A 3,4-benzo-1,2-germacyclobut-3-ene and related compounds. Palladium-catalyzed σ-metathesis, dehydrogenative coupling and hydrogermylation of the GeGe σ-bond

In the presence of catalytic amounts of Pd(PPh3)4, 3,4-benzo-1,1,2,2-tetraethyl-1,2-germacyclobut-3-ene (1) readily undergoes the reversible σ-bond metathesis to give a dimer, 1,2,5,6-dibenzo-3,4,7,8-tetragermacycloocta-1,5-diene (6) below 100°C. However, at 160°C in toluene containing catalytic amounts of Pd(PPh3)4, both 1 and 6 afford an unsymmetrical dimer, 1,2,4,5-dibenzo-3,6,7,8-tetragermacycloocta-1,4-diene (8) with two isomeric products. In the presence of Pd(PPh3)4, 1,2-bis(diethylgermyl)benzene (18) undergoes dehydrocouling to give 1, which is smoothly converted to 6 under these conditions. The metal-metal σ-bonds of 1 and its silicon analogue are highly susceptible to hydrogermylation but not to hydrosilylation in the presence of Pd(PPh3)4.