Arrhenius parameters for the addition of phenols to the silicon-silicon double bond of tetramesityldisilene

We have measured, for the first time for a disilene, the Arrhenius activation energies (E-a) and preexponential factors (In A) for an addition reaction. The addition reactions of both p-CH3OC6H4OH and P-F3CC6H4OH to tetramesityldisilene (1) have positive Arrhenius activation energies of 13.7 and 9.7 kcal/mol, respectively, and highly negative entropies of activation of -34.9 and -45.3 eu, respectively (In A = 13.0 and 7.8 M-1 s(-1), respectively). The more negative Delta S double dagger value for p-F3CC6H4OH is consistent with a more ordered type of addition in this case. The rates of addition of phenols to 1 (k approximate to 10(-4)-10(-2) M-1 s(-1)) are dramatically slower, i.e., by a factor of ca. 10(9)-10(12), compared with the rates of addition of alkyl alcohols to the less hindered (E)- and (Z)-1,2-dimethyl-1,2-diphenyldisilenes (3E and 3Z) and 1,2,2-trimethyl-1-phenyldisilene (4) (k approximate to 10(7)-10(8) M-1 s(-1)).(5) Steric protection of the Si=Si bond in 1 by the bulky mesityl substituents is probably responsible for this large reactivity difference. Competition experiments support this conclusion; EtOH reacts with 3E only 1.5 times faster than i-PrOH and 19 times faster than t-BuOH (similar competition ratios were measured for 3Z and 4),(5) while with 1 EtOH reacts 11 times faster than i-PrOH and at least 4000 times faster than t-BuOH. Ab initio quantum mechanical calculations (at MP3/6-31G*/HF6-31G*) for the addition of CH3OH and of CF3OH to Me2Si=SiMe2 reveal the following: for CH3OH,the rate-determining step is the nucleophilic attack of the alcohol on the disilene and the reaction proceeds via a zwitterionic alcohol-disilene intermediate; for CF3OH, the rate-determining step is concerted and the alcohol is involved both as a nucleophile and as an electrohile, with proton transfer being well advanced in the transition state.