Enantioselective 1,3-Dipolar Cycloaddition of Nitrones to Methacrolein Catalyzed by (η5-C5Me5)M{(R)-Prophos} Containing Complexes (M = Rh, Ir; (R)-Prophos = 1,2-bis(Diphenylphosphino)propane):  On the Origin of the Enantioselectivity

Daniel Carmona,*† M. Pilar Lamata,† Fernando Viguri,† Ricardo Rodríguez,† Luis A. Oro,† Fernando J. Lahoz,† Ana I. Balana,† Tomás Tejero,‡ and Pedro Merino‡
Contribution from the Departamento de Qumica Inorgnica and Departamento de Qumica Orgnica, Instituto Universitario de Catlisis Homognea, Instituto de Ciencia de Materiales de Aragn, Universidad de Zaragoza-Consejo Superior de Investigaciones Cientficas, 50009 Zaragoza, Spain
J. Am. Chem. Soc., 2005, 127 (38), pp 13386–13398
DOI: 10.1021/ja0539443
Publication Date (Web): August 31, 2005
Copyright © 2005 American Chemical Society
*

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

,
†

 Departamento de Química Inorgánica.

,
‡

 Departamento de Química Orgánica.

, dcarmona@unizar.es

Abstract

Abstract Image

The rhodium and iridium Lewis-acid cations [(η5-C5Me5)M{(R)-Prophos}(H2O)]2+ ((R)-Prophos = 1,2-bis(diphenylphosphino)propane) efficiently catalyze the enantioselective 1,3-dipolar cycloaddition of nitrones to methacrolein. Reactions occur with perfect endo selectivity and with enantiomeric excesses up to 96%. Intermediates [(η5-C5Me5)M{(R)-Prophos}(methacrolein)](SbF6)2 (M = Rh (3), Ir (4)) have been spectroscopically and crystallographically characterized. The nitrone complexes [(η5-C5Me5)M{(R)-Prophos}(nitrone)](SbF6)2 (M = Rh, nitrone = 1-pyrrolidine N-oxide (5), 2,3,4,5,-tetrahydropyridine N-oxide (6), 3,4-dihydroisoquinoline N-oxide (7); M = Ir, nitrone = 1-pyrrolidine N-oxide (8)) have been isolated and characterized including the X-ray crystal structure of compounds 6 and 8. The equilibrium between methacrolein and nitrone complexes is also studied. [Ir]-adduct complexes are detected by 31P NMR spectroscopy. A catalytic cycle involving [M]-methacrolein, [M]-nitrone, as well as [M]-adduct species is proposed, the first complex being the true catalyst. The absolute configuration of the adduct 4-methyl-2-N,3-diphenyl-isoxazolidine-4-carbaldehyde (9) was determined through its (S)-(−)-α-methylbenzylamine derivative diastereomer. Structural parameters strongly suggest that the disposition of the methacrolein in 3 and 4 is fixed by CH/π attractive interactions between the pro-S phenyl ring of the Ph2PCH(CH3) moiety of the (R)-Prophos ligand and the CHO aldehyde proton. Proton NMR data indicate that this conformation is maintained in solution. From the structural data and the results of catalysis the origin of the enantioselectivity is discussed.

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History

  • Published In Issue September 28, 2005
  • Received June 15, 2005

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