The encapsulation of copper inside ac yclodextrin capped with an N-heterocyclic carbene (ICyD) allowed both to catcht he elusive monomeric (L)CuH and ac avity-controlled chemoselective copper-catalyzed hydrosilylation of a,b-unsaturated ketones.R emarkably,( a-ICyD)CuCl promoted the 1,2-addition exclusively,w hile (b-ICyD)CuCl produced the fully reduced product. The chemoselectivity is controlled by the sizeo ft he cavity and weak interactions between the substrate and internal C À Hb onds of the cyclodextrin.

What happens when a C−H bond is forced to interact with unpaired pairs of electrons at a positively charged metal? Such interactions can be considered as "contraelectrostatic" H-bonds, which combine the familiar orbital interaction pattern characteristic for the covalent contribution to the conventional H-bonding with an unusual contra-electrostatic component. While electrostatics is strongly stabilizing component in the conventional C−H×××X bonds where X is an electronegative main group element, it is destabilizing in the C−H×××M contacts when M is Au(I), Ag(I), or Cu(I) of NHC−M−Cl systems. Such remarkable C−H×××M interaction became experimentally accessible within (a-ICyD Me )MCl, NHC−Metal complexes embedded into cyclodextrins. Computational analysis of the model systems suggests that the overall interaction energies are relatively insensitive to moderate variations in the directionality of interaction between a C−H bond and the metal center, indicating stereoelectronic promiscuity of fully filled set of d-orbitals. A combination of experimental and computational data demonstrates that metal encapsulation inside the cyclodextrin cavity forces the C−H bond to point toward the metal, and reveals a still attractive "contra-electrostatic" Hbonding interaction.