Tetrakis borate, reaction with

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). 

High catalytic activities have been achieved by the PYRPHOS- [18], PPCP [20], BICHEP- [21], Et-DuPHOS-Rh [19] complexes among others, allowing the reaction with a substrate-to-catalyst molar ratios (S/C) as high as 50,000. With a [2.2]PHANEPHOS-Rh complex, the reaction proceeds even at -45°C [27], Supercritical carbon dioxide, a unique reaction medium, can be used in the DuPHOS and BPE-Rh-catalyzed hydrogenation [43], A highly lipophilic counteranion such as tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BARF) or trifluoromethanesulfonate is used to enhance the solubility of the cationic Rh complexes. Under the most suitable reaction conditions of 102 atm of carbon dioxide, 1 atm of hydrogen, and 22°C, a-amino acid derivatives are produced with up to 99.7% ee. 

Benzylic silyl ethers couple with allylsilanes in the presence of trityl tetrakis [3,5-bis(trifluoromethyl)phenyl]borate (TFPB) catalyst leading to carbon-carbon bond formation (equation 77). The corresponding Z11CI2-catalyzed reactions with an allyl silyl ether lead to a mixture of regioisomers145,146. 

Komatsu et al. have reported the first cyclic 7t-conjugated silatropylium ion by hydride removal from 184 on reaction with triphenylmethyl tetrakis(pentafluorophenyl)borate in dichloromethane at -50 °C for 0.5 hours. The cation was characterized by H, 13C and 29Si NMR spectroscopy, with the observed chemical shifts suggestive of a degree of aromaticity not greatly less than that of the tropylium ion. The precursor 184 was made from 180 via the lithiated derivative 181, the dichloride 182, and the mesityl derivative 183 [01T3645], An extensive report on the synthesis and chemistry of enantiomerically pure dihydrodibenzo[i/]phosphine 5-oxides (e.g. 185) has been published by Wyatt et al. [01JCS(P 1)279],... 

The (N,O)-NiR(donor) complexes could effectively be activated by reaction with the triphenyl rrielhyliurti tetrakis(pentafluorophenyl)borate. With other reagents such as (COD)2Ni, N.N-dimethylaitihitium tetrakis(pentafluorophenyl)borate or MAO only traces of polymer were obtained. With tris(pentafluorophenyl)borane an activity below 50 g PE (g Ni h) was observed (experiments in toluene at 20 °C, 20 bar ethene 5 h). 

The cyclotetrasilenylium ion with the tetrakis(2,3,5,6-tetrafluorophenyhborate counter anion could also be synthesized by reaction with [EtaSiibenzene)]" TTFPB (TTFPB = tetrakis(2, 3,5,6-tetrafluorophenyl)borate) in toluene. Based on the chemistry of the lighter allyl cation congeners, the allylic character of the cyclotetrasilenylium ion may lead to interesting reactivity. 

Experience in PTC with cationic catalysts showed that, in general, the most suitable compounds have symmetrical structures, are lipophilic, and have the active cationic charge centrally located or sterically shielded by substituents. For anionic catalysis sodium tetraphenylborate fulfills these conditions, but it is not stable under acidic conditions. However, certain derivatives of this compound, namely sodium tetra-kis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB, 12.162) and sodium tetrakis[3,5-bis-(l,l,l,3,3,3-hexafluoro-2-methoxy-2-propyl)phenyl]borate (HFPB) are sufficiently stable to be used as PTC catalysts for azo coupling reactions (Iwamoto et al., 1983b 1984 Nishida et al., 1984). These fluorinated tetraphenylborates were found to catalyze strongly azo coupling reactions, some of which were carried out with the corresponding diazotization in situ. 

An unprecedented carbene insertion reaction was observed on reaction of the cationic re-arene ruthenium amidinates with trimethylsilyldiazo-methane (Scheme 145, TFPB = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate). 

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