Modular Synthesis of Functionalisable Alkoxy-Tethered N-Heterocyclic Carbene Ligands and an Active Catalyst for Buchwald-Hartwig Aminations

10.1002/adsc.201300903Modular syntheses of functionalised,alkoxy-tethered 1,3-bis(2,4,6-trimethylphenyl)imidazolium (IMes·H+) and 1,3-bis(2,6-diisopropylphenyl)imidazolium (IPr·H+) derivatives 1,3-bis(4-alkyloxy-2,4,6-trimethylphenyl)imidazolium (IXyOR·H+)and 1,3-bis(4-lkyloxy2,6diisopropylphenyl)imidazolium (IPrOR·H+) are reported. A reliable synthesis of the key starting material 4-amino3,5-diisopropylphenolis also described. Etherification of hydroxydecorated ligand intermediates before formation of the imidazolium core and subsequent modification,or direct etherification of the versatile synthonIPrOH·HCl, allowed access to various linker types including triethoxysilyl, primary amino and norbornenyl,which are not accessible by other methods.An IPrOR¿palladium(II) complex was prepared, and its catalytic activity was evaluated in challenging Buchwald¿Hartwig aminations of aryl chlorides. This precatalyst displayed excellent activity and selectivity under mild reaction conditions, achieving in some cases a 10-fold improvement in TOF relative to the IPr-based version. An unexpected activity profile was observed wherein sterically demanding anilines were coupled more easily than those lacking orthosubstitution