Abstract
N-Heterocyclic carbenes have found many applications in modern metal catalysis, due to the formation of stable metal complexes, and organocatalysis. Among a myriad of N-heterocyclic carbene metal complexes, gold complexes have gained a lot of attention due to their unique propensity for the activation of carbon-carbon multiple bonds, allowing many useful transformations of alkynes, allenes, and alkenes, inaccessible by other metal complexes. The present review summarizes synthetic efforts towards the preparation of chiral N-heterocyclic gold(I) complexes exhibiting C2 and C1 symmetry, as well as their applications in enantioselective catalysis. Finally, the emerging area of rare gold(III) complexes and their preliminary usage in asymmetric catalysis is also presented.
Highlights
Homogenous, enantioselective gold catalysis has witnessed growing attention of the synthetic community due to the element’s unique propensity to act as a soft, carbophilic Lewis acid
The synthesis of abnormal gold gold complexes 139 was accomplished via transmetalationofofsilver silver complexes
The first example of an intermolecular cyclopropanation reaction catalyzed by Chronologically, the first example of an intermolecular cyclopropanation reaction catalyzed by a chiral N-heterocyclic carbene gold(I) complex was reported by Espinet in 2010 (Scheme 38) [93]
Summary
Homogenous, enantioselective gold catalysis has witnessed growing attention of the synthetic community due to the element’s unique propensity to act as a soft, carbophilic Lewis acid. The proposed mechanism of nucleophile approach is believed to proceed via outer-sphere pathways [17] Due to these geometrical and conformational constraints, the transfer of chiral information from the ligand to the substrate in gold-catalyzed reactions becomes difficult and makes enantioselective gold(I) transformations a challenging field. The rigid structure of NHCs (N-heterocyclic carbenes) provides an excellent opportunity to form stable gold complexes with a well-defined chiral environment, as was proven in the case of other metals, such as palladium [26,27,28,29], ruthenium [30,31,32,33]. N-heterocyclic carbenes carbenes enable enable the able to create a chiral environment around the the development of enantioselective processes.
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