BARF anion

Scheme 8.4. Catalytic hydrogenation of imines using cationic iridium catalysts with the C02-philic BARF anion...

Barex 210 resin, l 450t Barex 218 resin, l 450t Barex resins, 1 449-452 properties of, l 450t BArF anions, 16 95 bar gene, 13 360, 36 It Barge transport, 25 327 Barite, 3 343, 351, 352-355 17 691... 

An iridium(I) complex with the l,2-bis(tcrt-butylmethylphosphino)ethane (4) and tetrakis(3,5-bis(trifluoromethyl)phenyl)borate as the counter anion catalyzes the hydrogenation of several acyclic aromatic Ai-arylimines under atmospheric hydrogen pressure at room temperature, giving the desired chiral amines with high-to-excellent enantioselectivities (up to 99%, Fig. 6) [19]. The authors also tested (S )-BINAP (Fig. 1) and (/ )-Ph-PHOX (PHOX = 2-[2-(diphenylphosphino) phenyl]-4,5-dihydrooxazole) hgands with lower enantioselectivities [19]. Both steric and electronic properties of the ligand and the combination with the BArF anion are in the base of the efficacy of this catalytic system. On the other hand, attempted hydrogenations of Ai-(2,2,2-trifluoro-l-phenylethylidene)aniline and M-(l,2,2-trimethyl-propylidene)aniline under the same conditions resulted in... 

The surface is initially covered by the BARF(-) anion and by an anilinium(+) cation and a floating cation is grafted here the coordination of the floating cation to surface oxygens should be avoided. Indeed results indicate the complexity of the surface. 

This occurs because the Si-H bond in free silanes is already polarized in the sense Si8+ Hs and coordination to an electrophilic M increases the positive charge on Si. This favors its effective elimination as a silylium cation, R3Si +, a powerful electrophile that can abstract OH from trace water to give R3SiOH and also extract fluoride from counteranions such as SbF6 [Eq. (28)] and even the BArf anion (104). As initially proposed by Crabtree (103) and supported by calculations discussed below, the cleavage in most cases is likely to be a concerted process, i.e., the nucleophile attacks the bound Si [Eq. (29)]. 

Although greater solubility in SCCO2 was observed with the borate (BArF) anion catalyst precursor than the triflate precursor, the more soluble precursor did not always give the greatest enantioselectivity. This article (60) was the first demonstration that asymmetric homogeneous catalysis could be performed in SCFs and the first demonstration that the technique could lead to greater enantioselectivity than that obtained in traditional solvents. 

Imines can also be hydrogenated enantioselectively in SCCO2. Leitner s group (111) reported that a cationic iridium complex with the BARF anion... 

Li and co-workers investigated the counter anion effect on the ability of a secondary ammonium cation to form host-guest complexes with per-ethylated pillar[5]arene (H5.3). The K value of the complex between the secondary ammonium cation with a hexafluorophosphate anion (G5.20) and H5.3 was 61 8 (run 20). In contrast, for the secondaiy ammonium cations with a tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF) anion (G5.21), the value was (3.4 0.4) X10 M (mn 21), which is 560 times larger than diat obtained for the hexafluorophosphate anion. The weakly coordinating BArF anion gave the free naked secondary ammonium cation therefore, the free secondary ammonium cation was very strongly included in the pillar[5]arene cavity. 

In complex salts, counterion choice is important to prevent undesired reactions. BF4, although useful, can form a B-F-M bridge or undergo F abstraction to give an M-F complex, particularly with metals. PFe is less reactive but can still give problems. BPh4 can form ri -PhBPhs complexes. The barf anion (4.25), one of our very best noncoordinating anions, ° permits isolation of such electro-... 

Fig. 7.24 Comparison of the likely active catalyst in the Pt(bpym)Cl2/H2S04 system with the weakly coordinating BARF anion.

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