Elusive transmetalation intermediate in copper-catalyzed conjugate additions: direct NMR detection of an ethyl group attached to a binuclear phosphoramidite copper complex.

Copper-catalyzed asymmetric conjugate addition reactions are a very powerful and widely applied method for enantioselective carbon-carbon bond formation. However, structural and mechanistic insight into these famous reactions has been very limited so far. In this article, the first direct experimental detection of transmetalation intermediates in copper-catalyzed reactions is presented.