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

[Cu(CH3CN)(4)](PF6) and the chiral C-2-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 [Cu-I(R,R-1)(CH3CN).Cu-I(R,R-1)(CH3CN)(2).Cu-2(I)(R,R-1)(3)](PF6)(4).2CH(2)Cl(2).3Et(2)O, which feature three distinct copper complexes in the crystal lattice. H-1 NMR and electrospray ionization MS (ESI-MS) studies establish that only [Cu-I(R,R-1)(CH3CN)](+) is present in solution in high yields. Upon addition of stoichiometric PhC(N-2)CO2Me, cations [Cu-I(R,R-1)(C(CO2Me)Ph)](+) and [Cu-I(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 [Cu-I(R,R-1)(CH3CN)](+), as suggested by the linearity of plots relating the enantiomeric excess tee) 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 degrees C) to 1.08 (25 degrees 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 In(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.