MO Study of Rearrangement of Ylides

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1.Bridged compounds and spiro compounds were obtained in the [2,3] sigmatropic rearrangement of 2-benzocycloammonium N-methylides. The product ratio depended on the ring size of the cyclic ammonium N-methylide. It was difficult that the calculated conformations of the starting ylides by AM1 predict the products. Prediction of the product selectivity required analysis of the transition states of the rearrangement.2.In the Stevens rearrangement of N-substituted -N,N-dimethylammonium N-benzylides, the yield of the rearrangement products was influenced by the substituent on the nitrogen atom. The relationship between the yield and the calculated dissociation energies of R-N^+ bond by AM1 or PM3 was not recognized.3.Mechanisms of the Sommelet-Hauser rearrangement and the Allyl rearrangement of ammonium N-methylides were simulated by AM1 or PM3. The two semi-empirical methods showed different potential energy surfaces. The AM1 method has suggested that the Allyl rearrangement has a concerted mechanism, on the other hand, the Sommelet-Hauser rearrangement has a stepwise mechanism. Expreimental data showed that the chemical behavior of S-methylides in the sigmatropic rearrangements is different from that of the N-methylides. Abinitio MO study of the ylides by B3LYP/6-31G^<**> level are progressing.