title: Enantioselective 1,3-dipolar cycloaddition of nitrones to methacrolein catalyzed by (eta5-C5Me5)M{(R)-prophos} containing complexes (M = Rh, Ir; (R)-prophos = 1,2-bis(diphenylphosphino)propane): on the origin of the enantioselectivity.
abstract: The rhodium and iridium Lewis-acid cations [(eta(5)-C(5)Me(5))M{(R)-Prophos}(H(2)O)](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 [(eta(5)-C(5)Me(5))M{(R)-Prophos}(methacrolein)](SbF(6))(2) (M = Rh (3), Ir (4)) have been spectroscopically and crystallographically characterized. The nitrone complexes [(eta(5)-C(5)Me(5))M{(R)-Prophos}(nitrone)](SbF(6))(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 (31)P 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)-(-)-alpha-methylbenzylamine derivative diastereomer. Structural parameters strongly suggest that the disposition of the methacrolein in 3 and 4 is fixed by CH/pi attractive interactions between the pro-S phenyl ring of the Ph(2)PCH(CH(3)) 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.
authors.affiliation: Departamento de Química Inorgánica, Instituto Universitario de Catálisis Homogénea, Universidad de Zaragoza-Consejo Superior de Investigaciones Científicas, 50009 Zaragoza, Spain. dcarmona@unizar.es
authors.forename: Daniel
authors.initials: D
authors.lastname: Carmona
authors.forename: M Pilar
authors.initials: MP
authors.lastname: Lamata
authors.forename: Fernando
authors.initials: F
authors.lastname: Viguri
authors.forename: Ricardo
authors.initials: R
authors.lastname: Rodríguez
authors.forename: Luis A
authors.initials: LA
authors.lastname: Oro
authors.forename: Fernando J
authors.initials: FJ
authors.lastname: Lahoz
authors.forename: Ana I
authors.initials: AI
authors.lastname: Balana
authors.forename: Tomás
authors.initials: T
authors.lastname: Tejero
authors.forename: Pedro
authors.initials: P
authors.lastname: Merino
mesh_terms: D000171:Acrolein / Q000031:analogs & derivatives* / Q000138:chemical synthesis; D002384:Catalysis; D018360:Crystallography, X-Ray; D003500:Cyclization; D007495:Iridium / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D009589:Nitrogen Oxides / Q000138:chemical synthesis / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D012238:Rhodium / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D009589:Nitrogen Oxides; D009942:Organometallic Compounds; C015388:nitrones; D007495:Iridium; D000171:Acrolein; C039175:methacrylaldehyde; D012238:Rhodium