Synthesis of New Chiral N-Heterocyclic Carbene−Imine Ligands and Their Application to an Asymmetric Allylic Alkylation Reaction

Abstract

Silver(I) and palladium(II) complexes of new chiral N-heterocyclic carbene (NHC)-imine ligands derived from trans-1,2-diaminocyclohexane have been prepared. These ligands have been applied to a palladium(II)-catalyzed asymmetric allylic alkylation reaction giving a maximum ee of 92%.Over the past decade N-heterocyclic carbenes (NHC) have emerged as one of the most important classes of compound used as ancillary ligands for a number of late transition metal mediated catalytic reactions. 1 In comparison to ubiquitous tertiary phosphine ancillary ligands, advantages of using NHC include increased thermal stability and that excess ligand is not required. 2 More recently a concurrent development has been the investigation of chiral NHC derivatives for asymmetric catalysis. 3 In comparison to tertiary phosphine chemistry, reported examples where chiral NHC complexes give good enantioselectivity are rare. However the recent reports of >99% ee for an iridium-catalyzed asymmetric hydrogenation of aryl alkenes 4 and ruthenium-catalyzed symmetry-breaking metathesis 5 exemplify the potential of chiral NHC ligands, indicating that further investigation is warranted. Even though the vast majority of reactions studied to date use NHC precatalysts of group 10 metals, 1 the highest enantioselective reaction to date by far is 76% ee for a palladium(II)-catalyzed intramolecular cyclization. 6 Here we wish to report the synthesis of new chiral NHC-imine ligands derived from trans-1,2-diaminocyclohexane and initial work on their application to a palladium(II)-catalyzed intermolecular asymmetric allylic alkylation reaction.We wished to prepare chiral chelating NHC precursors including imidazoles and imidazolium salts derived from chiral diamines such as trans-1,2-diaminocyclohexane, in part because of the success of ligand sets based on chiral diamines in several metal-mediated enantioselective catalytic reactions. 7 The most common method for the preparation of an imidazole or imidazolium salt is co-condensation between amines, paraformaldehyde, and a dione. 8 Initial attempts to prepare imidazoles from (1R,2R)-1,2-diaminocyclohexane using co-condensation routes led to the formation of oligomeric mixtures, and therefore an alternative strategy was sought. The base-induced 1,3-cycloaddition of tosylmethylisocyanide (TosMIC) to imines has sporadically been used for the preparation of imidazoles, 9 and therefore we investigated the reaction between TosMIC and diimine (1) shown in Scheme 1.Reaction between 1 equiv of TosMIC and 1 did give the imidazole-imine (2) in 95% yield as shown in Scheme 1. As noted by other workers, we found that the preparation of imidazoles from imines and TosMIC Park, H.; Kim, B. Y.; Lee, J. H.; Son, S. U.; Chung, Y. K. Organometallics 2003, 22, 618. (4) (a) Powell, M. T.; Hou, D. R.; Perry, M. C.; Cui, X. H.; Burgess, K. J. Am. Chem. Soc. 2001, 123, 8878. (b) Perry, M. C.; Cui, X. H.; Powell, M. T.; Hou, D. R.; Reibenspies, J. H.; Burgess, K.

Palladium Complexes of N‐Heterocyclic Carbenes as Catalysts for Cross‐Coupling Reactions—A Synthetic Chemist's Perspective

“…Douthwaite and co-workers disclosed a series of chiral, chelating NHC-imine ligands from trans-1,2-diaminocyclohexane. [229] Bulkier N-substituents at the imidazolium ring resulted in higher ee values. Ketimines derived from acetone were also more enantioselective than aldimines.…”

Aus der Sicht des Synthetikers: Palladiumkomplexe N‐heterocyclischer Carbene als Katalysatoren für Kreuzkupplungen

“…Douthwaite et al stellten eine Reihe von chiralen chelatisierenden NHC-Imin-Liganden mit trans-1,2-DiaminocyclohexanGerüst vor. [229] Je größer die Substituenten an den N-Atomen des Imidazoliumrings waren, desto höher waren die ee-Werte. Von Aceton abgeleitete Ketimine führten zu einer besseren Enantioselektivität als Aldimine.…”

[1,2]‐Rearrangement of Imino‐N‐heterocyclic Carbenes − Synthesis and Structures of Chelating Iminoimidazole Pd and Ni Complexes

“…pyridine and oxazoline, respectively. To the best of our knowledge, no other N-donor functionalities have been investigated as components of heterotopic [N,C] ligands for potential catalytic applications of their metal complexes, except for one very recent example, [10] published during the preparation of this manuscript. Conceptually, our motivation for the work reported here is based on the following considerations: We have a general interest in the development of new and improved olefin polymerization catalysts.…”