Rh(I)/(III)-N-Heterocyclic Carbene Complexes: Effect of Steric Confinement Upon Immobilization on Regio- and Stereoselectivity in the Hydrosilylation of Alkynes

Chemistry. 2021 Oct 21. doi: 10.1002/chem.202103099. Online ahead of print.ABSTRACTRh(I) NHC and Rh(III) Cp* NHC complexes (Cp* = pentamethylcyclopentadienyl, NHC = N -heterocyclic carbene = pyrid-2-ylimidazol-2-ylidene (Py-Im), thiophen-2-ylimidazol-2-ylidene) are presented. Selected catalysts were selectively immobilized inside the mesopores of SBA-15 having average pore diameters of 5.0 and 6.2 nm, respectively. Together with their homogenous progenitors, the immobilized catalysts were used in the hydrosilylation of terminal alkynes. For aromatic alkynes, both the neutral and cationic Rh(I) complexes showed excellent reactivity with exclusive formation of the β( E )-isomer. For aliphatic alkynes, however, selectivity of the Rh(I) complexes was low. By contrast, the neutral and cationic Rh(III) Cp* NHC complexes proved to be highly regio- and stereoselective catalysts, allowing for the formation of the thermodynamically less stable β-( Z )-vinylsilane isomers at room temperature. Notably, the SBA-15 immobilized Rh(I) catalysts, in which the pore walls provide an additional confinement, showed excellent β-( Z )-selectivity in the hydrosilylation of aliphatic alkynes, too. Also, in the case of 4-aminophenylacetylene, selective formation of the β( Z )-isomer was observed with a neutral SBA-15 supported Rh(III) Cp* NHC complex but not with its homogenous counterpart. These are the first examples of high β( Z )-selectivity in the hydrosilylation of alkynes by confinement ge...

Source: Chemistry - October 21, 2021 Category: Chemistry Authors: Pradeep K R Panyam Boshra Atwi Felix Ziegler Wolfgang Frey Michal Nowakowski Matthias Bauer Michael Rudolf Buchmeiser Source Type: research