Speciation and Mechanistic Studies of Chiral Copper(I) Schiff Base Precursors Mediating Asymmetric Carbenoid Insertion Reactions of Diazoacetates into the Si-H Bond of Silanes
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- 作者：Les A. Dakin ; Patricia C. Ong ; James S. Panek ; Richard J. Staples ; and Pericles Stavropoulos
- 刊名：Organometallics
- 出版年：2000
- 出版时间：July 24, 2000
- 年：2000
- 卷：19
- 期：15
- 页码：2896 - 2908
- 全文大小：547K
- 年卷期：v.19,no.15(July 24, 2000)
- ISSN：1520-6041
文摘
[Cu(CH3CN)4](PF6) and the chiral C2-symmetric diimine ligand (1R,2R)-bis((2,6-dichlorobenzylidene)diamino)cyclohexane (R,R-1) (1.2 equiv) mediate asymmetric carbenoid insertion of aryl diazoesters into the Si-H bond of silanes in good to high yields and levels ofenantiocontrol. Dichloromethane solutions of [Cu(CH3CN)4](PF6)/R,R-1 afford yellow crystalsof [CuI(R,R-1)(CH3CN)·CuI(R,R-1)(CH3CN)2·CuI2(R,R-1)3](PF6)4·2CH2Cl2·3Et2O, which feature three distinct copper complexes in the crystal lattice. 1H NMR and electrospray ionizationMS (ESI-MS) studies establish that only [CuI(R,R-1)(CH3CN)]+ is present in solution in highyields. Upon addition of stoichiometric PhC(N2)CO2Me, cations [CuI(R,R-1)(C(CO2Me)Ph)]+and [CuI(R,R-1)(C(CO2Me)Ph)(CH3CN)]+ are detected by ESI-MS, consistent with thepresence of a copper-carbenoid moiety. The catalytically active precursor is most likely amononuclear unit of the type [CuI(R,R-1)(CH3CN)n]+, as suggested by the linearity of plotsrelating the enantiomeric excess (ee) of product to that of the ligand (Kagan's analysis).Hammett plots correlate enhanced catalytic reactivities with stabilization of a sizable positivecharge on the carbenoid carbon and a smaller positive charge on the silicon atom, but thecorresponding enantioselectivities are insensitive to these electronic properties. The kineticisotope effect for carbenoid insertion into PhMe2Si-H(D) varies from 2.12 (-40 
C) to 1.08(25 C), in agreement with other small KIE values observed for processes in which Si-Hactivation is involved in the turnover-limiting step. A linear Eyring plot of ln(k(majorenantiomer)/k(minor enantiomer)) over a range of 80 K supports the notion of a single stepcontrolling the levels of enantioselection. No H/D scrambling is observed in competitivecarbenoid insertions into PhMe2Si-D/Ph2MeSi-H, indicating that the insertion reaction takesplace in a concerted fashion. These results are discussed in light of an early transition state,characterized by hydrogen-first penetration of the Si-H bond into the copper-carbenoidcavity, which is assumed to impart high levels of enantioselectivity due to intrinsicpreorganization under the influence of the specific ligand and aryl diazoesters employed.
C) to 1.08(25 C), in agreement with other small KIE values observed for processes in which Si-Hactivation is involved in the turnover-limiting step. A linear Eyring plot of ln(k(majorenantiomer)/k(minor enantiomer)) over a range of 80 K supports the notion of a single stepcontrolling the levels of enantioselection. No H/D scrambling is observed in competitivecarbenoid insertions into PhMe2Si-D/Ph2MeSi-H, indicating that the insertion reaction takesplace in a concerted fashion. These results are discussed in light of an early transition state,characterized by hydrogen-first penetration of the Si-H bond into the copper-carbenoidcavity, which is assumed to impart high levels of enantioselectivity due to intrinsicpreorganization under the influence of the specific ligand and aryl diazoesters employed.