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Published byFelicity Davidson Modified over 9 years ago
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Synthesis of Heterocyclic Rings 7 (Cycloaddition Reactions)
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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
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µ-electrons Reaction (Geometry) Cycloaddition 2+2 Chelotropic 1 + 2 = 3 2+2 2+ 2 = 4 2+4 Chelotropic 2 + 4 = 5 2+4 1,3-dipolar cycloaddition 2+ 3 = 5 2+4 Diels-Alder 2+4 = 6
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1,3-Dipolar Cycloaddition Reactions
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Resonance Structures of 1,3-Dipoles Each molecule has at least one resonance structure which indicates separation of opposite charges in 1,3- relationship.
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Mechanism of Cycloaddition: 1,3-Dipolar cycloaddition reactions were found to be stereoselective.
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Most of them are regioselective.
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Orbital picture of 1,3-dipoles and dipolarophiles
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Concerted mechanism: one step mechanism in which two bonds are formed at the same time (Huisgen)
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Biradical mechanism (Stepwise mechanism) (Firestone)
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Regiochemistry: orbitals of the same size interact
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Generation of 1,3-Dipoles Nitrile oxides
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Nitrile Sulfides
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Nitrile Imides (Nitrilimines)
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