Comparisons of the reactivities of the tri-t-butyl- and tris(trimethylsilyl)methyl-silicon compounds

Abstract

Comparisons of the reactions of t-Bu 3SiX (X usually I, but in some cases OSO 2CF 3 or Cl) and the corresponding TsiMe 2X species (Tsi = Me 3Si) 3)C) have led to the following conclusions: (a) Under conditions in which TsiSiMe 2I is thought to react via a silicocationic intermediate (mainly, solvolysis in CF 3CO 2H or reactions with silver salts), this iodide is much more reactive than t-Bu 3SiI. (b) The reactivity difference between the two iodides is markedly smaller in reactions with alkali metal salts, i.e. S N2 processes, the TsiSiMe 2I commonly being ca. 3–8 times the more reactive. (c) In methanolysis and hydrolysis under conditions in which an S N2-intermediate mechanism may operate, the differences in reactivity between TsiSiMe 2X and t-Bu 3SiX compounds (X = I or OSO 2CF 3) vary considerably with the medium, with the former usually, though not always, the more reactive, but again the differences are much smaller than those in the reactions mentioned under (a) in which the ease of breaking of the Si-X bond is the greatly dominant factor. (d) The differences between the reactivities of iodides and the corresponding chlorides in these highly crowded systems are smaller in reactions in which participation by the nucleophile is important than in those in which the ease of ionization of the Si-X bond is dominant; in reaction with NaN 3, KOCN or KSCN in MeCN, t-Bu 3SiCl is more reactive than t-Bu 3SiI.