Synthesis of 6-membered germacycles by intramolecular germylzincation of alkynes

Intramolecular addition of Ge-H bonds across internal alkynes was performed in excellent yields (from 78 to 94% according to the structures) for the first time in one operation with high regio- and stereoselectivities. The key step consists in intramolecular germylzincation of arylgermane cores bearing C-C triple bonds. We first studied the reactivity of dialkylzincs towards Ge-H bonds using hexane as solvent at 80 degrees C. The E configuration of the products suggested that the reaction proceeds following a syn addition to the alkyne. Tetrasubstituted vinylgermanes can be obtained in one pot, with retention of the C-C double bond geometry, through Cu-catalyzed electrophilic substitution of the C(sp(2))-Zn bond of the vinylzinc intermediates. In contrast, when the reaction was performed under radical conditions (AIBN), the Z isomer were obtained, and no further cross-coupling occurred.