Cationic mononuclear Ni(II) complexes of the chelating N-heterocyclic carbenes tBuCCmeth and tBuCCeth (where tBuCCmeth = 1,1‘-methylene-3,3‘-di-tert-butyldiimidazole-2,2‘-diylidene (1) and tBuCCeth = 1,2-ethylene-3,3‘-di-tert-butyldiimidazole-2,2‘-diylidene (2)) have been prepared and structurally characterized. Reaction between NiCl2(PMe3)2 and 1 equiv of 1 or 2 gives the monocationic salts [(tBuCCmeth)NiCl(PMe3)]Cl (3a) and [(tBuCCeth)NiCl(PMe3)]Cl (4a), which undergo salt metathesis with TlBPh4 to give [(tBuCCmeth)NiCl(PMe3)][BPh4] (3b) and [(tBuCCeth)NiCl(PMe3)][BPh4] (4b). A comparison of the X-ray structures of 3a and 4b shows that the bite angles at the nickel atoms, 84.92(18) and 88.4(4)°, respectively, are comparable to Ni(II) complexes of two-carbon-atom bridging chelating bis-phosphines. Reaction between NiCl2(PMe3)2 or NiBr2(DME) and 2 equiv of 1 gives the Ni(II) dicationic salts [Ni(tBuCCmeth)2][X]2 (X = Cl- (5a), X = Br- (5b)). The X-ray structure of 5b reveals a square-planar geometry at the nickel atom, and the dication adopts a trans double-boat like conformation. An analogous reaction between NiCl2(PMe3)2 and 2 equiv of 2 gives 4a and does not yield the dication [Ni(tBuCCeth)2][Cl]2. Reaction between 4a and 2 equiv of 1 gives 5a, whereas no reaction under analogous conditions occurs between 3a and 2. The influence of steric factors which determine the relative thermodynamic stability toward dicarbene substitution of complexes 3a and 4a is also discussed.

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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.

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.

A diimidazolium salt incorporating a secondary amine moiety has been used to prepare a palladium(II) di-N-heterocyclic carbene amino complex that can be deprotonated with NaH to give the first example of a transition metal NHC-amide.

A range of N-donor ligands based on the 1H-pyridin-(2E)-ylidene (PYE) motif have been prepared, including achiral and chiral examples. The ligands incorporate one to three PYE groups that coordinate to a metal through the exocyclic nitrogen atom of each PYE moiety, and the resulting metal complexes have been characterised by methods including single-crystal X-ray diffraction and NMR spectroscopy to examine metal-ligand bonding and ligand dynamics. Upon coordination of a PYE ligand to a proton or metal-complex fragment, the solid-state structures, NMR spectroscopy and DFT studies indicate that charge redistribution occurs within the PYE heterocyclic ring to give a contribution from a pyridinium-amido-type resonance structure. Additional IR spectroscopy and computational studies suggest that PYE ligands are strong donor ligands. NMR spectroscopy shows that for metal complexes there is restricted motion about the exocyclic C-N bond, which projects the heterocyclic N-substituent in the vicinity of the metal atom causing restricted motion in chelating-ligand derivatives. Solid-state structures and DFT calculations also show significant steric congestion and secondary metal-ligand interactions between the metal and ligand C-H bonds.

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The synthesis and structures of chiral N-heterocyclic carbene (NHC)-N-donor complexes of silver(I) and palladium(II) are reported. The X-ray structure of an NHC-imine silver(I) complex [((nPr)CN(CHPh))AgBr](2) exhibits an Ag(2)Br(2) dimer motif where the imine group is not coordinated to the silver atom. Reaction between 2 and [PdCl(2)(MeCN)(2)] gives the palladium(II) complex [(kappa(2)-(nPr)CN(CHPh))PdCl(2)](3) that contains a chelating NHC-imine ligand as shown by single-crystal X-ray diffraction. Slow hydrolysis of related complexes [(kappa(2)-(nPr)CN(CHPh))PdCl(2)](3) and [(kappa(2)-((Ph)(2)CH)CN(CHPh))PdCl(2)](4) using triethylammonium chloride and water lead to the precipitation of single crystals of insoluble NHC-amino palladium(II) complexes [(kappa(2)-(nPr)CN(H(2)))PdCl(2)](6) and [(kappa(2)-((Ph)(2)CH)CN(H(2)))PdCl(2)](7), respectively. In the solid state, complexes 6 and 7 both exhibit intermolecular hydrogen bonding between chlorine and an amino-hydrogen atom resulting in an infinite chain structure. Substitution of an amino hydrogen for an ethyl group gives the soluble complex [(kappa(2)-(iPr)CN((H)Et))PdCl(2)](12). Reaction between two equivalents of 2 and [PdCl(2)(MeCN)(2)] gives the di-NHC complex [(kappa(1)-(nPr)CN(CHPh))(2)PdCl(2)](5) that does not contain a coordinated imine as shown by single crystal X-ray diffraction. Conproportionation between 5 and an equivalent of [PdCl(2)(MeCN)(2)] to does not occur at temperatures up to 100 degrees C in CD(3)CN.