Cycloreversion reactions, solvent effects

Not only thermal [2 + 2] cycloaddition reactions but also the corresponding [2 + 2] cycloreversions are subject to large solvent effects. A good example is the thermal decarboxylation of the y9-lactone trani -3- -butyl-4-phenyloxetan-2-one, as described in Eq. (5-36). 

The [2 + 2] adduct from propenylidenecyclopropane and TCNE undergoes a cycloreversion at 1(X) °C to give 1,1-dicyanoethylene the solvent effect on this reaction is quite modest, a result not readily interpretable in terms of a zwitterionic transition state. ... 

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. ... 

Aryl substituted cyclobutenes undergo cycloreversion to arylalkynes but can also give addition products in hydroxylic solvents. These reactions are singlet state processes. The aryl derivatives of cyclobutene do not open to 1,3-butadienes, and based on substituent effects, the excited states appear to have zwitterionic character. These results suggest that the aryl substituent favors the formation of a zwitterionic excited state. 

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