25 years ago click chemistry changed science. What happened next?

Barry Sharpless and his team first popularised the concept of ‘clicking’ two molecules together back in 2001 – 25 years ago this month. The review paper that launched a new field has now been cited almost 20,000 times, making it one of the most influential chemistry papers ever written. And in 2022, Sharpless, along with Morten Meldal and Carolyn Bertozzi, won the Nobel prize in chemistry for establishing the field of click and bioorthogonal chemistry. But what are click reactions and what might the next 25 years of this field look like?

So, what is click chemistry?

Taking inspiration from how nature can combine small building blocks, Sharpless’s team first described a handful of ‘spring-loaded’ synthetic reactions that ‘click’ two molecules together in 2001. ‘[The name] was meant to call back that feeling that one gets when you snap together the two halves of a luggage strap – that satisfying click,’ says M G Finn at Georgia Tech in the US. Finn helped develop the click concept with Sharpless and Hartmuth Kolb at the Scripps Research Institute, US, leading to the seminal paper that established the field.

The team explained that such reactions must be modular, wide in scope, high-yielding and able to take place without a solvent or benign solvents such as water. Click reactions also had to be stereospecific and selective without the need to isolate the product using chromatography, as well as thermodynamically favourable.

Copper-catalysed cycloadditions between azides and alkynes to form triazole rings are the most well-known example of a click reaction. Sharpless’s team described this as the ‘cream of the crop’. However, other reactions fall under the umbrella of click reactions, including nucleophilic ring-openings of strained heterocycles such as epoxides and oxidative additions to carbon–carbon multiple bonds.

Why was click chemistry so revolutionary?

‘The idea of click chemistry has empowered non-organic chemists to be able to think about making new molecules,’ says Finn. ‘It allows biologists, material scientists [and others] to make bonds where they might not have thought that they could do that before,’ he says. ‘We sometimes refer to this as the democratisation of chemical synthesis.’ He adds that it has also challenged synthetic chemists to develop and expand the scope of such reactions. Chemists often use click reactions to label and modify cell surfaces, create antibody–drug conjugates or synthesise cross-linked polymers.

‘In my own area [click chemistry reactions] have been really revolutionary for the rapid synthesis of radio pharmaceuticals for imaging and therapy,’ says James Knight at Newcastle University in the UK. Radioactive isotopes often have short half-lives, ‘so the ability to use [these] reactions to shorten those reaction times and often remove the necessity of purification … they’ve been very useful in that area,’ Knight adds.

‘Some of the most useful and the most impactful are cycloaddition reactions, particularly a tetrazine ligation,’ says Finn. This involves a retro-Diels Alder reaction between a tetrazine – a six-membered aromatic ring with four nitrogen atoms – and a strained dienophile, such as trans cyclooctene. Release of dinitrogen gas drives the reaction and means that the reaction is irreversible, allowing scientists to create stable linkages. Finn explains that this reaction is ‘notable’ as the rates of reaction ‘are very high and selectivity is good’.

How has click chemistry evolved?

A major development within click chemistry has been bioorthogonal chemistry, pioneered by Carolyn Bertozzi in 2004. ‘A bioorthogonal reaction is a type of click reaction that is used on biological molecules, or in a biological context,’ says Finn. Staudinger ligation is an example of bioorthogonal click chemistry, he adds, where an azide and phosphine react to produce an iminophosphorane, which can then react with nearby esters to form stable amides.

Finn adds that reactions such as these allow scientists to probe biological systems by selectively forming bonds in complex chemical environments, by either attaching fluorescent dyes or chemical tags that can be identified in proteomic experiments.

‘The field, for example, of activity-based protein profiling allows us to explore and identify enzymes that act in particular ways,’ says Finn. ‘The field depends on the use of click chemistry in the workflow to identify what has bound to what.’

Chemists have developed a variety of other click reactions since Sharpless introduced the concept in 2001. In 2023, Sharpless’s team developed a way of exchanging phosphorus–fluorine bonds with alcohols or amines to form phosphorus–oxygen or phosphorus–nitrogen bonds. The method works in as little as 15 minutes and the reactions work alongside other click reactions. More recently, an allene–ketone addition catalysed by copper(I) formed a carbon–carbon bond, which was previously out of reach for click reactions.

Other researchers are also adapting the classic copper-catalysed azide–alkyne coupling to work without solvent. The mechanochemical approach shakes the reagents around a copper coil, with the ‘clicking’ taking place on the copper surface.

What might the next 25 years of click chemistry bring?

Knight thinks that a growing area will be ‘click-to-release’ drug systems. ‘You can imagine an antibody–drug conjugate where the antibody effectively ferries the drug to the target site – often a cell surface biomarker’. He adds that a second chemical can then cleave the click linker, allowing for ‘very precise deposition of the highly cytotoxic drug’. ‘I think this is something which is going to have a very important translational impact in medicine,’ he says.

‘Of course, click chemistry has never left polymer science,’ says Finn. ‘But I think there will be more and more applications and growth of making sophisticated materials using click reactions… That field has nowhere reached its maturity.’

Finn also explains that over the past decade, Sharpless has been working on click reactions ‘that can read the room’. These ‘undergo specific reactions depending on the chemical environment within the biological system’. ‘The best example of this is what we’ve called the SuFEx [sulfur fluoride exchange] reaction, which relies intensely on the ability of protons to activate the sulfur fluorine bond.’ He adds that these protons may come from specific residues within a protein’s active site or from a specific nucleotide.

As for his own research, Finn is exploring the use of click chemistry in molecular evolution. He explains that biology can transmit information through ‘highly reliable reactions with incredibly ornate machinery’, including the cell’s ribosome and DNA polymerases. ‘If you can already accomplish the high reliability aspect of the bond-forming reactions by choosing a non-biological click reaction, is there a way to turn that into systems that can evolve?’

‘This is something that I think a lot about, and we’re working on, but it’s got a long way to go,’ he says.