Initial Studies An Unexpected Anion Effect

Attempts to increase conversion by variation of the solvent, hydrogen pressure, or the catalyst and substrate concentration were unsuccessful. Coordinating solvents and additives such as amines, or coordinating anions such as halides, carboxylates. 

After extensive experimentation, a simple solution for avoiding catalyst deactivation was discovered, when testing an Ir-PHOX catalyst with tetrakis[3,5-bis (trifluoromethyl)phenyl]borate (BArp ) as counterion [5]. Iridium complexes with this bulky, apolar, and extremely weakly coordinating anion [18] did not suffer from deactivation, and full conversion could be routinely obtained with catalyst loadings as low as 0.02 mol% [19]. In addition, the BArp salts proved to be much less sensitive to moisture than the corresponding hexafluorophosphates. Tetrakis (pentafluorophenyl)borate and tetrakis(perfluoro-tert-butoxy)aluminate were equally effective with very high turnover frequency, whereas catalysts with hexafluorophosphate and tetrafluoroborate gave only low conversion while reactions with triflate were completely ineffective (Fig. 1). 

How can these bulky, extremely weakly coordinating anions prevent catalyst deactivation A comparative kinetic study of catalysts with different anions provided a plausible answer [19]. With PFg as a counterion, the rate dependence on olefin concentration was first order, whereas the rate order observed for the corresponding BArp complex was close to zero. This striking difference may be explained by the stronger coordination of PFs or formation of a tight anion pair, which slows down the addition of the olefin to the catalyst to such an extent that it becomes rate-limiting. In contrast, the essentially noncoordinating BArp ion does not interfere with olefin coordination, and the catalyst remains saturated with olefin even at low substrate concentration. The slower reaction of the PFg salt with the olefin could 

Virtually every iridium catalyst of the formula [Ir(L )(COD)] [X] for asymmetric olefin hydrogenation that has appeared after the initial counterion effect studies was based on BArp as the preferred anion [14]. The anion effect is broadly applicable in iridium-catalyzed reductions as experiments with a direct analog of the Crabtree catalyst of the formula [Ir(pyridine)(Cy3P)(COD)]BArp indicates (Fig. 2). 

Hydrogenation of 8-terpinene (Fig. 2) proceeded in higher conversion with Crabtree s catalyst with BArp counter ion rather than the normal PFs. The BArp counter ion performed better in all instances where the more coordinating PFg salt failed to reach complete hydrogenation [20]. 

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