Silica-supported manganese dioxide

R-i — p-MeOC0H4CO R2 — p-MeOCgH4 Scheme 8.45. Oxidation of alcohols by silica-supported manganese dioxide. 

A series of Michael adducts has been synthesized with high yields by solvent-free microwave promoted Michael addition of aza-nucleophiles to benzo[b]thiophen-2-yl-2-propenone (Pessoa-Mahana et al., 2009). In this method, aliphatic and aromatic amines act as Michael donors and on treatment with l-(4,7-dimetho2 benzo[b] thiophen-2-yl)-2-propen-l-one led to formation of P-aminoketones and azahetero-cyclic compounds. These aza-nucleophiles were impregnated on silica gel-manganese dioxide as solid inorganic support. 

Manganese dioxide (Mn02) supported on silica provides an expeditious and high-yield route to carbonyl compounds. Benzyl alcohols are selectively oxidized to carbonyl compounds by use of 35 % Mn02 doped silica under MW irradiation conditions (Scheme 6.28) [96]. 

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... 

Methods of oxidation of hydrazo to azo compounds and hydroxylamino to nitroso compounds have been reviewed. Reagents which oxidize aromatic primary amines to azo compounds are also suitable for the oxidation of aromatic hydrazo compounds, since the hydrazo compounds are intermediates in the oxidation of the amines. Thus, manganese dioxide, mercury(II) oxide and lead tetraacetate are all suitable oxidants. Silver carbonate on Celite rapidly oxidizes both diarylhydrazines and acylhydrazines to the corresponding azo compounds in good yield. Another supported oxidant which can convert hy-drazobenzene into azobenzene in high yield is sodium periodate 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). 

Several references have appeared on the use of solid-phase oxidants. Solid potassium permanganate-copper sulphate mixtures oxidize secondary alcohols to ketones in high yield, and pyridinium chromate or chromic acid on silica gel are described as convenient off-the-shelf reagents for oxidation of both primary and secondary alcohols. Anhydrous chromium trioxide-celite effects similar transformations only when ether is present as co-solvent. An excellent review, with over 400 references, on supported oxidants covers the use of silver carbonate-celite, chromium trioxide-pyridine-celite, ozone-silica, chromyl chloride-silica, chromium trioxide-graphite, manganese dioxide-carbon, and potassium permanganate-molecular sieve. 

Fiikuzumi and Cuo have very recently concluded that the termination reaction for oxidation of c miene with manganese dioxide or cobalt oxide supported on silica ( ) and during autoxidation of cumene initiated by reaction of cumene hydroperoxide with lead oxide ( ) is strictly first-order with respect to the concentration of cumylperoxy radicals. These workers proposed an unprecedented 1,3-methyl shift followed by 0-0 bond cleavage to account for these inusual kinetics. 

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). 

Alternatively, the oxidising or reducing solid reagent can be immobilised on a suitable support, as demonstrated in the spectrophotometric and chemiluminometric flow injection determinations of manganese in natural waters using mini-columns of lead(IV) dioxide or sodium bis-muthate immobilised on silica gel beads [112] and paper pulp [113], respectively. 

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