Indazaboles—synthesis and molecular structure

The reaction of 1‐trimethylsilyl‐indazole with boranes affords indazaboles accompanied by elimination of trimethysilane. Thus, the two isomers of parent indazabole are formed in a 1:1 ratio using borane in THF (BH₃/THF), characterized by NMR spectroscopy in solution (¹H, ¹¹B and ¹³C NMR). In contrast, the analogous reaction with 1,2‐bis(tetramethylene)diborane(6) proceeds to give a single isomer of the B‐alkylated indazabole via symmetric ring cleavage of the diborane(6), as shown by NMR in solution and X‐ray structural analysis in the solid state. The molecular structure is fluxional in solution. In the solid state, the central B₂N₄ ring adopts a distorted boat conformation. Calculated gas phase geometries of the parent indazaboles and of the B‐alkylated indazabole were optimized by DFT methods at the B3LYP/6‐311 + G(d,p) level of theory. Copyright © 2010 John Wiley & Sons, Ltd.