- Steric Effect of Carboxylate Ligands on Pd-Catalyzed Intramolecular C(sp2)–H and C(sp3)–H Arylation Reactions
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A bulky carboxylic acid bearing three cyclohexylmethyl substituents at the α-position, namely, tri(cyclohexylmethyl)acetic acid, is demonstrated to act as an efficient ligand source in Pd-catalyzed intramolecular C(sp2)?H and C(sp3)?H arylation reactions. The reactions proceed smoothly under mild reaction conditions, even at room temperature due to the steric bulk of the carboxylate ligands, which accelerates the rate-determining C?H bond activation step in the catalytic cycle.
- Tanji, Yutaka,Mitsutake, Naoya,Fujihara, Tetsuaki,Tsuji, Yasushi
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supporting information
p. 10314 - 10317
(2018/08/06)
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- Impregnated palladium on magnetite as catalyst for direct arylation of heterocycles
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Palladium impregnated on magnetite is an efficient, cheap and easy to prepare catalyst for the direct arylation of heterocycles. Good yields are afforded under relatively mild conditions and a broad substrate scope is evident. The catalyst is regioselective in many cases, affording arylated products, at the C2- or C3-position (depending of the heterocycle used). The methodology can be extended to prepare chromenes through an intramolecular direct arylation reaction. Some evidence is provided for two catalyst deactivation pathways, which prevents efficient recycling.
- Cano, Rafael,Pérez, Juana M.,Ramón, Diego J.,McGlacken, Gerard P.
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p. 1043 - 1050
(2016/07/06)
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- Catalytic direct arylation with aryl chlorides, bromides, and iodides: Intramolecular studies leading to new intermolecular reactions
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A catalyst for the intramolecular direct arylation of a broad range of simple and heterocyclic arenes with aryl iodides, bromides, and chlorides has been developed. These reactions occur in excellent yield and are highly selective. Studies with aryl iodides substrates revealed that catalyst poisoning occurs due to the accumulation of iodide in the reaction media. This can be overcome by the addition of silver salts which also permits these reactions to occur at lower temperature. The utility of the methodology is illustrated by a rapid synthesis of a carbazole natural product and by the synthesis of sterically encumbered tetra-ortho-substituted biaryls via ring-opening reactions of the direct arylation products. Mechanistic investigations have provided insight into the catalyst's mode of action and show the presence of a kinetically significant C-H bond cleavage in palladium-catalyzed direct arylation of simple arenes. Knowledge garnered from these studies has led to the development of new intermolecular arylation reactions with previously inaccessible arenes, opening the door for the development of other new direct arylation processes.
- Campeau, Louis-Charles,Parisien, Mathieu,Jean, Annie,Fagnou, Keith
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p. 581 - 590
(2007/10/03)
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- Intramolecular 9-membered hydrogen bonding of 2-arylmethylphenols having carbonyl groups at 2′-position
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Thermodynamic parameters of nine-membered intramolecular hydrogen bonding between carbonyl groups and phenolic hydroxyl groups of 2-arylmethylphenols having methoxycarbonyl, dimethylcarbamoyl, and formyl groups were determined by variable temperature 1H NMR studies and van't Hoff analysis. The enthalpy of the hydrogen bonding was related to the electron-withdrawing ability of the substituents on the phenol and the basicity of the carbonyl group. The entropy loss of the hydrogen bonding was dependent on the rotation freedom of the phenol group. The enthalpy of nine-membered intramolecular hydrogen bonding between carbonyl groups and phenolic hydroxyl groups of 2-arylmethylphenols was related to the electron-withdrawing ability of the substituents on the phenol and the basicity of the carbonyl group. The entropy loss was dependent on the rotation freedom of the phenol group.
- Yoshimi, Yasuharu,Maeda, Hajime,Hatanaka, Minoru,Mizuno, Kazuhiko
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p. 9425 - 9431
(2007/10/03)
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- Bu3SnH mediated oxidative radical cyclisations: Synthesis of 6H-benzo[c]chromen-6-ones
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Attempts to synthesise 6H-benzo[c]chromen-6-ones by Bu3SnH mediated cyclisation of o-(benzoyl)aryl radicals failed because of the preferred trans conformation of the ester. This problem was overcome by using cyclisation of o-(benzyloxy)aryl and o-[(aryloxy)methyl]aryl radicals to yield 6H-benzo[c]chromenes followed by oxidation to the 6H-benzo[c]chromen-6-ones. 3-Methoxy-6H-benzo[c]chromen-6-one 1, one of the main biologically active constituents of shilajit, a herbal medicine used in countries surrounding the Himalayan mountains, was synthesised using Bu3SnH mediated cyclisation of 1-benzyloxy-2,4-dibromo-5-methoxybenzene 31 to yield 3-methoxy-6H-benzo[c]chromene 25 followed by PCC oxidation of the 6-position. In order to avoid the problems of rearrangement, the aryl radical cyclisation must be designed such that whichever way the spirodienyl intermediate rearranges, the same product is obtained. For instance, the Bu3SnH mediated cyclisation of 1-iodo- and 1-bromo-2-(3-methoxy-phenyloxymethyl)benzenes 22 and 23 respectively gave both the isomers, 1-methoxy-6H-benzo[c]chromenes 24 and 3-methoxy-6H-benzo[c]chromenes 25 via rearrangement of the intermediate spirodienyl radical. The synthesised 6H-benzo[c]chromenes were oxidised in high yield to the corresponding 6H-benzo[c]chromen-6-ones. The mechanism of the 'oxidative' Bu3SnH mediated cyclisation is discussed.
- Bowman, Russell,Mann, Emma,Parr, Jonathan
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p. 2991 - 2999
(2007/10/03)
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