[(NHC)Cu^I–ER₃] Complexes (ER₃ = SiMe₂Ph, SiPh₃, SnMe₃): From Linear, Mononuclear Complexes to Polynuclear Complexes with Ultrashort Cu^I···Cu^I Distances

A series of complexes of the type [(NHC)­Cu–ER3] (NHC = IDipp, IMes, ItBu, Me2IMe, and ER3 = SiMe2Ph, SiPh3, SnMe3) and [(NHC)­Cu–R′] (NHC = IDipp, Me2IMe and R′ = Ph, CCPh) was synthesized in good yields by the reaction of the corresponding [(NHC)­Cu–OtBu] complex with the respective silylborane pinB–ER3 (pin = OCMe2CMe2O; ER3 = SiMe2Ph, SiPh3), the stannylborane ((C2H4)­(iPrN)2)­B–SnMe3, or a boronic acid ester pinB–R′ (R′ = Ph, CCPh). Solid structures of all complexes were systematically studied by X-ray diffraction analysis. The solid state structures of the complexes [(NHC)­Cu–ER3] show a dependence of the structural motif from the steric properties of the NHC ligand. The sterically demanding NHC ligands (IDipp, IMes, ItBu) lead to monomeric, linear complexes [(NHC)­Cu–ER3], while with the less demanding Me2IMe ligand, polynuclear, μ-ER3-bridged complexes with ultrashort Cu···Cu distances are observed. For the related complexes [(NHC)­Cu–R′] no analogous complexes with bridging anionic ligands are realized. Instead, irrespective of the NHC ligand, linear coordinated copper complexes of different types are formed. 29Si heteronuclear solution NMR spectroscopic data on [(NHC)­CuI–SiR3] exhibit distinctly different chemical shifts for the (in the solid state) monomeric and dimeric complexes suggesting different structure types also in solution. This agrees well with the observation of a trinuclear complex [(Me2IMe)­Cu–SnMe3]3 both in the solid state and in solution. Initial catalytic studies suggest that [(NHC)­Cu–OtBu] complexes (NHC = ItBu, Me2IMe) are, in addition to the established [(IDipp)­Cu–OtBu] complex, efficient precatalysts for the silylation of aldehydes and α,β-unsaturated ketones with pinB–SiMe2Ph

[(NHC)Cu^I–ER₃] Complexes (ER₃ = SiMe₂Ph, SiPh₃, SnMe₃): From Linear, Mononuclear Complexes to Polynuclear Complexes with Ultrashort Cu^I···Cu^I Distances

A series of complexes of the type [(NHC)­Cu–ER₃] (NHC = IDipp, IMes, I<i>t</i>Bu, Me₂IMe, and ER₃ = SiMe₂Ph, SiPh₃, SnMe₃) and [(NHC)­Cu–R′] (NHC = IDipp, Me₂IMe and R′ = Ph, CCPh) was synthesized in good yields by the reaction of the corresponding [(NHC)­Cu–O<i>t</i>Bu] complex with the respective silylborane pinB–ER₃ (pin = OCMe₂CMe₂O; ER₃ = SiMe₂Ph, SiPh₃), the stannylborane ((C₂H₄)­(<i>i</i>PrN)₂)­B–SnMe₃, or a boronic acid ester pinB–R′ (R′ = Ph, CCPh). Solid structures of all complexes were systematically studied by X-ray diffraction analysis. The solid state structures of the complexes [(NHC)­Cu–ER₃] show a dependence of the structural motif from the steric properties of the NHC ligand. The sterically demanding NHC ligands (IDipp, IMes, I<i>t</i>Bu) lead to monomeric, linear complexes [(NHC)­Cu–ER₃], while with the less demanding Me₂IMe ligand, polynuclear, μ-ER₃-bridged complexes with ultrashort Cu···Cu distances are observed. For the related complexes [(NHC)­Cu–R′] no analogous complexes with bridging anionic ligands are realized. Instead, irrespective of the NHC ligand, linear coordinated copper complexes of different types are formed. ²⁹Si heteronuclear solution NMR spectroscopic data on [(NHC)­Cu^I–SiR₃] exhibit distinctly different chemical shifts for the (in the solid state) monomeric and dimeric complexes suggesting different structure types also in solution. This agrees well with the observation of a trinuclear complex [(Me₂IMe)­Cu–SnMe₃]₃ both in the solid state and in solution. Initial catalytic studies suggest that [(NHC)­Cu–O<i>t</i>Bu] complexes (NHC = I<i>t</i>Bu, Me₂IMe) are, in addition to the established [(IDipp)­Cu–O<i>t</i>Bu] complex, efficient precatalysts for the silylation of aldehydes and α,β-unsaturated ketones with pinB–SiMe₂Ph

Cu^I‑Catalyzed Conjugate Addition of Silyl Boronic Esters: Retracing Catalytic Cycles Using Isolated Copper and Boron Enolate Intermediates

Copper­(I)-catalyzed conjugate additions of silyl boronic esters to α,β-unsaturated aldehydes, ketones, and esters are synthetically well-established reactions. For the first time central reactive intermediates as well as the boron enolates as the primary reaction products are isolated and employed in order to deduce catalytic cycles on an experimental basis. Employing an NHC Cu^I complex as a model catalyst, it is possible to perform efficient catalytic transformations as well as to isolate and characterize the formed copper enolate complexes as the key intermediates. It is shown that for this catalytic system the nature of this enolate<i>O</i>- or <i>C</i>-enolateis crucial for the catalytic process. For α,β-unsaturated aldehydes and ketones the <i>O</i>-enolate is formed predominantly, while for α,β-unsaturated esters the <i>C</i>-enolate is the major product. Catalytic turnover is only facile for copper <i>O</i>-enolates, as they react efficiently with the silyl boronic ester under (re)­formation of the catalytically active Cu–Si species and a thermodynamically favored boric acid ester. Thus, the formation of copper <i>C</i>-enolates is inhibiting the catalytic process, and effective turnover is possible only after solvolysis by an alcohol additive. The individual catalytic processes were retraced by performing stepwise stoichiometric reactions monitored by in situ NMR spectroscopy