The γ-silicon effect. IV. The solvolysis mechanism of 3-(aryldimethylsilyl)propyl p-toluenesulfonates

Article abstract of DOI:10.1246/bcsj.73.429

The solvolysis of 3-(aryldimethylsilyl)propyl p-toluenesulfonates was described based on the effects of substituents, solvents, and deuterium isotope, in comparison with the γ-silyl-assisted solvolysis of 3-(aryldimethylsilyl)-2,2-dimethylpropyl toluenesulfonates. The solvent effect on the simple γ-silyl assisted system showed the nucleophilic assistance of solvent, but failed to correlate linearly with the extended Winstein-Grunwald equation, substantiating that the reaction should not proceed through either the formation of the cation intermediate of the S_N2 mechanism. This suggested that the reaction occurs in competition between γ-silyl-assisted (k_Si) and solvent-assisted (k_s) pathways, and that the competition ratio varies with solvents and with aryl substituents. Product analysis revealed that the former pathway gave only cyclopropane and the latter gave only the substitution products. Using product ratios, the overall rate constant (k_t) value could be dissected into the partial rate constants k_Si and k_s for the two pathways. The effects of aryl substitutes at the γ-silyl atom on k_Si pathway were correlated with unexalted σ°(normal substituent constant) parameter, giving the ρ values of -1.0 in 60E and -1.32 in 97Tw, and reflecting the delocalization of incipient carbocationic charge by participation of the Si-C_γ bond. The substituent effects on the k_s pathway were negligible, which is in line with the remote reaction center in the concerted S_N2 mechanism.