Homogeneous hydrogenation iridium

In contrast to the generally accepted mechanism for the homogeneous hydrogenation of C=C double bonds catalysed by rhodium or iridium complexes, which is assumed to occur through M(III)/M(I) species, the postulated cycle for the hydrogenation of imines involves only Ir(III) species. Many aspects remain unclear for example the simple cycle neither explains the origin of the enantioselectivity nor the effect of acids and iodide used as promoters. 

The complex has enjoyed relatively little use in organic synthesis. For iridium-catalyzed homogeneous hydrogenation of alkenes, Crabtree s iridium complex ((1,5-Cycloocta-diene)(tricyclohexylphosphine)(pyridine)iridium(I) Hexafluoro-phosphate) is generally preferred, although this readily prepared Ir complex is active. It is more reactive than its rhodium counterpart in the catalytic isomerization of butenyl- to allylsilanes. ... 

Trihydridobis(triphenylphosphine)iridium (III), IrH3[P(C6H5)3]2 [1, 1228, before Triisopinocampheyldiborane]. Mol. wt. 731.79, m.p, 184-185°, dec. Preparation,1J Hydrogenation catalyst.3 This complex is an active catalyst for homogeneous hydrogenation of aldehydes (acetic acid is necessary). 

Complexes of the type [Ir(bzn)(cod)(L)](C104) (bzn = benzonitrile L = tricyclohexylphosphine, neomenthyldiphenylphosphine) catalyze the homogeneous hydrogenation of tetrasubstituted prochiral amido alkenes R R C=CR N. Under very mild conditions, the catalysis occurs for N = NHCOR, R = COjMe R = R = Me, R" = Me, Ph R = Me, R- = Ph, R = Me, Ph R = Ph, R = Me, R = Ph. [Ir(cod)(PCy3)(py)](PFg) serves as a catalyst in the hydroxyl-directed hydrogenation of cyclic and acyclic alkenic alcohols, wherein the reaction shows diastereoselectivity dependent on catalyst substrate stoichiometry. Park el have noted that the iridium(I)... 

M. Bikrani, L. Fidalgo, M. A. Garralda, and C. Ubide [J. Mol. Catal., 118, 47-53 (1997)] studied the catalytic activity of cationic iridium complexes for homogeneous hydrogen transfer from isopropanol to 2-cyclohexen-l-one to give cyclohex-anol. The reaction network contains consecutive, competitive, and reversible elements. In isopropanol these elements can be expressed as pseudo first-order processes ... 

In contrast to RhCl(PPh3)g, which is an effective catalyst, IrCl(PPh3)3 does not catalyse homogeneous hydrogenation. The explanation for this difference is the much greater reluctance of the iridium compound to lose a molecule of phosphine to generate a catalytically active species. ... 

Cationic iridium(I) complex Stereospecific homogeneous hydrogenation... 

Table 15.2. A comparison of turnover frequencies for the homogeneous hydrogenation of different alkenes catalyzed by rhodium, ruthenium, and iridium catalysts.

Cationic iridium and rhodium complexes of the formula [M(COD)(PPh3)2]+ (M = Ir, Rh) are catalyst precursors for the homogeneous hydrogenation of ben-zothiophene (BT) to 2,3-dihydrobenzothiophene (DHBT). These catalyst precursors react with BT and hydrogen to produce [MH2(j (S)-BT)2(PPh3)2]+, which then forms [MH2( f(C=C)-BT)(PPh3)2], followed by hydride transfer to the C2=Cs bond. Displacement of the hydrogenated dihydrobenzothiophene by BT restarts the cycle (55). 

Iridium. Whereas RhCl(PPh3)3 is a good catalyst for homogeneous hydrogenation, its iridium analogue IrCl(PPh3)3 is usually very much less... 

Semeniuchenko V, Khilya V, Groth U (2009) Nucleophilic homogeneous hydrogenation by iridium complexes. Synlett 2009 271-275... 

Many other complexes of rullienium, iridium, and other metals have also been used as homogeneous hydrogenation catalysts. The mechanisms with different precatalysts may differ substantially. Indeed, even between the mechanisms of reactions 5.1.1 and 5.1.2, there are some differences. 

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