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Oxidative cyclizations of olefinic alcohols to cyclic ethers may occur via [3+2], [2+2], [1] or epoxidation mechanisms. Insights into the mechanism is provided by structure-reactivity, implicating direct epoxidation by the chromate ester. [1] Subsequent epoxide opening and release of chromium leads to the observed products.
A chromate ester is a chemical structure that contains a chromium atom (symbol Cr) in a +6 oxidation state that is connected via an oxygen (O) linkage to a carbon (C) atom. The Cr itself is in its chromate form, with several oxygens attached, and the Cr–O–C attachment makes this chemical group structurally similar to other ester functional groups.
PCC is used as an oxidant.In particular, it has proven to be highly effective in oxidizing primary and secondary alcohols to aldehydes and ketones, respectively.The reagent is more selective than the related Jones' Reagent, so there is little chance of over-oxidation to form carboxylic acids if acidified potassium permanganate is used as long as water is not present in the reaction mixture.
Compounds of chromium(V) are rather rare; the oxidation state +5 is only realized in few compounds but are intermediates in many reactions involving oxidations by chromate. The only binary compound is the volatile chromium(V) fluoride (CrF 5). This red solid has a melting point of 30 °C and a boiling point of 117 °C.
Chromate salts contain the chromate anion, CrO 2− 4. Dichromate salts contain the dichromate anion, Cr 2 O 2− 7. They are oxyanions of chromium in the +6 oxidation state and are moderately strong oxidizing agents. In an aqueous solution, chromate and dichromate ions can be interconvertible.
Like conventional esters, the formation of this chromate ester is accelerated by the acid. These esters can be isolated when the alcohol is tertiary because these lack the α hydrogen that would be lost to form the carbonyl. For example, using tert-butyl alcohol, one can isolate tert-butyl chromate ((CH 3) 3 CO) 2 CrO 2), which is itself a good ...
Oxidation with chromium(VI)-amine complexes Collins reagent can be used as an alternative to the Jones reagent and pyridinium chlorochromate (PCC) when oxidizing secondary alcohols to ketones . PCC and pyridinium dichromate (PDC) oxidations have largely supplanted Collins oxidation.
The Cornforth reagent (pyridinium dichromate or PDC) is a pyridinium salt of dichromate with the chemical formula [C 5 H 5 NH] 2 [Cr 2 O 7].This compound is named after the Australian-British chemist Sir John Warcup Cornforth (b. 1917) who introduced it in 1962.