Novel synthesis of indazoles from (η6-arene)tricarbonylchromium complexes

Indazole chromium complexes and some of its derivatives were synthesised by two strategies: (1) by cyclisation of [η6-2-(2′-phenylhydrazine)-1,3-dioxolane]tricarbonylchromium (2) in acidic conditions which was converted into σ-complex (11); (2) by thermolysis of Cr(CO)6 with 1-bis(trimethylsilyl)methylindazole (3) and 2-bis(trimethylsilyl)methyl-3-trimethyl-silylindazole (4), using bulky protecting groups at N(1) or simultaneously at N(2) and C(3), to afford [η6-1-bis(trimethylsilyl)methylindazole]tricarbonylchromium (14) and [η6-2-bis(trimethylsilyl)methyl-3-trimethylsilylindazole]tricarbonylchromium (15), respectively. The deprotonation of complex 14 followed by electrophilic quench occurs at the C(4) and C(7) positions in a ratio of 3:1 and with complex 15 the deprotonation was completely regioselective at the C(7) position. The position of this substitution was confirmed by n.O.e. difference spectroscopy and X-ray crystal structure determination of the [η6-2-bis(trimethylsilyl)methyl-7-methyl-3-trimethylsilylindazole]tricarbonylchromium (5). These complexes were decomplexed to produce new substituted indazole derivatives in good yield.