Michael‐Type Reactions between Electron‐Deficient 1,3‐Dienes and Dimethylgermylene/Water

A new type of reaction between dimethylgermylene (Me2Ge) (1), generated in situ from a 7‐germanobornadiene, and electron‐deficient 1,3‐dienes RR′CCH – CHCH – Z [ZC(O)Me, CO2R″ 2a – f] or cyclic dienones 8, 9 in the presence of water is described. The Michael‐type behaviour of the system Me2Ge/H2O is clearly shown by the nucleophilic addition to the unsaturated systems resulting in the formation of β1‐ and δ1‐adducts, the new allylgermanes 3a – g, 4a –c, 10a, b, 11. The cis configuration of the resulting double bond of the δ adducts 4a–c is remarkable. With open‐chain dienes cycloaddition of 1 without participation of water, yielding germacyclopentenes 5a, b or germacyclopentanes 7a, b, competes with the Michael‐type reaction. A slight variation of the substitution pattern of the dienes has a strong influence on the reaction course.