Dipolar cycloaddition reactions are useful both for the synthesis of heterocyclic compounds and for carbon-carbon bond formation. The molecules that are capable of dipolar cycloaddition are called 1,3-dipoles which have $\pi$-electron systems that are isoelectronic with allyl anion consisting of two filled and one empty orbital.
The other reactant in a dipolar cycloaddition usually an alkene or alkyne is referred to as the dipolarophile. Other multiply bonded functional groups such as imine, azo and nitroso groups can also act as dipolarphiles. The transition states for 1,3-dipolar cycloadditions involve four $\pi$-electron from the 1,3-dipole and two from the dipolarophile.
The 1,3-dipolar cycloaddition reaction of azides and alkynes produces 1,4-disubmitted and 1,5-disubmitted 1,2,3-triazole compounds with interesting biological properties. The reaction is highly exothermic reaction and usually occurs at a low temperature even at room temperature.
The 1,3-dipolar cycloaddition reaction of an azide and an alkyne is shown below.
There is a large class of reactions known as 1,3-dipolar cycloaddition reactions that are analogous to the Diels Alder reaction in that they are concerted cycloadditons.
The mechanism of 1,3-dipolar cycloaddition mechanism is shown with the example ozonolysis.
Stereospecificity in the 1,3-Dipolar Cycloaddition Reaction
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The stereospecificity observed in many 1,3-dipolar cycloadditions is often considered to be compelling, if not conclusive evidence for concert in these reactions. However if the rate constant for rotation about bond in diradical intermediate was much smaller than the rate constant for cyclization, high stereospecificity would still be observed. Cycloaddition is thought to occur by a concerted process, because the stereochemistry (E or Z) of the alkene dipolarophile is maintained in the cycloadduct (a stereospecific aspect).