Abstract
Versatile syntheses of C2-linked and C2-symmetric carbohydrate-based imidazol(in)ium salts from functionalised amino-carbohydrate derivatives are reported. The novel NHCs were ligated to [Rh(COD)Cl]2 and evaluated in Rh-catalysed asymmetric hydrosilylation of ketones with good yields and promising enantioselectivities.
Highlights
Versatile syntheses of C2-linked and C2-symmetric carbohydrate-based imidazol(in)ium salts from functionalised amino-carbohydrate derivatives are reported
Complementary methodologies that enable the incorporation of cheap and diversifiable chiral building blocks onto N-heterocyclic carbenes (NHCs) scaffolds will likely accelerate the development of efficient ligand systems.1d
Anomeric reactivity has been exploited to append the NHC unit to C1 of the carbohydrate.5a,c,d,f,g Related C3- and C6-linked monosaccharide systems have been disclosed.5b,e,h In most cases, application to enantioselective transition-metal catalysis has not been pursued.5a–c,e–g A C1-linked carbohydrate-functionalized Ru catalyst was evaluated in asymmetric ring-opening cross-metathesis (AROCM) but high yields could only be achieved with modest enantioselectivities.5d More recently, elegant work by Sollogoub and coworkers has demonstrated that C6-linked NHC-capped cyclodextrins provide chiral “cavities” that mediate enantioselective gold-catalysed alkene cyclopropanation in up to 59% ee.5h
Summary
Versatile syntheses of C2-linked and C2-symmetric carbohydrate-based imidazol(in)ium salts from functionalised amino-carbohydrate derivatives are reported. The novel NHCs were ligated to [Rh(COD)Cl]2 and evaluated in Rh-catalysed asymmetric hydrosilylation of ketones with good yields and promising enantioselectivities.
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