Abstract
Methyl indolenines (4a-c) and (5a-c) were prepared in high yield by a Fischer indole synthesis reaction of o,m-tolylhydrazine hydrochlorides (1a-b) with isopropyl methyl ketone (2) and 2-methylcyclohexanone (3) in acetic acid at room temperature. o,p- Nitrophenylhydrazines (1c-d) were reacted with 2-methylcyclohexanone (3) in acetic acid at reflux to give nitroindolenines (5d-e), while the attempted reactions of o,p-nitrohydrazines with isopropyl methyl ketone (2) in acetic acid were not successful. Compounds (1c-d) were reacted with isopropyl methyl ketone (2) in acetic acid/HCl to give 2,3,3-trimethyl-5-nitro-indolenine (4e) and 2,3,3-trimethyl-7-nitroindolenine (4d).
Highlights
The Fischer indole synthesis is a chemical reaction that produces an aromatic heterocyclic indole from a phenylhydrazine and an aldehyde or ketone under acidic conditions
H-NMR spectrum of indolenine (4a) shows a singlet at δ = 1.16 ppm corresponding to the two methyl groups at C-3, a singlet methyl group for C-2 at δ = 2.24 ppm, and a methyl group singlet resonance for Ar-CH3 at δ = 2.1 ppm
2,3,3,6-Tetramethylindolenine (4b): TLC: RF 0.3 on silica gel eluting with CH2Cl2-CH3COCH3 (20:1); UV: (EtOH) λmax 219 (H, 2.292) 261 (H, 0.684), (EtOH, HCl) 210 (H, 1.513) 227 (H, 1.088) 280 (H, 0.625); IR: 2900, 1700, 1600, 1565, 1450, 1425, 1340, 1250, 1130, 800; 1H-NMR: 1.25 (s, 6H, 2×CH3), 2.19 (s, 3H, Ar-CH3), 2.33 (s, 3H, N=C-CH3), 6.7-6.9 (b, 2H, Ar-H), 7.1 (s, 1H, Ar-H); C-NMR: 19.9, 24.3, 25.1, 37.5, 123.6, 127.3, 127.4, 136.9, 149.8, 151.7, 164.6; C12H15N, Mol
Summary
The Fischer indole synthesis is a chemical reaction that produces an aromatic heterocyclic indole from a (substituted) phenylhydrazine and an aldehyde or ketone under acidic conditions. In the UV spectrum the maximum absorptions were noticed at 214 and 259 nm which after acid addition, a shift in the position of the band maxima was seen, which is indicative of the presence of non-bonding pair electrons in the reaction with H+ (bathochromic effect).
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