Synergistic catalysis: cis-cyclopropanation of benzoxazoles† †Electronic supplementary information (ESI) available. CCDC 1060573. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5sc03597j Click here for additional data file. Click here for additional data file.

In the last years huge efforts have been made to develop new activation modes in order to synthesize complex scaffolds in an enantiopure form. The most common approach has been the development of new catalysts to activate an “unreactive” compound A that will react with a highly reactive compound B to render C. However, recently a new approach pushes the boundaries of organic chemistry: synergistic catalysis, where the concurrent activation of both reactants allows the use of both unreactive substrates to form a new C–C bond. Earlier examples of synergistic catalysis merging transition metals and organocatalysts can be found in the pioneering works of Cordova who reported the a-allylic alkylation of unactivated aldehydes with allyl acetates by using both enamine and Pd chemistry. Later, Zhang reported an efficient asymmetric variant by using Cordova's synergistic strategy and Pd catalysts with C2-symmetrical chiral metallocene ligands. MacMillan and Sibi reported examples of asymmetric synergistic catalysis based on enamine catalysis and metal-bound electrophiles. On the other hand, synergistic catalysis with a,b-unsaturated carbonyl compounds has attracted much recent interest. The works of Cordova in b-silylation, b-arylation or b-borylation by using iminium and transition-metal activation demonstrated the importance of this approach. In this area, List also reported excellent results for allylation and Overman rearrangement reactions by using phosphoric acid derivatives and transitionmetal catalysts. Very recently, our research group became interested in synergistic catalysis.