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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Abstract

[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 of enantiocontrol. Dichloromethane solutions of [Cu(CH3CN)4](PF6)/R,R-1 afford yellow crystals of [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 ionization MS (ESI-MS) studies establish that only [CuI(R,R-1)(CH3CN)]+ is present in solution in high yields. 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 the presence of a copper−carbenoid moiety. The catalytically active precursor is most likely a mononuclear unit of the type [CuI(R,R-1)(CH3CN)n]+, as suggested by the linearity of plots relating 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 positive charge on the carbenoid carbon and a smaller positive charge on the silicon atom, but the corresponding enantioselectivities are insensitive to these electronic properties. The kinetic isotope 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−H activation is involved in the turnover-limiting step. A linear Eyring plot of ln(k(major enantiomer)/k(minor enantiomer)) over a range of 80 K supports the notion of a single step controlling the levels of enantioselection. No H/D scrambling is observed in competitive carbenoid insertions into PhMe2Si-D/Ph2MeSi-H, indicating that the insertion reaction takes place 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−carbenoid cavity, which is assumed to impart high levels of enantioselectivity due to intrinsic preorganization under the influence of the specific ligand and aryl diazoesters employed.

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