Tetrabutylammonium chloride chromate

The addition of chromium trioxide to solutions of alcohols in ether and dichloromethane in the presence of Celite furnishes ketones in 71-93% yields after 35 min at room temperature [535]. Oxidation can also be performed by refluxing the alcohols in solvents such as chloroform, ether, hexane, or benzene with chromium trioxide on an anion exchanger, Am-berlyst A26 [571]. Very good results are obtained when chromium trioxide is converted into tetrabutylammonium chromate by addition of catalytic amounts of tetrabutylammonium chloride in dichloromethane [617. ... 

To a stirred solution of 1.0 g (10 mmol) of chromium trioxide in 25 mL of water, an aqueous solution of 2.92 g (10.5 mmol) of tetrabutylammonium chloride in 50 mL of water is rapidly added at room temperature. A yellow-orange solid precipitates immediately. The mixture is cooled to 0 °C, and the solid is filtered with suction through a sintered-glass funnel. It is carefully washed with cold water, dried under vacuum over phosphorus pentoxide, and stored over calcium chloride. The yield of the tetrabutylammonium chromate is 2.76 g (77%). 

A solution of 1.58 g (5.68 mmol) of tetrabutylammonium chloride in 28 mL of water is added rapidly with stirring to a solution of 0.54 g (5.4 mmol) of chromium trioxide in 14 mL of water at room temperature. The tetrabutylammonium chromate is extracted with 200 mL of chloroform. The chloroform solution is concentrated to 6 mL, and a solution of 0.5 g (2.71 mmol) of benzhydrol in 4 mL of chloroform is added with stirring. After 3 h at 60 °C, the mixture is diluted with ether, and the solution is poured into 1 N sodium hydroxide. The ether layer is washed with a saturated solution of sodium chloride, dried with anhydrous sodium sulfate, and evaporated to give 0.450 g (91%) of benzophenone, mp 45-47 °C. 

Another chromium reagent for the oxidation of secondary alcohols to ketones is tetra-n-butylammonium chromate which is readily prepared by the addition of tetrabutylammonium chloride to an aqueous solution of CrOa. This reagent is very soluble in chloroform and, although only a limited number of substrates were studied, yields were good (80%) using only a small excess of oxidant. 

Tetrabutylammonium hydroxide 716 Tetraphenylarsonium chloride, 482 Thallium, D. of as thallium(I) chromate,... 

A modern version of dichromate oxidation in aqueous media is a phase-transfer reaction carried out in a two-phase system. Alkaline di-chromate is converted into tetraalkylammonium dichromate, which is soluble in organic solvents such as dichloromethane, chloroform, or benzene ( orange benzene ). The treatment of alcohols with a solution of potassium dichromate in acetic acid in the presence of Adogen 464 (Aldrich s trade name for methyltrialkyl [Cg-Cio] ammonium chloride) and benzene gives aldehydes at 55 °C [651. Similar results are obtained with a chloroform solution of tetrabutylammonium chromate at 60 °C [618]. ... 

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