共老我白发fumed Scheme35.
F.Alonso et al./Tetrahedron61(2005)11771–11835 11790
silica,followed by reaction with potassium diphenylarse-nide and palladium chloride and then reduction with hydrazine hydrate.140This complex exhibited a high activity in the stereoselective arylation of styrene and acrylic acid with aryl halides,affording a variety of trans -stilbenes and substituted trans -cinnamic acids in high yields (Scheme 36).The arylation of styrene with aryl iodides proceeded smoothly at 1008C,whereas that of aryl bromides took place at 1408C with added triphenyl-phosphane.The substituent effects in aryl iodides appeared to be less signi?cant than those in aryl bromides,the reactivity of the latter with electron-withdrawing sub-stituents being higher than that of aryl bromides with electron-donating substituents.The bidentate arsine ligand seems to ef?ciently prevent palladium leaching,since a yield variation of only 91to 89%was observed after four consecutive runs.Similar results to those described above were obtained in the coupling of aryl halides with acrylic acid.It is worthy of note that the catalyst activity did not remarkably decrease after exposure to air for 7days.The methodology was extended to the arylation of acrylonitrile,furnishing (E )-cinnamonitriles in high yields.141Some disadvantages include the fact that the starting 4-oxa-6,7-dichloroheptyltriethoxysilane and potassium diphenyl-arsenide are not commercially available and must be prepared,the potential toxicity of the arsenic products and waste,and the higher temperature and presence of Ph 3P required for aryl bromides.The same group prepared a silica-supported poly-g -methylselenopropylsiloxane-palladium(0)complex (from poly-g -chloropropylsiloxane and sodium methylselenolate,followed by reaction with PdCl 2and ?nal reduction with hydrazine hydrate),which was also a highly active and stereoselective catalyst for the arylation of conjugated alkenes.142
A palladium(II)–SCS-pincer complex was immobilised on porous silica and the resulting catalyst 5was applied to the reaction of iodobenzene and n -butyl acrylate with Et 3N in DMF at 1208C for 1.5h.143The catalyst was recycled three times with high yields (O 99,97and 94%,respectively),with induction periods between subsequent runs indicating that the silica-immobilised catalyst acted as a precatalyst,
with the real active species being generated in situ during the reaction.It would worthwhile knowing about the substrate scope and limitations of this catalytic
system.
Molna
′r et al.prepared two series of catalysts by modifying silica with various chlorohydrosilanes,followed by reaction with a saturated solution of PdCl 2or a 1.25!10K 3PdCl 2solution to obtain ?ve palladium-on-silica catalysts with different Pd loadings and another ?ve palladium catalysts with an equal palladium loading of 0.3wt%,respectively.144These catalysts were found to exhibit a high activity in the Heck coupling of aryl halides with styrene or methyl acrylate to afford the corresponding E isomers with good to excellent selectivities.The 0.3wt%Pd-silica catalysts showed a lower activity,but increased selectivities,with 0.3wt%Pd/SiO 2Ph being the best catalyst (Scheme 37).The catalytic performance seemed to be related to the organophilicity of the surface,since the catalysts prepared from precursors with a single organic group as the surface modi?er (Pd/SiO 2Me and Pd/SiO 2Ph)showed a higher activity than those containing two organic groups (Pd/SiO 2Me 2and Pd/SiO 2Ph 2).In a comparative study,the properties of these organically modi?ed silicas were considered to be identical with those of 10wt%Pd/C.Despite the reactions being carried out in a sealed tube at 1508C,the experimental procedure for the preparation of these catalysts is very straightforward and their performance is remarkable.In addition,the catalyst performance did not change in successive experiments in which no special precaution was taken to exclude air or moisture.
Rotello et al.reported a highly reactive,recyclable,heterogeneous catalyst in which palladium mixed mono-layer protected clusters were simultaneously used as building blocks and active catalytic units.145Palladium nanoparticle 6(obtained by place exchange of 11-mercapto-undecanoic acid onto 1nm octanethiol-covered particles)was dissolved in MeOH and,to this,was added silica particles 7and amine polymer 8.The active catalysts were prepared through calcination of the nanocomposite materials,providing total removal of organics
and
Scheme
36.
Scheme 37.
F.Alonso et al./Tetrahedron 61(2005)11771–1183511791
