Syntheses and Reactions of a Stable 1,2‐Dichloro‐1,2‐diborolane and Aromatic Tetraboranes

The 1,2‐dichloro‐1,2‐diborolane 1b was isolated in good yield after the reaction of the 1,2‐bis(dimethylamino)‐1,2‐diborolane 2c with BCl 3 . We prepared 2c from 1,2‐bis(dimethylamino)‐1,2‐dichlorodiborane(4) ( 3 ) and TMEDA‐free [1,3‐bis(trimethylsilyl)allyl]lithium ( 5a·Li ), which is accessible by deprotonation of 1,3‐bis(trimethylsilyl)propene ( 4a ) with n ‐butyllithium in diethyl ether. Upon its reaction with lithium in THF, the 1‐allyl‐2‐chlorodiborane(4) 6a obtained in the first step undergoes an unprecedented ring closure with 1,2‐migration of a trimethylsilyl substituent to yield the boron‐stabilized carbanion 7a . Reaction of 7a with either chlorotrimethylsilane or HCl gives 2c or 2d , respectively. The latter compound cannot be transformed into an isolable 1,2‐dichloro‐1,2‐diborolane; this observation demonstrates that a trimethylsilyl substituent next to each boron atom is essential for the stability of 1,2‐dichloro‐1,2‐diborolanes. Compound 1b is a versatile starting material, inter alia for the syntheses of the aromatic tetraboranes(6) cis ‐ 11 and trans ‐ 13 and the aromatic tetraborane(4) cis ‐ 12 . The isomerization cis ‐ 11 → trans ‐ 13 can be explained on the basis of reactions occurring under the conservation of two‐electron aromaticity. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)