Metal Confinement through N-(9-Alkyl)fluorenyl-Substituted N-Heterocyclic Carbenes and Its Consequences in Gold-Catalysed Reactions Involving Enynes
Résumé
A series of gold(I) and gold(III) complexes containing bulky bis-N, N '-(9-alkylfluorenyl) heterocyclic carbene (F-R-NHC) ligands have been prepared in high yields from appropriate imidazolinium, imidazolium and benzimidazolium salts. In all complexes, the carbene ligand provides high steric protection of the Au-X bond trans to the carbenic C atom. Irrespective of the metal oxidation state, the complexes showed high efficiency in a tandem 3,3-rearrangement/Nazarov reaction of an enynyl acetate. One of the Au-III complexes, [AuCl3(F-R-NHC)], was further found to be suitable for the efficient cyclisation of a propargylcarboxamide. Furthermore, unlike related NHC-gold(I) complexes based on conventional bulky N-heterocyclic carbenes (notably, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), 1,3-bis-(2,4,6-trimethylphenyl) imidazol-2-ylidene (IMes) and 1,3-(bis-tertbutyl) imidazol-2-ylidene (ItBu)), the studied [(AuCl)-Cl-I(F-R-NHC)] complexes catalysed the conversion of a 1,6-enyne in the presence of indole into a single product; this arises from the embracing character of the ligand, which prevents indole addition on one of the catalytic intermediates. A structure/selectivity relationship was established for all carbenes tested that took into account percent buried volumes and topographic steric maps. The results illustrate the high potential of confining NHCs in organic synthesis.