Carbene, -heterocyclic, chelating

NICKEL(II) COMPLEX OF N-HETEROCYCLIC SILYLENE (NHSi)-CARBENE (NHC) CHELATE LIGAND [L i(CH2)(NHC)NiBr2] L = CH(MeC = NAr>2. NHC = 3,4,5-TRIMETHYLIMIDAZ0L-2-YLIDEN-6-YL ... 

Bidentate NHC-Pd complexes have been tested as hydrogenation catalysts of cyclooctene under mild conditions (room temperature, 1 atm, ethanol). The complex 22 (Fig. 2.5), featuring abnormal carbene binding from the O carbon of the imidazole heterocycles, has stronger Pd-C jj, bonds and more nucleophilic metal centre than the bound normal carbene chelate 21. The different ligand properties are reflected in the superior activity of 22 in the hydrogenation of cyclooctene at 1-2 mol% loadings under mild conditions. The exact reasons for the reactivity difference in terms of elementary reaction steps are not clearly understood [19]. 

An extension of the research on silver complexes with Lewis base-functionalized mono(A-heterocyclic carbene) ligands has been made toward the better-studied and stronger coordinating phosphine systems. The reaction of a diphenylphosphine-functionalized imidazolium salt with silver oxide in dichloromethane affords a trinuclear silver carbene complex 50, as confirmed by electrospray-ionization mass spectrometry.96,97 Metathesis reaction of 50 in methanol using silver nitrate gives 51 in 33% yield. The crystal structures of 51 were found to be different when different solvents were used during crystallization (Scheme 12).97 One NO3- anion was found to be chelated to... 

Fig. 4. Tripodal N-heterocyclic carbene chelators with mesitylene (mes-carbene, left), carbon (TIME, center), and nitrogen (TIMEN, right) anchoring units.

A -Heterocyclic carbene complexes of Ir(I) and Ir(III) have also demonstrated high reactivity in transfer hydrogenation reactions of ketones (Scheme 2) [4]. Complex 4 catalyzed the reduction of a range of ketones into the corresponding alcohols, including the reduction of pinacolone 7 into alcohol 8 with a low catalyst loading and short reaction time [5]. The chelating bis(Af-heterocyclic carbene) complex 5 was shown to catalyze the reduction of ketones, and in the case of the reduction of benzophenone 9 to alcohol 10, the reaction was complete within 4 min [6]. 

Iminio-phosphonio dications have been obtained by N- and F-methylation of (imidazolylphenyl)diphenylphosphine. These dications are suitable precursors for C,C-chelating ligands. Deprotonation of the N-heterocycle leads to an NHC carbene which can be coordinated to a suitable metal center like Pd The subsequent deprotonation of the phosphonio moiety gave the complex with a bidentate chelating coordinated NHC-ylide ligand [207, 208]. 

Several catalytic systems based on palladium chelated with phosphine or W-heterocyclic carbene were evaluated for such a reaction (Table 1) [12]. 

Milstein et al. found that Pd complexes with chelating alkylphosphines such as bis(diisopropylphosphino)butane (dippb) efficiently catalyze the olefmation of aryl chlorides with styrenes in the presence of elemental zinc [29]. Unfortunately, these electron-rich phosphines are apparently incompatible with electron-poor olefins such as acrylic acid derivatives. The latter were successfully coupled with activated chloroarenes by Herrmann et al., who used palladacycles or Pd-catalysts with heterocyclic carbenes [30]. 

Other bidentate N-heterocyclic carbenes were used to form stable chelate complexes. A fine example is the use of palladium NHC complex 24 in the catalytic conversion of methane to methanol (Fig. 10) [111]. In this case the stability of the complexes is a requirement, since the reaction takes place in an acidic medium (trifluoroacetic acid) at elevated temperatures (80 °C) mediated by strong oxidizing agents (potassium peroxodisulfate). 

Peris E, Crabtree RH (2004) Recent homogeneous catalytic apphcations of chelate and pincer V-heterocyclic carbenes. Coord Chem Rev 248 2239-2246... 

Waltmann AW, Grubbs RH (2004) A new class of chelating A-heterocyclic carbene hgands and their complexes with palladium. Organometallics 23 3105-3107... 

Catalyst precursors modified with other types of hgands, such as bis(N-methyl-imidazole) 27 [56], N-heterocyclic carbene chelates 28 [57], and cahx[6]arene-derived... 

Structural and catalytic properties of chelating bis- and tris-N-heterocyclic carbenes 07CCR841. 

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