Iridium precursors

In general, two different kinds of iridium precursors are used. Isolated cationic [Ir(COD)2]A complexes and the in situ precursors formed from the neutral dimer [Ir(p-Cl)(COD)]2 by addition of the corresponding phosphorus ligand. 

Addition to linear 1,1-disubstituted allylic acetates is slower than addition to monosubstituted allylic esters. Additions to allylic trifluoroacetates or phosphates are faster than additions to allylic carbonates or acetates, and reactions of branched allylic esters are faster than additions to linear allylic esters. Aryl-, vinyl, alkynyl, and alkyl-substituted allylic esters readily undergo allylic substitution. Amines and stabilized enolates both react with these electrophiles in the presence of the catalyst generated from an iridium precursor and triphenylphosphite. 

Hartwig et al. demonstrated that the same combination of iridium precursor and phosphoramidite LI also catalyzes allylic etherifications (Scheme 9) [68]. Lithium and sodium aryloxides were shown to react with cinnamyl and hex-2-enyl carbonates to form the branched allylic ethers in high yield, with high branched-to-linear... 

Catalyhc systems based on the commonly used iridium precursor [ Ir(g-Cl)(cod) 2] and diferrocenyl dihalcogenides of L4 and L5 type were also studied in the asymmetric hydrosilylahon of acetophenone, giving a relahvely high yield of sec-phenetyl alcohol silyl ether (I) and a moderate ee of one stereoisomer [49]. 

Iridium siloxide complexes show a similar activity. Catalytic tests performed in the presence of [ Ir( 4-OSiMe3)(cod) 2], with the use of trimethylvinylsilane and dimethylphenylsilane as reactants [59], gave the same type of silicon derivatives as those obtained by Murai and coworkers [58], but the siloxide iridium precursor used appeared to be more efficient under milder conditions. When the [Ir(cod)(PCy3)(OSiMe3)] was used rather than the binuclear iridium siloxide complex, Z-Me3SiCH2CH=CHOSiMe2Ph was obtained exclusively [59],... 

In both cases 3C NMR showed the carbonyls attached to the rhodium or iridium precursor exhibited an important high field shift (31) upon CH.I addition, indicative of CO coordinated to high oxidation state cation. Furthermore it was shown that such carbonyl carbons were highly electron deficient thus particularly suited for a nucleophilic attack by species such as alkyls etc. 

Table 2 Some commercially available compounds of iridium precursors to inorganic...

From the other directions, there have been a number of studies of the cluster species that are formed when mononuclear iridium precursors are deposited onto inorganic supports and then subjected to carbon monoxide pressures. Gates and coworkers have shown that Ir(CO)2(acac) will form higher nuclearity iridium carbonyl clusters, the exact nature of which depends on the substrate and the carbonylation conditions. For zeolite NaY, they have observed that Ir(CO)2(acac) will yield both It4(CO)i2 (45) and fr6(CO)i6... 

Angelici and coworkers have demonstrated that coordination of thiophenes to an IrCp fragment (Cp = pentamethylcyclopentadienyl) in the mode leads to enhanced reactivity of the heterocycle as a donor. The monoiridium complex [Cp Ir(// -2,5-Me2C4H2S)] (33) reacts with triply-bonded [MoCp(00)2)2 to afford a cluster (34) in which the Mo-Mo bond is bridged by the thiophene sulfur.l l The iridium moiety remains attached to the thiophene in the same fashion as in the starting material. Reaction of the same iridium precursor with [Fc2(CO)9] or [Fe2(CO)8] also gives a product (35) with a fi2-S Fe2) thiophene. A... 

Three classes of catalysts have been studied for the asymmetric hydrogenation of imines. One class of catalyst is generated from late transition metal precursors and bisphosphines. These catalysts have typically been generated from rhodium and iridium precursors. A second class of catalyst is based on the chiral titanocene and zirconocene systems presented in the previous section on the asymmetric hydrogenation of unfunctionalized olefins. The third class of catalyst is used for the transfer hydrogenation of imines and consists of ruthenium or rhodium complexes containing diamine, amino tosylamide, or amino alcohol ligands. " ... 

In Albertin s system, the reaction follows different pathways depending on the nature of both the iridium precursor and the alkyne. Thus, the reaction of 347 with HC=CC(CH3)3 in the presence of H2O does not promote C-C bond breakage, but results in the formation of the acyl complex [ItCl2 -C(0)CH2CBu (PPh3)2] 62. Thermolysis of 62 in... 

A significant contribution to the use of iridium precursors for allylic alkylations has been provided by Takeuchi and co-workers, who demonstrated how the selectivity achieved by using iridium catalysts complements that obtained with palladium complexes. Fast combinatorial colorimetric screening has been used to individuate Ir(l) catalysts active for the allylic substitution reaction. Fundamental advancements in this field were achieved by Helmchen and co-workers who obtained high regio- and enantioselectivity in asymmetric allylic alkylations of achiral or racemic substrates with chiral phosphinooxazolines and phosphoramidites as... 

Iridium monohydrides may be prepared by the reaction of suitable iridium precursors with dihydrogcn via heterolytic splitting mediated intra- or intermolecularly by an external base or by oxidative addition of hydrogenated substrates (HX) to a low-valent iridium complex. Nucleophilic attack by hydride anion and / -elimination from coordinated ligand may also result in the formation of Ir-H bonds. 

The formation of iridium hydrides is a common feature in the many processes resulting in the C-H bond activation that are promoted by coordinatively unsaturated iridium precursors. A few examples are illustrated below, while other processes featuring C-H bond breakage of an organic compound arc mentioned in Section 3 of this review. 

The neutral lr(v) tetrahydride [Tp IrflJ 1000 has been prepared by hydrogenation of different pyrazolylborate iridium precursors. Reaction of [Tp Ir(C2H4)2] 370 with H2 affords the highest yield of 1000. Density functional calculations on the related and simpler [TpIrH4] complex predict that there are two minimum energy... 

Therefore, the reduction extent can be easily controlled by determining the amount of produced HCl this is done by a down-stream trap containing a basic solution after the reduction, the rest of basic solution is titrated key parameters are flow rate and composition of hydrogen/helium mixture to avoid redox decomposition of the precursor and formation of chlorine as it could be the case for platinum or iridium precursors H2PtCl6orH2lrCl6. 

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