Bifunctional Phebox Complexes for Asymmetric Catalysis

Abstract

Chiral bifunctional rhodium complexes bearing chiral bis(oxazolinyl) phenyl ligand (phebox) catalyzed asymmetric reactions were described. Meridional C 2 -symmetric environment around the metal center in the chiral phebox ligand is a crucial structure factor for determining the catalytic performance in terms of reactivity and selectivity. The phebox-Rh chloro complex serves as a mild Lewis acid catalyst for asymmetric allylation, hetero-Diels-Alder, and Michael reaction. Although the catalytic precursor, Rh(III) complex, with octahedral geometry is a quite stable molecule, it can be readily activated by hydrosilane to generate the putative Rh(I) species, which is a key intermediate in asymmetric hydrosilylation of alkenes and conjugate reduction of a,b-unsaturated carbonyl compounds. The Rh enolate species generated in situ can be used for the C-C bond formation reaction, asymmetric reductive aldol reactions, and direct aldol reactions.

Thio-Pybox and Thio-Phebox complexes of chromium, iron, cobalt and nickel and their application in ethylene and butadiene polymerisation catalysis

“…These ligands, referred to as Thio-Pybox and Thio-Phebox, are sulfur-based relatives of the well known oxygen-based Pybox and Phebox ligands. [32][33][34][35] Pybox ligands have been used previously for the preparation of ethylene polymerisation catalysts based on iron(II) and ruthenium(II), 36,37 and for propylene polymerisation catalysts based on iron(II) and vanadium(III), 15 but generally very low activities have been obtained. Chromium-based Pybox complexes have been used for the polymerization of ethylene, 38 and for the selective dimerisation of ethylene to butene.…”

Iron-Catalyzed Highly Enantioselective Hydrosilylation of Unactivated Terminal Alkenes

“…Chiral organosilanes have widespread applications in organic synthesis, silicon-based materials science, and silasubstitution in medicinal chemistry . Compared to other methods to prepare highly enantiopure organosilanes, such as desymmetrization of silanes for silicon-stereogenic silanes, and asymmetric silicon-hydride bond insertions, asymmetric alkene hydrosilylation is one of the most efficient methods for construction of nonracemic carbon-stereogenic silanes. , Although the recent earth-abundant transition metal-catalyzed hydrosilylation of alkenes offers an atom-economic and efficient method to produce organosilanes (Scheme a); however, the Markovnikov-type selective reaction of aliphatic alkenes is still a challenge . In 1991, Hayashi and co-workers reported the first successful highly enantioselective palladium-catalyzed hydrosilylation of aliphatic terminal alkenes (Scheme b) .…”

Highly Enantioselective Cobalt-Catalyzed Hydrosilylation of Alkenes

“…Chiral organosilanes have gradually emerged their utilities in organic synthesis, material science, and silasubstitution in medicinal chemistry . Several protocols have been generally developed to prepare highly enantiopure organosilanes, such as asymmetric hydrosilylation of double bond, desymmetrization of silanes, and asymmetric silicon–hydride bond insertions . To be an ideal synthetic strategy for the construction of carbon­(sp 3 )-silicon bonds, it should be enabled to tolerate various substituents available for further derivatization.…”