Evaluation of beta- and gamma-effects of group 14 elements using intramolecular competition

To evaluate beta-effects and gamma-effects of group 14 elements, we have devised a system in which the intramolecular competition between gamma-elimination of tin and beta-elimination of silicon, germanium, and tin can be examined. Thus, the reactions of alpha-acetoxy(arylmethyl)stannanes with allylmetals (metal = Si, Ge, Sn) in the presence of BF(3).OEt(2) were carried out. The reactions seem to proceed by the initial formation of an alpha-stannyl-substituted carbocation, which adds to an allylmetal to give the carbocation that is beta to the metal and gamma to tin. The beta-elimination of the metal gives the corresponding allylated product, and the gamma-elimination of tin gives the cyclopropane derivative. In the case of allylsilane, the cyclopropane derivative was formed as a major product, whereas in the case of allylgermane the allylated product was formed predominantly. In the case of the allystannane the allylated product was formed exclusively. These results indicate that the gamma-elimination of tin is faster than the beta-elimination of silicon, but slower than the beta-elimination of germanium and tin. The theoretical studies using ab initio molecular orbital calculations of the carbocation intermediates are consistent with the experimental results. The effect of substituents on silicon was also studied. The introduction of sterically demanding substituents on silicon disfavored the beta-elimination of silicon probably because of the retardation of nucleophilic attack on silicon to cleave the carbon-silicon bond.