| Summary: | 碩士 === 國立臺灣師範大學 === 化學研究所 === 87 === The π-facial selectivity in the reactions of 5-substituted-2-dicyanomethyleneadamantanes and 5-aza-2-dicyanomethyleneadamantane N-oxide with the Grignard reagents, CH3MgCl and/or PhCH2MgCl were examined using semiempirical and ab initio calculation. The substituents investigated include -F, -Cl, -Br, -OH, NH2, -OMgCl, -CH3 and -Si(CH3)3. The reactions for all substituents, except the -Si(CH3)3 group, favor syn attack. The calculated syn/anti product ratio for the substituents of -F, -Cl, -Br groups agree well with available experimental results. The results obtained from the PM3, HF and B3LYP methods show that the HF results agree with the experimental results better than the other two methods.
In addition, the results of employed NBO analysis show that the stabilization energy due to the hyperconjugative σCα-Cβ→σ*C-Nu interactions at the transition state correlates well withπ-facial selectivity in the reactions of 5-substituted-2-dicyanomethyleneadamantanes with CH3MgCl. The calculated results for the reaction with PhCH2MgCl appear that the strength of the hyperconjugative σCα-Cβ→π*C-C interactions at the transition state plays a significant role in π-facial selectivity for the substituents of -F, -Cl, -Br groups. Besides the hyperconjugation described above, the electrostatic interactions between the benzyl group of PhCH2MgCl and -OH and/or -NH2 groups in the 5-substituted-2-dicyanomethyleneadamantanes are also a significant factor in the π-facial selectivity. The calculations of 5-aza-2-dicyanomethyleneadamantane N-oxide also gave results that structural effects, as found in previous study, affect the syn/anti product ratio significantly in the π-facial selectivity.
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