[76899-34-8] · C10H9ClCrN2O3 · 2,2´-Bipyridinium Chlorochromate · (MW 292.64)
Alternate Name: BPCC.
Physical Data: mp not available.
Solubility: insol dichloromethane; sol acetone.
Form Supplied in: yellow crystalline solid; widely available.
Handling, Storage, and Precautions: the dry reagent is an air-stable and nonhygroscopic solid; it can be stored for extended periods protected from light and over calcium chloride. Solvents must be anhydrous and free of reducing impurities. 2,2´-Bipyridinium chlorochromate is reputed to be a toxic agent. The reagent should be handled in a fume hood.
2,2´-Bipyridinium chlorochromate (BPCC) is a synthetically useful variant of Pyridinium Chlorochromate (PCC), in which the presence of the 2,2´-bipyridyl cationic ligand seems to offer special advantages in certain instances. Treatment of primary and secondary alcohols with BPCC in dichloromethane or acetone at rt for 2-4 h affords aldehydes and ketones in good yield (eqs 1-3).2
No overoxidation of aldehydes to carboxylic acids has been observed. BPCC is weaker than PCC, and thus 2-4 equiv are required for good yields.3 However, the use of this reagent greatly simplifies purification of reaction products, since the chromium-containing byproducts are water-soluble granular materials, which can easily be removed by filtration through Celite. BPCC has proven to be efficient for the oxidation of alcohols in molecules containing acid-sensitive groups, due to the buffering effect of the 2,2´-bipyridyl system. Oxidation of citronellol gives citronellal in 89% yield without the cationic cyclization that occurs when unbuffered pyridinium chlorochromate is used.4 BPCC can also be buffered with sodium acetate, and this is especially useful for oxidations in systems prone to epimerization (eq 4).5,6
BPCC can be utilized for the oxidation of unsaturated primary alcohols in polyenes. Thus (2E,4Z)-2,4,11-dodecatrien-1-ol is smoothly oxidized in dichloromethane at rt to the corresponding unsaturated aldehyde without E/Z isomerization (eq 5).7
A particularly mild oxidant can be obtained by combination of BPCC with neutral Alumina. 1,2-Diols undergo oxidative carbon-carbon cleavage to aldehydes when treated with BPCC alone,8 but BPCC/alumina (1:1) converts the same 1,2-diol into an a-hydroxy ketone, albeit in moderate yield (eq 6).9
BPCC in conjuction with alumina as a buffering agent is effective in oxidizing a primary alcohol in the presence of a silyl ether protecting group (the acidic PCC reagent attacks silyl ethers). 5-(Benzyloxy)-3-[(t-butyldiphenylsilyl)oxy]pentan-1-ol is thus converted into the corresponding aldehyde in almost quantitative yield (eq 7).10
BPCC in acetone is able to perform benzylic oxidation of hydrocarbons to aryl ketones. A large excess of oxidant must be used (16 equiv) and these reactions require 24-28 h (eqs 8 and 9).11 PCC is also effective for these types of benzylic oxidation.
PCC is a useful reagent in oxidative ring opening of several furans, but the reagent is less effective in the reactions of reactive furans, such as 5-methoxy derivatives. By contrast, treatment of the same substrate with BPCC effects a smooth cleavage to an a,b-unsaturated g-keto ester (eq 10).12
BPCC (1 equiv) at rt can oxidize dialkyl sulfides to the corresponding sulfoxides (68-87% yield). However, dialkyl sulfones may be present as a byproduct, especially with primary alkyls (eq 11).4
University of Rome
La Sapienza and CNR, Italy