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

Abstract Indazole chromium complexes and some of its derivatives were synthesised by two strategies: (1) by cyclisation of [η 6 -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 [η 6 -1-bis(trimethylsilyl)methylindazole]tricarbonylchromium ( 14 ) and [η 6 -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 [η 6 -2-bis(trimethylsilyl)methyl-7-methyl-3-trimethylsilylindazole]tricarbonylchromium ( 5 ). These complexes were decomplexed to produce new substituted indazole derivatives in good yield.