3 Organometallic chemistry Part (i) Palladium- and nickel-catalysed methods 1 Introduction Palladium and nickel salts/complexes are exceptionally versatile and robust catalysts for the construction of carbon—carbon and carbon—heteroatom bonds. A number of novel palladium and nickel catalysts have appeared in the literature this year. These include the discovery of novel homogeneous catalysts— 1 and 2 and heterogeneous catalysts 3 including palladium grafted molecular sieves. Palladacycle 2 proves to be an extremely active and robust catalyst in the Heck arylation of alkenes with turnover numbers of up to 5 750 000 and turnover frequencies up to 300 000. Novel .uorinated phosphine palladium complexes in supercritical carbon dioxide have also been used as catalysts in carbon—carbon bond forming reactions. Reviewed herein are the recent developments in palladium- and nickel-catalysed carbon—heteroatom bond formation cascade cycloadditions and cascade cyclisation reactions.Visuvanathar Sridharan a. R = 1- naphthyl X = OAc b. R = 1- naphthyl X = I c. R = 1- naphthyl X = Br Department of Chemistry Leeds University Leeds UK LS2 9JT R R P Pd 1 Ar'O OAr' X t-Bu Pd P R N Si 3 O P O Cl Pd 2 X R t-Bu 2 P Ar' = 2,4 -(t-Bu)2C6H3 Me Pd Me O P O 97 Annu. Rep. Prog. Chem. Sect. B 1999 95 97—115 Me Me Me 2 Me PPh P 2 Fe P Me Me Me 4 5 2 2 Carbon–nitrogen bond formation Aromatic amines are an important class of compounds.They play a key role in a number of .elds which include pharmaceuticals agrochemicals photography and electronic materials. This section gives an update of the recent developments in palladium- and nickel-catalysed carbon—heteroatom bond formation as an extension to the last report. Hartwig’s and Buchwald’s groups have been engaged in developing novel second generation catalysts that can be used under mild conditions and have utilized them to increase the rate of carbon—nitrogen bond formation processes. Hartwig et al. have explored sterically hindered chelating phosphines 4–6 as ligands for palladium-catalysed carbon—nitrogen bond formation processes. P Me Me Me 2 Fe 2 P Me P Fe 2 6 Ligands 4–6 have two main advantages over conventional ligands that have been used in that they increase the rate of both oxidative addition (A) and reductive elimination (D) in the coupling processes (Scheme 1).The explanation for this increased activity with the ferrocenyl ligands (4–6) is that the sterically hindered alkylphosphines provide the required electron-rich metal centre while favouring ligand dissociation. This results in an increase in the rate of oxidative addition (A) the rate determining step in these catalytic processes. The catalytic cycle for the amination processes is presented in Scheme 1. Hartwig et al. have found remarkable rate enhancement for the coupling reactions with ligands 4–6. Unactivated aryl chlorides underwent coupling with aniline in excellent yield using 3mol% Pd(dba) and ligand 4 (Scheme 2).They also carried out amination of aryl toluene-p-sulfonates for the .rst time and synthesised mixed aryl—alkyl amines in high yield (Scheme 2). Ligand 4 allowed reactions between aryl bromides and aniline in excellent yield at room temperature (Scheme 3). Buchwald et al. have used the same hypothesis to develop ligand 7 for the amination of unactivated chlorides and bromides under mild conditions. Typical examples are shown in Scheme 4. 98 Annu. Rep. Prog. Chem. Sect. B 1999 95 97—115 P P Pd NHR P P P Pd P A D Ar P P Pd Pd NHR P P t-BuOH C B Ar P Pd RNH2 O t-Bu P Cl + PhNH2 + BuNH2 OTs + hexylNH2 Scheme 1 Pd(dba) / ligand 4 NaOt-Bu dioxane 110 °C 4 h Pd(OAc)2 / ligand 6 NaOt-Bu toluene 85 °C 2 h Pd(OAc)2 / ligand 5 NaOC6H3-2,4,6 -(t-Bu)3 toluene 110 °C 2 h Br + PhNH2 Scheme 2 Pd(dba) / ligand 4 NaOt-Bu toluene rt 20 h Scheme 3 PCy2 Me2N 7 Annu.Rep. Prog. Chem. Sect. B 1999 95 97—115 X Ar X NaOt-Bu H Ph N 93% N Bu H 87% H N hexyl 83% H N Ph 94% 99 Pd2(dba)3 ligand 7 O N Cl R + HN O NaOt-Bu toluene 80 °C 91% O N DME rt 97% Me Br Me O + HN N O DME rt Me Me 97% MeO NC Me Scheme 4 Pd(OAc)2 P(t-Bu)3 Me R = OMe R = CN X + N NaOt-Bu xylene 120 °C N H X = Cl Br I 99% Br H Scheme 5 Pd(dba)2 N + NH2 OH OH ligand 9 NaOt-Bu toluene 60 °C <5 min 100% Scheme 6 Yamamoto et al. have used bulky electron-rich trialkylphosphine P(t-Bu) as a ligand for the amination of aryl bromides iodides and chlorides with diarylamines (Scheme 5).In the above processes use of BINAP as a ligand gave poor results. Kocovsky has also developed O—P N—P ligands 8 9 for amination processes. He found that these ligands exhibit a dramatic accelerating e.ect on palladium-catalysed amination of aryl bromides (Scheme 6). Several chelating aromatic phosphine based ligands have appeared in the past year for palladium-catalysed amination processes. Typical examples include Buchwald’s bis[2-(diphenylphosphino)phenyl] ether 10 and Uemura’s N-1-[2-(diphenylphosphino)--phenyl]ethyl-N,N-dimethylamine complex 11 (LPPh ).Palladium-catalysed amination processes have been carried out in water—methanol 100 Annu. Rep. Prog. Chem. Sect. B 1999 95 97—115 R PPh PPh2 2 8 R = OMe 9 R = NMe2 PPh2 O 10 Me Br Me O + HN + X HN X = Cl Br mixtures using the water soluble chelating phosphine 12 in excellent yield (Scheme 7). Fort and Brenner have reported a liganded Ni catalyst for the amination of aryl bromides and chlorides under mild conditions (Scheme 8). Hartwig has further enhanced the scope of this process to synthesise azoles. Thus the combination of Pd(OAc) and DPPF catalysed the formation of N-arylazoles in the presence of Cs CO or NaOt-Bu with electron-rich neutral or electron-poor aryl halides in good yield (Scheme 9).For reactions of aryl halides possessing electrondonating substituents NaOt-Bu was a superior base to Cs CO . Beletskaya et al. have developed a similar Pd-catalysed process. Thus the arylation of benzotriazoles in water at 100 °C using TPPTS as the ligand proceeded in excellent yield (Scheme 10). Scheme 8 An intramolecular Pd-catalysed amination process has been successfully applied to the synthesis of functionalized pyrido[2,3-b]indoles in moderate yield (Scheme 11). One particular application of the amination process is found in the synthesis of indoles. Buchwald et al. have developed a novel Pd-catalysed cascade amination in combination with a Fischer indole process. This allows the synthesis of various substituted indoles in good yield (Scheme 12).Other applications are highlighted in the materials area. Polyaniline has attracted much attention in the .eld of organic conducting polymers due to its robust nature in the doped emeraldine state. Buchwald et Annu. Rep. Prog. Chem. Sect. B 1999 95 97—115 NMe2 PPh2 Cr(CO)2L 11 SO3Na Ar P Ar Ar P Ar SO3Na 12 Pd(OAc)2 / ligand 12 N Me Me O NaOH H2O / MeOH 75 °C Scheme 7 NaH t-AmOH 88% N 78% Ni(OAc)2 2,2' - bipyridine THF 65 °C 101 MeO NH Pd(OAc) or 2 / DPPF R Br+ Cs2CO3 or NaOt-Bu toluene 100 °C R = p-CN OMe t-Bu NC NH H N 2 N Ar = Ph p-MeC6H4 p-ClC6H4 SO3Na Cu(II) salt = P TPPTS = NaO3S Scheme 9 Pd(OAc)2 2 +N Ar2 IBF4 Cu(II) salt TPPTS NaOH H2O 100 °C SO3Na O O Br NH2 N Scheme 10 Pd2(dba)3 BINAP NaOt-Bu DMF 80 °C Me Scheme 11 al. have synthesised monodisperse controlled length and functionalized oligoanilines and triarylamine dendrimers via palladium-catalysed amination processes (Scheme 13).3 C–S C–P and C–C bond formation Zheng et al. have developed a palladium-catalysed C—S bond forming process using aryl tri.ates and thiols in the presence of Pd(OAc) and tolBINAP in toluene at 80 °C 102 Annu. Rep. Prog. Chem. Sect. B 1999 95 97—115 N 98% N 98% Ar N N N 93 - 96% O Cu O Ph 2 O O N N Me 51% NH2 R N + Ph Ph Br R = Cl Me BOC N H2N H2N (i) Pd(OAc)2 6 mol% BINAP 7 mol% NaOt-Bu 2.8 eq toluene Et3N 90 °C (ii) (BOC)2O 3 eq DMAP 0.1 eq THF reflux R OTf R = t-Bu OMe Me R to generate phenyl sul.des (Scheme 14).Similarly Parker et al. and Saa et al. have reported a palladium-catalysed C—P bond forming process using Pd(PPh ) Et N in toluene at 130 °C in which phosphonate esters were formed in good yield (Scheme 15). Hartwig’s and Buchwald’s groups have independently reported the palladiumcatalysed arylation of amides esters and ketones. Typical examples of inter- and intramolecular -arylation of amides are shown in Scheme 16. The choice of base was Scheme 14 R TsOH•H2O Cl Pd(OAc)2 NH NH N Ph Ph n-C5H11 Me or O n-C Me 5H11 ketone O Me BOC N Scheme 12 NH2 + BINAP NaOt-Bu toluene 80 °C N 95% NH NCPh2 2 eq Br BOC BOC N NCPh2 (i) (ii) N 74% BOC 5 Scheme 13 Pd(OAc)2 n-BuSH + 83% S n-Bu tolBINAP NaOt-Bu toluene 80 °C 78-93% 103 Annu.Rep. Prog. Chem. Sect. B 1999 95 97—115 Ar Ar O N N Pd(PPh3)4 + Me Me P H N N Ph EtO Et3N toluene 130 °C Br P Ph OEt O 60% O NMe2 O Scheme 15 Pd(OAc)2 Me + Me N Me BINAP KHMDS Me Me 72% Me Me MeO Me Me Br Pd(OAc)2 O O N Me N Me BINAP KHMDS 80% O O Br Me Ar = p-t-BuPh Br MeO Me Scheme 16 Pd(OAc)2 + t-Bu BINAP NaOt-Bu toluene 100 °C t-Bu Scheme 17 crucial for the success of the above process.Best results were obtained when potassium hexamethyldisilazide (KHMDS) or lithium tetramethylpiperidide (LTMP) were used in dioxane as solvent. Buchwald et al. have also reported an asymmetric palladium-catalysed arylation of ketone enolates in good yields with good ee’s (Scheme 17) during the creation of the quaternary carbon centres. Other regiospeci.c palladium-catalysed arylation on the -position of , unsaturated carbonyl compounds with aryl bromides and arylation on the benzylic position of 4-alkylnitrobenzenes with aryl bromides have been reported (Scheme 18). 104 Annu. Rep. Prog. Chem. Sect. B 1999 95 97—115 CHO Me + Me O2N Me+ R 2 2 1 X 1 one or two C component Table 1 Starter components for catalytic cascade cycloadditions Scheme 18 3' 2' 1 2 X two or three C component two C component 4 Cascade reactions The concept of cascade reactions involves careful design of multi-reaction ‘one-pot’ sequences so that the .rst step creates the functionality to trigger the second reaction and so on.This section is concerned with palladium- and nickel-catalysed processes in which two or more carbon—carbon/carbon—heteroatom bonds are formed. Cycloaddition cascades Grigg and Sridharan have proposed a new general scheme for palladium- and nickelcatalysed cascade cycloaddition processes in terms of ring sizes and applications. These processes involve combinations of a starter molecule which comprises a vinyl aryl allylic or benzylic halide tri.ate etc.with one (or more) acceptor molecules (alkene alkyne 1,2-diene 1,3-diene etc.). Carbon monoxide is also a valuable one carbon acceptor molecule and alkynes can function as the starter molecule via hydroor carbometallation. Some potential starter molecules are shown in Table 1. Five-membered rings. [41] processes. Several examples of nickel- and palladium-catalysed [41] processes have been reported by Negishi in which carbon monoxide was used as the one carbon component. Typical examples are shown in Scheme 19. Kundu et al. have successfully used alkynes as the one carbon component in the formal [41] cycloaddition palladium-catalysed process in which exocyc- Annu. Rep. Prog. Chem. Sect.B 1999 95 97—115 CHO Me Br Me Pd(OAc)2 PPh3 Cs2CO3 DMF 120 °C Br Pd(OAc)2 PPh3 Cs2CO3 H DMF 140 °C Y 70% 4 3 96% R O2N C 2 1 X X X three or four C component three atom component 105 CO2Et CO (40 atm) CO2Et CO2Et CO2Et I O 92% NiCl2(PPh3)2 Et3N MeCN 100 °C Pd(PPh3)2Cl2 82% CO2Et CO2Et CO (40 atm) CO2Et CO2Et n-Bu n-Bu I Ni(COD)2 PPh3 Et3N O 95% MeCN 100 °C H Ph I Scheme 19 Pd(0) / CuI HC CPh + O OH Et3N O O Scheme 20 lic alkenes were generated in moderate yields (Scheme 20). [32] processes. Most reported examples of .ve-membered ring formation have involved a [32] process. In this manner Larock et al. have developed a palladiumcatalysed regiospeci.c [32] cycloaddition process to synthesise 2,3-disubstituted indoles in excellent yield using disubstituted alkynes as acceptor molecules.Ujjainwalla and Warner have synthesised a 1H-pyrrolo[3,2-c]pyridine via a related process in good yield (Scheme 21). In this case Pd(dppf) Cl was found to be superior to Pd(OAc) as a catalyst. Back and Bethell have developed a palladium-catalysed [32] cycloaddition process to synthesise indolines in excellent yield using 1,3-dienes as the acceptor molecules (Scheme 22). The above process has been successfully transformed onto solid phase. Thus Wang and Huang synthesised indolines on Rink resin in good yield (Scheme 23). Alper and Yamamoto have developed novel palladium-catalysed [32] cycloaddition processes in which vinylic oxiranes react with unsymmetrical carbodiimides or activated ole.ns generating 1,3-oxazolidin-2-imine derivatives or tetrahydrofuran derivatives in excellent yield and high optical purity (Scheme 24).Reetz et al. have found nano structured nickel clusters catalyse the [32] cycloaddition of methylenecyclopropane and methyl acrylate in moderate yield (Scheme 25). A similar palladium-catalysed process has also been reported. Finally in the [32] theme 1,3-dipolar cycloadditions of nitrones and vinyl ethers have been found to be catalysed by palladium salts. Thus the diastereomeric adducts were obtained as a 1 1 mixture in 60% yield (Scheme 26). No reaction occurred without the catalyst in chloroform at 70 °C. 106 Annu.Rep. Prog. Chem. Sect. B 1999 95 97—115 N R NHX I CONH X = tosyl Six-membered rings. [42] processes. Larock et al. have used 1,2-dienes as acceptor molecules to prepare both novel nitrogen and oxygen containing heterocycles in excellent yield via palladium-catalysed [42] processes (Scheme 27). They have also synthesised both highly substituted isoquinolines and pyridines via a Pdcatalysed [42] process (Scheme 28). These processes utilise -iodoimines as the starter species and alkynes as the acceptor molecules. This process could also be adopted to synthesise analogous carbocycles via a [42] cycloaddition a typical Scheme 23 NHAc Pd(OAc)2 C C Me + I LiCl Na2CO3 NH2 DMF 100 ¡ÆC C C Si(Et)3 I I Scheme 21 Pd(OAc) Ts K2CO3 R2 Ts = tosyl + HOCH2CH2 R1 + NHCbz + Scheme 22 2 Pd(OAc) LiCl DIPEA DMF 100 ¡ÆC DMF / H2O H Annu.Rep. Prog. Chem. Sect. B 1999 95 97¡ª115 Ac N OH Pd(dppf)2 N Me 75% NH LiCl Na2CO3 SiEt3 Ts 60% R1 N R2 Cbz N DMF 120 ¡ÆC R 2 H N R = H R1 = R2 = H 83% R = CO2Me R1 = R2 = H 78% X O X N N 10% TFA / CH2Cl2 H O 90% 107 n-Bu N N O Cl Pd2(dba)3¡�CHCl3 N C N Cl + n-Bu O 92% ee 42% + Cl N O ¡© N n-Bu O L Pd+ H L 49% CN CN Pd(PPh3)4 + CN O O CN 90% (56:44) + 130 ¡ÆC THF rt CO2Me CO2Me 36% ¡© Me + N O + + OEt (S)¡©TolBINAP THF rt N O Scheme 24 Ni cluster Me Scheme 25 CHCl PdCl2(MeCN)2 3 Me N O OEt OEt 1 1 60% Scheme 26 example of which is shown in Scheme 29. Yamamoto et al. have published a series of papers concerning the palladium-catalysed [42] cycloadditions of enyne¡ªdiyne systems.These processes are regiospecic and occur characteristically in good yield (Scheme 30). They have also successfully developed an intramolecular version of the above process to prepare exomethylene paracyclophanes in excellent yield (Scheme 31). Palladium and nickel salts have also been found to catalyse intramolecular Diels¡ªAlder reactions (Scheme 32). A novel application of a palladiumcatalysed [42] cycloaddion process is reported in the synthesis of enantiopure 108 Annu.Rep. Prog. Chem. Sect. B 1999 95 97¡ª115 Me H Pd(OAc)2 N • + H H H I PPh3 Bu4NCl Na2CO3 DMF 80 °C O H H H Pd(OAc)2 OH • + MeO H Br H PPh3 Bu4NCl Na2CO3 DMF 80 °C t-Bu N EtO + I Me t-Bu N I HOH + 2C C C Scheme 27 Pd(OAc)2 2C C C PPh3 Na2CO3 DMF 100 °C Pd(OAc)2 PPh3 Na2CO3 DMF 100 °C X Pd(OAc)2 Scheme 28 C C + NaOAc Bu4NCl DMF 100 °C X = Br OTf TBSO 3)4 Bu + Scheme 29 Pd(PPh Bu (o-Tol)3P THF 100 °C Bu Hex Scheme 30 Annu. Rep. Prog. Chem. Sect. B 1999 95 97—115 Me N 72% O O OMe 72% N CO2Et 99% Me N CH2OH 95% 86% OTBS Hex Bu 83% 109 O (OCH2CH2)n n = 2 3 O Me EtO2C TMS Br Br + R R = H R = OMe 110 Annu.Rep. Prog. Chem. Sect. B 1999 95 97—115 O O ( ) n Pd(PPh3)4 / PPh3 DMSO 100 °C n = 2 100% n = 3 95% Me O H 87% H EtO2C TMS Scheme 31 Pd(OAc)2 PPh3 THF 50 °C 10 min Ni(acac)2 Et2AlOEt P(o-FC6H4)3 70 °C 67% OtBu Br Scheme 32 OtBu Pd(OAc)2 PPh3 R 61 - 66% R R P Pd OAc 2 OtBu Bu4NOAc DMF / MeCN H H H R R = H 99% R = OMe 99% Scheme 33 NiCl2(PPh3)2 C60 + X C60 X Zn PPh3 toluene 90 ¡ÆC X = C(CO2Me)2 68% X = O 47% X = NTs 50% Scheme 34 CO2Me I CO2Me CO2Me I CO2Me ( ) n ( ) n [O] n = 1-2 80-90% CO2Me CO2Me ( ) n 90% ( ) n ¡� Scheme 35 Pd(OAc)2 I + NHBu N Bu X = C(CO2Me)2 X = O ¡� X = NTs CO2Me Pd(OAc)2 Et3N DMSO rt NHTs 80% E/Z (55:45) N Ts + PPh3 Bu4NCl Na2CO3 DMA 100 ¡ÆC 3 Pd(OAc)2 Bu PPh 4NCl I Na2CO3 DMA 100 ¡ÆC 57% E/Z (78:22) Scheme 36 estrone.Tietze et al. have synthesised the estrone precusor via a double Heck reaction in excellent yield (Scheme 33). They also prepared an aza heterocycle via a similar process in good yield. [222] process. Cheng et al. have described a nickel-catalysed [222] conversion of several ene diynes to C . These processes occur in good yield (Scheme 34). de Meijere et al. have reported a double Heck reaction on 1,2-dihalocycloalkenes.The initial adducts subsequently underwent a 6-electrocyclisation forming 111 Annu. Rep. Prog. Chem. Sect. B 1999 95 97¡ª115 OH NH2 + Me I NH2 O OMe Me Br O O OMe OMe Me OMe CO2Me Br CO2Me Br Pd CO2Me – CO2Me 112 Annu. Rep. Prog. Chem. Sect. B 1999 95 97—115 Pd(OAc)2 LiCl i-Pr2NEt DMF 120 °C Me Scheme 37 Pd2(dba)3 (S)–BINAP Cs2CO3 Ag exchanged zeolite NMP 80 °C PdL O O Scheme 38 Pd(OAc)2 dppe KH THF 60 °C Pd Scheme 39 Me N 59% + NH2 OH Me 22% OMe Me O OMe 39% 63% ee (i) H2 Pd/C (ii) CAN O Me O O xestoquinone MeO2C CO2Me 70% MeO2C CO2Me Me AcO Me Me PdOAc H Br Br E E E = CO2Me benzene derivatives in good yield (Scheme 35).This is a formal [222] process leading to six-membered ring formation. Seven-membered rings. [52] processes. Larock et al. have described a palladium-catalysed [52] cycloaddition process to synthesise medium-size nitrogen heterocycles in good to excellent yields. In these examples 1,2-dienes were employed as the acceptor molecules (Scheme 36). Finally a formal palladium-catalysed [52] cycloaddition process has been developed by Dyker and Markwitz to synthesise benzazepine derivatives in moderate yield (Scheme 37). Scheme 41 Cyclisation cascades Shibasaki et al. synthesised ()-xestoquinone via asymmetric palladiumcatalysed cascade cyclisation reactions in moderate yield (Scheme 38).The use of silver O H Pd(PPh3)4 • Me AcOH CO 75 °C O O H Me O PdOAc PdOAc H O PdOAc Scheme 40 Pd(PPh3)4 E E E Bu4NCl xylene 80 °C E E E 68% 113 Annu. Rep. Prog. Chem. Sect. B 1999 95 97—115 exchange zeolite was found to be superior to Ag PO in obtaining high ee’s in this above process. A wide range of natural products have been synthesised utilising Pd-catalysed cyclisation processes as the key step. Such natural products include ()-physostigmine ()-physovenine, ()-laurequinone inhibitors of squalene synthase cp-225917 farnesyl transferase cp263114, antitumor antibiotic CC-1065, ()-crinamine, anatoxin, ()-valienamine, picrotoxinin, cardenolide and diazonamide analogues.Blame and Coudanne have reported in a series of papers a novel Pd-catalysed cyclisation process as an approach to the synthesis of hydrindane systems. These processes are similar to Wacker type cyclisation and occur in good yield (Scheme 39). In the cascade theme Yamamoto et al. have used allene and carbon monoxide as the relay species in their three-component cascade cyclisation process. These processes are regio- and stereospeci.c and occur in moderate yield (Scheme 40). Finally a novel bicyclic carbopalladation process with two geminal reaction centers has been used to construct bicyclic systems in good yield (Scheme 41). References 1 B.L. Shaw S. D. Perera and E. A. Staley Chem. 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