Lithium-7 and proton nuclear magnetic resonance spectra of methyllithium/dimethylmagnesium (zinc, cadmium) in tetrahydrofuran and methyllithium/dimethylcadmium in ether

Abstract

Lithium-7 and proton nuclear magnetic resonance spectra of ether solutions of methyllithium with dimethylcadmium have been examined over a range of temperatures. The spectra show that, like the magnesium and zinc systems, complexes of the form Li 2Cd(CH 3) 4 and Li 3Cd(CH 3) 5 are formed. No evidence was obtained for or against the formation of 1/1 complex. In contrast to the magnesium and zinc systems, methyl group exchange between methyllithium and complex appears to be faster than lithium exchange; both are more rapid than in the zinc system. Lithium exchange is apparently rate-determined by the dissociation of methyllithium tetramer to dimers. The mechanism of methyl exchange is uncertain. Evidence for rapid exchange of methyl groups between molecules of Li 2Cd(CH 3) 4 is discussed. Proton and lithium-7 nuclear magnetic resonance spectra of tetrahydrofuran (THF) solutions of methyllithium with dimethylmagnesium, dimethylzinc, and dimethylcadmium have been obtained over a wide temperature range. In contrast to ether solutions, no 3/1 complex is observed. The 2/1 complex predominates in the magnesium system, whereas spectra of the zinc system are consistent with an approximately equal molar mixture of 2/1 and 1/1 complexes. Finally, 1/1 complex predominates in the cadmium system. Lithium and methyl group exchange between methyllithium and complex in each system occur at comparable rates and activation energies; the rates increase in the order Mg < Zn < Cd. For magnesium and zinc, the exchange rates in THF and ether are similar, but for cadmium they are slower in THF. Exchange of methyl groups between (CH 3) 2M (M = Mg, Zn, Cd) and complex is very rapid. Evidence is presented for facile methyl group exchange between molecules of 1/1 complex in the cadmium system and between 2/1 and 1/1 complexes in the zinc system.