Base-catalysed hydrogen-exchange between organogermanium hydrides and methanol

Abstract

First order rate constants have been determined for the loss of tritium from tritiated germanium hydrides in MeOH/MeONa at 20–40°C. The rate constant for Ph 3GeT (T = 3H) is greater than that for exchange at the 9-position of fluorene under similar conditions. Values of k rel, the rate relative to that for Ph 3-GeH, for (XC 6H 4) 3GeT compounds at 30°C are: (X =) m-Cl, 380; p-Cl, 71; m-Me, 0.48; p-Me, 0.078; o-Me, 0.081; p-OMe, 0.025; o-Me, 0.018, and there is a good linear correlation between log k rel and σ. For the compounds (XC 6H 4)-Ph 2GeT the values of k rel are (X =) p-NO 2, 1280; p-CN, 680; m-Cl, 10; and p-F, 1.1, and a plot of log k rel against σ − constants is much better than that against σ constants. The results imply that there is substantial delocalization of charge from the anionic germanium centre into the aromatic rings, and thus na important degree of ( p p) π bonding between the germanium atom and these rings in the anion. The rates of exchange fall on going from Ph 3GeT to Ph 2GeHT, to PhGeH 2T (the values of k rel being 1, 0.11, and ca. 0.015 respectively), implying an order of kinetic acidities opposite to the reported order of equilibrium acidities in liquid ammonia. Replacement of Ph by Et groups also leads to a fall in kinetic acidity, values of k rel for EtPh 2GeT and Et 3GeT being 6 × 10 −2 and ca. 3 × 10 −3, respectively. Solvolysis, involving generation of hydrogen, is significant for PhGeH 3 under the reaction conditions, and important for Et 3GeH. The value of the inverse solvent isotope effect, k MeOD/ k MeOH is ca. 1.7 for both Ph 3GeT and ( m-ClC 6H 4) 3 GeT at 20°C. This value, and the activation parameters determined for some of the (XC 6H 4) 3GeT compounds are consistent with a transition state for the rate-determining step in which the hydrogen is roughly half transferred from germanium to methoxide ion, with partial liberation of the methanol molecules initially solvating the latter.