The methanolysis of the highly sterically hindered organosilicon compounds (Me3Si)3CSiMe2X with X = OSO2CF3, OCN or OClO3. A mechanistic enigma

Abstract

Rates of solvolysis have been determined for the compounds (Me₃Si)₃CSiMe₂X with X = OSO₂CF₃, 3, or OCN, 6, (Me₃Si)₃CSiMe(OMe)OSO₂CF₃, 5, and (Me₃Si)₂C(SiMePh₂)(SiMe₂OSO₂CF₃), 4, in MeOH alone or containing water or NaOMe. The methanolyses of the trifluoromethanesulfonates and the cyanate [like those of the perchlorate (Me₃Si)₃CSiMe₂OClO₃, 2, and iodide (Me₃Si)₃CSiMe₂I previously studied] are only slightly accelerated by the presence of NaOMe (which, however, promotes isomerization of the cyanate to the isocyanate). The rate and course of the reaction are remarkably sensitive to the presence of water; thus the cyanate 6 in the presence of only 0.05% v/v of water gives exclusively the hydroxide (Me₃Si)₃CSiMe₂OH, as do the perchlorate 2 and trifluoromethanesulfonate 3 in the presence of 1% of water, and addition of 1% of water increases the rate of disappearance of the substrate by factors of 6.0, 4.8 and 6.2, for 2, 6 and 3, respectively. The plot of the rate constant against the water concentration is linear for 3, but curved for 6, through tending towards linearity at the higher water concentrations. The approximate relative reactivities of the compounds (Me₃Si)₃CSiMe₂X towards MeOH at 35°C are (X=) OClO₃, 3.0; OSO₃CF₃, 1.0; OCN, 1.0; I, 10^–3. The results (and those previously observed for Bu^t₃SiOSO₂CF₃ and Bu^t₃Sil) are considered to rule out a mechanism involving a cationic intermediate [including the S_N2(intermediate) mechanism], but also appear to be inconsistent with bimolecular nucleophilic substitution via either a five- or six-coordinate transition state. No satisfactory mechanism can be suggested, but some of the observations can be tentatively rationalized in terms of preassociation of water with the substrate or an intermediate.

The rate of methanolysis of the iodide (Me₃Si)₃CSiMeFl has been shown to be very little affected by the presence of base; the estimated relative reactivities of this iodide, (Me₃Si)₃CSiMe₂I, and (Me₃Si)₃CSiMeHI towards MeOH at 60°C are 1, 10^–2 and 10⁴, respectively.