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

Abstract

AbstractThe 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 BCl3. 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)