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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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Oxidation reactions: transformations that deserve the oxygen of publicity

Nov. 7, 2018

Oxidation reactions are chemical transformations that result in a substrate losing electrons and increasing its oxidation state. They involve the use of an oxidizing agent or oxidant—a reagent in its higher oxidation state that is capable of gaining electrons, and is itself reduced in the reaction.

The term oxidation takes its name from the process of adding oxygen to compounds using oxygen gas (the first known oxidizing agent). But while the addition of oxygen in this way meets the modern definition of oxidation, oxygen gas is far from the only oxidizing agent. A wide range of oxidants exist, including reagents as diverse as hydrogen peroxide, permanganate salts, chromate salts and osmium tetroxide, to name just a few. Even reagents that don’t include oxygen atoms, such as halogens like fluorine and chlorine, can be powerful oxidants.

Amongst the earliest named oxidation reactions to be developed is the Tishchenko...

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Test your metal: How many of these transition metal-catalyzed couplings have you used?

Oct. 23, 2018

Transition metal-catalyzed coupling reactions are an important family of chemical transformations that are commonly used to create new carbon-carbon and carbon-heteroatom bonds from structurally useful building blocks. These convenient reactions offer chemists a synthetic short-cut to a broad range of pharmaceutical, agrochemical and industrial compounds.

Without transition metal-catalyzed coupling reactions, many of the chemicals used in everyday life couldn’t be produced anywhere near as quickly or cost-effectively (or even at all). They’re so valuable, in fact, that the 2010 Nobel Prize in Chemistry was awarded to Richard F. Heck, Ei-ichi Negishi and Akira Suzuki in recognition of their remarkable contributions to the development of palladium-catalyzed cross-coupling reactions (more on these award-winning transformations below!).

Transition metal-catalyzed coupling reactions fall into two broad categories: homocouplings and heterocouplings (also known as cross-coupling reactions). Homocoupling reactions bring two identical structures together to form a new product, while cross-couplings react two different reagents. Palladium is...

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