Chromium trioxide catalytic oxidation

Concentration Effects. The reactivity of ethyl alcohol—water mixtures has been correlated with three distinct alcohol concentration ranges (35,36). For example, the chromium trioxide oxidation of ethyl alcohol (37), the catalytic decomposition of hydrogen peroxide (38), and the sensitivities of coUoidal particles to coagulation (39) are characteristic for ethyl alcohol concentrations of 25—30%, 40—60%, and above 60% alcohol, respectively. The effect of various catalysts also differs for different alcohol concentrations (35). 

An intere.sting example in the context of waste minimization is the manufacture of the vitamin K intermediate, menadione. Traditionally it was produced by stoichiometric oxidation of 2-methylnaphthalene with chromium trioxide (Eqn. (8)), which generates 18 kg of solid, chromium containing waste per kg of menadione. Catalytic alternatives have been reported, but selectivities tend to be rather low owing to competing oxidation of the second aromatic ring (the. selectivity in the classical process is only 50-60%). The best results were obtained with a heteropolyanion as catalyst and O2 as the oxidant (Kozhevnikov, 1993). 

The oxidation of both linear and cyclic ethers to the corresponding acids and lactones by aqueous H202 as catalyzed by TS-1 and TS-2 was reported by Sasidharan et al. (241) (Scheme 17 and Table XXXV). The titanosilicates exhibited significantly better activity (about 55% conversion) and selectivity (98%) than chromium silicates, although vanadium silicates totally failed to catalyze the reaction. Such conversions are usually accomplished using either stoichiometric amounts of chromium trioxide, lead tetraacetate, or ruthenium tetroxide as oxidants (242) or catalytic amounts of Ru04 in the presence of... 

Addition of 4-pentenyllithium to the dione 65 gave the ds-diol 71 which was converted to the (Z)-l,2-disubstituted cyclododecene 72a. Hydroboration-oxidation and chromium trioxide oxidation provided the dialdehyde 72b whose McMurry ring closure, followed by partial catalytic hydrogenation gave the (Z)[10.10] precursor 73. Treatment of this (Z)-olefin 73 with HzS04-Ac0H in benzene was reported to effect conversion into [10.10]betweenanene (61b) of 95% purity and high yield. 

Oxidations with chromium trioxide.6 Secondary alcohols can be oxidized to ketones in good yields by Cr03 in the presence of catalytic amounts of tetraalkyl-ammonium halides. Yields from oxidation of primary alcohols are moderate. 

Halogenomethylpyrroles have been oxidized with lead(IV) salts or by chromium trioxide to yield the formylpyrroles, whilst catalytic hydrogenolysis or zinc-acetic acid reduction produces the 2-methylpyrroles (B-77MI30504). The methyl derivatives are also obtained by hydride reduction of trifluoromethyl-pyrroles and -indoles, and trifluoromethylindoles are converted into the carboxylic esters by ethanol under basic conditions (74JOC1836). 

The most important (and also the most expensive) grapefruit aroma compound is the bicyclic terpene nootkatone. It is manufactured by oxidation of valencene, which is extracted from Valencia oranges. Figure 1.23 shows two routes for this oxidation, a stoichiometric reaction using chromium trioxide, and a catalytic alternative using sodium hypochlorite (bleach) in the presence of 1 mol% osmium tetraoxide catalyst. 

Oxidation of 172 with chromium trioxide in pyridine gave the monoketone 176, whereas oxidation of spiradine F under the same conditions afforded the hydroxylactam 177. Catalytic hydrogenation of compound 176 afforded the a-ketol 178. The latter was treated with sodium methoxide in benzene to give an enolated a-diketone (179), which definitely fixed the position of the hydroxyl group at C-6 in spiradine G. Comparison of an ORD curve of compound 178 with that of 5a-cholestane-6-one established the indicated absolute configuration for these alkaloids. It is worth noting that these alkaloids bear many structural similarities to the earlier mentioned alkaloid ajaconine. 

A number of other chromium-based reagents have been developed for allylic oxidation for example that of stnoids by t-butyl hydroperoxide in the presence of a catalytic amount (0.0S-0.S mol equiv.) of chromium trioxide in dichloromethane solution at room temperature (equation 39). Yields vary from 32 to 69%. This modification is useful in terms of cost, operational simplicity and yields. 

While removal of an AT-alkyl substituent is not always a feasible process benzyl groups can be removed by reduction with sodamide or by catalytic hydrogenolysis. If such reductive methods fail, an oxidation with chromium trioxide in acid may be successful. Other groups are not so readily displaced, but a procedure involving transalkylation with benzyl chloride followed by debenzylation can be employed to convert 1-methylimidazole into imidazole (Scheme 137). The reaction is capable of extension and operates because the quaternary salt is in equilibrium with both 1-alkylimidazoles and the alkyl halides. Under conditions in which the more volatile alkyl halide can escape from the system the 1-benzylimidazole builds up. 

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. ... 

Thus, in general catalytic oxidations with molecular oxygen are not applicable to more complicated, less volatile molecules. For reactions in the liquid phase catalytic oxygen transfer constitutes a useful alternative that combines the advantages of stoichiometric oxidants (high selectivities and broad scope) with those of catalytic oxidation with 02 (inexpensive reagent and environmentally acceptable) [3,4], A reaction of commercial interest, to illustrate the point, is the oxidation of 2-methylnaphthalene to 2-methyl-l,4-naphthoquinone (menadione), an intermediate for vitamin K. Traditionally this oxidation was carried out with stoichiometric quantities of chromium trioxide in sulfuric acid and produced 18 kg... 

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