Chiral N-Alkylfluorenyl-Substituted N-Heterocyclic Carbenes in the Gold(I)-Catalyzed Enantioselective Cycloisomerization of 1,6-Enynes

A series of chiral A*Flu-NHC-gold(I) complexes, where A*Flu-NHC is an N-heterocyclic carbene (imidazolin-2-ylidene or benzimidazolin-2-ylidene) bearing a chiral 9-alkyl-9-fluorenyl N-substituent and a 2,6-diisopropylphenyl or benzyl N'-substituent, were straightforwardly prepared in few steps from readily available 2,6-diisopropylamine, imidazole or benzimidazole. The chirality of the N-substituent lies in the presence of a chiral alcoholic alkyl chain on the fluorenyl, which results from the opening of commercially available chiral styrene oxide, yielding to a 2-hydroxy-2-phenylethyl or a 2-hydroxy-1-phenylethyl group. Four [AuCl(A*Flu-NHC)] complexes were tested as precatalysts in an enantioselective cycloisomerization of a 1,6-enyne. Notably, the best inductions were observed with the benzimidazolin-2-ylidene derivative bearing a 2-hydroxy-1-phenylethyl group on the fluorenyl ring, showing that a constrained rotation around the N-Cfluorenyl bond and a chiral center in α position of the fluorenyl ring are determining factors. Interestingly, a strong improvement of the induction with up to 72 % ee was observed using AgOTf as activator. The presence of a hydrogen bond between the hydroxyl group and OTf- in the in situ generated active cationic gold(I) species probably stiffens its structure. This type of ligand-counteranion interaction represents a novel strategy for optimizing chirality transfer in asymmetric gold(I) catalysis.

Enantioselective Alkoxycyclization of 1,6-Enynes with Gold(I)-Cavitands: Total Synthesis of Mafaicheenamine C

Chiral gold(I)-cavitand complexes have been developed for the enantioselective alkoxycyclization of 1,6-enynes. This enantioselective cyclization has been applied for the first total synthesis of carbazole alkaloid (+)-mafaicheenamine C and its enantiomer, establishing its configuration as R. The cavity effect was also evaluated in the cycloisomerization of dienynes. A combination of experiments and theoretical studies demonstrates that the cavity of the gold(I) complexes forces the enynes to adopt constrained conformations, which results in the high observed regio- and stereoselectivities.

Chiral N-heterocyclic Carbene Gold Complexes: Synthesis and Applications in Catalysis

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.