Synthesis of Indoles by Reductive Cyclization of Nitro Compounds Using Formate Esters as CO Surrogates
Alkyl and aryl formate esters were evaluated as CO sources in the Pd- and Pd/Ru-catalyzed reductive cyclization of 2-nitrostyrenes to give indoles. Whereas the use of alkyl formates requires the presence of a ruthenium catalyst such as Ru3(CO)12, the reaction with phenyl formate can be performed by using a Pd/phenanthroline complex alone. Phenyl formate was found to be the most effective CO source and the desired products were obtained in excellent yields, often higher than those previously reported using pressurized CO. The reaction tolerates many functional groups, including sensitive ones like a free aldehydic group or a pendant pyrrole. Detailed experiments and kinetic studies allow to conclude that the activation of phenyl formate is base-catalyzed and that the metal doesn't play a role in the decarbonylation step. The reactions can be performed in a single thick-walled glass tube with as little as 0.2 mol-% palladium catalyst and even on a 2 g scale. The same protocol can be extended to other nitro compounds, affording different heterocycles.
Ahmed Fouad, Manar,Ferretti, Francesco,Formenti, Dario,Milani, Fabio,Ragaini, Fabio
supporting information
p. 4876 - 4894
(2021/09/20)
Pd-Catalyzed Reductive Cyclization of Nitroarenes with CO2 as the CO Source
A reductive amination process that constructs indoles, carbazoles or benzimidazoles from nitroarenes – irrespective of their electronic or steric nature – was developed that uses CO2 as the source of CO. The process is robust, tolerating common gaseous components of flue gas (H2S, SO2, NO and H2O) without adversely affecting the reductive cyclization.
Guan, Xinyu,Zhu, Haoran,Zhao, Yingwei,Driver, Tom G.
supporting information
p. 57 - 60
(2019/12/11)
Pd-catalyzed decarboxylative Heck vinylation of 2-nitrobenzoates in the presence of CuF2
A new protocol for the decarboxylative Heck vinylation of benzoic acids is disclosed. In the presence of a catalyst system generated in situ from Pd(OAc)2 (2 mol %), CuF2 (2 equiv), and benzoquinone (0.5 equiv) in NMP, a wide range of olefins were coupled with various 2-nitrobenzoates at 130 °C with the release of carbon dioxide to afford the corresponding vinyl arenes in good yields.
Goossen, Lukas J.,Zimmermann, Bettina,Knauber, Thomas
experimental part
(2010/08/19)
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