A simplified chemical reaction method recognized in 2001 by a group led by Nobel Laureate Prof. K. Barry Sharpless at the Scripps Research Institute. “Click” is meant to signify that joining molecular pieces is as easy as clicking together the two pieces of a buckle. The buckle works no matter what is attached to it, as long as its two pieces can reach each other and the components of the buckle can make a connection only with each other. Click reactions are easy to perform, give rise to their intended products in very high yields with little or no by-products, work well under many conditions (usually especially well in water), and are unaffected by the nature of the groups being connected to each other.
The central question posed by Sharpless and colleagues is whether desired functions can be obtained from molecules made using only click chemical reactions. In this deliberate eschewing of more sophisticated synthetic techniques is an implicit challenge to organic chemists that make complex structures by complex methods (can complex structures be made by simple methods?), and an explicit tip of the hat to the synthesis of functional polymers, which cannot occur without the use of reactions that meet the click chemistry standard. See also: Organic synthesis
Click reactions share the following attributes. (1) Many click components are derived from alkenes and alkynes, and thus ultimately from the cracking of petroleum. Carbon-carbon multiple bonds provide both energy and mechanistic pathways to be made into reactive structures for click connections. (2) Most click reactions involve the formation of carbon-heteroatom (mostly N, O, and S) bonds. This stands in contrast to much of synthetic organic chemistry of recent years, which has emphasized the formation of carbon-carbon bonds. (3) Click reactions are strongly exothermic by virtue of highly energetic reactants or strongly stabilized products. (4) Click reactions are usually fusion processes (leaving no by-products) or condensation processes (producing water as a by-product). (5) Many click reactions are highly tolerant of and often accelerated by the presence of water. See also: Alkene; Alkyne
These features are illustrated by the case of epoxide ring opening by a wide range of nucleophiles (Fig. 1). Since the epoxide is a strained three-membered ring, its ring opening is a strongly favored process. Yet that ring opening is required to occur in a particular way; that is, the nucleophile can attack the epoxide carbon atom only along the C-O bond axis, so by-products are avoided and yields are high. In addition, epoxides are relatively unreactive with water, while the hydrogen bonding ability and polar nature of water both serve to support the epoxide ring-opening reaction by other nucleophiles. See also: Electrophilic and nucleophilic reagents; Epoxide; Hydrogen bond
Most click reactions were discovered many years ago and have been widely appreciated, but have not been tapped for their full potential. They include the following classes of reactions, some of which are shown in Fig. 2.
Nucleophilic opening of spring-loaded electrophiles
The opening of the three-membered epoxide, aziridine, aziridinium, and episulfonium rings are all simple and versatile reactions. Included in this category are Michael addition reactions to α,β-unsaturated carbonyl compounds, which are also strongly favored processes.
Mild condensation reactions of carbonyl compounds
The formation of 1,3-dioxolane rings from aldehydes or ketones and of 1,3-diols, hydrazones, and oximes from aldehydes and hydrazines or hydroxylamine ethers, and of heterocycles from α- and β-carbonyl aldehydes, ketones, and esters are highly reliable and widely used reactions. See also: Aldehyde; Carbonyl; Heterocyclic compounds; Ketone; Oxime
Distinguished by the relatively nonpolar nature of their reactive groups, these fusion reactions comprise a wide range of processes, including the Diels-Alder reaction. Most useful are 1,3-dipolar cycloaddition reactions, with the cream of the crop being the reaction of organic azides with alkynes. See also: Azide; Diels-Alder reaction
The goals and philosophy of click chemistry have been most extensively explored with the uncatalyzed (Fig. 3a) and copper-catalyzed (Fig. 3b) versions of the cycloaddition reaction between azides and alkynes to form triazines. The former was first reported by A. Michael in 1893 and was later studied and popularized by the German chemist Rolf Huisgen in 1960s through the 1980s. The latter reaction was discovered independently in 2002 by the research groups of M. Meldal in Holland and K. B. Sharpless in the United States, and requires the use of a terminal alkyne. The abbreviation AAC (azide-alkyne cycloaddition) is used for the uncatalyzed process and CuAAC for the copper-mediated version.
These processes are uniquely useful because of the properties of the reactive substituents. Both azides and alkynes are high in chemical potential energy, and their fusion to make triazoles is exothermic by more than 45 kcal/mol. However, the rate of this reaction is quite slow, normally requiring prolonged heating for unactivated (not strongly electron-deficient or strained) alkynes. The azide and alkyne groups are stable in the presence of the nucleophiles, electrophiles, and solvents common to standard reaction conditions, with the azide being the only 1,3-dipolar reagent to have this quality. Most importantly, azides and alkynes are almost completely unreactive toward biological molecules. They are small and incapable of significant hydrogen bonding, and thus are unlikely to significantly change the properties of structures to which they are attached. Both can be easily introduced into organic compounds.
Click chemistry in situ
Because of their special reactivity—inert toward everything else and only sluggishly reactive with each other—azides and alkynes have been used to assemble molecules that bind very tightly to enzymes by using the enzyme itself as the reaction vessel (Fig. 4). The technique, known as click chemistry in situ, involves the tagging of molecules that bind to adjacent locations on an enzyme with azide and alkyne groups. If the derivatized molecules can interact simultaneously with the target in the proper orientation to hold the azide and alkyne units in proximity, a triazole may form to link the enzyme-binding components to each other. Since attachments to any target by two arms is better than either arm alone, the result is invariably a molecule of much higher binding affinity. Prior knowledge of the structure of the target is not required, nor is an assay for enzyme activity necessary. Since no reaction occurs between azides and alkynes at the concentrations used in these experiments without templating by the target enzyme, the formation of a new product is easy to detect by mass spectrometry and is proof of the creation of an excellent enzyme inhibitor. See also: Enzyme
Click chemistry in situ has been used to create novel high-affinity binders to the important neurotransmitter enzyme acetylcholinesterase, the metabolic enzyme carbonic anhydrase, and HIV protease. It has become apparent in these and other studies that the triazole linkage has advantageous properties for drug discovery. Having a strong dipole moment, the capacity for significant hydrogen bonding, and the ability to engage in π-stacking interactions, triazoles can engage in productive interactions with proteins. The creation of this unit from two “invisible” pieces in a pocket of an enzyme is an event that influences the selectivity of bond formation during in-situ drug discovery efforts. The click chemistry in-situ technique is used in many laboratories and pharmaceutical companies around the world as a complement to traditional methods of synthesis and screening. See also: Dipole moment
While not yet used directly in living cells because of the cytotoxicity and attendant bioregulation of copper, the copper-catalyzed reaction has achieved extraordinarily wide use in organic chemistry and materials science. Applications include the synthesis of biologically active compounds, the preparation of conjugates to proteins and polynucleotides, the synthesis of dyes, the elaboration of known polymers and the synthesis of new ones, the creation of responsive materials, and the covalent attachment of desired structures to surfaces.
Click chemistry expands the scope of structures that can be made by expert chemists and nonchemists alike. The rationale is simply that the more tolerant is the connection reaction between chemical pieces, the more diverse the pieces that can be brought to bear on any problem. Chemists possess neither the ability to control reactions as well as living cells do, nor the ability to manufacture large structures such as proteins for every purpose. Therefore, while nature can inspire us, making such complex molecules comes at a fearful cost of time and expense. If indeed it is chemical function that is the goal, keeping it simple is a useful rule.