The methanolysis of the highly sterically hindered organosilicon compounds (Me3Si)3CSiMe2X with X = OSO2CF3, OCN or OClO3. A mechanistic enigma
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 But3SiOSO2CF3 and But3Sil) are considered to rule out a mechanism involving a cationic intermediate [including the SN2(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.