Tris-perfluoroaryl Borane Derivatives B C6F5 3 and Related Compounds

TRIS-PERFLUOROARYL BORANE DERIVATIVES B(C6F5)3 AND RELATED COMPOUNDS 

The high Lewis acidity, thermal stability and resistance to protic B-C bond cleavage conspire to make B(C6F5)3 an extremely effective activator of organometallic precatalysts for olefin polymerization and other reactions. As a result, other related boranes incorporating perfluoro biphenyl (Chart 1, III,93 IV94), naphthyl (V95) or fluorinated 9-borafluorene (VI96) frameworks have been prepared in an effort to increase the Lewis acidity of the boron center and studied in the context of olefin polymerization. 

The LA strength of B(C6F5)3 has been assessed using the Childs method by three groups, with some variation in the values obtained evident. We find a Childs acidity of 0.68 0.02, while Erker and co-workers assign a value of 0.7297 and Marks and Luo report 0.77.14 In light of the fact that Erker s value appears to be relative to BC13 = 1.00 

Incorporation of bulkier, more highly fluorinated aryl groups increases the Childs Lewis acidity, but calorimetric measurements on the reactions of III and V with simple Lewis bases or metallocenes reveal that back-strain upon pyramidalization at boron limits the eflFective Lewis acidity of these compounds. This notion is supported computationally.98 As an alternative to bulky, exhaustively fluorinated aryl groups, incorporation of the boron atom into an anti-aromatic 9-borafluorene ring, VI, results in comparable LA strength (Childs acidity = 0.70 0.02) to B(C6F5)3, despite the loss of two fluorine atoms. 

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