Cheletropic reactions, cyclic sulfones

Cheletropic Cycloadditions. If an atom can both donate an electron pair and accept an electron pair to move to a higher covalency, it can act as a dienophile to a diene. Such is the case with the sulfur atom in sulfur dioxide and with phosphoms in tricovalent halides. This type of cycloaddition is called cheletropic, and results in the formation of 5-membered rings. The reaction of butadiene with SO2 is particularly important it is conducted on a commercial scale to produce the cyclic sulfone 5.18 (called sulfolene). Its hydrogenation product 5.19 (sulfolane) is widely used as a nonaqueous highly polar solvent. Substituted dienes also participate in the cycloaddition. Here, the sulfur can donate its electron pair while accepting an electron pair to form a new carbon-sulfur bond. 

The cheletropic reaction between conjugated dienes and sulfur dioxide yields cyclic sulfones. However, the isolated adducts of selenium dioxide and dienes possess the selenalactone structures (174). The only apparent explanation for the different stabilities of the cheletropic products (175) is that selenium is softer and tends to avoid a higher oxidation state. 

D-A-substituted a-quinquethiophene 2.110 capped by an anthracene and Ceo unit has been synthesized via a [4-1-2] cycloaddition reaction of Ceo with anthrylquinquethieno-o-quinodimethane intermediate, which was generated in situ by cheletropic extrusion of SO2 from the precursor cyclic sulfone 2.109 (Scheme 1.19) [212]. Energy transfer from the anthracene donor to the fullerene acceptor has been observed for the triad. Photoinduced electron transfer processes in the triad were observed in low-temperature ESR measurements [213]. 

Cycloadditions Interestingly, sulfur dioxide participated as a dienophile in the [4+2] cycloaddition reaction with 1,3-dienes. In this manner, sulfur dioxide reacts similarly to the related selenium dioxide and the other sulfur dienophiles RN=S=0, RN=S=NR and R2C=S=0 (sulfines). However, the [4+2] cycloadducts derived from 1,3-dienes and sulfur dioxide are only obtained at low temperatures (—80 °C) in a kinetically controlled reaction and the cycloaddition reactions often require the presence of a Lewis acid (CF3COOH or BF3). Above —50 °C the Diels-Alder adducts undergo a cycloreversion and a cheletropic addition of the generated sulfur dioxide to the diene occurs with formation of the corresponding 2,5-dihydrothiophene-1,1-dioxides (sulfolenes). According to ab-initio computations, electrostatic solvent effects are predicted to be of importance in the control of the selec-tivities in this reaction . From linear dienes, the [4+1] cycloadducts are usually obtained. For example, from 1,3-butadiene and SO2 at -20 °C, the cyclic sulfone 25 is obtained in 95% yield. ... 

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