Enantioselective hydrogenation of alkenes with iridium-PHOX catalysts: a kinetic study of anion effects.
In the asymmetric hydrogenation of unfunctionalized olefins with cationic iridium-PHOX catalysts, the reaction kinetics and, as a consequence, catalyst activity and productivity depend heavily on the counterion. A strong decrease in the reaction rate is observed in the series [Al[OC(CF3)3]4]- >BArF- >[B(C6F5)4]- >PF6- >>BF4- >CF3SO3-. With the first two anions, high rates, turnover frequencies (TOF >5000 h(-1) at 4 degrees C), and turnover numbers (TONs) of 2000-5000 are routinely achieved. The hexafluorophosphate salt reacts with lower rates, although they are still respectable; however, this salt suffers from deactivation during the reaction and extreme water-sensitivity, especially at low catalyst loading. Triflate and tetrafluoroborate almost completely inhibit the catalyst. In contrast to the hexafluorophosphate salt, catalysts with [Al[OC(CF3)3]4]-, BArF-, and [B(C6F5)4]- as counterions do not lose activity during the reaction and remain active, even after all the substrate has been consumed. In addition they are much less sensitive to moisture and, in general, rigorous exclusion of water and oxygen is not necessary. A first-order rate dependence on the hydrogen pressure was determined for the BArF- and the PF6- salts. At low catalyst loading, the rate dependence on catalyst concentration was also first order. The rate dependence on the alkene concentration was strikingly different for the two salts. While the reaction rate observed for the BArF- salt slightly decreased with increasing alkene concentration (rate order -0.2), a rate order of approximately 1 was determined for the corresponding hexafluorophosphate at low alkene concentrations.