NHC-Ir complex

Li and co-workers described the cleavage of a ketone substituent from NHC-Ir complex 53 resulting in an unusual iV-H carbene complex 54 in 30% yield after purification with silica gel (Scheme 3.20). This provides an interesting method for the preparation of A-H N-heterocyclic carbenes, since compound 54 did not undergo tautomerization to the A-bound imidazole form. ... 

During their study on the (3-alkylation of alcohols. Peris et al. observed an oxidative dehydrogenation of the final alcohol to the corresponding ketone under the reaction conditions (toluene, reflux). This result was interpreted in the light of the previous report by Fujita and Yamaguchi where a [Ir(Cp )] complex was efficient for the oxidation of alcohol in an oxidant-free environment (with release of H2), and demonstrated that [(NHC)Ir] complexes are good candidates for this promising reaction. 

N-Alkylation of aniline with [(NHC)Ir] complexes was first reported with catalyst 36a. Employing KOH or t-BuOK as stoichiometric base, better yields of the monoalkylated produet were obtained than with the parent complex [IrCl2Cp ]2. Furthermore, upon using the pyrimidine-containing complex 37 as catalyst, a weak base such as NaHCOs was sufficient to promote the reaction. The combination of complex 35a with AgOTf afforded a very active catalytic system, which did not require any added base, and was also very active in the benzylation of the secondary amine N-methylaniline. The resulting drawback was a certain lack of... 

Mono- and bis-NHC iridium(i) complexes were applied in the C-H bor-ylation of aromatic carbons with bis(pinacolato)diboron or pinacolborane under microwave irradiation. The bis-NHC complexes gave better yields, mainly because of the lower stability of the mono-NHC Ir complexes compared to the bis-NHC Ir complexes. Finally, picoline- and pyridine-functionalised NHC-iridium complexes of general formula [(C N)IrCl(Cp )]Cl were moderately active catalysts for the polymerisation of norbornene in the presence of methylaluminoxane (MAO) as co-catalyst. ... 

Although these systems will not be discussed further here, it should be noted that NHC-Ir complexes were also used to catalyze the Oppenauer-type oxidation of alcohols. ... 

Studied zwitterionic NHC-Ir complexes 35, which proved to be particularly active, even for the hydrogenation of hindered alkenes in nonpolar solvents or of neat alkenes. Neat hydrogenation of alkenes could be performed with catalyst loadings as low as 10 ppm with 8 atm of H2- ... 

Immobilized NHC-Ir complexes 88 and 89 with carbon nanotubes or polyoxometalates were also successfully applied to TH of ketones (Figure 13.8). The carbon nanotube-supported NHC-Ir materials were active in the TH of cyclohexanone and TOF values < 5550 h" obtained. 

Peris and co-workers used glycerol instead of i-PrOH as the hydrogen source and the reaction medium in a TH process. In this context, they described the use of a series of NHC-Ir " complexes 99 and 100 for the TH of double bonds using glycerol as hydrogen donor under microwave, ultrasound and oil bath conditions (Figure 13.9). These complexes were found to be active for TH of carbonyl compounds, moderately active in the reduction of olefins and alkynes and, more remarkably, showed excellent chemoselec-tivity in the reduction of the alkenic double bond of o,p-unsaturated ketones. 

A bidentate phosphine-functionalized NHC-Ir complex 130 was developed and applied to the N-monoalkylation of aromatic amines with a wide range of primary alcohols in good to excellent yields (Figure 13.14). The authors also reported that a variety of secondary amines could be prepared in high isolated yields under solvent-free conditions, even at room temperature. 

A mesoporous SBA-15-supported pyrimidine-substituted NHC-Ir " complex 132 was successfully applied to the N-alkylation of amines with alcohols. 

Alkenyl-functionalized NHC-Ir complexes 149 showed a high selectivity for the p-(Z)-isomers (up to 0 100 0, p-( )/p-(Z)/a), with no a-isomers or dehydrogenative silylation products observed (Figure 13.17). Similar Rh complexes with an amino-alkyl NHC, instead of alkenyl, displayed high regio- and stereoselectivities (up to 2 98 0, p-( )/p-(Z)/a) in the hydrosilylation of 1-hexyne with PhMe2SiH. Viciano et al. reported the hydrosilylation... 

Chiral hydroxy-amide-functionalized NHC-Ir complexes 199 facilitated efficient asymmetric hydrosilylation of ketones using HSiMe(OEt)2 under ambient conditions (Figure 13.25). A chiral NHC containing an isobutyl stereodirecting group was found to be the best ligand in the examined functionalized NHC-Ir complexes and <96% ee was obtained for the hydrosilylation of aromatic ketones. 

Complexes 59 and 60 catalyse the hydrosilylation of phenylacetylene (but not other terminal alkyl alkynes) with HSi(Me)jPh. Generally, the Rh analogue is more active than the relative Ir. Both catalysts gave mixtures of all regioisomers, with a preference for the p-Z-isomer, in contrast to what has been reported with other non-NHC cationic complexes of Rh, where the p-f isomers are predominating. Here also the exact nature of the catalytic species is unclear [48],... 

The Rh and Ir complexes 85-88 (Fig. 2.14) have been tested for the intramolecular hydroamination/cyclisation of 4-pentyn-l-amine to 2-methyl-1-pyrroline (n = 1). The reactions were carried out at 60°C (1-1.5 mol%) in THF or CDCI3 The analogous rhodium systems were more active. Furthermore, the activity of 87 is higher than 85 under the same conditions, which was attributed to the hemilabihty of the P donor in the former complex, or to differences in the trans-eSects of the phosphine and NHC ligands, which may increase the lability of the coordinated CO in the pre-catalyst [75,76]. 

More success has been had with Ir complexes incorporating permelhylcyclopentadiene and NHC ligands. Complexes 18-20 (Fig. 4.7) were evalnated for norbomene polymerisation following activation with MAO [22]. Complex 19 was the most active, giving a TOF of 12 220 h over 10 min, followed by 18 (TOF = 3 220 h" ), while 20 was inactive, indicating that a hemilabile pendant group seems essential. Analysis (NMR) of the polymers formed with 18 and 19 shows that polymerisation proceeds via an addition (coordination-insertion) mechanism. 

Cp Ir(NHC) complexes are a very versatile type of catalyst, with a wide range of applications. In a chemoenzymatic application, Cp Ir complexes activated by fluo-rinated and nonfluorinated NHC ligands were shown to be catalysts for racemiza-tion in the one-pot chemoenzymic dynamic kinetic resolution (DKR) of secondary... 

The C2 - H oxidative addition of imidazolium salts to metal complexes was recently proved for metals other than low valent group 10. The reaction of a ferrocenyl-bisimidazolium salt to [IrCl(cod)]2 in the presence of NEt3 provided the first evidence of the preparation of a stable NHC-Ir(III)-H complex by direct oxidative addition of the imidazolium salt [96]. It was proposed that the ferrocenyl fragment may be sterically protecting the M - H from further reductive elimination, but later it was shown that this fragment was not necessary in order to obtain the desired NHC-Ir(III)-H complexes (Scheme 43) [159]. The role of the weak base (NEt3 in this case) had to be reconsidered in order to explain the overall metallation process, and it was proposed that a mechanism as that shown in Scheme 44 may better explain the process. The oxidative addition of the C2 - H bond of the imida-... 

NHC - Ir(I) (long linker), depends on whether the oxidative addition product yields the trans (short linker) or cis (long linker) products, since only the latter would be ready to undergo the reductive elimination of HC1 (Scheme 45). The trans products are the thermodynamically favored complexes, but in the case of the ligands with long linker lengths, the cis complexes are ki-netically favored, thus providing the bis-NHC - Ir(I) reductive elimination products [159]. 

Crabtree and coworkers have recently proved that imidazolium-2-carbox-ylates can be used as efficient precursors to NHC - M complexes of Rh, Ir, Ru, and Pd, which are obtained under mild conditions, short reaction times, and very high yields [166]. The imidazolium-carboxilates can be obtained by reaction of an imidazolium salt with CO2 in the presence of KOBuf (Scheme 49). Alternatively, an imidazolium esther (prepared from reaction of an imidazolium salt with NaH and isobutyl chloroformate) can be used in the NHC transfer reaction. This method provides some of the mildest reaction condi-... 

Nolan and coworkers have described the use of their cationic Ir complex 27 as an effective catalyst for transfer hydrogenations [73]. While complex 27 was found to be effective, the analog bearing the NHC ICy was found to be su-... 

The transmetallation process was found competent for several metals - for instance, Crabtree and coworkers reported the transmetallation of [Ag(NHC)2](AgCl2) to Rh(I) and Ir(i) 155 Owing to its mild reaction conditions, this procedure for the synthesis of NHC-containing complexes has found... 

The novel complexes of N-heterocyclic carbene ligands (NHC), /ac-[Re(CO)3(N C)X] (N C = l-phenyl-3-(2-pyridyl)imidazole or 1-quinoli-nyl-3-(2-pyridyl)imidazole X = Cl or Br), were found to undergo a solvent-dependent photochemical CO dissociation under excitation to their lowest excited state. The key step of the mechanism is the formation of a photo-chemically active tricarbonyl solvato-complex, following the dissociation of the CO in trans to the strongly cr-donatingNHC moiety. The photoinduced CO dissociation of c-[Re(CO)3(NHC)L] complexes, upon excitation at 370 nm, has been proven by the change in both IR and NMR spectra, as well as by the red shift in the emission profile after photolysis. Photochemical studies suggest that Re-C bond cleaves from a MLCT-type excited state. ... 

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