Synthesis of Heterocyclic Rings 7 (Cycloaddition Reactions)

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Presentation transcript:

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] pericyclicchemical reaction [1] a pericyclic reaction is a type of organic reaction wherein the transition state of the molecule has a cyclic geometry,organic reactiontransition state

µ-electrons Reaction (Geometry) Cycloaddition 2+2 Chelotropic = = Chelotropic = ,3-dipolar cycloaddition 2+ 3 = Diels-Alder 2+4 = 6

1,3-Dipolar Cycloaddition Reactions

Resonance Structures of 1,3-Dipoles Each 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)

Regiochemistry: orbitals of the same size interact

Generation of 1,3-Dipoles Nitrile oxides

Nitrile Sulfides

Nitrile Imides (Nitrilimines)