engleski

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 phenylethyne in H2O and D2O solutions of perchloric acid were found to be proportional to acid concentration, giving the hydroniuj ion rate constant kH+=1.85x10-2M-1s-1 and the isotope effect kH+/kD+=3.10. Magnitude of this isotope effect suggests that the reaction occurs by rate-determining hydron transfer to the substrate, producing a vinyl carbocation ; this carbocation then loses its tributhylstannyl group, giving phenylacetylene as the only detectable aromatic product. The hydronium ion rate constant, when compared to the rates of protonation of phenylethyne and 1-(trimethylsilyl)-2-phenlyethyne, gives a beta-stannyl stabilizing effect of delta Δ G≠ = 6.6 kcalmol-1 and a differential beta-stannyl/beta-silyl effect of delta Δ G≠ = 3.2 kcalmol-1. These stabilizations are very much smaller than proviously reported beta-stannyl effects. Possible reasons for the difference are suggested.

beta-stannyl effect; beta-silyl effect; carbocation stabilization; protodemetalation

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