Enantioselective 1,3-Dipolar Cycloaddition of Nitrones to Methacrolein Catalyzed by (5-C5Me5)M{(R<

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• 作者：Daniel Carmona ; M. Pilar Lamata ; Fernando Viguri ; Ricardo Rodrí ; guez ; Luis A. Oro ; Fernando J. Lahoz ; Ana I. Balana ; Tom&aacute ; s Tejero ; and Pedro Merino
• 刊名：Journal of the American Chemical Society
• 出版年：2005
• 出版时间：September 28, 2005
• 年：2005
• 卷：127
• 期：38
• 页码：13386 - 13398
• 全文大小：318K
• 年卷期：v.127,no.38(September 28, 2005)
• ISSN：1520-5126

文摘

The rhodium and iridium Lewis-acid cations [(mages/gifchars/eta.gif" BORDER=0 >⁵-C₅Me₅)M{(R)-Prophos}(H₂O)]²⁺ ((R)-Prophos= 1,2-bis(diphenylphosphino)propane) efficiently catalyze the enantioselective 1,3-dipolar cycloaddition ofnitrones to methacrolein. Reactions occur with perfect endo selectivity and with enantiomeric excesses upto 96%. Intermediates [(mages/gifchars/eta.gif" BORDER=0 >⁵-C₅Me₅)M{(R)-Prophos}(methacrolein)](SbF₆)₂ (M = Rh (3), Ir (4)) have beenspectroscopically and crystallographically characterized. The nitrone complexes [(mages/gifchars/eta.gif" BORDER=0 >⁵-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 andcharacterized including the X-ray crystal structure of compounds 6 and 8. The equilibrium betweenmethacrolein and nitrone complexes is also studied. [Ir]-adduct complexes are detected by ³¹P NMRspectroscopy. A catalytic cycle involving [M]-methacrolein, [M]-nitrone, as well as [M]-adduct species isproposed, 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)-(-)-mages/gifchars/alpha.gif" BORDER=0>-methylbenzylaminederivative diastereomer. Structural parameters strongly suggest that the disposition of the methacrolein in3 and 4 is fixed by CH/mages/gifchars/pi.gif" BORDER=0 > attractive interactions between the pro-S phenyl ring of the Ph₂PCH(CH₃) moietyof the (R)-Prophos ligand and the CHO aldehyde proton. Proton NMR data indicate that this conformationis maintained in solution. From the structural data and the results of catalysis the origin of theenantioselectivity is discussed.