| Popis: |
Donor-acceptor-substituted vinylcyclobutanes 1 rearrange to cyclohexenes 2 at temperatures between 20 and 195°C. In solvents of low polarity, [2+2] cycloreversion competes with ring enlargement. Under acidic conditions, addition of nucleophiles to vinylcyclobutanes 1 with ring opening is observed. Stereospecificity of rearrangement and [2+2] cycloreversion are functions of temperature, solvent polarity and acid catalysis. Strong dependence on solvent polarity is found for the rates of both reactions. The mechanisms of the thermally induced and acid-catalyzed reactions are discussed on the basis of stereospecificity, kinetic parameters and trapping of intermediates. Two structurally different zwitterions 13 and 14 are proposed as intermediates, one of which is responsible for rearrangement, the other for diastereomerization and [2+2] cycloreversion. |
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