Furans palladium catalyst

The NHC-palladium catalyst system can also promote the ring closure of o-bromobenzyl ketones 170 (Scheme 5.45) [50]. Benzo[b]furans 171 were successfully prepared using this methodology. 

Several syntheses of furans are based upon the use of acetylenes to form carbon-carbon linkages. Of two that employ palladium catalysts,7172 one is outlined in Scheme 11,71 A simpler approach is to treat a 1,3-diketone with... 

A palladium catalyst is used in the transformation of a siloxyfuran to a phenyl substituted furanone <00JCS(P1)3350>. Similar products, furan-2(5//)-one derivatives, are afforded through the reaction of tetra-n-butylammonium fluoride with the corresponding substituted 2-siloxyfuran <00S1878>, as well as the oxidation of 3,4-disubstituted furans by singlet oxygen . 

Pure decarbonylation typically employs noble metal catalysts. Carbon supported palladium, in particular, is highly elfective for furan and CO formation.Typically, alkali carbonates are added as promoters for the palladium catalyst.The decarbonylation reaction can be carried out at reflux conditions in pure furfural (165 °C), which achieves continuous removal of CO and furan from the reactor. However, a continuous flow system at 159-162 °C gave the highest activity of 36 kg furan per gram of palladium with potassium carbonate added as promoter. In oxidative decarbonylation, gaseous furfural and steam is passed over a catalyst at high temperatures (300 00 °C). Typical catalysts are zinc-iron chromite or zinc-manganese chromite catalyst and furfural can be obtained in yields of... 

Hydrogenation of dibenzofuran over a platinum catalyst in acetic acid at 50°C and moderate pressure affords perhydrodibenzofuran. At higher temperatures and pressures with platinum or palladium catalysts the product is 2-biphenylol. When dibenzofuran is hydrogenated in ethanol over Raney nickel at 190°C and 200 atm for 23 h, the products isolated were perhydrodibenzofuran (36%), trans-2-cyclohexylcyclohexanol (27%), cis-2-cyclohexylcyclohexanol (20%), and dicyclohexyl (3%). When the hydrogenation, under these conditions, was terminated after the absorption of only 3 mol equiv of hydrogen, the only product detected was perhydrodibenzo-furan. ° Hydrogenation of dibenzofuran over a sodium-rubidium catalyst,... 

Catalytic hydrogenation of furan Catalytic hydrogenation of furan with a palladium catalyst gives tetrahydrofuran, which is a clear, low-viscosity liquid with a diethyl-ether-like smell. 

The formation of 2 furane rings was achieved in one transformation by Ma and co-workers. Allenoic acids and allenyl ketons were reacted in the presence of a palladium catalyst to give the unsymmetrical bifuryl product, arising from the cyclization of both allene derivatives mediated by the same palladium centre followed by their coupling (3.73.) 91... 

Furans, thiophenes, thiazoles and pyrroles all reacted smoothly with alkylydenecyclopropanes in the presence of a palladium catalyst and added tributylphosphone, to give the allylated heterocycles as product. Thus furane-2-carboxylic ester on treatment with the butyl substituted alkylydenecyclopropane gave the 5-allyl-2-furanecarboxylate in good yield (6.96.).128... 

An equimolar mixture of 3,4,5-trimethoxy phenyl iodide 157, lithium propargyl alkoxide 158, and diethyl ethoxymethylene malonate 159 was stirred at room temperature in the presence of a palladium catalyst. Then, to the resulting intermediate 161 potassium t-butoxide was added, and the ensuing base-promoted decarboxylative aromatization afforded tetrahydrofuran MCR adduct 162 in good yield. The ester was first reduced and the furan ring was hydrogenated with Raney nickel to furnish a diastereomeric mixture of products 163 in high yield. Further synthetic manipulations then provided a known precursor to the natural product. 

According to Freifelder, in most instances ruthenium catalyst is superior to nickel catalyst for the hydrogenation of furans to tetrahydrofurans the hydrogenation can be carried out at 70-100°C and 7 MPa H2.185 He refers to an example in which 2-furfu-rylamine was hydrogenated without solvent over ruthenium dioxide at 100°C and 8 MPa H2 in 10 min, compared to the temperatures of < 150°C and a reaction time of 30 h at 7 MPa H2 that were required for hydrogenation with Raney Ni. Hydrogenation of P-(2-furyl)alkylamines and an A-ethyl-2-furylalkylamine to the corresponding tetrahydro compounds was performed satisfactorily over palladium catalyst in ethanol in the presence of hydrochloric acid at room temperature and 0.62 MPa H2 (eq. 12.102)197 and over 5% Rh-C in neutral solvent at room temperature and 0.15 MPa... 

Another cascade reaction using conjugated enynals as starting materials with a palladium catalyst was reported. Good to high yields of 2,3,4-trisubstituted furans were afforded in the reaction. An example is shown in the following scheme 070L1191>. 

The experiments were run with a view to determining how four experimental variables influence the yield in the catalytic hydrogenation of furan over a palladium catalyst. The variables were x2, the amount of palladium catalyst x2, the hydrogen pressure x3, the reaction temperature and x4, the stirring rate. The factorial design, 24, shown in Table 2 was used to fit a second-order intraction model to the yields obtained ... 

Palladium catalysts were able to catalyze the allylation of furans with alkylidenecyclopropanes, presumably via an allylpalladium intermediate, to furnish 2-allylated products, as illustrated in Equation (31) <2000JA2661>. 

The cyciization of o-alkynyl phenol derivatives under the influence of a base or copper or palladium catalysts is a simple and reliable route to benzo[3]furans (Equation 107) <1996H(43)101>, given that the substrates are easily... 

As a consequence of the mild reaction conditions in the sequence to 3-halo furans 55 the palladium catalyst should be still intact to trigger another Pd-catalyzed coupling in the sense of a sequentially Pd-catalyzed process [31]. As a consequence, a sequential Sonogashira-deprotection-addition-cyclocondensation-Suzuki reaction, where the same catalyst system is applied in two consecutive significantly different cross-coupling reactions in the same reaction vessel should be feasible. Therefore, upon consecutive reactions of (hetero)aroyl chlorides 7 and THP-protected propargyl alcohols 54, Nal and PTSA, and finally, addition of 1.05 equiv of boronic acids 60 and sodium carbonate, the substituted 3-aryl furans 61 can be obtained in decent yields (Scheme 35). 

Several benzologs of furan and thiophene are conveniently formed by procedures of ring closure over dehydrogenation catalysts. o-Ethylphenol is cyclized at 620° over a palladium catalyst to benzofuran (11%). Chromium oxide on alumina at 450° converts o-ethylthiophenol to benzo-thiophene (42%). Alkyl groups in the alpha and beta positions are obtained by suitable variation of structure in the alkyl side chain. For the... 

Aromatic hydrocarbons, such as benzene add to alkenes using a ruthenium catalyst a catalytic mixture of AuCVAgSbFs, or a rhodium catalyst, and ruthenium complexes catalyze the addition of heteroaromatic compounds, such as pyridine, to alkynes. Such alkylation reactions are clearly reminiscent of the Friedel-Crafts reaction (11-11). Palladium catalysts can also be used to for the addition of aromatic compounds to alkynes, and rhodium catalysts for addition to alkenes (with microwave irradiation). " Note that vinyhdene cyclopropanes react with furans and a palladium catalyst to give aUylically substituted furans. ... 

Aryl halides which are rather inert in usual organic reactions can undergo reactions by means of palladium catalysts. Thus, styrene and stilbene derivatives are obtained by reaction of olefins with aryl bromides at 125 °C using Pd(0Ac)2 (1 mol%) and tri-(o-tolyl)phosphine (2 mol%)83. The palladium-catalyzed vinylic substitution reaction is applicable to a variety of heterocyclic bromides including pyridine, thiophene, indole, furan, quinoline and isoquinoline84. Thus, reaction of 3-bromopyri-dine with l-(3-butenyl)phthalimide at 100 °C gives l-[4-(3 -pyridyl)-3-butenyl]-phthalimide (yield of mixed amine 57%, selectivity 68%) at 100 °C. This phthalimide is subsequently converted to nornicotine (188) (Scheme 59). The reaction of acrylic... 

The experiments were run to determine how four experimental variables influence the yield of tetrahydrofuran in catalytic hydrogenation of furan over a palladium catalyst. The variables and the experimental domain are shown in Table 5.10. 

In further studies of asymmetric hydrocyanation the phosphinite [(+ )-(35,4S )-tetrahydro-furan-3,4-diyl]bis(diphenylphosphinite [( + )-DIPHIN] was prepared in several steps from (+)-(/ ./ )-tartaric acid via the corresponding diol, and used as a chiral ligand. With a palladium catalyst precursor prepared from Pd(BDA)2 and this ligand, hydrocyanation of norbornene was achieved with only 7% op and 10% yield. Under the same conditions Diop gives 90% yield and 24% ee21. 

Palladium catalysts can also be used for furan reductions. Depending on the reaction conditions, civ-substituted tetrahydrofurans (e.g., 8)46, as well as ring-cleaved products, can be obtained. Palladium on barium sulfate or calcium carbonate preferentially gives the tetrahydro-furan derivatives46. 

Hydroalkoxylation of alkynes, or the addition of alcohol to alkynes, is a fundamental reaction in organic chemistry that allows the preparation of enol ethers and a variety of oxygen-containing heterocycles such as furan, pyran, and benzofuran derivatives. Bergbreiter et al. found that a Mnear poly-(A-isopropylacrylamide) (PNIPAM) polymer exhibited inverse temperature solubility in water (i.e., soluble in cold water but insoluble in hot water). A recoverable homogeneous palladium catalyst was prepared based on the polymer. The PNIPAM-bound Pd(0) catalyst was effective for the reaction of 2-iodophenol with phenylacetylene in aqueous THE media to give the target product... 

A highly efficient synthesis of a wide range of 2,5-disubstituted furans from enyne acetates was described. The reactions were conducted by using Lewis acid and palladium catalysts, and provided symmetrical and unsym-metrical products in good-to-excellent yields, with broad substrate scope, including with a variety of aromatic and aliphatic substituents at the 2- and 5-positions of the furan (14SL2341). 

Pyridine N-oxides undergo a regioselective C—H coupling with triazoles, thiophenes, and furans in the presence of a palladium catalyst and silver (I) carbonate additive (Scheme 34). While C-H/C-H palladium-catalyzed... 

The cycloisomerization of a-allenyl ketones to the corresponding substituted furans was the first example of a gold-catalyzed addition of an oxygen nucleophile to an allene (Scheme 4-86). Traditionally, silver or palladium catalysts were employed for cyclizations of this type advantages of gold catalysis incluiie shorter reaction times, milder conditions, an or lower catalyst loadings. Variable amounts of... 

Intermolecular direct arylations of heteroarenes, such as indoles, pyrroles or (benzo)furans, were, thus far, predominantly achieved with palladium catalysts (see Chapter 10). However, rhodium complexes proved also competent for the direct functionaUzations of various valuable heteroarenes with comparable or, in some cases, improved catalytic performance. Thus, rhodium-catalyzed C—H bond functionalizations of various N-heterocycles, were elegantly developed by Bergman, Ellman and coworkers. Here, the use of a catalytic system comprising [RhCl(coe)2]2 and PCys led to direct arylations of unprotected benzimidazoles with aryl iodides... 

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