A Practical and Robust Zwitterionic Cooperative Lewis Acid/Acetate/Benzimidazolium Catalyst for Direct 1,4‐Additions
Andreas Hans, Patrick Becker, Johanna Haußmann, Simon Suhr, Daniel M. Wanner, Vera Lederer, Felix Willig, Wolfgang Frey, Biprajit Sarkar, Johannes Kästner, René Peters
Abstract
Abstract A catalyst type is disclosed allowing for exceptional efficiency in direct 1,4‐additions. The catalyst is a zwitterionic entity, in which acetate binds to Cu II , which is formally negatively charged and serving as counterion for benzimidazolium. All 3 functionalities are involved in the catalytic activation. For maleimides productivity was increased by a factor >300 compared to literature (TONs up to 6700). High stereoselectivity and productivity was attained for a broad range of other Michael acceptors as well. The polyfunctional catalyst is accessible in only 4 steps from N ‐Ph‐benzimidazole with an overall yield of 96 % and robust during catalysis. This allowed to reuse the same catalyst multiple times with nearly constant efficiency. Mechanistic studies, in particular by DFT, give a detailed picture how the catalyst operates. The benzimidazolium unit stabilizes the coordinated enolate nucleophile and prevents that acetate/acetic acid dissociate from the catalyst.