Activated manganese dioxide-silica

Manganese dioxide on bentonite clay has also been used for oxidation of phenols to quinones (30-100%) [97] and Mn02 on silica effects the dehydrogenation of pyrrolidines (58-96%) [98]. 

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Silica reinforced rubber, 22 703 Silica sheets, 22 383-385 Silica-silane system, 22 377-378 Silica sol-gel fiber processing, 23 80 Silica sols, 22 383, 473-474 applications of, 22 394 modification of, 22 393-394 preparation of, 22 392-393 properties of, 22 391-392 purification of, 22 393 Silica, solubility in steam, 23 212-213 Silica-supported activated manganese dioxide, 76 568... 

To 5-hydroxymethyl-2, 3 -isopropylideneuridine (20 g, 64 mmoles) in dichloromethane (500 ml) is added active manganese dioxide (100 g), and the suspension is stirred at room temperature for 48 hr. It is filtered over Celite with the aid of a suction pump and washed well with chloroform-methanol (1 1, v/v). The filtrate and washings are combined and evaporated under reduced pressure. The residue is dissolved in chloroform and applied to a silica gel column (50 X 6 cm) which is eluted with 1 liter of chloroform, 1.5 liters of chloroform-methanol (98 2, v/v), and then with chloroform-methanol (96 4, v/v). Fractions of 200 ml are collected and are monitored by TLC. Those fractions containing pure 5-formyl-2, 3 -isopropylideneuridine are combined and evaporated to dryness under reduced pressure to yield 7.2 g of the desired compound as a white crystalline residue, m.p. 157°-159°, 280 nm (13,600) and 235 nm (9000). 

Very active manganese dioxide (320 mg) is added to a solution of the quinone imine complex (63 mg, 0.152 mmol) in dichloromethane (3 mL) and the heterogeneous reaetion mixture is stirred for 2 h at room temperature. By addition of very active Mn02 (80 mg) and stirring for additional 2 h at room temperature, the reaction is completed. The mixture is filtered through a short path of silica gel/Celite that is subsequently washed with ethyl acetate. The solvent is removed in vacuo and the residue is subjected to flash chromatography (hexane/ethyl acetate 5 1) on silica gel to afford the carbazolone iron complex as yellow crystals mp > 165 °C (decomp.) 49 mg (78%). ... 

Pyridinium chlorochromate has also been used (Chakraborty and Bordoloi, 1999) under microwave irradiation for the oxidation of alcohols to the corresponding carbonyl functions with an efficient and mild methodology. Microwave irradiation of alcohols with silica supported active manganese dioxide in solvent-free condition provides rapid and selective oxidation of alcohols to the corresponding carbonyl compounds (Varma et al., 1997). 

Ya.B. considered that the most important adsorbents—porous coal, silica gel, and the powdered manganese dioxide which he had studied experimentally—are amorphous substances, i.e., they do not have clearly articulated crystalline structure. Only thus is it possible to obtain a large developed surface—the most important feature of an adsorbent. In this case, it is natural to consider all the possible values of adsorption activity and a smooth distribution function of surface sectors according to their level of activity. 

The time required to achieve complete reaction varies from 20-48 hr depending on the activity of the manganese dioxide. The progress of the oxidation is easily monitored by TLC analysis on silica gel. 

Silver permanganate and potassium permanganate on silica gel, or mixtures of silver oxide and manganese dioxide on the same support, show good activity in the absence of oil and water vapor (15), and a mixture of silver permanganate and manganese dioxide was rather active in the presence of water vapor (15). 

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