Six-Membered, Chiral NHCs Derived from Camphor: Structure–Reactivity Relationship in Asymmetric Oxindole Synthesis

Abstract

A series of three chiral, expanded six-membered NHC−palladium(II) complexes was prepared with successively increased sterical demand, while retaining natural d-(+)-camphor as a chiral motif. The catalysts showed different reaction profiles in the asymmetric, intramolecular α-arylation of amides. The molecular structure of two N-heterocyclic and one nitrogen acyclic carbene palladium isonitrile complex was unequivocally determined by X-ray crystallographic analysis. The results reported herein account for a correlation of catalytic activity and enantiodiscrimination in relation to the degree of chiral substitution and steric congestion at the metal center. The modular and convergent synthetic route of these air-and moisturestable palladium isonitrile complexes underlines the usefulness of this approach.

New catalysts with unsymmetrical N-heterocyclic carbene ligands

“…This procedure is efficient and also displays a promising synthetic access towards unsymmetrical Au-and Pt-based compounds bearing five-and six-membered 8 uNHCs. Sterically demanding boronic acids were successfully coupled with aryl bromides and with even more challenging chlorides in a Suzuki reaction.…”

Metal-Mediated Addition of N-Nucleophiles to Isocyanides: Mechanistic Aspects

“…As a result of the interaction between K 2 [PtCl 4 ] and C≡NMe and the following addition of hydrazine, the first complex of a heteroatom-stabilized carbene was obtained [ 46 , 47 , 48 , 49 ]. Since that time, the metal-mediated addition of the monofunctional O- [ 50 , 51 , 52 , 53 , 54 ], N- [ 34 , 35 , 36 , 37 , 41 , 43 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 ], and other [ 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 ] nucleophiles as well as bifunctional nucleophiles [ 27 , 34 , 53 , 62 , 64 , 79 , 80 , 81 , 82 , …”

Recent Developments in Inter- and Intramolecular Enolate Arylation

“…While many of the ligands employed are based on five-membered imidazoles, imidazolines (or oxazoles), the use of expanded NHCs and furthermore unsymmetrical substituted chiral NHC-metal complexes has rarely been reported or investigated for this type of transformation. Trapp and coworkers 49 presented the synthesis of three six-membered (hexahydropyrimidine core), camphor-derived NHC-Pd-isonitrile complexes and also their application as catalysts in the asymmetrical a-arylation of amides (Scheme 25). All catalysts showed good conversions of bromo-and chloro-functionalized substrates.…”