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Unravelling the reaction path of Rhodium–MonoPhos-catalysed olefin hydrogenation

Alberico, Elisabetta and Baumann, Wolfgang and G. de Vries, Johannes and Drexler, Hans-Joachim and Gladiali, Serafino Gabriele and Heller, Detlef and Henderickx, Huub J. W. and Lefort, Laurent (2011) Unravelling the reaction path of Rhodium–MonoPhos-catalysed olefin hydrogenation. Chemistry: a European Journal, Vol. 17 (45), p. 12683-12695. eISSN 1521-3765. Article.

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DOI: 10.1002/chem.201101793

Abstract

The mechanism of the asymmetric hydrogenation of methyl (Z)-2-acetamidocinnamate (mac) catalysed by [Rh(MonoPhos)2(nbd)]SbF6 (MonoPhos: 3,5-dioxa-4-phosphacyclohepta[2,1-a:3,4-a′]dinaphthalen-4-yl)dimethylamine) was elucidated by using 1H, 31P and 103Rh NMR spectroscopy and ESI-MS. The use of nbd allows one to obtain in pure form the rhodium complex that contains two units of the ligand. In contrast to the analogous complexes that contain cis,cis-1,5-cyclooctadiene (cod), this complex shows well-resolved NMR spectroscopic signals. Hydrogenation of these catalyst precursors at 1 bar total pressure gave rise to the formation of a bimetallic complex of general formula [Rh(MonoPhos)2]2(SbF6)2; no solvate complexes were detected. In the dimeric complex both rhodium atoms are ligated to two MonoPhos ligands but, in addition, each rhodium atom also binds to one of the binaphthyl rings of a ligand that is bound to the other rhodium metal. Upon addition of mac, a mixture of diastereomeric complexes [Rh(MonoPhos)2(mac)]SbF6 is formed in which the substrate is bound in a chelate fashion to the metal. Upon hydrogenation, these adducts are converted into a new complex [Rh(MonoPhos)2{mac(H)2}]SbF6 in which the methyl phenylalaninate mac(H)2 is bound through its aromatic ring to rhodium. Addition of mac to this complex leads to displacement of the product by the substrate. No hydride intermediates could be detected and no evidence was found for the involvement at any stage of the process of complexes with only one coordinated MonoPhos. The collected data suggest that the asymmetric hydrogenation follows a Halpern-like mechanism in which the less abundant substrate–catalyst adduct is preferentially hydrogenated to phenylalanine methyl ester.

Item Type:Article
ID Code:6903
Status:Published
Refereed:Yes
Uncontrolled Keywords:Asymmetric catalysis, dimers, phosphoramidites, reactive intermediates, rhodium
Subjects:Area 03 - Scienze chimiche > CHIM/04 Chimica industriale
Divisions:001 Università di Sassari > 01 Dipartimenti > Chimica
Publisher:Wiley
eISSN:1521-3765
Deposited On:29 Dec 2011 11:19

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