Pentafluorophenyl borate

The authors conducted a similar investigation of precatalysts 7 and 11 using TiBA and trityl tetrakis(pentafluorophenyl)borate as the cocatalyst. They concluded that this material contained no fraction that could be characterized as blocky. It was therefore proposed that reversible chain transfer occurred only with MAO or TMA and not with TiBA. This stands in contrast to the work of Chien et al. [20] and Przybyla and Fink [22] (vida supra), who claim reversible chain transfer with TiBA in similar catalyst systems. Lieber and Brintzinger also investigated a mixture of isospecific 11 and syndiospecific 12 in attempts to prepare iPP/sPP block copolymers. Extraction of such similar polymers was acknowledged to be difficult and even preparative temperature rising elution fractionation (TREF) [26, 27] was only partially successful. 

Rieger et al. described a heteroatom-containing C, symmetric metallocene 13 whose stereoselectivity depended on the activator [28, 29], The resulting PP contained fewer stereoerrors when activated with a combination of TiBA and trityl tetrakis(pentafluorophenyl)borate than with MAO. In addition, the molecular weight was lower with MAO. To explain this, it was proposed that some of the stereoerrors arise by reversible chain transfer to aluminum. 

Solution-phase DPV of Au144-C6S dispersed in 10 mM [bis(triphenylpho-sphoranylidene)-ammoniumtetrakis-(pentafluorophenyl)-borate (BTPPATPFB)/ toluene] [acetonitrile] 2 1 revealed well-behaved, equally spaced and symmetric quantized double-layer charging peaks with AE - 0.270 0.010 V. Applying the classical concentric spheres capacitor model (8) reveals an individual cluster capacitance of 0.6 aF [334, 335]. 

Indeed, a variety of Lewis acids have been shown to effed glycosylation with hemiacetal donors. Ernst and coworkers have used 5mol% of [Rh(III)(MeCN)3 (triphos)] tris(triflate) with 4 A molecular sieves to prepare glycoconjugates 18 and 19 [26]. Mukaiyama s group has used trityl tetrakis(pentafluorophenyl)borate (3-5 mol%) with Drierite in the preparation of disaccharides 20 and 21 [27,28]. In the synthesis of 21, the a-seledivity was shown to arise from in situ anomerization of the (1-pyranoside over time. 

Tetrakis(pentafluorophenyl)borate (TPFB), in silicone network preparation,... 

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). 

In 1998 the groups of Jprgensen and Helmchen reported the preparation of the chiral silyl cationic salt 2 (Scheme 3) [30]. This was the first time that a chiral silyl cation was used as a catalyst in an enantioselective reaction, hi order to ensure that the silyl salt had a high reactivity, the almost chemically inert and non-coordinating anions tetrakis[pentafluorophenyl]borate [TPFPB] and tetrakis[3,5-bis (trifluor-omethyl)phenyl]borate [TFPB] were chosen as counter anions. 

Since silyl cations are highly reactive and moisture sensitive, the salts (S)-2a and (S)-2b were prepared in situ from the air and moisture stable precursor (S)-5 via a hydride transfer [34, 35] with trityl tetrakis[3,5-bis(trifluoromethyl)phenyl]borate [Tr][TFPB] or trityl tetrakis[pentafluorophenyl]borate [Tr][TPFPB], The authors showed by Si-NMR studies that the desired salts were formed. The silyl salt (5)-2a was then tested in the Diels-Alder reaction as shown in Scheme 5. A good reactivity was found, and the product was obtained in 95% yield with higher than 95% endo selectivity at -40 °C in 1 h. However, only 10% ee was achieved. 

The catalytic activity of the oxoisoindolium salt 54 and 55 was compared to that of trityl tetrakis[pentafluorophenyl]borate salts in the addition reaction of enol acetate to benzaldehyde and glycosylation reaction (Scheme 59) [151, 152]. 

The utilization of compound 54 in the aldolization showed higher yield of the product (92%) after 30 min, compared to that (73%) of a trityl catalyzed reaction. The similar results were obtained in the glycosylation reaction 85% (o/p ratio 9 91) and 72% (cx/p ratio 10 90) respectively. The application of the highly hindered tetrakis[pentafluorophenyl]borate anion is remarkably advantageous for the stabilization of the positive charge in the carbocation 54 and at the same time promotion of its accessibility to the interaction with a carbonyl species. 

The silicon analogs of carbocations have been reported a number of times [131], but there is only one confirmed sighting to date, the highly hindered trimesitylsilicenium ion as its tetrakis(pentafluorophenyl)borate salt [132] ... 

It is generally adopted that the catalytically active species in the metallocene-catalysed polymerization is a 14-electron cation. As an example, the mechanism of activation of an unbridged zirconocene catalyst is presented in Fig. 9.5-4, top. In the first two steps the activation by MAO, resulting in the 14-electron cation, is shown. The same cation can be generated by N,N -dimethylanilinium-tetrakis(pentafluorophenyl)borate and methylated metallocenes. As side-products methane and an amine are formed. TiBA can also be involved in the activation, which is not shown in Fig. 9.5-4, bottom. On the other hand, TiBA acts as a scavenger in the polymerization. The above-mentioned reactions take place in the absence of the monomer and are performed before the catalyst is used in the polymerization process. 

Recently, a variety of (3-silylated carboxonium ions have been prepared and characterized by NMR spectroscopy.541 Kira et al.631 used the Corey hydride transfer method, whereas Olah, Prakash, and co-workers applied triphenylmethyl tetrakis (pentafluorophenyl)borate to silylate esters,632 ketones, enones, and carbonates633 [Eq. (3.91)]. The ions thus produced are resonance hybrids of oxocarbenium (327b) and carboxonium (327a) ions with the latter as the major contributors. Calculated (DFT/IGLO) 29 Si NMR chemical shifts agree well with the experimental data. 

The reaction of l,3,2(Az)-diazasilole 83 with strong Lewis acids such as tris(pentafluorophenyl)borate B(C6F5)3 yields a Lewis acid-base adduct 94 which can be isolated at room temperature and which transforms slowly into silylborane 95 (Scheme 5) <1996CC2657>. 

Irradiation of isoquinolinium hydroxytris(pentafluorophenyl)borate 232 resulted in C,T S transfer to the isoquino-linium cation to yield 2-m cthy 1-1-(2,3,4,5,6-pentafluoropheny I)-1,2-dihydroisoqu incline 233 <2005JOC10653>. The mechanism is proposed to be due to a photoinduced electron transfer from the singlet state of the iV-methylisoqui-nolinium cation, confirmed using fluorescence quenching (Scheme 41). 

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