Iridium hydride intermediates

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)). 

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. 

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. 

Although phosphinite- and NHC-based iridium catalysts show very similar enantiose-lectivities in the hydrogenation of various olefins, replacement of the phosphinite group by an A-heterocyclic carbene (NHC) unit results in particularly effective catalysts which are much better suited for the hydrogenation of acid-sensitive substrates beeause of the lower acidity of iridium hydride intermediates produced. The new NHC-pyridine ligands are also likely to prove useful for other applications in asymmetric catalysis." ... 

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... 

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. 

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. 

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. 

Neutral formyl complexes which contain ligating CO often decompose by decarbonylation however, several exceptions exist. For instance, the osmium formyl hydride Os(H)(CO)2(PPh3)2(CHO) evolves H2(54). Although the data are preliminary, the cationic iridium formyl hydride 49 [Eq. (14)] may also decompose by H2 evolution (67). These reactions have some precedent in earlier studies by Norton (87), who obtained evidence for rapid alkane elimination from osmium acyl hydride intermediates Os(H)(CO)3(L)(COR) [L = PPh3, P(C2H5)3], Additional neutral formyls which do not give detectable metal hydride decomposition products have been noted (57, 65) however, in certain cases this can be attributed to the instability of the anticipated hydride under the reaction conditions (H2 loss or reaction with halogenated solvents). 

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). 

Review "cis-Akyl and cis-Aoylrhodium and Iridium Hydrides Model Intermediates in Homogeneous Catalysis"... 

Studies of hydrogen-deuterium exchange reactions of some of these iridium hydrides show that two mechanisms are apparently exhibited, one possibly involving seven- or eight-coordinate intermediates. 

Intensive studies also showed that many transition metal complexes are able to catalyze aromatic C-H borylation of various arenes (Scheme 7), e.g., Cp Ir(H)(Bpin)(PMe3) [64,65], Cp Rh(Ti4-C6Me6) [65,66], ( 75-Ind)Ir(COD) [67], (776-mesitylene)Ir(Bpin)3 [67], [IrX(COD)]2/bpy (X = Cl, OH, OMe, OPh) [68-70]. A very recent study by Marder and his coworkers showed that [Ir(OMe)(COD)]2 can also catalyze borylation of C-H bonds in N-containing heterocycles [71]. For the Ir-catalyzed borylation reactions, it is now believed that tris(boryl)iridium(III) complexes [67,69], 40c, [72] are likely the reactive intermediates and a mechanism involving an Ir(III)-Ir(V) catalytic cycle is operative [67,69]. A recent theoretical study [73] provided further support for this hypothesis. A mechanism, shown in Scheme 8, was proposed. Interestingly, there are no cr-borane complexes involved in the Ir-catalyzed reactions. The very electropositive boryl and hydride ligands may play important roles in stabilizing the iridium(V) intermediates. 

Like complex XXXVII, the phosphine complex XXXIX reacts with 2,3-dimethyl-l,3-butadiene to give the expected complex XL. Hydridodiene complexes are intermediates. Using an unreactive diene, 2,3-dimethylbutadiene and (Ph3P)2Co(N2)H, one such complex can be isolated, XLI. Rhodium and iridium hydrides also react with dienes. 

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