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1,3- Dipolar Cycloaddition

May 20, 2013

A chemical reaction between a 1, 3-dipole and a dipolarophile is known as 1,3-dipolar Cycloaddition which results the formation of a five-membered ring. Generally the dipolar cycloaddition is observed in between an organic azide and an alkyne to form a 1, 2, 3-triazole as product which is a cyclic compound. These are region-selective and stereo-selective synthesis which generates five-membered heterocyclic compounds.

There are two possible 1 3 Dipolar Cycloaddition Mechanism; one is the concerted pericyclic addition which is proposed by Rolf Huisgen while the second one is a stepwise mechanism which involve the formation of a diradical as an intermediate and this mechanism is proposed by Firestone.

Generally the first mechanism is used to describe the 1 3 Dipolar Cycloaddition. In this mechanism; one 1, 3-dipole and a dipolarophile reacts in a concerted manner to form a five-membered ring. It is an asynchronous and symmetry-allowed process which occurs in π4s + π2s fashion by the formation of a Huckel aromatic transition state which contains six-electrons.

There are many example of this type of reaction such as ozonolysis of alkene to from the carbonyl compounds. In ozonolysis; ozone molecule is added on double bond by cycloaddition to form an intermediate called as ozonide which is un-stable and decompose readily by cycloreversion to form carbonyl compound; Aldehydes or ketones with carbonyl oxide.

Because of positive inductive effect of alkyl group; the more alkylated part of alkene involve in intermediate while less alkylated end involves in the formation of carbonyl compound. The carbonyl oxide further involves in cycloaddition forming secondary ozonide which oxidised by hydrogen peroxide and hydrolysed to form carbonyl compounds.

The cycloaddition involves the combination of 4 π-electrons of the diene with 2 π-electrons of the dienophile to form a cyclic ring. Overall it involves the conversion of pi-bond to sigma bond which is basically driving force of the reaction as sigma bonds are more stable and strong compare to pi-bonds. Both reactants of reaction can be substituted and the product will form according to that only.

Therefore; it has wide applications in synthetic organic chemistry for the preparation of unsaturated six-membered rings. In these reactions; the highest occupied molecular orbital of the diene gets overlap with the lowest unoccupied molecular orbital of the dienophile and generates degenerate orbitals which are at same energy level.

If there is any electron-withdrawing group is bonded on the dienophile, it can react quickly as it decreases the energy of lowest unoccupied molecular orbital of them. Some common substituent on dienophile are -COOR, -Ph, -CN, -COR >C=C<, or -CHO. On the contrary; the diene must be electron-rich, therefore any electron-donating group activate them.


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