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 (H-1, B-11 and C-13 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 (C) 2010 John Wiley & Sons, Ltd.