Indoles 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). 

Bromination of the diphenyl indole derivative 316 with bromine in DMF or trimethylammonium bromide afforded the 7-bromo derivative 317. Reaction with allyl bromide or its derivatives gave A-allyl derivatives 318 that upon cyclization with palladium acetate gave 7,9-dimethoxy-l,2-diphenylpyrrolo[3,2,l-// ]quinoline derivatives 319 (92T7601) (Scheme 57). 

A novel route to indoles and quinolines has been developed by sequential Wiltig and Heck reactions <96CC2253>. Thus, treatment of o-bromo- or iodo-lV-lrifluoroaceiylanilines (86) with a stabilized phosphorane affords the corresponding enamines 87 as a mixture of isomers. Cyclization to 88 is effected by heating with palladium acetate, tri phenyl phosphine, and bu.se. 

Kostic el al. discovered that Pd11 complexes, when attached to tryptophan residues, can rapidly cleave peptides in acetone solutions to which a stoichiometric amount of water is added, for hydrolysis.436 The indole tautomer in which a hydrogen has moved from the nitrogen to C(3) is named indolenine. Its palladium(II) complexes that are coordinated via the nitrogen atom have been characterized by X-ray crystallography and spectroscopic methods.451 Binuclear dimeric complexes between palladium(II) and indole-3-acetate involve cyclopalladation.452 Bidentate coordination to palladium(II) through the N(l) and the C(2) atoms occurs in binuclear complexes.453 Reactions of palladium(II) complexes with indole-3-acetamide and its derivatives produced new complexes of unusual structure. Various NMR, UV, IR, and mass spectral analyses have revealed bidentate coordination via the indole carbon C(3) and the amide oxygen.437... 

Another compound 9 with three heterocyclic rings linearly fused (5 5 5) with two heteroatoms has been prepared from 1,1 -carbonyl diindole 297 <2001T5199>. Palladium-mediated coupling of the 2- and 2 -positions of 297 afforded the 1,1 -carbonyl-2,2 -biindolyl 9. 1,1 -Carbonyl diindole 297 was in turn obtained in 41% yield from 1,1 -carbonyldiimidazole 296 by reaction with indole in DMSO at 125 °C. The palladium-catalyzed coupling step afforded the desired product 9 in low yield and required a stoichiometric amount of palladium acetate. Therefore, it was felt prohibitively expensive. Addition of various co-oxidants (Ac20, Mn02, and Cu(OAc)2, etc) to make the reaction catalytic in palladium did not result in any improvement of the yield of 18 (Scheme 53). 

Dialkylindolines and 1,3-dialkylindoles are formed in poor yield (<10%) from the reaction of ethyl- or phenymagnesium bromide with 2-chloro-N-methyl-N-allylaniline in the presence of catalytic quantities of (bistriphenylphosphine)nickel dichloride.72 In a modification of this procedure, the allyl derivatives can be converted by stoichiometric amounts of tetrakis(triphenylphosphine)nickel into 1,3-dialkylindoles in moderate yield72 (Scheme 43) an initial process of oxidative addition and ensuing cyclization of arylnickel intermediates is thought to occur. In contrast to the nickel system,72 it has proved possible to achieve the indole synthesis by means of catalytic quantities of palladium acetate.73 It is preferable to use... 

A type Ilac synthesis of functionalized pyrroles was developed that adapted the Larock indole synthesis <06OL5837>. For example, treatment of iodoacrylate 19 and trimethylsilylphenylacetylene 20 with palladium acetate led to the formation of pyrrole-2-carboxylate 21 with excellent regioselectivity. 19 was prepared by iodinating (N-iodosuccinimide) the corresponding commercially available dehydroamino ester. 

The bromo derivative of A -mcthylsuccinimide did also undergo Suzuki coupling when treated with naphthylboronic acid in the presence of palladium acetate, triphenylphosphine and potassium carbonate (6.3.). The coupled product was deprotected under the reaction conditions and an indole derivative was isolated in good yield, which was successfully converted into the hexacyclic naphthopyrrolo[3,4-c]carbazole structure. Using the analogous trimethylstannyl-naphthalene derivative and optimised Stille coupling conditions the desired product was isolated only in 56% yield.5... 

By using an olefin embedded into the parent molecule Stoltz developed the oxidative annulation of indoles. The optimal catalyst consisted of palladium acetate and ethyl nicotinate, and molecular oxygen was used as the oxidant in the process. The reaction proceeded equally well irrespective of the attachment point of the alkyl chain bearing the pendant olefin bond on the five membered ring, and the formation of five and six membered rings were both effective (6.95.),127... 

Another interesting application of microwave irradiation is found in the selective dehalogenation of the iodinated indole alkaloid Plakohypaphorine F, described by E. Fattorusso et al. [117,118] (Scheme 32). The bis-halogenated compound was treated with potassium formate and palladium acetate under controlled microwave irradiation, resulting in selective deiodination. The choice of the solvent, in this case DMSO, was found to be crucial. 

A few recent examples of related C-C bond-forming reactions, all involving a palladium-catalyzed C-H activation step at arenes, will be mentioned. Salts are produced in these reactions, or acetic acid, as in the first example. Allylation of indoles at the 3-position was achieved by using palladium acetate, and bipyridine and allylic acetates as the reactants (Scheme 5) [19]. 

Applications of this reaction are not limited to advanced materials, but can be applied to natural product synthesis. Indeed, indoles have quite recently (in 2008) been arylated in the presence of palladium acetate and silver oxide (Scheme 10.52).84... 

Several new routes involve formation of one carbon-carbon bond in pre-formed substrates. Palladium-catalyzed cyclization of /3-hydroxyenamine derivatives has been employed in a route to substituted pyrroles and 4,5,6,7-tetrahy-droindoles with multiple substituents by formation of the C-3-C-4 bond as the key feature, as illustrated by construction of the molecule 534 (Equation 146) <2006T8533>. Zinc perchlorate-catalyzed addition of alcohols to the nitrile functionality of a-cyanomethyl-/3-ketoesters, followed by annulation gave access to a series of substituted ethyl 5-alkoxypyrrole-3-carboxylates <2007T461>. Similar chemistry has also been used for synthesis of a related set of pyrrole-3-phosphonates <2007T4156>. A study on preparation of 3,5,7-functionalized indoles by Heck cyclization of suitable A-allyl substituted 2-haloanilines has also appeared <2006S3467>. In addition, indole-3-acetic acid derivatives have been prepared by base induced annulation of 2-aminocinnamic acid esters (available for instance from 2-iodoani-lines) <2006OL4473>. 

Polycyclic indoles. The treatment of the 3-benzoylindoles 1 and 3 with palladium acetate (0.5 equiv.) in acetic acid leads to 2 and 4, respectively, by intramolecular oxidative ring closure. ... 

At the early stage of Heathcock s biomimetic total syntheses of discorhabdins [108], a 5-ejco Heck cyclization was employed for the synthesis of 3,6,7-functionalized indole. As highlighted in Scheme 42, when precursor 237 was exposed to catalytic palladium acetate, tri-o-tolylphosphine, and stoichiometric base, indole 238 was smoothly produced in 89% yield. Subsequently, the total syntheses of discorhabdin C (239) and discorhabdin E (240) were accomplished using indole 238 as the common intermediate. 

In 1981, Itahara reported the palladium acetate-mediated direct arylation of pyrroles and indoles with arenes [59], With (sub)stoichiometric Pd(OAc)2 in benzene and acetic acid, arylation of 1-benzoylpyrrole and 1-acetylindole occurred with moderate to excellent conversions, but gave generally low yields of the desired cross-coupling products (Scheme 14). Direct arylation of isoxazole in the presence of stoichiometric amount of Pd(OAc)2 was also reported by Nakamura et al. [60]. 

Arylation of indoles can also be carried out via arylpalladium acetates generated from boronic acids or trifluoroborate salts " and palladium acetate. The reactions are catalytic in palladium, cycling of the Pd(ll) being effected by the use of a re-oxidant (Cu(ll)/air). The reaction works well on NH and A-methyl indoles but fails with the A-acetyl derivative. 

A novel 2-arylation of A -substituted indoles has been reported <04OL2897>. Treatment of indole substrates with palladium acetate, triphenylphosphine, cesium acetate and aryl iodides led to the formation of 2-arylindoles. 

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