The preparation of a series of complexes of the type CpIrX2(NHC) provides effective catalysts for the H/D exchange of a wide range of organic molecules in methanol-d4. The reaction proceeds with higher yields under milder reaction conditions than previous CpIr systems reported thus far. For comparative purposes, we also studied the catalytic activity of CpIrCl2(PMe3) under the same reaction conditions. The molecular structures of two of the new CpIr(NHC) complexes are described.

A comparative study on the catalytic activity of a series of [IrCl(2)Cp*(NHC)] complexes in several C-O and C-N coupling processes implying hydrogen-borrowing mechanisms has been performed. The compound [IrCl(2)Cp*(I(nBu))] (Cp*=pentamethyl cyclopentadiene; I(nBu)=1,3-di-n-butylimidazolylidene) showed to be highly effective in the cross-coupling reactions of amines and alcohols, providing high yields in the production of unsymmetrical ethers and N-alkylated amines. A remarkable feature is that the processes were carried out in the absence of base, phosphine, or any other external additive. A comparative study with other known catalysts, such as Shvo's catalyst, is also reported.

The reaction of 1,2,4-trimethyltriazolium tetrafluoroborate ([LH 2 ](BF 4 ) 2 ) with several metal precursors provides a series of homo-and heterodimetallic compounds of Rh and Ir. The reaction of [LH 2 ](BF 4 ) 2 with [Cp*IrCl 2 ] 2 in the presence of NaH in MeOH yields the monocarbene complex [Cp*Ir(LH)Cl 2 ](BF 4 ), which is a very convenient synthon to the preparation of heterodimetallic species through the deprotonation of the remaining NCHN group. Three crystal structures of dimetallic compounds of di-iridium(III), iridium(III)/iridium(I), and iridium(III)/rhodium(I) are described. The complexes obtained have been tested in a tandem catalytic reaction implying the consecutive oxidative cyclization of 2-aminophenyl ethyl alcohol and the alkylation of the resulting imidazole with a series of primary alcohols. The new catalysts show high activity in the overall reaction process, and the selectivity in the production of the bisindolylmethane or monoalkylated indole is tuned by changing the indole/alcohol molar ratio.⊥ This article is dedicated to Prof. Robert H. Crabtree on the occasion of his 60th birthday.

We recently discovered that hydroboration of 1,2-disubstituted aryl olefins can be catalyzed by chiral Cu-based bidentate N-heterocyclic carbene (NHC) [1] complexes. [2, 3] Reactions proceed to deliver homobenzylic CÀB bonds exclusively, [4] which is in contrast to transformations with alkylboranes or those catalyzed by the more costly Rh-or Irbased complexes, where formation of benzylic C À B bonds is preferred.[3] We later showed that the NHC À Cu-catalyzed transformations proceed efficiently with b-vinylboronates to afford acyclic vicinal diboronates with complete site selectivity (< 2 % geminal).[5] The above protocols are enantioselective, delivering boron-substituted stereogenic centers at secondary carbon atoms in 86:14-99:1 enantiomeric ratio (e.r.). Despite such advances, as well as seminal findings reported by other research groups, [3] several shortcomings persist. One long-standing problem relates to catalytic enantioselective hydroborations of 1,1-disubstituted alkenes.[6] The use of stoichiometric quantities of chiral boranes in reactions of 1,1-disubstituted alkenes has been outlined; [7] however, high enantioselectivity (e.r. > 90:10) is only observed when there is a significant size difference between the olefin substituents. Rh-and Ir-catalyzed processes with catecholborane typically furnish low enantioselectivity with 1,1-disubstituted alkenes and, in some cases, control of site selectivity is problematic. [8] Recently, an Ircatalyzed process with pinacolatoborane was found to afford complete site selectivity and e.r. ! 90:10 in the case of two amethylstyrene derivatives (overall range of e.r. = 66:34-96:4).[9] Herein, we present an NHCÀCu-catalyzed process for the site-and enantioselective catalytic hydroboration of 1,1-disubstituted aryl olefins (Scheme 1);[10] a-alkyl-b-pinacolatoboranes are formed with > 98 % site selectivity, in up to > 98 % yield and e.r. = 96.5:3.5. Reactions involving a range of acyclic 1,1-disubstituted aryl olefins, including those that contain alkyl substituents other than the typically utilized methyl unit, have been developed. Also included are transformations of exocyclic alkenes, which, to the best of our knowledge, have not been previously reported.In considering the mechanism of catalytic 1,1-disubstituted alkene hydroborations (Scheme 1), we surmised that transformations would likely be initiated by [(NHC)Cu{B-(pin)}][11] i [formed from reaction of bis(pinacolato)diboron (1) with an NHC-Cu-alkoxide]. Subsequent Cu À B addition would afford b-alkylborane ii, which contains a quaternary Cu-substituted stereogenic center. Protonation of the CuÀC bond, [12] likely occurring with retention of configuration, [4] can deliver the desired product and regenerate iii, which reacts with 1 to form i. An uncommon feature of the catalytic processes is that the eventual stereochemical outcome does not depend only on the enantioselectivity of the Cu À B addition: the stereochemistry of the intermolecular protonation of the enantiomerically enriched alkylcopper species wit...

The metalation of a series of C2-Me-substituted monoimidazolium and bisimidazolium salts to [Cp*IrCl2]2 is described. The reaction of the monoimidazolium salt provides the species Cp*Ir(aNHC)Cl2, in which the NHC shows an abnormal coordination mode. The use of the bisimidazolium salt provides different reaction patterns depending on the linker length between the two azolium rings. For the methylene-linked bisimidazolium salt, the only compound obtained shows an unusual type of coordination in which the chelating ligand is coordinated through an abnormal NHC and a methylene group resulting from the CH activation of the C2-Me group. For the ethylene-linked bisimidazolium salt, a similar product is obtained, together with the chelating bis-abnormal-NHC species. All compounds have been fully characterized by usual spectroscopic techniques, and X-ray molecular structures are described. The formation of the reaction products, in the case of the methylene linker, has been rationalized by means of DFT calculations with inclusion of solvent effects (PCM). The calculations could not discriminate the nature of the first metalation between direct deprotonation of the ligand by the base and metalation through C−H activation at Ir. However both cases point to a kinetic preference for first metalation at the C2-Me group. The second metalation process is the result of kinetically preferred C−H activation at the C5 position.

This microreview focuses on the preparation of homogeneous catalysts of group 8-11 metals containing Nheterocyclic carbene ligands. The current status of the design of mono-, bi-and tridentate NHC ligands is reviewed through the description of the authors' own work. The cata-

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