The acid-catalyzed demetalation of 1-(tri-n-butylstannyl)-2-phenylethyne. A surprisingly small β-stannyl effect

• Published in: Canadian journal of chemistry 1996-07, Vol.74 (7), p.1366-1368
• Main Authors: Egle, I, Gabelica, V, Kresge, A.J, Tidwell, T.T
• Format: Article
• Language: English
• ISSN: 0008-4042; 1480-3291
• DOI: 10.1139/v96-153

Rates of conversion of 1-(tri-n-butylstannyl)-2-phenylethyne to phenylethyne in H 2 O and D 2 O solutions of perchloric acid were found to be proportional to acid concentration, giving the hydronium ion rate constant and the isotope effect . The 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 tributylstannyl 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-phenylethyne, gives a β-stannyl stabilizing effect of δΔG ≠  = 6.6 kcal mol −1 and a differential β-stannyl/β-silyl effect of δΔG ≠  = 3.2 kcal mol −1 . These stabilizations are very much smaller than previously reported β-stannyl effects. Possible reasons for the difference are suggested. Key words: β-stannyl effect, β-silyl effect, carbocation stabilization, protodemetalation.