Pentiptycene-based concave NHC–metal complexes

Abstract

The reaction of 1-amino,4-hydroxy-pentiptycene with diacetyl or acenaphthene-1,2-dione gave the respective diimines, followed by alkylation of the hydroxyl groups, and cyclization of the alkylated diimines to the respective bispentiptycene-imidazolium salts NHC·HCl. The azolium salts, being precursors to N-heterocyclic carbenes, were converted into metal complexes [(NHC)MX] (MX = CuI, AgCl, AuCl) and [(NHC)IrCl(cod)] and [(NHC)IrCl(CO)2] in good yields. In the solid state [(NHC)AgCl] displays a bowl-shaped structure of the ligand with the metal center buried within the concave unit.

Bispentiptycenyl‐N‐Heterocyclic Carbene (NHC) Gold Complexes: Highly Active Catalysts for the Room Temperature Hydration of Alkynes

“…Complex 4 contains one pentiptycenyl and one triptycenyl substituent. The bulkiest gold complexes are the previously reported complexes 5 and 6 , which possess two N‐pentiptycenyl substituents, with different backbones attached to the heterocyclic unit.…”

“…With a view to the observed modest influence of substrate sterics, this raises questions with respect to the established mechanism of the gold catalyzed hydration,, which typically involves the side‐on coordination/activation of the alkyne to the catalytically active gold center. Based on the X‐ray crystal structure of the related complex [AgCl( 6 )] this side‐on coordination of bulky alkynes should be much less facile for the very bulky NHC ligands employed here – nonetheless, the bulkiest NHC ligands provide the most active hydration catalyst and the very bulky 2,4,6‐triisopropylphenyl acetylene is easily converted into the respective ketone. However, the coordination of the alkyne to the cationic gold is normally not considered to be a rate limiting step in alkyne alkoxylation.…”

BIAN‐NHC Ligands in Transition‐Metal‐Catalysis: A Perfect Union of Sterically Encumbered, Electronically Tunable N‐Heterocyclic Carbenes?

“…In 2016, Plenio and co‐workers reported the synthesis of a BIAN‐based Au I complex 35 featuring extremely bulky N ‐pentiptycene wingtip substituents (Scheme 40). [78] This catalyst was tested in the electrophilic cyclization of diethyl 2‐allyl‐2‐(prop‐2‐ynyl)malonate, affording the desired product in much higher yield and selectivity than the structurally related (IMes)AuCl .…”

Acenaphthene-Based N-Heterocyclic Carbene Metal Complexes: Synthesis and Application in Catalysis

“…It should be noted that bisimine 11 plays a dual role-a protecting group for the amine Among the sterically hindered NHC precursors, special attention was given to pentipticene derivatives. Plenio and co-workers [47] have developed a scalable synthetic route to those derivatives bearing a unique rigid skeleton (Scheme 4). The resulting salt 13 could be easily accessed via bisimine formation starting from aminophenol 10 (available via double cycloaddition of hydroquinone and anthracene on a multigram scale), and subsequent O-alkylation and ring closure, performed in the usual manner.…”