Use of Pyridinium Chlorochromate
and Reusable Polyaniline Salt Catalyst Combination for the Oxidation
of Indoles

Chebolu Naga Sesha Sai Pavan Kumar; Chebrolu Lavanya Devi; Vaidya Jayathirtha Rao; Srinivasan Palaniappan

doi:10.1055/s-2008-1077974

LETTER

Use of Pyridinium Chlorochromate and Reusable Polyaniline Salt Catalyst Combination for the Oxidation of Indoles

Chebolu Naga Sesha Sai Pavan Kumar^a, Chebrolu Lavanya Devi^a, Vaidya Jayathirtha Rao^a, Srinivasan Palaniappan*^b
^a Organic Chemistry Division II, Indian Institute of Chemical Technology, Hyderabad 500007, India
^b Organic Coatings & Polymers Division, Indian Institute of Chemical Technology, Hyderabad 500007, India
Fax: +91(40)27193991; e-Mail: palani74@rediffmail.com;

Abstract

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Abstract

A novel method is described herein for the simple, convenient and efficient oxidation of indoles to isatins using pyridinium chlorochromate with the aid of polyaniline salt catalyst at room temperature or at reflux in dichloroethane. Interestingly, oxidation of 3-alkyl indoles by this procedure gave 3-hydroxy 3-alkyl oxindoles. On the other hand, indol-3-alkanols gave mixtures of isatins and 3-formyl indoles. This is the first example of use of polyaniline as a catalyst in oxidation reaction.

Key words

indoles - oxidation - PCC - isatin - polyaniline

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References and Notes

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Representative Experimental Procedure: In a typical experiment, PCC (2.5 mmol, 537 mg) was added to a solution of indole (1 mmol, 117 mg) in 1,2-DCE (5 mL). Polyaniline salt catalyst [15 wt% with respect to indole (17 mg)] was added and the reaction mixture was stirred at r.t. or at reflux. Completion of the reaction was monitored by TLC. After completion, the reaction mixture was filtered through a Celite^® pad and washed with EtOAc (2 × 5 mL). The combined organic extracts were dried and evaporated under vacuum. The crude product was purified by silica gel column chromatography (n-hexane-EtOAc, 3:1) to furnish isatin (129 mg, 88%). A similar procedure was adopted for the preparation of other isatins, 3-hydroxy oxindoles and the authenticity of the products was confirmed from their ^¹H NMR and mass spectral data.

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Spectral Data for New Compounds: 1-Butyl-5-chloroindoline-2,3-dione (Table 2, entry 8): TLC: R _f 0.5 (30% EtOAc-hexanes); orange solid; mp 62-63 ˚C. IR (KBr): 3089, 2953, 1737, 1606, 1448, 1339, 1185, 1116, 844, 720 cm^-¹. ^¹H NMR (200 MHz, CDCl₃): δ = 7.49 (m, J = 8.1 Hz, 2 H), 6.80 (d, J = 8.1 Hz, 1 H), 3.70 (t, J = 7.3 Hz, 2 H), 1.67 (quint, J = 7.3 Hz, 2 H), 1.43 (m, J = 7.3 Hz, 2 H), 0.98 (t, J = 7.3 Hz, 3 H). ^¹³C NMR (200 MHz, CDCl₃): δ = 182.5, 157.6, 149.4, 137.5, 129.3, 125.1, 118.3, 111.8, 40.07, 29.08, 20.0, 13.5. MS (ESI): m/z = 238 [M + H⁺]. HRMS (ESI): m/z [M + H⁺] calcd for C₁₂H₁₃NClO₂: 238.0634; found 238.0630.
1-Methyl-5-nitroindoline-2,3-dione (Table 2, entry 9): TLC: R _f 0.3 (50% EtOAc-hexanes); yellowish orange solid; mp 203 ˚C. IR (KBr): 2924, 1743, 1610, 1465, 1331, 1289, 1109, 1074, 845, 747, 710, 600 cm^-¹. ^¹H NMR (200 MHz, CDCl₃): δ = 8.47 (dd, J = 8.8 Hz, 1 H), 8.34 (d, 1 H), 7.06 (d, J = 8.8 Hz, 1 H), 3.29 (s, 3 H). ^¹³C NMR (200 MHz, CDCl₃): δ = 182.1, 157.3, 150.3, 140.5, 128.0, 118.3, 116.7, 111.5, 26.2. GC-MS: m/z = 206 [M⁺]. Anal. Calcd for C₉H₆N₂O₄: C, 52.44; H, 2.93; N, 13.59. Found: C, 52.40; H, 2.78; N, 13.42.
1-Octyl-1 H -pyrrolo[2,3- b ]pyridine-2,3-dione (Table 2, entry 15): TLC: R _f 0.5 (30% EtOAc-hexanes); yellow solid; mp 74-76 ˚C. IR (KBr): 3063, 2925, 2855, 1737, 1592, 1456, 1353, 1263, 1085, 783, 738 cm^-¹. ^¹H NMR (200 MHz, CDCl₃): δ = 8.41 (d, 1 H), 7.78 (d, 1 H), 7.05 (t, 1 H), 3.83 (t, 2 H), 1.74 (t, 2 H), 1.25-1.35 (m, 10 H), 0.87 (t, 3 H). ^¹³C NMR (200 MHz, CDCl₃): δ = 180.9, 163.3, 157.3, 154.9, 131.8, 118.1, 110.9, 38.9, 30.7, 28.6, 28.1, 26.5, 25.8, 21.6, 13.03. MS (ESI): m/z = 261 [M + H⁺]. HRMS (ESI): m/z [M + H⁺] calcd for C₁₅H₂₁N₂O₂: 261.1603; found: 261.1606.