共老我白发Hara et al.obtained quasi 2-dimensional palladium nanoparticles encapsulated into graphite,which proved to be active catalysts for the Heck reaction.119Thus,an 82%yield of the coupled product,stilbene,was isolated by reacting 1mmol of styrene with 2mmol of iodobenzene in the presence of 4mmol of potassium carbonate,2mmol of tetra-n -butylammonium bromide and 10.2mg of the quasi two-dimensional palladium particles encapsulated in graphite (37%)in DMF using a sealed tube at 1008C for 4days (Scheme 31).No coupling product was observed with chlorobenzene.The nanoparticles remained inside the
carbon lattice after the Heck reaction and could not be washed out,the catalyst therefore being stable.Despite this catalytic system completely avoiding palladium leaching,its preparation is rather time consuming and somewhat complex.In addition,harsh reaction conditions and long reaction times are required.
For the application of Pd/C in combination with ionic liquids,see Section 6.2.
5.2.2.Metal oxides and other inorganic materials.The catalytic activity and selectivity of palladium supported on various metal oxides in the Heck reaction have attracted the interest of many research groups,both the nature of the oxide support and the Pd dispersion playing a crucial role in the activity of the catalytic system.120On the other hand,there is a considerable interest in determining the heterogeneous or homogeneous nature of the mechanisms involved in these reactions.121
Silica,within the family of metal oxides,has been,by far,the most utilised support in the Heck reaction.122In one of the ?rst reports on this topic,Strauss et 5a98184be45c3b3567ec8bfced 0.18%palladium on porous glass tubing for Heck reactions conducted continuously or batchwise.123This support offered resistance to oxidative deterioration and could be re-used several times for repeating,or for different,reactions,but the regio-and stereoselectivity was lower than expected.In parallel with this work,Mirza et al.developed a supported liquid-phase catalytic system for the Heck reaction of iodobenzene and methyl acrylate in a batch reactor,based on a combination of sulfonated triphenyl-phosphine–palladium complexes (Na-TPPMS and PdCl 2)supported upon porous glass beads and solvated with ethylene glycol (Scheme 32).Under these conditions,the catalyst complex was held in solution,whilst the reactants and products were restricted to a nonmiscible solvent phase.124Leaching was eliminated to the limits of detection,but the activity of the supported liquid-phase catalyst decreased signi?cantly after recycling.With regard to the nature of the catalysis occurring within the system,evidence suggests that this is predominantly homogeneous via the formation of an organic-soluble catalyst complex.Alter-natively,potassium acetate was used by Arai et al.as an inorganic base instead of triethylamine in a very
similar
Scheme
30.
Scheme
31.
Scheme 32.
F.Alonso et al./Tetrahedron 61(2005)11771–11835
11788
catalytic system,125whilst tri-n -butylamine gave higher reaction rates than triethylamine.126
A supported palladium catalyst on glass beads,generated from a guanidinium phosphane and palladium acetate,showed a high activity and low leaching of catalyst in the Heck reaction of aryl iodides and alkenes (Scheme 33).127The low catalyst loading,as well as the high yields achieved and the low leaching maintained over four reaction cycles,are clear advantages of this catalytic system.The main inconvenience is the preparation of the ligand which involves four steps,that is,Grignard reaction of com-mercially available 2-(N ,N -bis-trimethylsilylamino)phenyl-magnesium chloride with phosphorus trichloride,nitrogen deprotection,hydrochloride formation and condensation with dimethyl cyanamide.
The same research group prepared a new catalyst by treatment of reverse-phase silica beads with Pd(OAc)2and Ph 3P in cyclohexane.128The reverse-phase bead catalyst formed was assumed to contain (Ph 3P)2Pd(OAc)2and was stable in air and easy to handle.This catalyst was applied to the Heck reaction between iodobenzoic acid and acrylic acid,the expected product being obtained in a high yield and with low palladium leaching (Scheme 34).Heteroaromatic halides were also coupled with methyl acrylate with lower yields and low palladium leaching.The reverse-phase beads could be recovered from the reaction mixture and were
re-used seven times without any apparent loss of activity.Some reactions could be performed in water or with no added solvent.The real advantage of using the reverse-phase silica support seems to be that more polar substrates can be employed.
The catalysts shown in Chart 1are all stable and active heterogeneous catalysts based on chemically modi?ed silica,prepared by building up a suitable ligand structure,containing an aminopropyl moiety,on the surface of the silica,followed by complexation to the M(II).All of these catalysts have been recently reported and shown to be effective in Heck reactions,complex 1furnished 82%conversion in the coupling of iodobenzene and methyl acrylate with Et 3N in MeCN at 828C for 24h.129This catalyst was successfully re-used without a noticeable loss of activity and with no detectable amounts of palladium leached.The same complex 1,but prepared from PdCl 2instead of Pd(OAc)2on ordered mesoporous silica FSM-16,was active for the coupling of aryl iodides and bromides with methyl acrylate,giving 100%conversion in 1–5h at 1308C.Aryl bromides with electron-donating substituents were particularly sluggish in their reactivity,but the reactivity could be increased by the addition of TBAB.Seven reaction cycles were applied in the vinylation of 4-bromoacetophenone with methyl acrylate without a signi?cant loss of reactivity.130Complex 2was prepared by heating 3-(4,5-dihydroimidazol-1-yl)-propyltriethoxy-silanedichloropalladium(II)with mesoporous silica nano-tube particles.131A 1.5%palladium-imidazoline complex
2
Scheme
33.
Scheme
34.
Chart 1.
F.Alonso et al./Tetrahedron 61(2005)11771–1183511789
with Cs2CO3in dioxane at808C for2–3h gave excellent yields(79–94%)of coupled products obtained for a wide array of bromides or iodides with styrene.In all cases where recycling of the catalyst was attempted,however,small decreases in activity were observed from one run to the next.
A higher reaction temperature(1408C)was required for catalyst3in the reaction of iodobenzene and n-butyl acrylate using sodium carbonate as base in NMP.132High TONs were obtained and the catalyst was recycled three times with high activity,whereas a lower reactivity was observed for styrene,with both E/Z stilbenes being formed in a7:1ratio.A higher loading of catalyst was necessary in the case of4-bromotoluene and a lower yield was found after three runs.Zheng et al.prepared silica-supported poly-g-aminopropylsilane-transition metal complexes4,which were also active and stereoselective for the Heck vinylation reaction of aryl iodides with ole?ns at120–1508C.133All of the complexes were treated with KBH4–EtOH before use. The supported Ni complex furnished the expected products in86–98%yield and the Co and Cu complexes in71–96% yield,although the reaction of iodobenzene and styrene gave stilbene in only40%yield with the latter complexes. An induction period of O2h was observed for these catalysts.The reaction temperature could be reduced by up to70–1008C using a similar,but new,silica-supported poly-g-aminopropylsilane-Cu2C-Pd2C complex.134 Although there is no doubt about the ef?ciency and the possibility of recycling of the above-described catalysts,a comment must be made concerning their preparation.A minimum of three reaction steps are involved in the preparation of the catalysts(10steps for catalyst3)and, in addition,other multiple operations such as?ltration, washing,drying under vacuum or high temperature,or conditioning by re?uxing in a solvent,as well as long reaction times,are needed for some of the steps(up to 4days).Therefore,taking into account the yield reduction when increasing the number of steps,as well as the waste produced along the synthesis of the catalysts and the price of the starting materials and metallic salts,the catalysts4seem to be the most convenient from a practical point of view and for the larger-scale preparations.
In relation to the above catalytic system described by Zheng et al.this research group prepared silica-supported palladium(0)complexes from g-chloropropyl-and g-aminopropyltriethoxysilane via immobilisation on fumed silica,followed by reaction with ethylenediamine and salicylaldehyde and then reaction with PdCl2and reduction with KBH4.135In this case,the reaction temperature was reduced to908C for the Heck reaction of aryl iodides(p-RC6H4I,R Z H,MeO,CO2H)with acrylic acid,methyl acrylate and styrene.The catalysts could be recovered and re-used without loss of activity.A compar-able performance was observed for the silica-supported palladium(0)complex similarly prepared from poly-g-cyanopropyltriethoxysilane.136
N,N-di(pyrid-2-yl)norborn-2-ene-5-ylcarbamide was sur-face grafted or coated onto various silica-based carriers through a ring-opening metathesis polymerisation(ROMP) in the presence of Mo(N-2,6-Me2C6H3)(CHCMe2Ph) [OCMe(CF3)2]2.137The silica-based materials were used in slurry reactions under standard conditions,as well as under microwave irradiation,the latter case leading to a drastic reduction of the reaction times.A quantitative conversion was achieved for iodoarenes,while activated bromoarenes required prolonged reaction times.Typically, less than2.5%palladium was leached into the reaction mixture.Nonetheless,the preparation of the supported catalysts is very sophisticated in order to?nally achieve similar results to those obtained with simpler supported catalytic systems.An example using0.3mol%of the silica-based slurry is depicted in Scheme35.
In1997,Cai et al.described the preparation of a silica-supported poly-g-mercaptopropylsiloxane-palladium(0) complex that gave high yields(86–96%)in the arylation of styrene and acrylic acid(Bu3n N,xylene,1008C,6h)and could be recovered and re-used with a low decrease in activity.138The same ligand,but supported on FSM-16 mesoporous silica,was shown to be an active and stable catalyst for the Heck reaction of4-bromoacetophenone and ethyl acrylate(KOAc,NMP,1308C,92%yield),the catalyst being re-used at least?ve times with no indication of catalyst deactivation.The less reactive electron-rich aryl bromides,4-bromoanisole and bromobenzene,however, resulted in moderate yields(58and62%,respectively), whereas4-chloroacetophenone gave a low yield(18%).139 Based on the above original methodology and,more recently,Cai’s group has prepared a silica-supported bidentate arsine-palladium(0)complex from4-oxa-6,7-dichloroheptyltriethoxysilane via immobilisation on
