Mechanism of the Ziegler Dilithiomethane Synthesis. An ab Initio Study of Methyllithium Pyrolysis

Ziegler's disproportionation reaction of methyllithium to give dilithiomethane and methane upon heating to 225 °C is predicted by ab initio theory (MP2/6-31+G*//6-31G*) to proceed endothermically through a polar hydrogen-transfer mechanism within a tetrameric methyllithium aggregate, (CH₃Li)₄→ CH₂Li₂·(Li/CH₃)₂ + CH₄. A free energy of reaction of +19.6 kcal/mol and an activation barrier (ΔG^‡) of 53.8 kcal/mol at 500 K are computed. The alternative reactions within dimeric and trimeric aggregates are endothermic by 31.1 and 21.4 kcal/mol at MP4SDTQ/6-31+G*//6-31G* with activation barriers of 64.3 and 58.2 kcal/mol, respectively. Under the reaction conditions, methyllithium trimers should be in equilibrium with tetramers, but tetramers are still expected to be the predominant species.