Oxidation with chromium(VI) complexes

Chromium(VI)-amine oxidation reactions involve the conversion of alcohols to carbonyl compounds or more highly oxidized products through the action of chromium(VI) oxide-amine adducts and salts. Representative members of this family of reagents include Collins reagent, pyridinium chlorochromate (PCC), and pyridinium dichromate (PDC).

Introduction

Sarrett identified the adduct of pyridine and chromium(VI) oxide (Collins reagent) as a selective compound for the oxidation of primary and secondary alcohols to carbonyl compounds. Collins reagent suffers from difficulties associated with its preparation, stability, and efficiency, however. The milder adducts pyridinium chlorochromate (PCC) and pyridinium dichromate (PDC) are more easily handled and more selective than Collins reagent in oxidations of alcohols. These reagents, as well as other, more exotic adducts of nitrogen heterocycles with chromium(VI), facilitate a number of oxidative transformations of organic compounds, including cyclization to form tetrahydrofuran derivatives and allylic transposition to give enones from allylic alcohols.

There are two primary limitations of oxidation by chromium(VI) amines. Operationally, the tarry byproducts of chromium oxidations cause reduced yields and reagent sequestration. In addition, Cr(VI)-amines (particularly PCC) may react with acid-labile functionality. Thus, they have been employed in oxidations of relatively simple substrates, often in excess to account for reagent trapping and decomposition.

Mechanism and Stereochemistry

Prevailing Mechanisms

Chromate esters have been implicated in most oxidations of alcohols by chromium(VI)-amines. After formation of the chromate ester, either deprotonation or hydride transfer leads to the product carbonyl compound. Isotopic labeleing studies have shown that C-H bond cleavage is involved in the rate-determining step.

Oxidative annulation of alkenols to form six-membered rings may be accomplished with PCC. This is postulated to occur via initial oxidation of the alcohol, attack of the alkene on the new carbonyl, then re-oxidation to a ketone. Double-bond isomerization is likely under the acidic conditions of the reaction.

An important process mediated by chromium(VI)-amines is oxidative transposition of tertiary allylic alcohols to give enones. The mechanism of this process likely depends on the acidity of the chromium reagent. Acidic reagents such as PCC may cause ionization and recombination of the chromate ester, while the basic reagents (Collins) likely undergo direct allylic transposition via sigmatropic rearrangement.

Oxidative cyclizations of olefinic alcohols to cyclic ethers may occur via [3+2], [2+2], or epoxidation mechanisms. The exact mechanism has been debated, although there is evidence for a direct epoxidation by the chromate ester. Subsequent epoxide opening and release of chromium(IV) leads to the observed products.

Scope and Limitations

Buffering agents may be used to protect acid-labile protecting groups from removal during chromium(VI)-amine oxidations. However, buffers will also slow down oxidative cyclizations, leading to selective oxidation of alcohols. Citronellol, for instance, which cyclizes to pugellols in the presence of PCC, does not undergo cyclization when buffers are used.

Oxidative cyclization can be used to prepared substituted tetrahydrofurans. Cyclization of dienols leads to the formation of two tetrahydrofuran rings in a syn fashion.

Enones can be synthesized from tertiary allylic alcohols through the action of a variety of chromium(VI)-amine reagents. The reaction is driven by the formation of a more substituted double bond. (E) enones form in greater amounts than (Z) isomers due to chromium-mediated geometric isomerization.

Suitably substituted olefinic alcohols undergo oxidative cyclization to give tetrahydrofurans. Further oxidation of these compounds to give tetrahydropyranyl carbonyl compounds may occur.

Comparison with Other Methods

Methods employing dimethyl sulfoxide (the Swern and Moffatt oxidations) are superior to chromium(VI)-amines for oxidations of substrates with heteroatom functionality that may coordinate to chromium. Dess-Martin periodinane (DMP) offers the advantages of operational simplicity, a lack of heavy metal byproducts, and selective oxidation of complex, late-stage synthetic intermediates. Additionally, both DMP and manganese dioxide (MnO₂) can be used to oxidize allylic alcohols to the corresponding enones without allylic transposition. When allylic transpositions is desired, however, chromium(VI)-amine reagents are unrivaled.

Catalytic methods employing cheaper terminal oxidants in conjunction with catalytic amounts of chromium reagents offer the benefit of small amounts of metal byproducts. However, undesired side reactions mediated by stoichiometric amounts of the terminal oxidant may occur.

Experimental Conditions and Procedure

Typical Conditions

Reagent-grade pyridine is usually sufficient for the preparation of PDC and PCC. Although these reagents may darken over time, their loss in activity is minimal. Isolated reagents should be stored in a desiccator in the dark. Care should be taken when adding chromium trioxide to pyridine, as ignition of pyridine has been known to occur.

Reduced chromium residues can be removed from glassware with concentrated HCl or 10-15% aqueous HF. Solid chromium waste should never be thrown away, as residual CrO₃ may ignite. Chromium(VI) reagents are toxic and should be handled with care in a well-ventilated fume hood.

Example Procedure

To a suspension of 4-(dimethylamino)pyridinium chlorochromate (1.24 g, 4.8 mmol) in dry dichloromethane (7.0 mL) was added p-(3-hydroxypropyl)-benzyl alcohol (200 mg, 1.2 mmol). Stirring was continued (2 hours) at room temperature under a nitrogen atmosphere. The reaction mixture was diluted with ethyl acetate (10 mL) and the brown granular chromium reduction products were removed by vacuum filtration through a Celite® pad. Concentration of the solvent and column chromatography on silica gel (hexane/ethyl acetate, 2:1) furnished 121 mg (62%) of p-(3-hydroxypropyl)benzaldehyde as an oil. The spectral data were the same as those of the product reported above with imidazolium dichromate.