eq. 2). Based on this report, it is expected that the Lewis acidity of the organoborane may play an important role in the process leading to homolytic substitution.
BRRROR'OR'BRR+ R•(2) ArSO2• + OBORR• + OBOArSO2(1)
Scheme 1. Nucleohomolytic reaction at boron.
9-Alkyl-9-borafluorenes represent a fascinating class of organoboranes due to the 4π antiaromatic nature of the central borole ring since the vacant p-orbital at boron is conjugated with the π-system. Interactions with a Lewis base breaks the planarity, removing the p-orbital from the conjugation.10 Therefore, 9-borafluorenes are strong Lewis acids and highly reactive. They were first synthesized by Köster and Benedikt in 196311,12 and by Narula and Nöth in 1985,13 but the most efficient synthesis was developed only recently by Holthausen, Wagner and coworkers (Scheme 2).14,15,16 2,2’-Dibromobiphenyl 1 underwent double lithium-bromine exchange, followed by lithium-boron exchange15 delivering desired 9-chloro-9-borafluorene 2. Then, 9-H-9-borafluorene 3 could be generated by reduction of 9-chloro-9-borafluorene 2 or 9- bromo-9-borafluorene in the presence of triethylsilane in benzene.16 Borane 3 is particularly unstable due to its antiaromatic character and it tends to reversibly form a dimer and to rapidly degrade, affording isomers and oligomers that exclude boron’s empty p-orbital from the π- plane.14,16,17,18 The mechanisms of isomerization and polymerization have been studied.16,17,18 Unsymmetrical borafluorenes, synthesized by boron insertion in C–H19 and C–C20 bonds, and fluorinated 9-borafluorenes21 were also studied.
BrBr1) BuLi (2 eq), hexane 0 °C to rt, 3 daysBCl12 (90%) HSiEt3 (2.0 eq) benzene, rtBH32) BCl3 (1 eq), hexane 0 °C to rt, overnight(3)2
Scheme 2. Reported synthesis of 9-alkyl-9-borafluorenes.
