Oxidation with chromium(VI) complexes involves the conversion of alcohols to carbonyl compounds or more highly oxidized products through the action of molecular chromium(VI) oxides and salts. The principal reagents are Collins reagent, PDC, and PCC. These reagents represent improvements over inorganic chromium(VI) reagents such as Jones reagent. Inventory of Cr(VI)-pyridine and pyridinium reagents Cr(VI)-pyridine and pyridinium reagents have the advantage that they are soluble in organic solvents as are the alcohol substrates. One family of reagents employs the complex CrO3(pyridine)2. Sarett's reagent: a solution of CrO3(pyridine)2 in pyridine. It was popularized for selective oxidation of primary and secondary alcohols to carbonyl compounds. Collins reagent is a solution of the same CrO3(pyridine)2 but in dichloromethane. The Ratcliffe variant of Collins reagent relates to details of the preparation of this solution, i.e., the addition of chromium trioxide to a solution of pyridine in methylene chloride. The second family of reagents are salts, featuring the pyridinium cation (C5H5NH+). pyridinium dichromate (PDC) is the pyridium salt of dichromate, [Cr2O7]2-. pyridinium chlorochromate (PCC) is the pyridinium salt of [CrO3Cl]−. These salts are less reactive, 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 afford enones from allylic alcohols. Practical considerations Oxidation with chromium(VI) has two primary limitations. Operationally, the tarry byproducts lead to lowered yields. In addition, some such reagents (particularly PCC) react with acid-labile functionality. Thus, these agents have been employed in oxidations of relatively simple substrates, often in excess to account for reagent trapping and decomposition. The use of adsorbents such as Celite or silica gel facilitates the removal of chromium byproducts and eliminates many of the operational difficulties associated with chromium-mediated oxidations. (1) Mechanism and stereochemistry Chromate esters are implicated in these reactions. The chromate ester decomposes to the aldehyde or carbonyl by transfer of a alpha proton. Large kinetic isotope effects are observed. (2) Oxidative annulation of alkenols to form six-membered rings may be accomplished with PCC. This process 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 may occur upon treatment with base as shown in equation (3) below. (3) An important process mediated by chromium(VI)-amines is