Large yet Flexible N-Heterocyclic Carbene Ligands for Palladium Catalysis

• Published in: Chemistry : a European journal Vol. 19; no. 51; pp. 17358 - 17368
• Main Authors: Meiries, Sebastien, Le Duc, Gaëtan, Chartoire, Anthony, Collado, Alba, Speck, Klaus, Arachchige, Kasun S. Athukorala, Slawin, Alexandra M. Z., Nolan, Steven P.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 16-12-2013; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: bulky; Carbenes; Catalysis; Chemistry; Congeners; Couplings; Cross coupling; Electronics; flexible; Ligands; N-heterocyclic carbene; Nickel; Palladium; Synthesis; tentacular; Xylene; N-heterocyclic carbene; catalysis; palladium; tentacular; bulky; flexible
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201302471

A straightforward and scalable eight‐step synthesis of new N‐heterocyclic carbenes (NHCs) has been developed from inexpensive and readily available 2‐nitro‐m‐xylene. This process allows for the preparation of a novel class of NHCs coined ITent (“Tent” for “tentacular”) of which the well‐known IMes (N,N′‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene), IPr (N,N′‐bis(2,6‐di(2‐propyl)phenyl)imidazol‐2‐ylidene) and IPent (N,N′‐bis(2,6‐di(3‐pentyl)phenyl)imidazol‐2‐ylidene) NHCs are the simplest and already known congeners. The synthetic route was successfully used for the preparation of three members of the ITent family: IPent (N,N′‐bis(2,6‐di(3‐pentyl)phenyl)imidazol‐2‐ylidene), IHept (N,N′‐bis(2,6‐di(4‐heptyl)phenyl)imidazol‐2‐ylidene) and INon (N,N′‐bis(2,6‐di(5‐nonyl)phenyl)imidazol‐2‐ylidene). The electronic and steric properties of each NHC were studied through the preparation of both nickel and palladium complexes. Finally the effect of these new ITent ligands in Pd‐catalyzed Suzuki–Miyaura and Buchwald–Hartwig cross‐couplings was investigated.
Tentacular NHCs: A straightforward, scalable eight‐step synthesis of N‐heterocyclic carbenes (NHCs) has been developed using inexpensive and readily available 2‐nitro‐m‐xylene, allowing the preparation of a class of NHCs designated ITent (“Tent” for “tentacular”). The electronic and steric properties of each NHC were studied through the preparation of both nickel and palladium complexes, and the effect of these new ITent ligands in Pd‐catalyzed Suzuki–Miyaura and Buchwald–Hartwig cross‐couplings was investigated.