Overview of Cheletropic Reactions

The cycloaddition of a conjugated it system to an electrophilic molecule by the formation of two new a bonds to an atom of the electrophile in a concerted manner is known as cheletropic addition reaction and its reverse process in which two a bonds are broken from the same atom of the adduct is known as cheletropic elimination reaction. In cheletropic elimination, the driving force is often from the entropic benefit of gaseous elimination of N2, CO, and SO2. For example, the cycloaddition of 1,3-butadiene and its derivatives with SO2 and of alkene with a carbene are cheletropic addition reactions. 

The mechanism of this cycloaddition can be explained by FMO theory, in which one component acts as a HOMO and other as LUMO in a favorable low-energy TS to afford a stereoselective product. The Woodward-Hofifmann mles for electro-cyclic reactions are also applied to this cycloaddition reaction. The reaction of an alkene with a carbene is considered as a 4n electron process and of a conjugated diene with an electrophilic molecule as a 4n+2 electron process. Therefore, for thermal reaction of 4n electron process, conrotatory motion of the substituents from the termini of the n system will favor a low-energy TS to afford the product and in photochemical process, the reverse disrotatory mode of motion will be the favored path. Similarly, for a 4n + 2 electron process, disrotatory mode is a symmetry allowed process in thermal reaction and conrotatory mode for its photochemical reaction. 

The orbital symmetry allowed HOMO and LUMO interaction for the reactions of 4n+2, and 4n processes can be explained as follows (Fig. 3.14) 

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