Synthesis of Constrained-Geometry Chiral Di-N-Heterocyclic Carbene Ligands and Their Silver(I) and Palladium(II) Complexes

Abstract

Silver(I) and palladium(II) complexes of constrained-geometry chiral di-N-heterocyclic carbenes have been prepared from trans-1,2-diaminocyclohexane via the base-induced 1,3-cycloaddition of tosylmethyl isocyanide to diimines.Since the discovery of stable N-heterocyclic carbenes (NHC), increasing attention has been focused on using these compounds as ancillary ligands for a number of late-transition-metal-mediated catalytic reactions. 1 In general, it appears that catalytic reactions which employ transition-metal complexes of tertiary phosphines may also be catalyzed using complexes of NHC, and many of the precatalysts studied to date exhibit excellent thermal stability and the need for excess NHC ligand is not required. 1-5 In addition, further interest lies in the unusual structural motif rendered at the metal atom by NHC ligands, and in this respect a concurrent development has been the investigation of chiral NHC derivatives to induce enantioselective reactivity. 6-21 In comparison to tertiary phosphine chemistry, reported examples where chiral NHC complexes give good enatioselectivity are rare, in most cases primarily due to the flexibility of the ligand systems investigated. However, the recent reports by Burgess et al. of >99% ee for the iridium-catalyzed asymmetric hydrogenation of aryl alkenes, using a constrained-geometry NHC-oxazoline ligand, exemplify the potential of chiral NHC ligands. 22,23 Excellent levels of enantioselectivity have been observed for symmetry-breaking metathesis reactions using NHC ligands that exhibit a well-defined geometry. 24,25 also demonstrating that further investigation into chiral NHC ligands is justified.By far the most common method for the synthesis of chelating NHC ligands and their hybrids is from reaction between an imidazole and a suitable precursor. In part, the application of NHC ligands to asymmetric catalysis has been limited by the paucity of synthetic routes to their preparation.Here we wish to report the synthesis of constrainedgeometry chiral di-NHC ligands derived from trans-1,2-diaminocyclohexane. The synthesis of silver(I) and palladium(II) complexes and an X-ray crystal structure of a palladium(II) complex are also presented.Our aim was to prepare chiral chelating NHC ligand precursors derived from chiral diamines, including the readily accessible enantiomers of trans-1,2-diaminocyclohexane, which we envisaged would exhibit a welldefined geometry, as shown in Figure 1. We were motivated in part by the availability of structurally diverse chiral diamines and in particular by the success of ligand sets based on chiral diamines in several metalmediated enantioselective catalytic reactions. Park, H.; Kim, B. Y.; Lee, J. H.; Son, S. U.; Chung, Y. K. Organometallics 2003, 22, 618. (22) Powell, M. T.; Hou, D. R.; Perry, M. C.; Cui, X. H.; Burgess, K. J. Am. Chem. Soc. 2001, 123, 8878. (23) Perry, M. C.; Cui, X. H.; Powell, M. T.; Hou, D. R.; Reibenspies, J. H.; Burgess, K.

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

“…[216] Subsequently the chiral diimidazolium-ligand precursor 123 [217] was prepared. The last group of chiral NHC ligands or ligand precursors contains oxazoline groups.…”

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

“…The aim of this section is [95][96][97][98] The strength of the PdÀNHC bonds renders ligand exchange (Schemes 20-25) an excellent general route to Pd-NHC complexes starting from the preformed carbene and Pd II or Pd 0 complexes with alkenes, [99] phosphanes, [91] nitrogen ligands, [45] or bridging chloride [37,100] or acetate ligands. [101] Pure [102] Another approach is to use an azolium salt precursor in the presence of a base to form in situ the NHC, which is captured by Pd (Schemes 16 and 27-29).…”

“…Similarly, Douthwaite and co-workers observed 90 % yield, but only 11 % ee of 309 when they used complex 76 (Scheme 22). [45] …”

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

“…[95][96][97][98] Der Ligandenaustausch, angetrieben durch die starke Pd-NHC-Bindung, bietet eine ausgezeichnete Möglichkeit, Pd-NHC-Komplexe ausgehend von Pd II -oder Pd 0 -Komplexen mit Alkenen, [99] Phosphanen, [91] Stickstoffdonorliganden [45] oder verbrückenden Chlorid- [37,100] oder Acetationen herzustellen (Schema 20-25).…”

“…In einem ähnlichen Beispiel beobachteten Douthwaite et al mit Komplex 76 für 309 90 % Ausbeute bei nur 11 % ee (Schema 22). [45] 4.8. Die Buchwald-Hartwig-Aminierung und ähnliche C-N-Kupplungen…”