Efficient Access to Substituted Silafluorenes by Nickel-Catalyzed Reactions of Biphenylenes with Et2SiH2.

The reaction of biphenylene ( 1) with Et2SiH2 in the presence of [Ni(PPhMe2)4] results in the formation of a mixture of 2-diethylhydrosilylbiphenyl [ 2(Et2HSi)] and 9,9,-diethyl-9-silafluorene ( 3). Silafluorene 3 was isolated in 37.5 % and 2(Et2HSi) in 36.9 % yield. The underlying reaction mechanism was elucidated by DFT calculations. 4-Methyl-9,9-diethyl-9-silafluorene ( 7) was obtained selectively from the [Ni(PPhMe2)4]-catalyzed reaction of Et2SiH2 and 1-methylbiphenylene. By contrast, no selectivity could be found in the Ni-catalyzed reaction between Et2SiH2 and the biphenylene derivative that bears tBu substituents in the 2- and 7-positions. Therefore, two pairs of isomers of tBu-substituted silafluorenes and of the related diethylhydrosilylbiphenyls were formed in this reaction. However, a subsequent dehydrogenation of the diethylhydrosilylbiphenyls with Wilkinson's catalyst yielded a mixture of 2,7-di- tert-butyl-9,9-diethyl-9-silafluorene ( 8) and 3,6-di- tert-butyl-9,9-diethyl-9-silafluorene ( 9). Silafluorenes 8 and 9 were separated by column chromatography. [ABSTRACT FROM AUTHOR]