Hydride iridium

A stoichiometric reaction, leading to a similar product distribution, lends support to a C-H activation process, giving rise to an iridium hydride intermediate (Equation (41)). 

The reaction is thought to proceed via an iridium hydride, with the olefin group acting as a directing group. Metallacycle intermediates have also been implicated in this reaction (Scheme 22).96... 

Iridium hydride complexes effectively catalyze addition of nitriles or 1,3-dicarbonyl compounds (pronucleophiles) to the C=N triple bonds of nitriles to afford enamines.42S,42Sa Highly chemoselective activation of both the a-C-H bonds and the C=N triple bonds of nitriles has been observed (Equation (72)). To activate simple alkane dinitriles, IrHs(P1Pr3)2 has proved to be more effective (Equation (73)). The reaction likely proceeds through oxidative addition of the a-C-H bonds of pronucleophiles to iridium followed by selective insertion of the CN triple bonds to the Ir-C bond. 

One might ask the question why a reaction involving such a small dihydrogen molecule can lead to such enormous differences in rate for the diastereomeric alkene adducts present (major and minor). Tentative answers were developed by Brown, Burk and Landis [9], Their studies included the use of iridium instead of rhodium since the iridium hydride intermediates can be studied spectroscopically. Consider the oxidative addition in Figure 4.10. 

Pincer complexes catalyze a variety of other organic reactions [49-51]. Hence, this work is currently being extended to other metals, and other more readily accessible PCP systems. For example, as shown in Scheme 3, lO-Rfs can be converted to the iridium hydride chloride complex 15-Rfs. Closely related dihydride complexes catalyze dehydrogenations of alkanes at high temperatures [52], However, no efforts to develop recoverable catalysts have been reported to date. 

The same authors further developed the catalyhc system described above, wherein the chiral Ir catalytic system generated in situ from the iridium hydride complex [IrH(cod)Cl2]2 and chiral diaminodiphosphine ligand 83 was employed in the ATH of aromahc ketones PhC(0)R (R = Me, Et, Pr, cy, Bu, etc.) to produce... 

Although the precise mechanism has not yet been clarified, a possible mechanism is shown in Scheme 5.2. First, the iridium alkoxide 3 is produced from 1 and an alcohol, this step being stimulated by a base (K2CO3). A ]3-hydride elimination of 3 then yields a carbonyl product and the iridium hydride 4. The insertion of acetone into the iridium-hydride bond in 4, giving metal isopropoxide 5, is followed by exchange of the alkoxy moiety to regenerate 3. 

A possible mechanism for the N-alkylation of primary amines is shown in Scheme 5.21. The first step of the reaction involves the oxidation of an alcohol to a carbonyl intermediate, accompanied by the generation of an iridium hydride. 

The carbonyl intermediate then reacts readily with a primary amine to afford an imine and water. A subsequent addition of the iridium hydride to the C=N double bond of the imine, followed by amide-alkoxide exchange, would then occur to release the product. 

Hydrogen transfer oxidation of an alcohol to give an aldehyde and an iridium hydride. 

Transfer hydrogenation of the arylacrylonitrile by the iridium hydride to give the product. 

The dimerization of functional alkenes such as acrylates and acrylonitrile represents an attractive route to obtain bifunctional compounds such as dicarboxylates and diamine, respectively. The head-to-tail dimerizahon of acrylates and vinyl ketones was catalyzed by an iridium hydride complex generated in situ from [IrCl(cod)]2 and alcohols in the presence of P(OMe)3 and Na2C03 [26]. The reaction of butyl acrylate 51 in the presence of [IrCl(cod)]2 in 1-butanol led to a head-to-tail dimer, 2-methyl-2-pentenedioic acid dibutyl ester (53%), along with butyl propionate (35%) which is formed by hydrogen transfer from 1-butanol. In order to avoid... 

The reaction may proceed as follows (see Scheme 10.4). An acrylate 51 coordinates to the iridium-hydride complex generated in situ, and then inserts into the Ir-H bond to form a o-lr complex 55 the coordination and insertion of another acrylate to 55 leads to an iridium complex, 56. A (5-hydride elimina-hon of the iridium-hydride from the intermediate 56, followed by isomerization of the double bond to a more stable internal aUcene, results in a head-to-tail dimer. 

The use of CO2 as a reagent for synthetic purposes would be highly desirable, due not only to the vast availabiUty of this gas but also its environmental concerns. The stoichiometric activation of CO2 has been achieved with the iridium-PCP complex 29 comprising an alkyl rather than an aryl skeleton (Scheme 12.12) [32]. The addition of CO2 to the dihydride complex results in C=0 insertion into the iridium-hydride bond, and affords the formate complex 30. However, this complex is not stable and disproportionates spontaneously into the virtually insoluble bicarbonate complex 31 and the carbonyl dihydride 32. Such disproportionation is suppressed when the iridium metal center is replaced by rhodium [33], which is generally assumed to have a lower hydride affinity than iridium. 

TABLE 4.3. Strength of Intramolecular Dihydrogen Bonding in Iridium Hydride Complexes (Structure 4.3) as a Function of the Nature of the frans-Ligand Y... 

The x-ray structure obtained for one of two possible isomers of the cationic iridium hydride complex [Structure 5.9(a)] exhibited two short N-H- -H-lr contacts of 1.75 0.05 A. These distances are significantly smaller than the sum... 

This enhancement could be attributed to an increase in the nucleophiUcity of the iridium-hydride intermediate, due to the good electron donor abihty of this type of hgand, which leads to the acceleration of the hydride transfer to acetone as the hydrogen acceptor. 

Screening several amine racemization catalysts, we found that the SCRAM and the Shvo catalyst would both racemize the (S)-enantiomer at temperatures above 11() G. Interestingly, no dimeric products were found. The best racemization conditions were found to be using toluene or TBME at 150°C in a pressure vessel with 1 mol% SCRAM or 5 mol% Shvo catalyst over 24 h, providing quantitative conversion. In the presence of (R, R)-dibenzoyltartaric acid the racemization slowed, possibly because of unfavorable coordination of the alkylammonium substrate or acid quenching of the iridium hydride catalyst intermediate. 

The studies that were conducted by other workers are reviewed, and our previously published work on ruthenium and iridium hydrides is presented. This... 

To determine the nature of the photoactive excited state in these hydride complexes, we examined the electronic absorption spectra of a series of iridium hydride complexes. The spectra showed only a few shoulders on a rising absorption into the uv, and no definitive excited state assignments could be made. 

If ethylene is present during the carbonylation of methanol catalyzed by IrCl4, once again with Mel as promoter, methyl propionate is formed.416 The reaction depends on the presence of iridium hydride species in solution, and a rhodium analogue of the reaction exists. The full details of the mechanism are not known but the basic steps are shown in Scheme 34. The intermediates are all believed to be complexes of iridium(IIl). 

Iridium-catalyzed reductive coupling of acrilates and imines has been reported to provide trans (3-lactams with high diastereoselection [142], The use of electron-deficient aryl acrylates resulted in improved product yields. The mechanism, proposed by the authors, started from an in situ generated iridium hydride reacting with the acrilate to provide an iridium enolate that, then, reacted with the imine to give a (3-amido ester. Subsequent cyclization furnished the p-lactam and an iridium alcoxide. 

Homogeneous catalytic hydrogenation of imines has been carried out using cationic iridium hydride catalysts.50 The mechanistic possibilities are compared and contrasted with C=0 hydrogenations. 

Binding energy, pentacarbonyliron, 6, 3 Binuclear complexes bis-Cp titanium halides, 4, 522 with Ni-M and Ni-C cr-bonds heterometallic clusters, 8, 115 homometallic clusters, 8, 111 Binuclear dicarbonyl(cyclopentadienyl)hydridoiron complexes, with rand C5 ligands, 6, 178 Binuclear iridium hydrides, characteristics, 7, 410 Binuclear monoindenyl complexes, with Ti(IV), 4, 397 Binuclear nickel(I) carbonyl complexes, characteristics, 8, 13 Binuclear osmium compounds, with hydrocarbon bridges without M-M bonds, 6, 619... 

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