Synthesis of 6-membered germacycles by intramolecular germylzincation of alkynes

Abstract

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 °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(sp²)–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.