Iridium Compounds in Homogeneous Hydrogenation

Abstract

Recently, we reported the new complexes [Ir(cod) LL’]PF6 la (\( \mathop{1}\limits_{\sim } \), L = Pi-Pr3, \( \mathop{a}\limits_{\sim } \); PCy3, b. L’ = pyridine, cod = 1, 5-cyclooctadiene.), which appear to be the most active homogeneous hydrogenation catalysts yet described2. They readily reduce even tri-and tetra-substituted olefins, such as 1-methylcyclohexene or 2, 3-dimethyl-2-butene, at rates (0°, H2: 62 cmHg) of ca.4000 catalytic cycles per hour. Less hindered olefins, such as cyclohexene and 3, 3-dimethyl-1-butene are reduced somewhat more rapidly**: 4500 and 8300 cycles h-1, respectively 2. For comparison, the rates (cycles h-1) of reduction of 2, 3-dimethyl-2-butene and cyclohexene by RhCl (PPh3)3 (1:1 benzene-ethanol, 0°) are <0.1 and 70, and by RuHCl (PPh3)3 are <0.1 and 7, respectively lb, 3. While less active, the related and previously described 4a complexes [Ir (cod) L2]PF6 (\( \mathop{2}\limits_{\sim } \), L = PMePh2, \( \mathop{2}\limits_{\sim } \); PPh3, b) share many of the characteristics of the catalysts \( \mathop{1}\limits_{\sim } \), including a high activity for the reduction of polysubstituted olefins (for \( \mathop{2}\limits_{\sim } \mathop{a}\limits_{\sim } \) at 0°, rates: 2, 3-dimethyl-2-butene, 50; cyclohexene, 3800). The active catalysts are formed after (cod) has been lost from the metal coordination sphere by reduction to cyclooctane (coa).2