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 S. 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.