PERICYCLIC REACTIONS

In organic chemistry, a pericyclic reaction is the type of organic reaction wherein the transition state of the molecule has a cyclic geometry, the reaction progresses in a concerted fashion, and the bond orbitals involved in the reaction overlap in a continuous cycle at the transition state. Pericyclic reactions stand in contrast to linear reactions, encompassing most organic transformations and proceeding through an acyclic transition state, on the one hand and coarctate reactions, which proceed through a doubly cyclic, concerted transition state on the other hand. Pericyclic reactions are usually rearrangement or addition reactions.

The major classes of pericyclic reactions are: 1. Cyclo addition 2. Electro cyclic reaction 3. Sigmatrophic rearrangement reaction

Characteristics of Pericyclic reactions: * The pericyclic reactions occur in single step and hence there is no intermediate formed during the reaction. * The breaking and making of bonds (both σ & π) occur simultaneously in a cyclic transition state. * The configuration of the product depends on 1) the configuration of reactants 2) the number of electron pairs undergoing reorganization and 3) the reaction conditions (like thermal or photochemical).

CYCLOADDITION REACTIONS

Cyclo addition reactions involve the formation of a cyclic product due to addition of two different π bond containing components, which are joined by newly formed two σ bonds at their ends at the expense of two π bonds. Otherwise we can explain it as, A cycloaddition is a 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”. It is usually reversible and the backward reaction is also referred to as retro-cycloaddition or a cycloreversion.

Examples of cycloaddition reaction

  1. The classic example of cycloaddition is Diels-Alder reaction between a Diene and a Dienophile to give a cyclic adduct.

2. Vinyl acetate reacts with butadiene via the Diels-Alder cycloaddition to form a six membered ring product

3. The Huisgen cyclo addition reaction is a 1,3-dipolar process and provides a tri-azole as the product The Huisgen cyclo-addition reaction is an example of a 1,3-dipolar cycloaddition, A 1,3-dipolar cyclo-addition involves a 1,3-dipole reacting with a dipolarophile (lover of dipoles) to form a ring. The type of ring formed depends on the nature of the 1,3-dipole and the dipolarophile. A 1,3-dipole is a reactive intermediate that contains both a positive and a negative charge, which are in a 1,3 relationship to one another. It’s very similar to a magnet which has a positive pole and a negative pole. The positive and negative charges on the intermediate make it very reactive toward dipolarophiles. The Huisgen cyclo-addition involves an organic azide (a compound containing three nitrogen atoms all bonded together) reacting with an alkyne (a compound containing a carbon-carbon triple bond). The product of the reaction is a five membered ring containing three nitrogen atoms as part of the ring, called a triazole.

4. The Cheletropic Cycloaddition Reaction A cheletropic cycloaddition is a reaction that occurs in which both new bonds are formed to the same atom on one of the reactants. The classic example of this type of cycloaddition is the reaction between butadiene and sulfur dioxide. Notice that the sulfur atom is forming a bond to each of the two terminal carbons in butadiene. This is the characterizing feature of a cheletropic cycloaddition reaction and makes it fairly easy to recognize amongst other types of cycloaddition processes.

Butadiene reacts with sulfur dioxide in a cheletropic cycloaddition reaction

ELECTROCYCLIC REACTIONS

In organic chemistry, an electrocyclic reaction is a type of pericyclic rearrangement where the net result is one pi bond being converted into one sigma bond or vice versa, otherwise electrocyclic reactions are intramolecular pericyclic reactions which involve the rearrangement of π-electrons in an open conjugated system leading to formation of a cyclic product with a new σ bond at the expense of a π-bond. However the electrocyclic reactions not only involve ring-closure but also ring opening, which are referred to as retro-electrocyclic reactions.

Examples 1.The formation of Cyclohexa-1,3-diene by heating Hexa-1,3,5-triene is an example of ring-closure electrocyclic reaction.

2. Some other reactions

SIGMA-TROPIC REARRANGEMENTS

A sigma-tropic reaction in organic chemistry is a pericyclic reaction wherein the net result is one σ-bond is changed to another σ-bond in an uncatalyzed intramolecular reaction. The name sigmatropic is the result of a compounding of the long-established sigma designation from single carbon–carbon bonds and the Greek word tropos, meaning turn. In this type of rearrangement reaction, a substituent moves from one part of a π-bonded system to another part in an intramolecular reaction with simultaneous rearrangement of the π system. True sigmatropic reactions are usually uncatalyzed, although Lewis acid catalysis is possible. Sigmatropic reactions often have transition-metal catalysts that form intermediates in analogous reactions. The most well-known of the sigmatropic rearrangements are the [3,3] Cope rearrangement, Claisen rearrangement, Carroll rearrangement, and the Fischer indole synthesis.

Sigmatropic rearrangements are concerted unimolecular isomerization reactions characterized by the overall movement of a σ-bond from one position to another with an accompanying rearrangement of π-electrons of conjugated system so as to accommodate the new σ-bond.

EXAMPLES 1. The [3,3] Cope rearrangement. The σ-bond undergoing movement is shown as red thick line.

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