Synthesis of Heterocyclic Rings 7 (Cycloaddition Reactions)

A cycloaddition is  a pericyclic chemical reaction,  in which "two or more unsaturated molecules (or parts of the  same molecule)  combine with  the  formation  of  a cyclic  adduct  in which  there  is  a  net  reduction  of  the bond multiplicity."[1] 

a pericyclic reaction is a type of organic reaction wherein the transition state of the molecule has a cyclic geometry,

µ-electrons Reaction(Geometry)

Cycloaddition

2+2 Chelotropic1 + 2 = 3

2+2 2+22+ 2 = 4

2+4 Chelotropic2 + 4 = 5

2+4 1,3-dipolar cycloaddition2+ 3 = 5

2+4 Diels-Alder2+4 = 6

1,3-Dipolar Cycloaddition Reactions

Resonance Structures of 1,3-DipolesEach molecule has at least one resonance structure which indicates

separation of opposite charges in 1,3-relationship.

Mechanism of Cycloaddition:1,3-Dipolar cycloaddition reactions were found to be stereoselective. 

Most of them are regioselective.

Orbital picture of 1,3-dipoles and dipolarophiles

Concerted mechanism: one step mechanism in which two bonds are formed at the same time (Huisgen) 

Biradical mechanism (Stepwise mechanism) (Firestone)

XY

z

R

H H

R

Y

RR

zX

YX

RR

zY

X

RR

z

Y

RR

zX

+

-+

.

..

.

+

+

Regiochemistry: orbitals of the same size interact

Generation of 1,3-DipolesNitrile oxides

Nitrile Sulfides

Nitrile Imides (Nitrilimines)