Catalysis, metal-organic cooperative

Iminium-based organocatalysis is somewhat less explored than enamine-based organocatalysis and has been mostly used in the activation of a,/S-conjugated aldehydes and ketones. Therefore, this type of catalysis has unsurprisingly been the subject of a limited number of studies under the umbrella of the metal-organic cooperative catalysis concept. In 2011, the Cdrdova group [55] reported the first enantioselective and chemoselective /3-silyl addition to a./S-unsaturated aldehydes using copper salts and chiral pyrrolidine derivatives as catalysts. As proposed, the chiral secondary amine forms an iminium salt with... 

Metal-Organic Cooperative Catalysis in C-C Bond Activation... 

Park YJ, Park J-W, Jun C-H (2008) Metal-organic cooperative catalysis in C-H and C-C bond activation and its concurrent recovery. Acc Chem Res 41(2) 222-234. doi 10.1021/ar700133y... 

This volume contains eight chapters from leading experts in the field, and is expected to provide an overview of the latest developments in C-C bond activation. It begins with mechanistic studies of transition metal-mediated C-C bond action by Prof. W.D. Jones, which laid the foundation for further discovery of catalytic reactions. The following chapters from Profs. Y. Nakao, C.-H. Jun, and C. J. Douglas focus on different strategies to activate unstrained C-C bonds, namely, C-CN bond activation, metal-organic cooperative catalysis, and activation of 8-acylquinolines. 

Scheme 7.6 Asymmetric CDC reaction of Af-aryl THIQs under metal-organic cooperative catalysis.

Kinetic investigations have shown that during heterogeneous catalysis performed with metal oxides the associated acid site (metal cation) can also activate the carbonyl group, leading to cooperative acid-base catalysis. With organic bases attached to a mineral support such an effect would not be so pronounced. 

Keywords C-C bond activatiOTi Chelation-assistant strategy Cooperative catalysis Ketone Organic catalyst Retro-Mannich fragmentation Transition metal... 

Considering the rapid growth of asymmetric construction of oxindoles, Sun et al. recently reported their assembly of chiral spirooxindoles by combining secondary amine and palladium catalysis in a cascade reaction [55]. The reaction was initiated by the reversible Michael addition of 3-substituted oxindole to enal, which was followed by a metal/organic-cocatalyzed carbocyclization of the aUcyne tether (Scheme 9.60). Similar to the aforementioned dynamic kinetic asymmetric transformations, this chemistry highlighted the cooperative effects of the two catalysts in the same reaction vessel, while either catalyst could not solely promote the overall reaction, and unsatisfactory results were observed when this reaction was conducted in a two-step mode. 

If we first consider purely organic systems, one can mention the asymmetric reduction of ketmies with borane promoted by a chiral oxazaborolidine 1 developed by Corey, Bakshi, and Shibata (CBS reduction. Scheme 1) [13,14]. In this system, the nitrogen atom of the oxazaborolidine serves as Lewis base and coordinates BH3 thus improving its nucleoplulicity, while the endocyclic boron atom acts as the Lewis acid and activates the ketone toward the reduction. This seminal work constimtes an early example of metal-free catalysis and shows that cooperative effects can emerge from ambiphilic Lewis acid/base catalytic system. 

---