Hydrogen iodide, deactivation

It is apparent that substitution of fluorine for hydrogen initially causes a reduction in reactivity towards the electrophilic oxy n atom, but tetra-fluoroethylene is anomalous. A further study has indicated that substitution by trifluoromethyl has a strong deactivating effect compared with methyl, which has an activating effect. A study of the reactions of nearly thermal i F atoms, produced by F(n,2n) F and moderated by collisions with an excess of sulphur hexafluoride, with fluoro-oleflns (modes of addition were identified by scavenging the radicals produced with hydrogen iodide) has indicated that F atoms react preferentially with less-fluorinated olefins, and at the less-fluorinated end of a particular olefin. ... 

Only a few detailed studies of the reaction mechanism of the homogeneous hydrogenation of imines have been published until now. A generalization seems to be very difficult for two reasons. First, rather different catalyst types are effective and probably act by different mechanisms. Second, the effect of certain additives (especially iodide or iodine and acid/base) is often decisive for ee and rate, but a promoter in one case can be a deactivator in another case. 

The next breakthrough was obtained when iridium was used instead of rhodium. This idea was inspired by results from Crabtree who had described an extraordinarily active Ir-tricyclohexylphosphine-pyridine catalyst that was able to hydrogenate even tetra-substituted C=C bonds. For the MEA imine hydrogenation very good ee values were obtained with an Ir-bdpp catalyst in the presence of iodide ions (ee 84% at 0°C) but the activity was disappointing. Turnover numbers (ton) of up to 10000 and tof numbers of 250/h (100 bar and 25 °C) but somewhat lower ee values were obtained with Ir-diop-iodide catalysts [10, 11], A major problem with these new Ir-diphosphine catalysts was an irreversible catalyst deactivation. 

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