Iridium complexes hydride transfer

The mechanism operating in rhodium-catalyzed and iridium-catalyzed hydrogen transfer reactions involves metal hydrides as key intermediates. Complexes such as [ M(p.-C1)(L2) 2], [M(cod)L2](Bp4) (M = Rh, Ir L2 = dppp, bipy), and [RhCl(PPh3)3] are most likely to follow the well-established mechanism [44] via a metal alkoxide intermediate and elimination to generate the active hydride species, as shown in Scheme 2. 

Promise is held in MPV reactions carried out under catalytic conditions. Instead of, for example, stoichiometric amounts of aluminum as the metal ion activator, catalytic quantities of complexes of rhodium and iridium can sometimes be used to bring about the same reactions. Although the catalytic mechanisms have not been established, postulation of the usual six-membered transition state in the critical step of hydride transfer appears reasonable. The strongly basic conditions of the MPV reaction are avoided. Reductions of aryl ketones (69 equation 30) using (excess) isopropyl alcohol as hydrogen donor and at partial conversions have led to the formation of alcohol (70) in modest enantiomeric excesses with various chiral ligands. " ... 

Asymmetric reductions of aiylalkylketones by hydride transfer from i-PrOH are catalyzed by iridium complexes, and the most efficient chiral ligands are bis-oxazolines 3.57 (R = i-Pr) and pyridine derivatives 3.63 and 3.64 [339, 873,942],... 

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). 

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