1,3-Dipolar cycloaddition

Example of 1,3-dipolar cycloaddition

General mechanism of 1,3-dipolar cycloaddition

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18 species of second-row 1,3-dipoles.tif

Major resonance structures of diazomethane and hydrazoic acid

Solvent effect

The solvent effect on the reaction between enamines and diazomalonate.tif

1,3-Dipolar cycloaddition

1,3-Dipolar cycloaddition is a type of cycloaddition reaction in organic chemistry where a 1,3-dipole reacts with a dipolarophile to form a five-membered ring. This reaction is a subset of the more general pericyclic reactions and is a valuable method for constructing heterocycles.

Mechanism

The mechanism of 1,3-dipolar cycloaddition involves the interaction between a 1,3-dipole and a dipolarophile. The 1,3-dipole is a molecule with a delocalized set of electrons over three atoms, typically represented as X=Y+-Z−. The dipolarophile is usually an alkene or alkyne that can participate in the cycloaddition. The reaction proceeds through a concerted mechanism, meaning that the bond formation and bond breaking occur simultaneously in a single step. This results in the formation of a five-membered ring, which can be either a pyrrolidine, isoxazoline, or other heterocyclic structures depending on the nature of the 1,3-dipole and dipolarophile.

Types of 1,3-Dipoles

Common 1,3-dipoles include:

• Azides (R-N3)
• Nitrile oxides (R-C≡N+-O−)
• Nitrile imines (R-C≡N+-N−)
• Nitrile ylides (R-C≡N+-C−)
• Ozone (O3)

Applications

1,3-Dipolar cycloaddition reactions are widely used in the synthesis of natural products, pharmaceuticals, and other complex organic molecules. They are particularly valuable for constructing heterocycles, which are common motifs in many biologically active compounds. One notable application is the synthesis of triazoles via the Huisgen cycloaddition, which involves the reaction of an azide with an alkyne. This reaction is a cornerstone of click chemistry, a concept introduced by K. Barry Sharpless.

Stereochemistry

The stereochemistry of 1,3-dipolar cycloaddition can be complex, as the reaction can produce multiple stereoisomers. The selectivity of the reaction can be influenced by the nature of the substituents on the 1,3-dipole and the dipolarophile, as well as by the reaction conditions.

See also

• Cycloaddition
• Pericyclic reaction
• Click chemistry
• Huisgen cycloaddition
• Heterocycle