[81121-61-1] · C7H11ClCrN2O3 · 4-(Dimethylamino)pyridinium Chlorochromate · (MW 258.65)
(mild chromium oxidant, selective for the oxidation of benzylic, allylic, and sterically hindered alcohols to the corresponding aldehydes or ketones)
Alternate Name: DMAPCC.
Physical Data: mp 92-94 °C.
Solubility: sol H2O; sl sol dichloromethane, chloroform.
Form Supplied in: yellow powder; commercially available.
Analysis of Reagent Purity: elemental analysis.
Preparative Method: by addition of 4-Dimethylaminopyridine to a cooled solution of Chromium(VI) Oxide oxide in aqueous HCl.1
Handling, Storage, and Precautions: nonhygroscopic, shelf stable at rt indefinitely; slightly light sensitive over a period of days; preferably should be stored for long periods in a brown glass container. Treat as a typical chromium(VI)-amine reagent: oxidizer and cancer suspect agent. The reagent is most conveniently used as a suspension in CH2Cl2 or CHCl3.
DMAPCC was developed in an investigation of the effects of varying the amine ligand on chlorochromate oxidations of alcohols.1 Compared to other pyridinium ligands, the (dimethylamino)pyridinium species oxidizes primary alcohols only very slowly. For example, when n-heptanol is treated with 4.0 equiv of DMAPCC in dichloromethane for 4 h, 94% of the starting alcohol is recovered (5% oxidation to heptanal).
In contrast to these results, DMAPCC is a useful reagent for the oxidation of allylic and benzylic alcohols to the corresponding aldehydes or ketones (eqs 1-3).1 Like Pyridinium Chlorochromate, the reagent is acidic enough to isomerize initially formed (Z)-unsaturated aldehydes to the corresponding (E)-isomers (eq 4). The benzylic selectivity is illustrated in the oxidation of 3-[4-(hydroxymethyl)phenyl]propanol using DMAPCC (eq 5). Treatment of this diol with excess reagent affords the benzaldehyde derivative in 62% yield. Less than 2% dialdehyde is obtained under these conditions. In contrast, oxidation of this diol using PCC under analogous conditions affords a mixture of mono- and dialdehyde in 45% and 32% yield, respectively.1
In the attempted selective oxidation of the reduction product of andrenosterone with 3 equiv of DMAPCC under normal conditions, 11-oxotestosterone was obtained in 49% yield (eq 6). Less than 3% andrenosterone was obtained under these conditions.1 Similarly, treatment of the triol in eq 7 with 4 equiv of the reagent for 3 h affords the more hindered monoketone in 36% yield.2 Allowing the reaction to proceed for 22 h affords the tricarbonyl compound in 23% yield. It is likely that these observed selectivities are due to differential breakdown of the initially formed, more sterically hindered, secondary chromate ester.
As with most amine-modified chromium(VI) reagents, attempted oxidations of unprotonated basic nitrogen-containing molecules with DMAPCC are generally unsuccessful.3 In the attempted oxidation of 2-pyridylcarbinol with this reagent, no aldehyde is observed, nor can starting alcohol be recovered from the reaction mixture. 4-(Dimethylamino)pyridine is the only organic product isolated from the reaction.3
Manganese Dioxide has been widely used for the selective oxidations of allylic and benzylic alcohols; however, the reagent is most effective when freshly prepared, large excesses of the reagent must be used, and long reaction times are generally necessary for successful oxidations.4 Barium Manganate has also proved to be an effective reagent for similar oxidations.5 A number of amine-modified chromium(VI) reagents have also been reported to display allylic and benzylic oxidation selectivity.3
Frank S. Guziec, Jr.
New Mexico State University, Las Cruces, NM, USA