Structure and Dynamic Behavior of Selected Organolithium Compounds

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This project in the Organic Dynamics Program by Professor Gideon Fraenkel of the Ohio State University is aimed at the elucidation of the structure of organolithium compounds by Nuclear Magnetic Resonance Spectroscopy. In addition, the Ohio State group will investigate the way that organolithium compounds group together in solution (aggregation). This work is highly significant in that organolithiums are widely used by organic chemists as intermediates in the synthesis of new and important compounds. A variety of NMR techniques including line-shape analysis and NOE experiments will be applied to the study of structure and dynamic behavior of model solvated organolithium compounds enriched in Li-6, and if necessary C-13 at the alpha carbon, selected to reveal specific effects and interactions. Use of very low temperature, down to 120 degrees K, and low concentrations to ensure that C-Li bond exchange processes do not perturb the spectra, will allow observation of specific structural features. These include a) possible interactions of (RLi)n species with isolated double bonds; b) nature of ligand solvation and accompanying thermodynamics; c) the degree of non-planarity of allyllithiums; and d) structure of theoretically interesting dilithium compounds. Cisoid diamines and triamines, some chiral, will be synthesized to see if they stabilize different (RLi)n aggregates, and also to ensure slow lithium ligand exchange. The architecture of ion pairs will be studied via NOE experiments including ion pairs in liquid crystal media making use of the structural dependence of the dipolar couplings. Exchange averaging in the latter spectra will allow measurement of very fast rates of exchange of ion among ion pairs. The signal averaging of C-13/Li-6 couplings and shifts among species seen in NMR data for RLi compounds will be analyzed to determine exchange rates, and via kinetics and structural information, to learn about exchange mechanisms at equilibrium.