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

In the presence of catalytic amounts of Pd(PPh3)(4), 3,4-benzo-1,1,2,2-tetr aethyl-1,2-germacyclobut-3-ene (1) readily undergoes the reversible sigma-b ond metathesis to give a dimer, 1,2,5,6-dibenzo-3,4,7,8-tetragermacycloocta -1,5-diene (6) below 100 degrees C. However, at 160 degrees C in toluene co ntaining catalytic amounts of Pd(PPh3)(4), both 1 and 6 afford an unsymmetr ical 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(diethylgermy l)benzene (18) undergoes dehydrocouling to give 1, which is smoothly conver ted to 6 under these conditions. The metal-metal sigma-bonds of 1 and its s ilicon analogue are highly susceptible to hydrogermylation but not to hydro silylation in the presence of Pd(PPh3)(4). (C) 2000 Elsevier Science S.A. A ll rights reserved.