Alcohols oxidation with chromium derivatives

Evidence for the structure (CXXIII) of the hemiacetal is based on the extremely hindered nature of the derived aldehyde (CXXV) and carboxylic acid (CXXVII). Thus, the aldehyde exhibited a negative Cotton effect in methanol, which remained unchanged upon the addition of hydrochloric acid, indicating great resistance toward acetal formation. Attempts to prepare carbonyl derivatives of this aldehyde were unsuccessful. The acid CXXVII was prepared by oxidation of alcohol CXXIV with chromium trioxide in acetic acid. Comparison of the apparent dissociation constant of this acid (pX cs 9.45) with that for... 

In 1946, Jones discovered that secondary alcohols could be efficiently oxidized to ketones by pouring a solution of chromium trioxide in diluted sulfuric acid over a solution of the alcohol in acetone.13 This procedure, which has proved to be quite safe, allows a sufficient contact of the alcohol with chromium oxide derivatives for a reaction to take place. Jones oxidation marked the beginning of the highly successful saga of chromium-based oxidants. 

The periodinane (10) may also be prepared from o-iodobenzoic acid by oxidation with potassium bromate and then treatment with acetic anhydride18 (see Expt 6.36 for detailed formulation). It should be noted that the organic derivatives of pentacoordinate iodine(v) are termed periodinanes.18b This compound (the systematic name is l,l,l-triacetoxy-2,l-benzoxiodol-3(3//)-one) has found use as an oxidant of primary alcohols to aldehydes and alicyclic ketones to secondary alcohols it is claimed to have advantages over the chromium-based oxidation reagents. 

The oxidation of alcohols is not limited to chromium derivatives, DMSO derivatives, or hypervalent iodine compounds. There are several metal-based reagents that can be very effective, particularly with sensitive functionality. This section will examine several of the more important regents. 

Another more environmentally friendly class of oxidants than chromium species involve hypervalent iodine compounds, such as the reagent commonly referred to as the Dess-Martin periodinane (DMP). Hypervalent means a compound with a greater electron count than predicted by the octet rule. The name DMP derives from the two chemists that developed its reactivity, and its use is called the Dess-Martin oxidation. Once again, primary alcohols are oxidized to aldehydes, while secondary alcohols are oxidized to ketones, and tertiary alcohols are unreactive. Hence, DMP performs the same transformations as PCC and the Swern oxidation (same products as in Example 10.11). 

It is well known that tertiary ally lie alcohols such as 21 can be oxidized to the corresponding enone 23 with chromium reagents. Yoshiharu Iwabuchi of Tohoku University observed (J. Org. Chem. 2008, 73, 4750) that the oxammonium salt 22 derived from TEMPO effected the same transformation. David E. Richardson of the University of Florida found (Tetrahedron Lett. 2008, 49, 1071) that could be used to oxidize N-methyhnorpholine in situ to the N-oxide, that in turn reoxidized catalytic OsO. In the presence of the Sharpless ligand, the dihydroxylation proceeded withhighee. This approach could offer cost and waste stream advantages over currently used oxidants. 

When methanol was used to rinse a pestle and mortar which had been used to grind coarse chromium trioxide, immediate ignition occurred due to vigorous oxidation of the solvent. The same occurred with ethanol, 2-propanol, butanol and cyclo-hexanol. Water is a suitable cleaning agent for the trioxide [1]. For oxidation of sec-alcohols in DMF, the oxide must be finely divided, as lumps cause violent local reaction on addition to the solution [2]. Use of methanol to reduce the Cr(VI) oxide to a Cr(III) derivative led to an explosion and fire [3], The ignitability of the butanols decreases from n -through sec- to iert-butanol [4],... 

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