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The most radical transformations in organic synthesis? Our top five reactions involving free radicals

May 2, 2019

 

The existence of a carbon species with a single unpaired electron was considered a radical idea in the nineteenth century. However, that changed in 1900, when Russian chemist Moses Gomberg first synthesized the carbon radical triphenylmethyl. Nowadays, we know that organic free radicals have tremendous biological and synthetic significance.

Key to the chemistry of organic radicals is their structure. The carbon atom, with its single unpaired electron, is desperate to bond with another atom to obtain its full complement of electrons. As a result, free radicals are extremely reactive and have been implicated in a wide range of biological processes including aging and cancer development. Indeed, our growing knowledge of organic free radicals has helped us to understand many natural phenomena, from DNA synthesis to sunburn.

Organic free radicals also play many vital roles in industry, as they are instrumental in producing various materials such as plastics and synthetic rubber. Reactions involving...

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Electrophilic addition: do you know your cyclopropanation from your hydrozirconation?

April 12, 2019

 

Electrophilic addition reactions are a bit like the coming together of the leading characters in a romantic comedy. On the one hand, a substrate possessing an electron-rich carbon–carbon double or triple bond. On the other, an electron-deficient electrophile that can’t wait to form new bonds of its own. Despite their differences, they’re a perfect match: the reaction resulting in the addition of the electrophile across the carbon–carbon bond.

Electrophilic addition reactions come in all shapes and sizes, with many playing a key role in the synthesis of natural products and industrially significant chemicals. Among the earliest electrophilic addition reactions to be identified was the Prilezhaev reaction, named after the Russian chemist Nikolai Alexandrovich Prilezhaev who, in 1909, was the first to oxidize alkenes to oxiranes using peroxycarboxylic acids. Today, peroxycarboxylic acids such as meta-chloroperoxybenzoic acid (mCPBA) are commonly used to prepare epoxides, which are vital for the manufacture of...

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Rearrangement reactions: why shifting bonds is so synthetically useful

March 5, 2019

 

Rearrangement reactions are among the most useful transformations in organic synthesis. Why? Well, these reactions do exactly what you might expect them to – rearrange the carbon skeleton of a molecule to produce a structural isomer of the original.

Most rearrangement reactions involve the relocation of a substituent between adjacent atoms, but some more adventurous rearrangements involve migrations over longer distances.

One of the earliest named rearrangement reactions is the Lossen rearrangement, in which O-acyl hydroxamic acids are converted to their corresponding isocyanates. Discovered in 1872 by German chemist Wilhelm Lossen, this reaction is still widely used today, due to the fact that the versatile isocyanate products generated by this transformation can be converted into a wide range of useful compounds, including amines, carbamates, ureas and amides.

Since the Lossen rearrangement was discovered, a whole host of other rearrangements have been developed – and some have proved particularly useful. We’ve taken a...

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The best organic reactions on offer: Five great reductions

Feb. 4, 2019

Reduction reactions play a vital role in organic synthesis. Historically, a reaction was considered a reduction if hydrogen was gained or oxygen lost. Nowadays, reduction is more broadly defined as a transformation when a compound gains one or more electrons – basically, a generalization of the original idea.

In these reactions, a ‘reducing agent’ donates electrons to another chemical species. Particularly generous electron donors include the earth metals and metal hydrides. Since the reducing agent loses electrons, it is itself oxidized in the process. Therefore reduction and oxidation always occur together, as electrons lost by one species must be gained by another.

Amongst the earliest named reduction reactions is the Tishchenko reaction, first reported in the early 1900s by the Russian organic chemist Vyacheslav Tishchenko. This reaction is still industrially relevant today as it is used to convert acetaldehyde into the commercially important solvent ethyl acetate.

Since Tishchenko’s discovery,...

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Carbonyl chemistry: have you used all five of these important reactions?

Jan. 3, 2019

The carbonyl bond is one of the most synthetically important functional groups in the whole of organic chemistry, and is present in aldehydes, ketones, esters, amides and many other carboxylic acid derivatives. Consisting of a carbon atom joined to an oxygen atom by a double bond, the carbonyl group is key to the versatile chemistry these compounds can undergo.

What makes carbonyl chemistry so interesting?

It’s all about the carbonyl bond. The more electronegative oxygen atom draws electron density from the carbon, increasing the polarity of the bond. As a result, the carbon atom is a good electrophile and is more vulnerable to attack by marauding nucleophiles such as negatively charged ions or molecules with a lone pair of electrons. This electrophilic nature means carbonyl groups can be involved in a range of transformations, including simple addition reactions (where the double bond is broken) and addition–elimination reactions (where the double bond...

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