BASE-CATALYZED HYDROGEN-EXCHANGE BETWEEN ORGANOGERMANIUM HYDRIDES AND METHANOL

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 Ph3GeT (T = 3H) is greater than that for exchange at the 9-position of fluorene under similar conditions. Values of krel, the rate relative to that for Ph3-GeH, for (XC6H4)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 krel and σ. For the compounds (XC6H4)-Ph2GeT the values of krel are (X =) p-NO2, 1280; p-CN, 680; m-Cl, 10; and p-F, 1.1, and a plot of log krel 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 (pp)π bonding between the germanium atom and these rings in the anion. The rates of exchange fall on going from Ph3GeT to Ph2GeHT, to PhGeH2T (the values of krel 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 krel for EtPh2GeT and Et3GeT being 6 × 10-2 and ca. 3 × 10-3, respectively. Solvolysis, involving generation of hydrogen, is significant for PhGeH3 under the reaction conditions, and important for Et3GeH. The value of the inverse solvent isotope effect, kMeOD/kMeOH is ca. 1.7 for both Ph3GeT and (m-ClC6H4)3 GeT at 20°C. This value, and the activation parameters determined for some of the (XC6H4)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. © 1979.