The Collins oxidation is an

organic reaction Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, Mechanistic Organ ...

for the oxidation of primary alcohols to aldehydes. It is distinguished from other chromium oxide-based oxidations by the use of Collins reagent, a complex of

chromium(VI) oxide Chromium trioxide (also known as chromium(VI) oxide or chromic anhydride) is an inorganic compound with the formula CrO3. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name. This compound is a dark-purple s ...

with pyridine in

dichloromethane Dichloromethane (DCM or methylene chloride, methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odour is widely used as a solvent. Although it is not miscible with ...

Mechanism

The mechanism of the Collins oxidation is a relatively simple oxidation process.

History

The collins oxidation first came about in 1968 when J.C. Collins used pre-formed CrO3•2Pyr dissolved in

to oxidize alcohols. Although difficult, it was beneficial at the time because it provided an alternative to the Sarett oxidation, that used pyridine as a solvent. The Collins oxidation allowed for a less basic reagent, which in turn provided a useful option for oxidation of primary alcohols to aldehydes. A safer variant of the Collins oxidation was discovered in 1970 by Ratcliffe and Rodehorst. The variant featured an ''in situ'' preparation of the Collins reagent by adding one equivalent of CrO3 over two equivalents of pyridine in

Benefits

The Collins oxidation is also very useful because it is cheap in comparison to its oxidizing counterparts, PCC and

PDC PDC may refer to: In science and technology Chemistry, biology and medicine * Phosducin, a human protein and gene in the retina * Pyridinium dichromate (Cornforth reagent), a chromium-based oxidant * Pyruvate dehydrogenase complex, an enzyme ...

. However, it is more difficult experimentally because of its required

anhydrous A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...

conditions. The Collins oxidation is a good option when using uncomplicated substrates because it produces good yields of aldehyde and

ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bo ...

products. However, as the complexity of the substrates increases, the usefulness of the Collins oxidation decreases because it lacks the selectivity that other reagents have.

Uses

One of the main uses of the Collins oxidation is the transformation of alkenes to enones by adding carbonyl groups to

allylic In organic chemistry, an allyl group is a substituent with the structural formula , where R is the rest of the molecule. It consists of a methylene bridge () attached to a vinyl group (). The name is derived from the scientific name for garlic, . ...

positions. While this process is very slow, it allows for alcohols to be oxidized to aldehydes or

ketones In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bon ...

without alkene interference. The Collins oxidation can also be used to form cyclic chromate esters from 1,2-

diols A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. The most common industrial diol is e ...

in order to them intramolecularly oxidize alkenes. This process can then result in the formation of highly stereoselective tetrahydrofuran.

Related oxidation reactions

Several chromium oxides are used for related oxidations. These include Jones oxidation and Sarett oxidation.

References