Chemo- and Stereospecific Solid-State Thermal Dimerization of Sodium <i>trans</i>-2-Butenoate and γ‑Ray-Induced Single-Crystal-to-Single-Crystal Dimerization of Hexaaquamagnesium <i>trans</i>-2-Butenoate Dihydrate: Both Give <i>rel</i>-(3<i>S</i>,4<i>R</i>)‑1-Hexene-3,4-dicarboxylate but by Different Mechanisms. Stereospecific γ‑Ray-Induced Trimerization of Sodium <i>trans</i>-2-Butenoate

γ-Irradiation of crystalline hexaaquamagnesium trans-2-butenoate dihydrate affords rel-(3S,4R)-1-hexene-3,4-dicarboxylate by a single-crystal-to-single-crystal reaction. The reaction proceeds by a radical chain mechanism with anti addition to the butenoate double bond, as established by deuterium labeling. The product structure is that expected from the orientation of trans-2-butenoates in the pristine crystal. The same dicarboxylate is formed by heating crystalline sodium trans-2-butenoate at 300 °C, but the thermal ene reaction was shown by deuterium labeling to proceed by syn addition to the butenoate double bond as expected for a concerted ene reaction. γ-Irradiation of sodium trans-2-butenoate forms a single trimer chemo-, regio-, and stereospecifically. The structure of sodium trans-2-butenoate was determined, and the crystal packing is consistent with both the observed ene dimerization and γ-ray-induced trimerization.