Platinum-Catalyzed Asymmetric Alkylation of a Secondary Phosphine: Mechanism and Origin of Enantioselectivity

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• 作者：Corina Scriban ; David S. Glueck ; James A. Golen ; Arnold L. Rheingold
• 刊名：Organometallics
• 出版年：2007
• 出版时间：March 26, 2007
• 年：2007
• 卷：26
• 期：7
• 页码：1788 - 1800
• 全文大小：294K
• 年卷期：v.26,no.7(March 26, 2007)
• ISSN：1520-6041

文摘

The catalyst precursor Pt((R,R)-Me-Duphos)(Ph)(Cl) (1) mediated asymmetric alkylation of the secondaryphosphine PHMe(Is) (2; Is = 2,4,6-(i-Pr)₃C₆H₂) with benzyl bromide in the presence of the base NaOSiMe₃to yield enantioenriched PMeIs(CH₂Ph) (3). A mechanism for the catalysis has been proposed, on thebasis of studies of the individual stoichiometric steps. The terminal phosphido complex Pt((R,R)-Me-Duphos)(Ph)(PMeIs) (4) was formed by proton transfer from 2 to the silanolate ligand in Pt((R,R)-Me-Duphos)(Ph)(OSiMe₃) (5), which was generated from 1 (or from Pt((R,R)-Me-Duphos)(Ph)(Br) (7)) andNaOSiMe₃. The silanolate complex 5 reacted with water to yield Pt((R,R)-Me-Duphos)(Ph)(OH) (6);both 6 and 7 were crystallographically characterized. The stoichiometric reaction of 4 and benzyl bromidein toluene gave the bromide 7 and 3. In more polar solvents these compounds were in equilibrium withthe cation [Pt((R,R)-Me-Duphos)(Ph)(PMeIs(CH₂Ph))][Br] (8-Br), the major Pt complex present duringcatalysis, which was isolated as the BF₄ salt. Treatment of 8-BF₄ with 2 and NaOSiMe₃ yielded phosphine3 and regenerated phosphido complex 4. This reaction does not appear to proceed via phosphine ligandsubstitution on 8-BF₄ to yield the secondary phosphine complex cation [Pt((R,R)-Me-Duphos)(Ph)(PHMe(Is))][BF₄] (12). Instead, treatment of 8-BF₄ with NaOSiMe₃ gave phosphine 3 and 5, which then reactedwith 2 to yield 4. The crystal structure of the major diastereomer of 8-BF₄ showed that the major enantiomerof 3 formed by catalyst precursor 1 had an R_P absolute configuration. Low-temperature NMR studies onthe major diastereomer of phosphido complex 4 were consistent with the R_P solid-state structure. Thus,the major enantiomer of phosphine 3 appeared to be formed from the major diastereomer of intermediate4, and enantioselectivity was determined mainly by the thermodynamic preference for one of the rapidlyinterconverting diastereomers of 4, although their relative rates of alkylation were also important (Curtin-Hammett kinetics).