Confinement-induced Z-selectivity in the rhodium N-heterocyclic carbene-catalyzed hydroboration of terminal alkynes

Abstract

The N- and O-chelating N-heterocyclic carbene (NHC) based Rh(I) and Rh(III) complexes [RhCl(1-(quino-8-yl)-3-(trimethoxysilylpropyl)imidazol-2-ylidene)(COD)] (Rh1, COD = 1,5-cyclooctadiene), [RhCl(1-(mesitylaminocarbonylmethyl)-3-(trimethoxysilylpropyl)imidazol-2-ylidene)(COD)] (Rh2), [Rh(1-(mesitylaminocarbonylmethyl)-3-(trimethoxysilylpropyl)imidazol-2-ylidene)(COD)⁺] [BF₄⁻] (Rh3), [Rh(1-(mesitylamidocarbonylmethyl)-3-(trimethoxysilylpropyl)imidazol-2-ylidene)(COD)] (Rh4), RhCl(1-(quino-8-yl)-3-(trimethoxysilylpropyl)imidazol-2-ylidene)(Cp*)⁺] Cl⁻ (Rh5, Cp* = pentamethylcyclopentadienyl), [RhCl(1-(quino-8-yl)-3-(trimethoxysilylpropyl)imidazol-2-ylidene)(Cp*)⁺] [BF₄⁻] (Rh6), [RhCl₂(1-(mesitylaminocarbonylmethyl)-3-(trimethoxysilylpropyl)imidazol-2-ylidene)(Cp*)] (Rh7), and [RhCl(1-(mesitylaminocarbonylmethyl)-3-(trimethoxysilylpropyl)imidazol-2-ylidene)(Cp*)] [BF₄⁻] (Rh8) were prepared. The solid-state structures of Rh3, Rh5 and Rh6 are presented. Selected complexes were used in the hydroboration of terminal aliphatic alkynes under homogeneous conditions using HBpin (pin = pinacolate) as hydroboration reagent. As expected, only very low β(Z)-selectivity (1–27%) was observed under homogeneous conditions; by contrast, Rh1, Rh4, Rh5, and Rh7 immobilized inside (hexagonally) ordered mesoporous silica (OMS) with pore sizes of 6.0 and 3.5 nm, respectively, showed improved β(Z) selectivity up to 30%. Most important, reactions carried out with Rh1, Rh4 and Rh7 supported on OMS_35Å and additionally confined in a thin (1 nm) layer of the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM⁺ BF₄⁻) using “solid catalyst with ionic liquid layer” (SCILL) conditions allowed for a pronounced increase in Z-selectivity up to 67%. Overall, the β(Z)/β(E) isomeric ratio was successfully increased up to a factor of 22 when going from homogenous to SCILL conditions. A mechanistic picture is presented.