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-di-ethyl-9-silafluorene (8) and 3,6-di-tert-butyl-9,9-diethyl-9-silafluorene (9). Silafluorenes 8 and 9 were separated by column chromatography.