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−C fluorenyl 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.