88,42 derived from scandium triflate, ( R )-( + )- l,l'-bi-2-naphtho17 and cis- 172,6-trimethylpiperidine, was employed in catalytic amounts in the enantioselective Diels-Alder reaction of the dienophile 89 and cyclopentadiene (2) to afford the cycloadduct 90.Evans and co-workers have employed catalysts based upon copper( 11) triflate and the diimine 9 1 to catalyse Diels-Alder reactions on similar substrates with 83-94% e.e.43 90 (92% e.e.) 89:ll edo:exo H WOiN< 0\ I H Ar FN N Y A r 91 Corey and co-workers have employed the titanium catalyst 92 in the enantioselective Diels-Alder reaction between 2-bromoacrolein (93) and cyclopentadiene (2) which affords the cycloadduct 94 in 93% e.e.44 =(cHo Q &CHO \ Br 10 mot% 92 - Br -78%.CHZCIZ 94 (93%e.e.) 67:l exomdo 94% yield 93 4.4 Carbonyl-ene reactions Terada and Mikami have used the p-0x0 complex 95 to catalyse the carbonyl-ene reaction between a-methylstyrene (96) and methyl glyoxylate to afford the product 97 with very high enantio~electivity.4~ The same group have also further developed the use of the complex 98 to a more extensive range of ~ubstrates.~~ 0 f l C O * M e HKC02Me 0.2 md% 95 Ph 96 a%, CHg12. 2h 88% yield 97 (98.7% e.e.) Ph Mikami and Matsukawa have described an aldol-type reaction between the ketene silyl acetal99 and the aldehyde 100 catalysed by titanium complex 98 to afford the product 10 1 with high enantio~electivity.~' Whilst the outcome of this reaction indicates an aldol reaction, these authors postulate that the reaction pathway may in fact involve a silatropic ene reaction.OTMS oms BnOJH EtS L O B n 101 (94%e.e.) 0% toluene 2h Ets 819/0 yield 100 Q ' o,-p 98 5 Coupling reactions There have been many hundreds of examples of metal-catalysed coupling reactions reported recently, which is a testament to the synthetic utility of these reactions. Palladium has a dominant position as the main metal of choice for conventional coupling reactions, such as the Stille reaction and carbonylation 70 Contemporary Organic Synthesisreactions. However, other transition metals have been examined, and as described here provide useful methods for what have been collected together as ‘coupling reactions’.5.1 Heck reactions Various methods for optimizing Heck reactions have been described. For example, it has been reported that Heck reactions are accelerated by high pressure condition^.^^ Jeffery has reported that Heck reactions can take place in water in the presence of added tetrabu t ylammonium salt sJY of their work on the enantioselective intermolecular Heck reactions0 The reaction of phenyl triflate (102) with 2,3-dihydrofuran (103) in the presence of catalytic amounts of palladium acetate and ( R )-BINAP affords the products 104 and 105 with configurations opposite to each other. Japanese workers have provided a detailed account 103 3 eq. PtZNEt 104 105 82V’e.e. 92 18 60%e.e. 57%yield 6% yield Achiwa and co-workers have achieved good enantioselectivity in a related reaction employing phenyl triflate ( 102) and norbornene 6 1 as coupling partners to afford the product of hydroarylation 106 in the presence of a palladium catalyst and the ligand 107.” Ozawa, Hayashi, and co-workers looked at a similar reaction using the vinyl triflate 108 as the coupling partner to afford the product of hydroalkenylation 109 in 93% enantiomeric excess? PhOTf 102 1.2 m ~ l % Pd(0AC)z EbN, 2.4 md% 107 65%.DMSO. 20h 61 106 (71.4% 0.e.) 73.6% yield NHS02M0 p+&PPk 107 Me H 111 piperidine. HCOd 113 70’C, DMF. 6.5h HBr I AcOH r.t., 22h 74% yield H 1 I W’ H 110 transformed into ( k )-epibatidine (1 10) upon treatment with HBr/acetic acid. The Heck reaction has provided some useful examples of cyclization reactions on complex substrates.sJ For example, Masters and co-workers obtained a 52% yield in the Heck cyclization of precursor 114 into the taxol analogue 1 15.5s Overman and co-workers have provided another example of an enantioselective intramolecular Heck reaction: treatment of compound 116 with an enantiomerically pure palladium catalyst, followed by acidic hydrolysis afforded the cyclization product 117 in high enantiomeric excess.56 This compound was converted into the naturally occurring alkaloid ( - )-physostigmine ( 1 18).0 114 0 115 116 10% Pdz(dba)&HCb (S )-BINAP.100 “c 84% yield OzCHNMe 1 108 L ___c Me 61 MezN 109 (93% 0.0.) I 117 (95% 0.0.) 63% yield Clayton and Regan have reported the synthesis of racemic epibatidine ( 1 lo), an alkaloid which has received much synthetic attention recently.The key step in their synthesis is a reductive palladium-catalysed Heck-type coupling.s3 The reaction between compounds 1 1 1 and 1 12 using a palladium catalyst with piperidine and formic acid afforded the coupled product 113, which was Me 118 5.2 Suzuki coupling Wallow and Novak have reported increased catalytic efficiency by using phosphine-free palladium sources in Suzuki coupling reaction^.^' Soderquist and Colberg have shown that the silylated vinylborane 119 could be converted into trans-vinylsilanes 120 via Suzuki coupling in good yields.58 Frost and Williams: Catalytic applications of transition metals in orgunic synthesis 71R RBr BBN 120 SiMe3 rdlux.THF, 1% 2.5% Pdddba)~ = p w co (56 p.s.1.) LEI 80% yield 70%. NMP 584996 yield Johnson and Braun have employed a Suzuki coupling reaction in the preparation of the prostaglandin PGE, methyl ester 12 1 .5y Treatment of a suitable vinyl iodide and borane with a palladium catalyst afforded the coupled product, which was converted into prostaglandin PGE methyl ester 12 1 in two further steps (73% yield).0 0 ~TBDMS 25%. D M ~ T H F M ~ ~TBDMS Jl BBN = 9-borabiidononane HO . ' W C s H 4 I 121 OH 5.3 Stille coupling The Stille coupling process has provided efficient synthetic routes to many natural products. Indeed there have been several recent examples which demonstrate the versatility of this powerful reaction. Overman and co-workers employed a Stille coupling between the vinyl stannane 122 and the aryl iodide 123 to afford the coupled product 124 which they use in the total synthesis of ( - )-strychnine.b0 Falck and co-workers converted the stannane 125 into the ketone 126 on treatment with benzoyl chloride and a palladium catalyst? Stereoselective reduction and debenzylation afforded the natural product goniofufurone ( 12 7).Similar acyclic examples were also reported to proceed with retention of configuration.6 0 MeN NMe L N ) b' 123 OTI PS OBU' 0 H 126 H 127 Danishefsky and co-workers have reported an extraordinary palladium-catalysed coupling between (2)-bis( trhethylstanny1)ethylene ( 128) and the bis( iodoalkyne) 129 to afford the cyclic enediyne 130 in an incredible 80% yield!63 10?6Pd(PPh& 60% DMF, l h OTBS 80% yleld 129 1 Om 130 Boden and Pattenden have reported a macrocyclization strategy based on the intramolecular Stille coupling of a vinylstannane with an ally1 ~hloride."~ Thus, substrate 13 1 undergoes macrocyclization on treatment with a palladium catalyst to afford the product 132 in 38% yield.131 132 Kilburn and co-workers have described an unusual Stale-type 4,4'-biaryl formation as a macrocyclization step.65 Treatment of substrate 133 with a palladium catalyst afforded the macrocycle 134, albeit in modest yield. 7 2 Contemporary Organic SynthesisBr 98% yield 139 140 An application of zinc reagents to the preparation of homophenylalanine derivatives has been provided by Jackson and co-workers.6Y The reaction of zinc reagent 142 with the aryl iodide 143 in the presence of a palladium catalyst affords the homophenylalanine derivative 144 in 65% yield.Casson and Kocienski have shown that a-alkoxyalkenylzinc reagents such as 145 are suitable for palladium-catalysed reactions with various coupling partners, including the vinyl triflate 146 to give the coupled product 147.70 133 lMmo L 2 w2(dba)3 4 ml% Ph&s r.t., MF 6796 yield 134 In a process somewhat related to the Stille coupling, Hartwig66 and B~chwald,~~ with their respective co-workers, have independently reported the coupling between arylbromides and aminostannanes. For example, the in situ conversion of the secondary amine 135 into the aminostannane 136 upon treatment with ( N,N-diethylamino)tributyltin, and subsequent reaction with the aryl bromide 137 in the presence of a palladium catalyst affords the tertiary amine product 138.r 1 rPh Et#tSnBu3 Me H-N, L J 135 136 1 molx WCl2 (P(o -Tol)3)2 137 105% toluene 2h l - E t O & e N, rPh Me 138 5.4 Coupling of other organometallics Untiedt and de Meijere have shown that the palladium-catalysed coupling of the unusual zinc reagent 139 proceeds efficiently to afford the phenyl derivative 140 in excellent yield, when treated with iodobenzene (141) and a palladium catalyst."* 145 147 Two groups have employed ortho-directing groups to prepare zinc reagents, and coupled these organometallics with aryl triflates. Koch and co-workers ortho-lithiated the aryl oxazoline 148, generated the zinc reagent via transmetallation, and coupled this to iodobenzene ( 14 1 ) using palladium catalysis, thereby obtaining the product 149 in 75% yield.7 N, 0 * -6 (iii) 141.a. Pd(PPh44 75% yield 148 149 A similar strategy was employed by Snieckus and co-workers to form the coupled product 150 from the carbamate 15 1 and the aryl triflate 152. Both groups found that this strategy was effective with other ortho-directing groups.72 OCONEti &Me 672 EtZNOCO On 151 OMe 150 85% yleld Frost and Williams: Catalytic applications of transition meta Is in organic synthesis 73An unusual coupling involving cyclopropyl Grignard reagents 153 with benzylic dithioacetals 154 has been reported.73 The rearrangement of an intermediate cyclopropylcarbinylnickel intermediate accounts for the observed product 155. bromide 165 with potassium triisopropylsilanethiolate (166) to afford the vinyl silane 167.77 Coupling reactions between aryl halides and 166 were also achieved.Me, An unusual palladium-catalysed decarbonylative coupling reaction has been investigated by Tsuji and co-worker~.~~ Benzoyl chloride (168), hexamethyldisilane (169), and butadiene (170) are coupled together by a palladium catalyst to afford the allylsilane 17 1 in which decarbonylation has occurred. Hiyama and co-workers have employed alkyltrifluorosilanes as coupling partners.74 For example, the palladium-catalysed reaction between hexyltrifluorosilane ( 156) and p-bromoacetophenone ( 157) is promoted by tetrabutylammonium fluoride to give the coupled product 158 in 63% yield. 0 PhACI 157 5 md% Pd(PPh& 100°C. THF. 37h 03% Yield C6H13-SiF3 156 C6H13 158 5 m ~ l % W(dba)o 168 169 Me3Si-SiMe3 c /\//\/SiMe3 80% toluene 4h ph 171 86% yield w 170 The regiochemistry of coupling reactions involving an allyltrifluorosilane 1 59 with p-bromoacetophenone (157) was found to be highly dependent upon the ligand employed.75 The use of triphenylphosphine as the ligand afforded the y-product 16 1, whereas the use of the bidentate ligand dppp [Ph2qCH2),PPh2 (162)] afforded predominantly the a-product 160.5.5 Allylic substitution The main interest in allylic substitution has been in enantioselective palladium-catalysed allylic substitution. The conversion of acetate 172 into the substitution product 173 upon treatment with dimethylmalonate and a palladium catalyst with a suitable ligand has been achieved with high enantioselectivity by a number of research groups. 1 57 Me bsiF3 5mof96PdCI& 159 5 m ~ l % L.BuANF 120°C. MF 6OMe AOMe 160 161 L = PPha 0 100 = Ph*P(CH&PPk (162) 99 1 Brown and co-workers have developed the QUINAP ligand 174,7y whilst Wills and co-workers have shown that a monodentate ligand 175 is also effective.80 Koga and co-workers employed the C2 symmetric bis( pyrrolidines) 1 76,81 whereas Tanner and co-workers have used C, symmetric bis( aziridines) 1 77.82 Williams and co-workers have examined the use of ligands 1 7 v 3 and 1 79.84 The diamine ligand 180g5 has also been applied to this process, whilst Togni and co-workers have demonstrated that the chelating diphosphine ligand 181 is a useful ligand for a number of catalytic processes, including palladium-catalysed allylic substitution.86 A French group has reported an efficient palladium-catalysed reaction between terminal alkynes and vinyl and aryl halides.76 Iodobenzene ( 14 1 ) was coupled to the alkyne 163 to afford the product 164 in 96% yield.The added base was critical to the success of the reaction. Piperidine as the added base afford this high yield, whereas the use of either triethylamine or diethylamine gave no product! OH I ) PhI 141 -- f 5 m0R6 W(PPhd4 3 PPh2 1 74 / \ Ph piperidine 96% yield 163 Et2NH 0% yield 164 Soderquist and co-workers have demonstrated that thiolates may be employed as coupling partners in the palladium-catalysed cross-coupling between the vinyl affords up to 98% 8.8. affords up to 91 5% 8.8. 74 Contemporary Organic Synthesispalladium catalysis in the presence of these ligands, afforded high levels of enantioselectivity in the product 190.affords rq to 91% 8.8. affords ~96% e .e . MeHN 'hxph NHMe 180 affords up to 88% 8.8. affords Up t0 959/0 8.8. affords Up t0 93% 8.8. Helmchen, Pfaltz, and co-workers have extended the use of the ligand 182 to palladium-catalysed allylic amination?' Thus, treatment of the same allylic acetate 172 with a variety of nitrogen nucleophiles, including the sodium salt of p-toluenesulfonamide, afforded the allylic substitution product 183 with excellent levels of asymmetric induction. Ph+rPh TsNHNa TsHN PhqPh 1 mol% [(allyl)PdCI]2 172 183 (97% e.e.) Trost and Bunt have shown that the ligand 184 is particularly effective in enantioselective reactions involving 3-( acyloxy )cycloalkenes. For example, the reaction of the allyl acetate 185 with potassium phthalimide in the presence of a palladium catalyst and an enantiomerically pure ligand afforded the substitution product 186 with very high enantioselectivity.88 These workers have also shown that, in some cases, the addition of some tetraalkylammonium salts can have a remarkably beneficial effect on product enantioselectivity." 0 CH2C12 84% yield 186 (98% e.e.) 185 184 Hayashi and co-workers have also employed the Treatment of the allyl ligands 187 and 188 in the palladium-catalysed reduction of allylic carbonate 189 with formic acid and a base, under 188 1 87 with 187; 95% yield, 76% e.e.with 188; 99% yield, 85% 8.8. The cyclization of allyl acetates 191 and 192 under palladium-catalysed allylic substitution conditions has been shown to afford either the benzazepinium salt 193 or the alternative five-membered ring compound 194.92 The preference for one regioisomer over the other was found to be due to thermodynamic control.OAc I 191 R = H 192 R=Ph 194 R = Ph 81 % yield Whilst allylic substitution reactions catalysed by metals other than palladium continue to receive less attention, Takahashi and co-workers have provided examples of a regioselective carbon-carbon bond-forming reaction between an allyl ether 195 and a Grignard reagent catalysed by zirconocene dichloride to afford the product 196.93 Kobayashi and Ikeda have shown that the allylic carbonate 197 reacts with 2-furylborate 198 in the presence of a nickel catalyst to give 199.94 Frost and Williams: Catalytic applications of transition metals in organic synthesis 755.6 Carbonylation and related reactions The aryloxy carbonylation of 4-bromobipheny1(200) with the phenoxide 20 1 has been reported as an efficient synthesis of the hindered esters 202.95 carbonylation protocol in the preparation of 2-arylbenzimidazoles."" For example, the palladium-catalysed reaction between iodobenzene (14 1) and o-phenylenediamine (203) affords 2-phenylbenzimidazole (204).The reaction proceeds via a monoamide, which cyclizes in situ. Perry and Wilson have used a palladium-catalysed Bu! P h o B r 200 R 202 PhI 141 co (95 p.s.1.) - c k P h ' N NH2 1.2 ea. 2.6-btidine . I 1.5 ml% PdCb(PPh& - 140 C,' M%NCOMe rl 204 203 75% yield An interesting variant on standard carbonylation procedures has been devised by Grushin and A l ~ e r .~ ~ The carbon monoxide is generated in situ from chloroform and hydroxide. In a typical reaction, iodobenzene (141) and chloroform are treated with a palladium catalyst and potassium hydroxide, which after acidic work-up affords benzoic acid (205) in 72% yield. cat. PdC&(PPh& PhCO2H 205 60% Z&KOH Ph1+CHC'3 22%. CHCb 24h * 141 then H1O+ 5.7 Cyclizations There are many possible cyclization reactions which rely upon the previously described coupling processes. A few examples are given here. Murai and co-workers have reported the cyclization of the enyne 206 with a ruthenium catalyst to afford the product 207 in which a skeletal rearrangement has taken place.98 206 207 The palladium-catalysed oxaspirocyclizations of substrates such as 208 has been reported by Swedish workers.y9 The reaction with a palladium catalyst affords the spiro-product 209 with high diastereoselectivity using acetate as the external nucleophile.Hong and Overman have used a palladium-catalysed cyclization reaction in the construction of the pentacyclic opiate 2 1 1 (which can be converted into morphine) from the precursor 2 10.'O0 Ho\ b 2eq. o+ - * 3 eq. L I f i 5 m ~ l % Pd(0AC)z HOAc:Me&O (1 a) 24h. 86%yie# A&'* 209 208 OH 21 0 21 1 Takacs and Chandramouli have reported a highly diastereoselective palladium-catalysed tetraene cyclization.lo1 The enantiomerically pure substrate 2 12 was cyclized to the product 2 13 as a single diastereomer. c 0.1 5 eq. Ph3P 65%, THF 21 2 65% yield 21 3 A highly stereoselective cyclization of the unsaturated aldehyde 2 14 into the cyclopentanone 2 15 using a cationic BINAP-derived rhodium complex with > 99% e.e.and high trans-selectivity has been reported.'O* The use of a neutral rhodium complex afforded a cis-selective reaction. 76 Contemporary Organic SynthesisOHC * $Me 214 215 cat. RhCl/ (R MINAP 97 3 cat. Rh+ClOd-/ (R)-BINAP 3 97 In a related study, Bosnich and co-workers have cyclized the substrate 2 16 into the cyclopentanone 2 17 with > 99% e.e. using a similar cationic rhodium complex.103 217 (99%e.e.) The trans-selective zirconocene-catalysed cyclization of the diene 2 18 into the cyclopentane 2 19 with the trans-isomer predominating has been reported by Knight and Waymouth.lo4 Hiemstra, Speckamp, and co-workers have examined a copper-catalysed cyclization of trichloroacetates which takes place via a chlorine-transfer process.105 Treatment of the trichloroacetate 220 with the copper catalyst 22 1 affords the eight-membered lactone 222 as a single diastereomer in 74% yield. then H+M& -w 21 9 21 8 220 reflux.CICHCHGI. 18h- 222 .CI 5.8 Cyclizations to form aryl rings Cyclizations in which aryl rings are formed are particularly impressive. In retrosynthetic planning, most research groups consider modification rather than construction of aryl groups, although with the increasing number of transition metal catalysed aryl-forming reactions this situation may start to change. Hidai and co-workers have carbonylated the dienyl acetate 223 in the presence of acetic anhydride and triethylamine, and isolated 2-acetoxybiphenyl (224) in 74% yield.lo6 The reaction proceeds via the carbonylated, cyclized intermediate 2 25, which tautomerizes and is acetylated under the reaction conditions.An aromatization reaction involving the incorporation of two equivalents of carbon monoxide has been reported by Murai and c o - w ~ r k e r s . ~ ~ ~ Thus, the diyne 226 is cyclized to the aromatic compound 227 upon treatment with a ruthenium catalyst in the presence of carbon monoxide and a silane. OAc CO (50 atm) A@ NEb 140°C. benzene.3h ~ i Z ( P P h d 2 - \ / \ / 74% yield OAc 224 223 via Ph 0 225 EtO2C COpEt II n I I 226 EtO& CO2Et HSIBU~M% ~ U d W 1 2 my3 -8 CO (50 atm) 140°C. MeCN. Mh 74% yield Me2Bu'SiO OSiBu'Me2 227 Barry and Kodadek have designed an aromatization of the bis-vinyltriflate 228.Treatment with tributylvinylstannane and a palladium catalyst affords the aromatic compound 229. The reaction proceeds via a Stille coupling to one vinyl triflate and a Heck coupling to the other vinyl triflate to give the intermediate 230 which tautomerizes to the product .l O8 - OTf OTf via 228 e S n B u 3 4 md% Pd(PPh& - 75°C. NMP 55% yield 230 8 229 -Me 5.9 Tandem and cascade reactions Tandem and cascade reactions provide an opportunity for transition metal catalysts to effect remarkable transformations. The standard coupling reactions can be linked together in a sequence to afford products in which many new bonds have been created. Some of the synthetic applications recently achieved are reviewed here.Palladium catalysis has been exploited by Balme and Bouyssi for the cyclization of the alkene 23 1, which affords the tricyclic product 232, which was subsequently conberted into capne1lene.lo9 Frost and Williams: Catalytic applications of transition metals in organic synthesis 77C02Me 6 ' KH I25 % I M F 5 m ~ l % Pd(0Ac)z 64% yield 232 Weinreb and co-workers have examined the three component coupling between the vinyl bromide 233, the alkene 234, and dimethylmalonate (235), which is deprotonated under the reaction conditions.' l o Initially, the bromide and alkene undergo a Heck coupling to form a palladium ally1 intermediate 236, which undergoes nucleophilic addition by the malonate to afford the products 237 and 238. In an intramolecular variation of this reaction, where the nucleophilic component is a tethered sulfonamide 239, the reaction proceeds to afford the bicyclic product 240.' +C4HQ 233 Pd(OAc), P( o -T0l)3 237 Bu~NCI I NaH / DMF @C4Hg loo%, 42h I 234 58% yield I II 5 mol% Pd(0AC)t Ye 10 mow P(o -Tol)i mHTs loo%, N a g 3 DMF, Bu,NCI 24h * 84% yield 239 240 A catalytic tandem oxy-palladation, vinylation reaction has been reported by Semmelhack and Epa.'l2 The hydroxy alkene 24 1 is treated with catalytic palladium acetate in the presence of copper chloride and air as re-oxidant.This affords the intermediate 243, which is trapped with the alkene 242 in Heck fashion to afford the reaction product 244. MeOC 0 2 89%yield qMB r.t., DMF, a 0 243 Grigg and co-workers have reported more examples of palladium-catalysed reactions leading to polycyclic products.113 For example, treatment of the indole 245 with a palladium catalyst afforded the spiro product 246 obtained in a remarkable 91% yield.li4 The starting material undergoes initial oxidative addition with the Pdo catalyst (formed in situ), followed by the stitching indicated to provide the expected product.S C H O A scHo 3 10 m ~ l % Pd(0Ac)z c 20 m ~ l % PPh3 KOAc 130%. PhOMe, 24h 91% yield 245 246 Overman and co-workers have designed an impressive palladium-catalysed bis-Heck cyclization which takes place with complete stereocontrol.' l 5 The vinyl iodide 247 cyclizes as indicated to afford the tricyclic product 248 as a single diastereomer in 82% yield. \ (i) 10 mot% Pd(0Ac)z (ii) TBAF 2O%PhP 1 23% THF 3 P T H F I 248 OH 6 Reactions involving metal carbenoids Rhodium and copper are the most commonly used metals for effecting reactions which proceed via metal carbenoids.Ironically, the metal carbenoids of copper and rhodium are too unstable to isolate, and so the exact structures of such species can only be assumed. Nevertheless, the behaviour of catalytic reactions involving metal carbenoids is now becoming fairly well rationalized. An interesting synthesis of epoxides from aldehydes has been provided by Aggarwal and co-workers.'16 Dirhodium tetraacetate catalyses the formation of sulfur ylides from diazo-compounds and sulfides. The so-formed sulfur ylide reacts with the aldehyde to form an epoxide and regenerates the sulfide, which can therefore be used catalytically. For example, benzaldehyde (249) was converted into stilbene oxide (250) in 74% yield.2 0 d % M G 1 m ~ l % R ~ ~ O A C ) ~ * & Ph 86 :12 trans :as N&HPh Ph 74% yield PhCHO 249 BubMe I CH2C12 24h 250 78 Contemporary Organic SynthesisSeveral diastereoselective rhodium-catalysed transformations controlled by chiral auxiliaries have been reported."7 Davies and co-workers used the pantolactone ester 25 1 in a diastereoselective cyclopropanation reaction, to afford the product 252.' l 8 0 0 Landais and Planchenault have examined diastereoselective rhodium-catalysed insertions into the Si-H bond. The rhodium-catalysed reaction of the diazoacetate 253 with a silane 254 affords the product 255 with moderate diastereoselectivity.' l y -,SiMe2Ph PhMe,SiH (254) 73% yield A C-H insertion reaction which proceeds with very high diastereocontrol and enantiocontrol has been reported by Doyle, Muller, and co-workers.''O The diazoacetate 256 cyclizes to the product 257 on treatment with the and 97% e.e. 256 Rhp4 = 258 rhodium catalyst 258 with 98% d.e.cat. Rh& reflux, CH2Cb 70% yield H 257 (97% e.e.) ( ) Q 4 O 2 4 ? ,Rh Rh 7 Conjugate addition and substitution Van Koten and co-workers have further developed the use of enantiomerically pure arenethiolatocopper( I ) complexes 26 1 as catalysts for enantioselective conjugate addition.122 In the presence of catalyst 26 1, methylmagnesium iodide can be added to the a,P-unsaturated ketone 262 to afford the product of 1,4-conjugate addition 263 with high enantioselectivity.261 M s c u Ph-0 c Ph?o Me 0%, MeMgI EtzO Me Me 262 97% yield 263 Zhou and Pfaltz have used the related copper catalyst 264 to provide asymmetric induction in the addition of isopropylmagnesium chloride to 2-cycloheptenone (265), which affords the product 266.123 (y 265 s - c u 264 * Pr'MgCI 55% yield -78 %, THF/HMPA 266 (87%e.e.) Backvall, van Koten, and co-workers have reported the extraordinary finding that treatment of geranyl acetate 267 with the achiral catalyst 268 and butylmagnesium iodide (addition over 120 min.) affords the product of y-substitution 270 when conducted in diethyl ether at O T , whilst the product of a-substitution, 269, was obtained when the reaction was conducted in THF at - 30°C (with Grignard addition over 5 min.).124 The enantioselective cyclopropanation of alkenes with diazoacetates catalysed by the ruthenium complex 259 has been reported.121 Styrene is converted into the cyclopropane 260 with high enantioselectivity and diastereoselectivity.268 270 259 Ph v C O 2 B u ' 94% d.e. MYo 8.8. N&HCO~BU~ Ph- cat. 259 20%, CH&&.8h 260 65% yield 267 269 269 : 270 0 "C Et20 2h addition 0 : 100 -30 "C M F 5 min. addition 100 : 0 S C U 8 Catalysed nucleophilic additions Miura and co-workers have reported the copper-catalysed reaction of aryl iodides with active methylene compounds.'*' For example, the reaction between iodobenzene ( 14 1) and ethylcyanoacetate (27 1) affords the substituted aromatic compound 272 in 89% yield. Frost and Williams: Catalvtic amlications of transition metals in nrmnir wnthmiv 7910 mot% PhI 141 CuBr * Ph4'" CN 120%.DMSO COPE1 89%yie# - 271 272 Rhodium catalysts have been employed to effect nucleophilic additions to imines.126 Thus, the reaction between the imine 273 and the methylmalonitrile 274 affords the product of nucleophilic addition 275 using a rhodium catalyst. A palladium(o) catalyst was also found to be effective. Y 273 27s 9 Metathesis Grubbs and co-workers have extended their ring-closing metathesis methodology to include a wider range of substrates. The reaction of enol ether 276 with the molybdenum catalyst 277 affords the ring-closed product 278 in 88% yield.127 The ruthenium catalyst 279 was also found to be able to catalyse ring-closing metathesis reactions, including the conversion of 280 into 281.12* Martin and co-workers have used similar methodology to afford fused nitrogen-heterocycles." Thus, treatment of the diene 282 with catalyst 277 affords the products 283 in 80-90% yields. Me I cPh * o O n P h 12mot%m 20%.pentane, 3.91 88% yleld 278 276 277 * oco2H 2mOpx279 20%. C&. lh 281 3c02H 280 87% yield 283 n = 1,2 x = 1 9 Hoveyda and co-workers have provided a further example of a metathesis/cyclization reaction, using the ruthenium catalyst 279.I3O The diene 284 is cyclized to the racemic pyran 285. By treatment of the so-formed reaction mixture with an enantiomerically pure zirconium catalyst and ethylmagnesium chloride, a kinetic resolution process takes place, allowing recovery of the pyran 285 as a single enantiomer. 1 mdsc279 %Me Then: 10 mi% zr conpiex EtMgCl Me 25to70'C.THF 285 (>Woe.e.) 10 Miscellaneous Frauenrath and Kaulard have examined an interesting asymmetric isomerization of the achiral substrate 286 into the enantiomers 287 and er~t-287.'~~ Using an enantiomerically pure ruthenium catalyst, the product is obtained with modest enantioselectivity. Trost and co-workers have continued to find synthetic applications for the unusual ruthenium-catalysed addition of alcohols and a~ety1enes.l~~ For example, the reaction of the alkyne 288 with ally1 alcohol 289 with a ruthenium catalyst affords the addition product 290, which was further elaborated to rosefuran (291).133 286 287 h O x 0 Ph H ent-287 291 A Japanese group has reported the remarkable ruthenium-catalysed dimerization of norbornadiene (292) to pentacyclotetradecadiene (293) in 85% yield.134 It has been assumed that this reaction proceeds by repeated activation of one or more C-C bonds.292 NMP 293 80 Contemporary Organic Synthesis11 Conclusion Transition metal catalysed reactions have continued to grow in importance. The areas of asymmetric catalysis, and the development of tandem and cascade sequences are examples which demonstrate the efficiency of transition metal catalysed reactions, both in terms of potential and of the tremendous achievements already attained. 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