Alcohol oxidation

Alcohol oxidation is an important organic reaction. Primary alcohols (R-CH₂-OH) can be oxidized either to aldehydes (R-CHO) or to carboxylic acids (R-CO₂H), while the oxidation of secondary alcohols (R¹R²CH-OH) normally terminates at the ketone (R¹R²C=O) stage. Tertiary alcohols (R¹R²R³C-OH) are resistant to oxidation.

Quotes

• The classical procedure for oxidizing primary alcohols to aldehydes and secondary alcohols to ketones involves treatment of the appropriate alcohol with a chromium(VI) reagent. Oxidation of primary alcohols to aldehydes requires anhydrous conditions. In the presence of water, the resultant aldehyde can form the hydrate, which may be further oxidized to the carboxylic acid.
  • George S. Zweifel and Michael H. Nantz, Modern Organic Synthesis (2006), Ch. 4 : Functional Group Transformations: Oxidation and Reduction.

• The Jones reagent is an excellent reagent for the oxidation of secondary alcohols that do not contain acid-sensitive groups such as acetals. Oxidation of primary alcohols with Jones reagent may result in the conversion of the aldehydes initially formed to the corresponding carboxylic acids. … Chromic acid oxidation may also be performed in the presence of water-immiscible solvents.
  • George S. Zweifel and Michael H. Nantz, Modern Organic Synthesis (2006), Ch. 4 : Functional Group Transformations: Oxidation and Reduction.

• CrO₂·C₅H₅N is a mild reagent for the oxidation of alcohols that contain acid-sensitive groups. … Primary and secondary alcohols are readily oxidized in CH₂Cl₂ utilizing 1 to 1.5 equivalents of PCC. … PDC is soluble in H₂O, DMF, and DMSO but sparingly soluble in CH₂Cl₂ or CHCl₃. The reagent is less acidic than PCC. Hence, oxidations in CH₂Cl₂ can be carried out under nearly neutral conditions. This permits the conversion of primary alcohols containing acid-sensitive groups into the corresponding aldehydes or ketones, as illustrated below.
  • George S. Zweifel and Michael H. Nantz, Modern Organic Synthesis (2006), Ch. 4 : Functional Group Transformations: Oxidation and Reduction.

• The Swern oxidation proceeds rapidly at low temperatures and thus can be employed for the preparation of a-keto aldehydes and acylsilanes, which are hyperactive carbonyl compounds and prone to hydration, polymerization, and air oxidation. … The Dess-Martin oxidation of alcohols has proven to be an efficient method for the conversion of primary and secondary alcohol to aldehydes and ketones, respectively. The rate of oxidation is markedly accelerated in the presence of water. … [TPAP(Pr₄N⁺RuO₄^-)] is commercially available and environmentally friendly since it is used in catalytic amounts in the presence of a co-oxidant such as N-methylmorpholine-N-oxide (NMO).
  • George S. Zweifel and Michael H. Nantz, Modern Organic Synthesis (2006), Ch. 4 : Functional Group Transformations: Oxidation and Reduction.

• MnO₂ is a highly chemoselective oxidant—allylic, benzylic, and propargylic alcohols are oxidized faster than saturated alcohols. The oxidation takes place under mild conditions in H₂O, acetone, or CHCl₃. … Ba[MnO₄]₂ possesses similar chemoselectivites as MnO₂ in oxidations of alcohols, but it is more readily available and does not require special treatment for its activation. … Silver carbonate is especially useful for small-scale oxidations since the products usually are recovered in high purity by simply filtering the Ago and evaporating the solvent. … TEMPO is a commercially available nitroxyl radical-containing reagent that catalyzes the oxidation of primary and secondary alcohols in conjunction with co-oxidants.… Chemoselective oxidation of a secondary OH group in the presence of a primary OH group has been achieved with (NH₄)₂Ce(NO₃)₆, NaBrO₃. Note, however, that the reagent does not tolerate the presence of double bonds. … Triphenylcarbenium salts (Ph₃C⁺X^-) selectively oxidize secondary t-butyl or triphenylmethyl (trityl) ethers derived from alcohols. The oxidation proceeds via initial hydride abstraction followed by loss of the group on oxygen. … Chemoselective oxidation of a secondary OH group in the presence of a primary OH group is possible with NaOCl in aqueous acetic acid.
  • George S. Zweifel and Michael H. Nantz, Modern Organic Synthesis (2006), Ch. 4 : Functional Group Transformations: Oxidation and Reduction.

• A carbonyl transposition can be effected via the addition of a vinyl or an alkyl Grignard reagent to an α,β-unsaturated ketone. Acid-catalyzed rearrangement of the resultant allylic alcohol during oxidation with PCC affords the transposed α,β-unsaturated carbonyl substrate. This reaction represents a useful alternative when Wittig olefination of the ketone is problematic.
  Tertiary bis(allylic) alcohols are oxidized by PCC or PDC to the carbonyl transposed dienones.
  Addition of silica gel (SiO₂) to the PCC reaction greatly facilitates the workup, and application of ultrasound enhances the rate of the reaction and the yield of the product.
  • George S. Zweifel and Michael H. Nantz, Modern Organic Synthesis (2006), Ch. 4 : Functional Group Transformations: Oxidation and Reduction.