Carbenium borate

Figure 17.7 Catalytic activity of Cp TiOMe3 with carbenium borate...

Figure 3.7 Carbenium borate as cocatalyst (polymerization conditions styrene, lOml Cp Ti(OMe)3, 5 x 10" mol borate, 5 x ICr mol TIBA, 3 x 10" mol polymerization temperatnre 70°C time 4h).

In order to enhance the catalytic activity of a carbocationic center, the novel Lewis acid 54 was designed by Mukaiyama [149-152]. The 1-oxoisoindolium-based carbenium salt 54 [149], possessing a weak coordinating borate counter anion, proved to be a very active catalyst in the aldoUzation (Scheme 58) [150]. The Mukaiyama aldol reaction was catalyzed by 1 mol% of salt 54 and proceeded in up to 97% yield in 30 min. 

The hydride transfer from a hydrosilane to the carbenium ion leads to the transient formation of a complex of tricoordinate silicon cation with solvent. Lambert et al. showed that if a stable counterion of very low nucleophilicity is used, the complex of silylenium ion with solvent is a persistent species [12]. Tetrakis(pentafluorophenyl)borate was successfiilly explored for this purpose, and this system was later used by Olah ef al. [1] for the generation of trisilyloxonium ions derived from D4... 

Preparation of elastomeric polypropylenes was also reported by Chien et al. [64]. Two metallocene catalysts of different stereospecificities were used. The isospecific catalyst precursors were either rac-ethylene bis-(l-r -indenyl)zircoifium dichloride or rac-dimethylsilylene bis(l-T] -indenyl)zirco-nium dichloride. The unspecific one was ethylene bis(9-T] -fluorenyl)zirconium dichloride. The precursors were activated with triphenyl carbenium tetrakis(pentafluorophenyl)borate and triisobutylaluminum. The resultant catalysts exhibit very high activity, yielding products that range from tough plastomers to weak elastomers [64]. 

Use as Ligand in Ziegler-Natta Polymerization of Ethylene in Solution. A medium pressure process for the polymerization of ethylene has been developed in the presence of a catalytic system involving a monocyclopentadienyl titanium species, containing TIPSthiolate as the heteroligand and two activable ligands (2C1 or 2Me), associated with an ionic activator such as triphenyl-carbenium tetrakis(pentafluorophenyl)borate. ... 

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