Rearrangement from the heteroantiaromatic borole to the heteroaromatic azaborine motif.

Treatment of 9-chloro-9-borafluorene with N,O-bis(trimethylsilyl)hydroxylamine results in 10-trimethylsilyloxy-9-aza-10-boraphenanthrene 6b. NMR spectroscopy shows that the expected antiaromatic 9-(trimethylsilyloxyamino)-9-borafluorene 5b rearranges to the formally aromatic phenanthrene 6b at room temperature.