Unimolecular Solvolysis of Some Organosilicon Perchlorates and Iodides

foadmsm's picture
Journal Title, Volume, Page: 
J. Chem. Soc., Perkin Trans. 2, 1309-1315
Year of Publication: 
Foad M. S. Mahmoud
Current Affiliation: 
Department of Chemistry, Faculty of Science, An-Najah National University, Nablus, Palestine
Colin Eaborn
Preferred Abstract (Original): 

Kinetic studies have been made of the solvolysis (mainly the methanolysis) of the highly sterically hindered compounds TsiSiMe2OClO3[Tsi =(Me3Si)3C], (Me3Si)2(Ph2MeSi)CSiMe2OClO3, TsiSiHPhX (X = I, Br, ONO2), and TsiSiHMel. The rate of methanolysis of TsiSiMe2OClO3 is increased by only ca. 20% upon addition of 0.1MNaOMe, and further additions of base have smaller effects. Additions of LiCl or LiNO3 cause even smaller rate increases, but in the presence of LiNO3 substantial amounts of TsiSiMe2ONO2 are formed, the amounts being greater than would correspond to the rate increases. Water has a very large accelerating effect, and even with only 1 vol % of water present the product is very predominantly TsiSiMe2OH. The solvolysis is slower in EtOH and PriOH, and occurs only very slowly, if at all, in CF3CH2OH. The methanolysis of (Me3Si)2(Ph2MeSi)CSiMe2OClO3 is slower than that of TsiSMe2OClO3. The methanolysis of TsiSiHPhl and TsiSiHMel are also accelerated only to a small extent by NaOMe. The above results are interpreted in terms of an SN1 mechanism involving anchimerically assisted ionization of the Si–OClO3 or Si–I bond to give a methyl-bridged cation with unusual solvation requirements. In contrast, the methanolyses of TsiSiHPhX (X = Br or ONO2) are markedly accelerated by base, and it seems that, at least in the presence of base, attack by the nucleophile is involved in the rate-determining step in these cases.

Unimolecular_Solvolysis_of_Some_Organosilicon_Perchlorates_and_Iodides.pdf1.33 MB