Iridium, tetrakis catalyst

Iridium-Phosphinooxazoline Catalysts. Asymmetric hydrogenation of simple olefins with chiral Rh or Ru catalysts generally shows low reactivity and unsatisfactory enantioselectivity (198,248,249). However, several unfunctionalized olefins can be hydrogenated in high yields and excellent enantioselectivity by using iridium catalysts with chiral phosphinooxazoline ligands (60,186,187, 189,191-194,250) (Fig. 56). To avoid catalyst deactivation, the extremely weakly coordinating anion tetrakis[3,5-bis(trifiuoromethyl)phenyl]borate has to be used (182,251). 

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). 

Many organometallic catalysts and especially many chiral catalysts are cationic, and modification of the anion has been found to be very effective for enhancing their solubility in SCCO2 without the requirement for ligand alteration. Tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (BARF) was an early example of such an anion which has proven extremely useful for this purpose [28], and very pronounced anion effects on the activity and selectivity of the catalysts are observed in many other cases [27, 29]. Using the BARF-modified "C02-philic chiral iridium catalyst... 

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