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

Abstract

Cu(CH 3 CN) 4 ](PF 6 ) 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(CH 3 CN) which feature three distinct copper complexes in the crystal lattice. 1 H NMR and electrospray ionization MS (ESI-MS) studies establish that only [Cu I (R,R-1)(CH 3 CN)] + is present in solution in high yields. Upon addition of stoichiometric PhC(N 2 )CO 2 Me, cations [Cu I (R,R-1)(C(CO 2 Me)Ph)] + and [Cu I (R,R-1)(C(CO 2 Me)Ph)(CH 3 CN)] + 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)(CH 3 CN) 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 PhMe 2 Si-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 PhMe 2 Si-D/Ph 2 MeSi-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.

Vinylogous Aldol Products from Chiral Crotylsilanes Obtained by Enantioselective Rh(II) and Cu(I) Carbenoid Si−H Insertion

“…Although the idea of Cu(I) catalysis has been used to promote Si-H insertions prior to the application of rhodium catalysis, 7, 8 the field remains under developed and few cases of asymmetric variants have been reported. 9 In that regard, we reported earlier that useful levels of selectivity with α -diazophenylacetates 10 and anticipated that we could extend the Cu(I) catalysis to α -diazovinylacetates (Table 1, entries 1 and 2), thereby complementing Landais and Davies’ earlier contributions, 11 who were the first to describe examples of Rh(II) promoted asymmetric Si-H insertion to α -diazovinylacetates. In this paper we report an efficient synthesis of chiral allylic silanes with C-centered chirality, and these experiments also allow a comparison of chiral Cu(I) vs Rh(II) catalysis.…”

Diverse reactivities of aromaticity–unsaturation coexisted metalladithiolene rings

“…Dimethyl diazomalonate (N 2 C(CO 2 Me) 2 ), which is less nucleophilic than the other diazoalkanes, is hard to react with [CpM(dithiolene)] (M ¼ Co, Rh) complexes but moderately reacts with it to form the corresponding alkylidene complexes (26d and 27d) while early activation of N 2 C(CO 2 Me) 2 is supplied with a Cu catalyst (20e28% yield) (Scheme 1(c)) [71], because the early activation can generate a nucleophilic Cu-carbenoid intermediate [72]. The Rh alkylidene adduct 26d was structurally characterized in this work (see CCDC).…”

Diacetonitrile[N,N′-bis(3,4,5-trimethoxybenzylidene)ethylenediamine]copper(I) perchlorate

“…For related literature, see: Amirnasr et al (2006); Chowdhury et al (2000); Dakin et al (2000); Khalaji et al (2007).…”