Versatile Reactivity of Phosphagermaallene Tip(t‑Bu)GeCPMes* with α‑Ethylenic Esters

The phosphagermaallene Tip­(t-Bu)­GeCPMes* (1) (Tip = 2,4,6-triisopropylphenyl, Mes* = 2,4,6-tri-tert-butylphenyl) reacts with methyl crotonate, methyl cinnamate, and methyl fumarate according to a [2+2] cycloaddition between the GeC and CO double bonds to afford oxagermacyclobutanes 2–4. With methyl maleate, a [2+4] cycloaddition is observed between the GeC double bond and the OC–CC moiety, leading to oxagermacyclohexene 5. With methyl acrylate, phosphagermaallene 1 behaves as a 1,3-dipole, giving a transient five-membered-ring germa­(phosphino)­carbene, which rearranges by insertion of the carbenic carbon atom into a CH bond of a tert-butyl group of the Mes* group to afford the tricyclic derivative 8. These results have been supported by a theoretical approach in order to describe the reaction pathways and explain the different experimental results.