Conversion of indoline-2~thione and oxindole to isatin- some new observations

Indoline-2-thione (1) and oxindole, when treated separately with ceric ammonium nitrate adsorbed on silica gel (CAN-SiO 2 ) in acetonitrile, furnished isatin (2) as the sole product, whereas Montmorillonite K10 clay alone converted oxindole to isatin in a dry reaction.

electron oxidant used in organic synthesisl. 2 . The strong oxidising power of cerium(IY) salts, however, leads to ·mdersired and overoxidised products 3 . The concept of moderating the effect of reagents by using them adsorbed on inorganic solid supports has consequently been developed since long and applied s:tccessfully in organic syntheses 4 . It has been earlier demonstrated that CAN adsorbed on silica gel (CAN-Si0 2 ) is a more selective and efficient oxidant and nitrating agent than CAN alone due to its mildness and ease of work-up 5 . In continuation of our ongoing interest in the study of CAN-mediated reactions of various heterocyclic systems with or without adsorption (of CAN) on solid supports 6 , we report herein the action of CAN-Si0 2 on indoline-2-thione (1) and oxindole in acetonitrile solution. The conversion of ox indole on an acidic clay i~ also being reported herein.
Results and discussion lndoline-2-thione (1) was prepared from oxindole by La wesson's reagent 7 . When a solution of lin dry acetonitrile was treated with two equivalents of CAN-Si0 2 at 60° for a few hours, isatin (2) was isolated as the only product, although in low yield (Scheme 1 ). In this connection, our attention was drawn to a recent report on the conversion of thioamides to amides by CAN in acetic acid at RT 8 . When the progress of our reaction was being monitored by TLC, a spot initially appeared, and, with passage oftime, it slowly disappeared with the simultaneous appearance of a spot conesponding to isatin. This observation coupled with the aforesaid report induced us to think that CAN -Si0 2 may have initially dethionated l to oxindole which subsequently underwent CAN-SiOrmediated oxidation to 2. This notion of ours proved to be correct when a somcwh .t similar treatment of ox indole with CAN-Si0 2 in acetonitrile furnished 2 in 25% yield (Scheme 2).
Since the conversion of ox indole to isatin on the acidic surface of silica gel alone by autoxidation 9 was a distinct possibility, we repeated the above reaction but without using CAN. But there was no change, which demonstrated the inability of silica gel itself to bring about the observed oxidation.
Since the commercially available Montmorillonite KIO clay is reported 10 to have a larger specific area (500-760 m 2 /g) and higher Bronsted acidity (Hammel acidity function, H 0 = -6 to -8) than those of silica gel, we became curious to know if this acidic clay could induce autoxidation of oxindole to isatin. As planned, oxindole was adsorbed on M.KlO clay and kept at room temperature until it was fully consumed. Leaching ofthe clay with a suitable solvent furnished a single product which was identified as isatin by usual comparisons (Scheme 3). Our results demonstrated for the first time the vulnerability of indoline-2-thione and oxindole to CAN-Si0 2 and also of oxindole to Montmorillonite KlO clay. These findings may be of use to the community of chemists.

Experimental
Lawesson's reagent was purchased from Aldrich, U.S.A. The NMR spectra were recorded on a Bruker DRX 500 spectrometer and the low resolution mass spectrum in a AEI MS 30 mass spectrometer. Silica gel G (Merck, India) was used for TLC and silica gel (60-120 mesh, Qualigens, India) for column chromatography was used for the CANmediated reactions. M.K10 was purchased from Fluka.

Reaction of indoline-2-thione
(1) with CAN-Si0 2 : A solution of CAN (2 mmol, 1.1 g) in CH 3 CN (4 ml) was adsorbed on silica gel (2 g) and the solvent was allowed to evaporate at RT. This CAN-Si0 2 was added to 1 (1 mmol, 150 mg) in dry acetonitrile (7 ml). The reaction mixture was stirred at 600 for 5.5 h. On completion of the reaction (TLC), the above mixture was adsorbed on fresh silica gel and the solvent was allowed to evaporate at RT. Leaching with CH 2 CI 2 (3 x 20 ml) and distilling off the solvent gave a residue which was crystallised from pet. ether-CH 2 CI 2 to furnish isatin (2) C-7). The 13 C NMR data agreed well with those reported in litera-ture13.
Reaction of oxindole with CAN-Si0 2 :The same procedure as applied to I was adopted, except CAN (I mmol, 170 _ ~8 mg) and oxindole (0.5 mmol, 67 mg) were used. The reaction took 5 h and, after usual work-up, isatin (2) was obtained in 25% overall yield.
Oxidation of oxindole on M.KJO clay surface: Ox indole (l mmol, 133 mg) was dissolved in CH 2 Cl 2 (5 ml) and M.K10 clay (3 g) was added to the solution. The solvent was allowed to evaporate at RT and kept for 24 h. On completion of the reaction (TLC), leaching with CH 2 Cl 2 (3 x 20 ml) furnished isatin in 82% overall yield.