SUBSTITUENT AND SOLVENT EFFECTS IN THE [2-CYCLOADDITION REACTION BETWEEN OLEFINS AND ISOCYANATES(2])

Ab initio calculations provide with the models to explain the main fea tures of the cycloaddition between olefins and isocyanates to yield 2- azetidinones (beta-lactams). It is found that the reaction takes place via concerted transition structures involving retention of configurat ion in the starting olefins. These transition structures have zwitteri onic character. The presence of olefins having pi-donating groups and/ or isocyanates with electron-withdrawing groups diminish the synchroni city as well. as the activation energy of the reaction, yielding exclu sively the 4-substituted regioisomers. The solvent enhances the asynch ronicity of the reaction and can modify its profile from a concerted t o a two-step process, thus explaining the loss of stereospecificity ob served in the reaction between sulfonyl isocyanates and vinyl ethers.