Unimolecular solvolysis of some organosilicon perchlorates and iodides

Abstract

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 S_N1 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.