In the presence of pyridine, the oxidizing property of Cr(VI) is lowered and now it can be used to selectively to oxidize primary alcohols to the corresponding aldehydes. The chromium trioxide- pyridine complexes, CrO₃·py₂ (Collins reagent) and pyH⁺CrO₃Cl⁻ (Sarrett reagent) are also known to oxidize allylic methylene group (Scheme 3).

F. Freeman, Encyclopedia of Reagents for Organic Synthesis , John Wiley and Sons, Inc., L. A. Paquette, Ed., New York, 1995, 5 , 2272.

 

Scheme 3

Another useful application of PCC is in the conversion of allylic tertiary alcohols to their transposed α,β-unsaturated ketones. The reaction is thought to proceed by rearrangement of the chromate ester of the allylic alcohol to give a new allyl chromate ester that is oxidized to the ketone (Scheme 4).

 

Scheme 4