Efficient Access to Substituted Silafluorenes by Nickel‐Catalyzed Reactions of Biphenylenes with Et 2 SiH 2

The reaction of biphenylene ( 1 ) with Et 2 SiH 2 in the presence of [Ni(PPhMe 2 ) 4 ] results in the formation of a mixture of 2‐diethylhydrosilylbiphenyl [ 2 (Et 2 HSi)] and 9,9,‐diethyl‐9‐silafluorene ( 3 ). Silafluorene 3 was isolated in 37.5 % and 2 (Et 2 HSi) 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(PPhMe 2 ) 4 ]‐catalyzed reaction of Et 2 SiH 2 and 1‐methylbiphenylene. By contrast, no selectivity could be found in the Ni‐catalyzed reaction between Et 2 SiH 2 and the biphenylene derivative that bears t Bu substituents in the 2‐ and 7‐positions. Therefore, two pairs of isomers of t Bu‐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.