The effects of ion-pairing on the rates of fragmentation of alkali-metal salts of tertiary alcohols

Abstract

The lithium, sodium, and potassium salts of 1,2,3-triphenylpropan-2-ol (1) decompose in DMSO solution at convenient rates yielding deoxybenzoin and toluene. Under conditions of excess of base, deoxybenzoin is rapidly converted into its enolate, whose u.v. absorption permits spectroscopic rate determinations. Following expectations from earlier work, rates for (1) are in the order 1:122:1 330 for the lithium, sodium, and potassium salts. The effects of added cryptands and common ion metal iodide have been studied and are shown to be consistent with an earlier proposed reaction scheme involving differential reactivity of associated and ‘free’ alkoxide anions. Use of cryptands has allowed estimate of the rate of decomposition of the unassociated alkoxide of (1), 8.5 × 10³ s^–1 at 18.6°, and this is at least 100-fold faster than its ion-pair with potassium. Reactions are strongly inhibited by added iodides and again, this is shown to be consistent with the reaction scheme. The salts of 3-methyl-1,2,3-triphenylbutan-2-ol (2) have also been studied. Under similar conditions these are 10³ times more reactive than those of (1), fragmenting to deoxybenzoin and cumene. Steric inhibition of ion-pairing and steric enhancement of reactivity of the free alkoxide both contribute to the observed reactivity. Steric effects alone appear to be responsible for the regioselectivity of its fragmentation.