Proton Magnetic Resonance Spectroscopy. Configurational Stability of Neohexyl(3,3-dimethylbutyl) Organometallic Compounds

The proton resonance spectra of dilute ether solutions of neohexyl(3,3-dimethylbutyl) organometallic compounds have been investigated as a function of temperature as a means of determining their configurational stabilities. Neohexyllithium, dineohexylmagnesium (free from magnesium chloride), and dineohexylzinc appear to undergo the inversion of configuration at the a-methylene groups at various temperatures. The activation energies for inversion are approximately 15, 20, and 26 kcal. for the lithium, magnesium, and zinc compounds, respectively. Inversion appears to be a first-order process for the lithium and magnesium compounds. This fact and the general dependence of the inversion rate on the nature of the metal suggest a dissociation-recombination mechanism (SE1). Trineohexylaluminum and dineohexylmercury were found to be configurationally stable up to 150-160º. The chemical shifts of the CH_2-CH_2 groupings in these organometallic compounds can be correlated with the electronegativity of the metal atoms.