THE ACID-CATALYZED DEMETALATION OF 1-(TRI-N-BUTYLSTANNYL)-2-PHENYLETHYNE - A SURPRISINGLY SMALL BETA-STANNYL EFFECT

Rates of conversion of 1-(tri-n-butylstannyl)-2-phenylethyne to phenyl ethyne in H2O and D2O solutions of perchloric acid were found to be pr oportional to acid concentration, giving the hydronium ion rate consta nt k(H+)=1.85x10(-2) M(-1) s(-1) and the isotope effect k(H+)/k(D+)=3. 10. The magnitude of this isotope effect suggests that the reaction oc curs by rate-determining hydron transfer to the substrate, producing a vinyl carbocation; this carbocation then loses its tributylstannyl gr oup, giving phenylacetylene as the only detectable aromatic product. T he hydronium ion rate constant, when compared to the rates of protonat ion of phenylethyne and 1-(trimethylsilyl)-2-phenylethyne, gives a bet a-stannyl stabilizing effect of delta Delta G double dagger=6.6 kcal m ol(-1) and a differential beta-stannyl/beta-silyl effect of delta Delt a G double dagger=3.2 kcal mol(-1). These stabilizations are very much smaller than previously reported beta-stannyl effects. Possible reaso ns for the difference are suggested.