Cycloadditions
Cycloadditions - Related Videos
01:16
Cycloaddition Reactions: Overview
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2023
Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition. The feasibility of cycloaddition reactions under thermal and photochemical conditions can be predicted using...
01:12
Cycloaddition Reactions: MO Requirements for Photochemical Activation
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2023
Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden. Thermally-induced [2 + 2] cycloadditions are symmetry forbidden. This is because the ground state HOMO of one ethylene molecule and the LUMO of the other ethylene are out of phase, preventing a concerted suprafacial-suprafacial overlap. Absorption...
01:16
Cycloaddition Reactions: MO Requirements for Thermal Activation
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2023
Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical example of a thermally-allowed cycloaddition is the Diels–Alder reaction, which is a [4 + 2] cycloaddition. In contrast, a [2 + 2] cycloaddition is thermally forbidden. The reaction occurs between the highest occupied molecular orbital (HOMO) of one π component and the lowest unoccupied molecular orbital (LUMO) of the other. These are known as...
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01:16
[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction
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2023
The Diels–Alder reaction is an example of a thermal pericyclic reaction between a conjugated diene and an alkene or alkyne, commonly referred to as a dienophile. The reaction involves a concerted movement of six π electrons, four from the diene and two from the dienophile, forming an unsaturated six-membered ring. As a result, these reactions are classified as [4+2] cycloadditions. From a molecular orbital perspective, the interacting lobes of the two π systems must be in phase to permit the...
09:06
Efficient and Site-specific Antibody Labeling by Strain-promoted Azide-alkyne Cycloaddition
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Cited by 3 •
2016
Here, we present a protocol to site-specifically introduce chemical probes into an antibody fragment by genetically incorporating an azide-containing amino acid, and subsequently coupling the azide with a chemical probe by strain-promoted azide-alkyne cycloaddition (SPAAC).
FAQs
Orbital symmetry determines whether the interacting π systems can overlap in a way that supports simultaneous bond formation. In a [4+2] reaction, the diene and dienophile must align their orbitals appropriately, while some [2+2] reactions may depend on light to access a compatible pathway. This principle helps chemists distinguish feasible reaction conditions from unfavorable ones.
Substituents alter the electron distribution and shape of the reacting unsaturated components, affecting which atoms bond to one another. Their electronic effects can favor one orientation over another. Evaluating these factors before reaction helps chemists anticipate major regioisomers and design a transformation that builds the desired molecular framework.
The spatial relationships present in the starting unsaturated components can be transferred into the newly formed ring because bond formation occurs through coordinated π-bond changes. As a result, the reaction can preserve meaningful stereochemical information rather than generating an arbitrary mixture. This feature is especially valuable when synthesis requires a defined three-dimensional arrangement in a complex molecular framework.
Planning starts by matching the unsaturated components to the ring framework sought in the product. The chemist then examines orbital symmetry, substituent effects, and possible regioselective orientations, followed by choosing conditions involving heat or light when appropriate. This sequence connects molecular design to the expected bond pattern and helps prioritize a selective transformation rather than trial-and-error assembly.
Their ability to form rings efficiently makes them useful for constructing frameworks found in pharmaceuticals and natural products. The same bond-forming strategy also supports preparation of polymers and advanced materials. Because the reactions can be stereospecific, they are relevant when a target requires controlled molecular architecture, not merely creation of a new ring.
Ring-forming efficiency and controlled connectivity make these reactions relevant beyond small-molecule synthesis. In polymer chemistry, they can support construction of larger molecular architectures, while advanced-materials research benefits from the ability to build defined frameworks. The same stereospecificity and selectivity that aid organic synthesis can help control structural features in these larger systems.