Thiophenes palladium® acetate

There are reports of an increasing number of palladium-assisted reactions, in some of which the palladium has a catalytic function. Thus furan and thiophene undergo facile palladium-assisted alkenylation giving 2-substituted products. Benzo[6 Jfuran and TV- acetyl-indole yield cyclization products, dibenzofurans and carbazoles respectively, in addition to alkenylated products (8UOC851). The arylation of pyrroles can be effected by treatment with palladium acetate and an arene (Scheme 86) (81CC254). 

Four solvent-soluble poly(aniline-co-thiophene) materials have been prepared by coupling thiophene derivatives with 4-n-butyl aniline using palladium acetate as catalyst. Mn s of up to 17,500 daltons with a PDI of 1.1 were obtained. 

Under ligandless conditions, 3-thiophenetrifluoroborate (23) was treated with 2-bro-mopyridine and 3-broniopyridine in the presence of 1% palladium acetate to afford the coupled thiophenes 121a and 121b in 60 and 68% yield, respectively [84],... 

Palladium-mediated oxidative arylation of thiophene has also been reported <8SJ0CS272>. Thus, treatment of 2-formylthiophene with palladium acetate in a mixture of acetic add and benzene gave 2-formyl-4-phenylthiophene (30%), 2-formyl-5-phenylthiophene (5%) and 5,5 -diformyl-2,2 -bithienyl (16%). It has been suggested that preliminary palladation of benzene would lead to 4-phenylation of the thiophene, while palladation of the thiophene would produce the 5-phenylated product. 

Regioselective carboxylation of thiophene by Pd-mediated reaction with CO in the presence of mercuric chloride, mercuric acetate or thallium(III) acetate has been reported <90JMOC23>. It is preferable to carry out the reaction in two steps— iiiitial mercuration, followed by reacting the thienylmercury derivative with CO in the presence of palladium acetate. 

Applications of these C-H activation processes in polymerization reactions have also been reported in the literature. For example, a palladium-catalyzed C-H arylation polycondensation was carried out between a thiophene unsubstituted at the 2- and 5- positions and its 2,5-dibromo derivative. The transformation was effected in the presence of palladium acetate, a phosphine, and cesium carbonate in DMF at 100 °C for 48 h (eq 23). A survey of different phosphines showed that similar yields, molecular weights (Mn), and polydispersity indexes (PDI) could be obtained using di-tert-butyl(methyl)phosphonium tetrafluoroborate or tri-cyclohexylphosphonium tetrafluoroborate as the ligand. ... 

Palladium-catalyzed Direct Arylation of Indoles and Thiophenes. Five-membered ring heterocycles possessing only one heteroatom and no Af-oxide function can also be arylated using palladium(II) complexes, a phosphine ligand, and an inorganic base. In one example, a tandem palladium-catalyzed Heck coupling reaction and direct intramolecular C2 arylation reaction on a )V-(2-chlorobenzyl)-5-bromoindole was reported (eq 30). The procedure, which is catalyzed by palladium acetate, uses tn-tert-butylphosphonium tetrafluoroborate as a ligand and ferf-butyl acrylate as the alkene for the Heck reaction (eq 30). ... 

The Ar-H functionalization approach has also been used in the preparation of polymers. For instance, a diketopyrrolopyrrole-based polymer was prepared via a palladium-catalyzed direct C-H (hetero)arylation reaction between a di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-l,4-dione and 4,7-dibromo-2,l,3-benzothiadiazole (eq31). The reaction was conducted using palladium acetate as the catalyst and pivalic acid as an additive. A survey of different phosphines demonstrated that molecular weights of the same order and similar polydispersity indexes (PDI) were obtained using tri-terf-butylphosphonium and tri-ferf-cyclohexyl tetrafluoroborate. ... 

Cyclododecene may be prepared from 1,5,9-cyclododecatriene by the catalytic reduction with Raney nickel and hydrogen diluted with nitrogen, with nickel sulfide on alumina, with cobalt, iron, or nickel in the presence of thiophene, with palladium on charcoal, with palladimn chloride in the presence of water, with palladium on barium sulfate, with cobalt acetate in the presence of cobalt carbonyl, and with cobalt carbonyl and tri- -butyl phosphine. It may also be obtained from the triene by reduction with lithium and ethylamine, by disproportionation, - by epoxidation followed by isomerization to a ketone and WoliT-Kishner reduction, and from cyclododecanone by the reaction of its hydrazone with sodium hydride. ... 

Alkenylation of thiophenes with substituted alkenes (styrene, acrylic ester, acrylonitrile) in the presence of equimolar amounts of palladium(II) acetate results in mono- and... 

To a solution of 2-iodo-5-(4-fluorophenylmethyl)thiophene (5.30 g, 16.6 mmol), in anhydrous DMF (5.0 ml) was added (R)-N-hydroxy-N-(3-butyn-2-yl)urea (2.12 g, 16.6 mmol), triphenylphosphine (84.0 mg, 0.32 mmol), bis(acetonitrile)palladium(II) chloride (40.0 mg, 0.16 mmol), copper(I) iodide (16.0 mg, 0.08 mmol), and diethylamine (5.6 ml). The mixture was stirred under nitrogen at room temperature for 22 h and concentrated in vacuum at 32°C. The residue was subjected to chromatography on silica eluting with 2-7% MeOH in CH2CI2, crystallization from ethyl acetate-hexane and trituration in CH2CI2 to afford (R)-N- 3-[5-(4-fluorophenylmethyl)thien-2-yl]-l-methyl-2-propynylVN-hydroxyurea as a cream-colored solid 0.94 g (18%), melting point 135°-136°C, (dec). 

Thiophene 1,1-dioxide with a shielded sulfur atom was hydrogenated without difficulty to the tetrahydro derivative, sulfolane, over palladium catalyst in acetic acid at room temperature and atmospheric hydrogen pressure (eq. 12.121).249... 

Since sulfur is the most effective of all catalyst poisons, the hydrogenation of sulfur containing heterocycles is not easily accomplished unless there are no unshared electron pairs on the sulfur atom or the catalyst used is not affected by the poison. The hydrogenation of the cyelie sulfone, 58, takes palace over an excess of palladium in acetic acid at room temperature and atmospheric pressure (Eqn. 17.57). Thiophene, itself, can be hydrogenated to tetrahydrothiophene over rhenium heptasulfide at 250°C and 300 atmospheres of hydrogen or over a large excess of palladium in methanolic sulfuric acid at room temperature and 3-4 atmospheres. No hydrogenolysis of the carbon-sulfiir bond was observed in these reactions. 

Direct catalytic oxidative carbonylation to form acid anhydrides was achieved with arylmercuiy acetate. Palladium(II) salts are taken as catalysts. Aromatic compounds such as benzene, toluene, anisole, furan, thiophene and naphthalene can be converted catalytically with palladium(II) acetate and an excess of 1,2-dibromoethane to acid anhydrides in yields of 30- %. 

We attempted the palladium-catalyzed ring closure of 27a (Table 5). Treatment of 27a with palladium(Tf) acetate [Pd(OAc)2] (30 mol%) in the presence of triethylphosphite [( tO)3P] and sodium bicarbonate (NaHCOs) in aqueous tetrahydrofuran (THF) afforded the desired cyclized product 28a albeit in a poor yield (30%) (Table 5, Entry 1). Both the lack of base or the use of a strong base (NaH) in place of NaHCOs were detrimental, yielding thiophene 44 or 2-oxazolidinone derivative 45, respectively (Table 5, Entries 2 and 3). 

The relative sensitivity of furan to electrophilic attack stands in the order benzene < naphthalene < ferrocene < furan as found from competition experiments with styrene palladium(II) acetate.441 Relative rales for acylation by trifluoroacetic anhydride (no catalyst) arc observed to be thiophene (1), selenophene (6.5), furan (1.4 x 102), pyrrole (5.3 x I07), and... 

The commercial catalyst used for hydrogenation was a palladium on charcoal catalyst (type 37), manufactured by Johnson Matthey (UK)) with metal content 4.89% and supplied as powder. The Pd metal was d osited on the exterior sur ce of the charcoal The Pd/Ii02 catalyst was prepared using sodium tetrachloropalladate (II), supplied by Johnson Matthey (UK) and titanium dioxide, firom Degussa (Germany). The hydrogen gas was supplied by BOC (UK), with >99.98% purity and was used directly fi om cylinder. The reactant, butyne-l,4-diol 99%, was procured firom Aldrich (UK) and water and 2-propanol obtained firom Fisons (UK), were used as solvents. Additives such as lead acetate, quinoline, thiophene and triethyl phosphite were supplied by Aldrich (UK) and cupric acetate, zinc su hate, ferric nitrate and potasaum hydroxide were provided by Fisons (UK). 

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