Palladium-Catalyzed Asymmetric Phosphination. Scope, Mechanism, and Origin of Enantioselectivity

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• 作者：Natalia F. Blank ; Jillian R. Moncarz ; Tim J. Brunker ; Corina Scriban ; Brian J. Anderson ; Omar Amir ; David S. Glueck ; Lev N. Zakharov ; James A. Golen ; Christopher D. Incarvito ; Arnold L. Rheingold
• 刊名：Journal of the American Chemical Society
• 出版年：2007
• 出版时间：May 30, 2007
• 年：2007
• 卷：129
• 期：21
• 页码：6847 - 6858
• 全文大小：222K
• 年卷期：v.129,no.21(May 30, 2007)
• ISSN：1520-5126

文摘

Asymmetric cross-coupling of aryl iodides (ArI) with secondary arylphosphines (PHMe(Ar'), Ar'= (2,4,6)-R₃C₆H₂; R = i-Pr (Is), Me (Mes), Ph (Phes)) in the presence of the base NaOSiMe₃ and a chiralPd catalyst precursor, such as Pd((R,R)-Me-Duphos)(trans-stilbene), gave the tertiary phosphines PMe(Ar')(Ar) in enantioenriched form. Sterically demanding secondary phosphine substituents (Ar') and aryliodides with electron-donating para substituents resulted in the highest enantiomeric excess, up to 88%.Phosphination of ortho-substituted aryl iodides required a Pd(Et-FerroTANE) catalyst but gave lowenantioselectivity. Observations during catalysis and stoichiometric studies of the individual steps suggesteda mechanism for the cross-coupling of PhI and PHMe(Is) (1) initiated by oxidative addition to Pd(0) yieldingPd((R,R)-Me-Duphos)(Ph)(I) (3). Reversible displacement of iodide by PHMe(Is) gave the cation [Pd((R,R)-Me-Duphos)(Ph)(PHMe(Is))][I] (4), which was isolated as the triflate salt and crystallographically characterized. Deprotonation of 4-OTf with NaOSiMe₃ gave the phosphido complex Pd((R,R)-Me-Duphos)(Ph)(PMeIs)(5); an equilibrium between its diastereomers was observed by low-temperature NMR spectroscopy.Reductive elimination of 5 yielded different products depending on the conditions. In the absence of a trap,the unstable three-coordinate phosphine complex Pd((R,R)-Me-Duphos)(PMeIs(Ph)) (6) was formed.Decomposition of 5 in the presence of PhI gave PMeIs(Ph) (2) and regenerated 3, while trapping withphosphine 1 during catalysis gave Pd((R,R)-Me-Duphos)(PHMe(Is))₂ (7), which reacted with PhI to give 3.Deprotonation of 1:1 or 1.4:1 mixtures of cations 4-OTf gave the same 6:1 ratio of enantiomers of PMeIs(Ph) (2), suggesting that the rate of P inversion in 5 was greater than or equal to the rate of reductiveelimination. Kinetic studies of the first-order reductive elimination of 5 were consistent with a Curtin-Hammett-Winstein-Holness (CHWH) scheme, in which pyramidal inversion at the phosphido ligand wasmuch faster than P-C bond formation. The absolute configuration of the phosphine (S_P)-PMeIs(p-MeOC₆H₄)was determined crystallographically; NMR studies and comparison to the stable complex 5-Pt wereconsistent with an R_P-phosphido ligand in the major diastereomer of the intermediate Pd((R,R)-Me-Duphos)(Ph)(PMeIs) (5). Therefore, the favored enantiomer of phosphine 2 appeared to be formed from the majordiastereomer of phosphido intermediate 5, although the minor intermediate diastereomer underwent P-Cbond formation about three times more rapidly. The effects of the diphosphine ligand, the phosphidosubstituents, and the aryl group on the ratio of diastereomers of the phosphido intermediates Pd(diphos*)(Ar)(PMeAr'), their rates of reductive elimination, and the formation of three-coordinate complexes wereprobed by low-temperature ³¹P NMR spectroscopy; the results were also consistent with the CHWH scheme.