The transition metal catalyzed synthesis of nitrogen-containing heterocycles have become a powerful tool in organic chemistry. In particular, the catalytic reductive cyclization of ortho-nitrostyrenes by palladium complexes using gaseous CO as the reductant is a powerful method for the synthesis of substituted indoles.1 From a laboratory scale point of view, the use of CO is often complicated and inconvenient. In facts its employment implies the installation of expensive safety measures and often the use of high-pressure special reactors. A way to overcome this issue involves the use of organic molecules capable of releasing carbon monoxide in situ. In this perspective the use of formates esters is considered a promising alternative to the carbon monoxide. Ru3(CO)12 is able to acts as catalyst for the decarbonylation of butyl and methyl formate to give CO and the corresponding alcohols.2 Since many years, [Pd(Phen)2][BF4]2 was used as catalyst in the nitrostyrenes cyclization using gaseous CO as the reductant.3 Herein we report the results on the use of a bimetallic catalytic system based on [Pd(Phen)2][BF4]2 and Ru3(CO)12 for the synthesis of substituted indoles from ortho-nitrostyrenes using formate esters (both butyl and methyl) as CO sources. All the reactions were performed in an economical pressure tube reaction vessel in order to avoid the use of an expensive and CO-line -dependent autoclave. The reactions were optimized using methyl 2-nitrocinnamate and 2-nitrostylbene and the substrate scope was extendend for the synthesis of various 2,6-substituted indoles.
Formate esters as CO sources: synthesis of indoles by reductive cyclization of ortho-nitrostyrenes / D. Formenti, F. Ferretti, E. Storer, F. Ragaini. ((Intervento presentato al 11. convegno Congresso Interdivisionale di Chimica Organometallica della Società Chimica Italiana tenutosi a Milano nel 2014.
Formate esters as CO sources: synthesis of indoles by reductive cyclization of ortho-nitrostyrenes
D. FormentiPrimo
;F. FerrettiSecondo
;F. RagainiUltimo
2014
Abstract
The transition metal catalyzed synthesis of nitrogen-containing heterocycles have become a powerful tool in organic chemistry. In particular, the catalytic reductive cyclization of ortho-nitrostyrenes by palladium complexes using gaseous CO as the reductant is a powerful method for the synthesis of substituted indoles.1 From a laboratory scale point of view, the use of CO is often complicated and inconvenient. In facts its employment implies the installation of expensive safety measures and often the use of high-pressure special reactors. A way to overcome this issue involves the use of organic molecules capable of releasing carbon monoxide in situ. In this perspective the use of formates esters is considered a promising alternative to the carbon monoxide. Ru3(CO)12 is able to acts as catalyst for the decarbonylation of butyl and methyl formate to give CO and the corresponding alcohols.2 Since many years, [Pd(Phen)2][BF4]2 was used as catalyst in the nitrostyrenes cyclization using gaseous CO as the reductant.3 Herein we report the results on the use of a bimetallic catalytic system based on [Pd(Phen)2][BF4]2 and Ru3(CO)12 for the synthesis of substituted indoles from ortho-nitrostyrenes using formate esters (both butyl and methyl) as CO sources. All the reactions were performed in an economical pressure tube reaction vessel in order to avoid the use of an expensive and CO-line -dependent autoclave. The reactions were optimized using methyl 2-nitrocinnamate and 2-nitrostylbene and the substrate scope was extendend for the synthesis of various 2,6-substituted indoles.Pubblicazioni consigliate
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