I. The structure and reactivity of chelated organolithium reagents. II. Application of the equilibrium isotope effect to organolithium reagents

I. Chelation, despite its importance in organolithium chemistry, is poorly understood. We attempted a systematic study of the effects of chelation on the structure and reactivity of organolithium reagents. The chelation was intramolecular, five-membered ring chelation with thiopyridine, N-methylimidazole, thiocarbamate and silyl pyridine as the chelating group. With low temperatures Nuclear Magnetic Resonance techniques we were able to examine the ion pairing, lithium electrophilicity and configurational stability of chelated organolithium reagents and compare them to model compounds. In the course of our studies, we found that chelation caused quite dramatic effects in the structures of chelated organolithium reagents. They were much stronger contact ions, some not showing any ion separation, even upon the addition of large excesses of hexamethylphosphoramide (HMPA). Chelated lithium reagents also had a tendency to form triple ions ([R-Li-R]--//Li(HMPA) 4+) upon the addition of 1--2 equivalents of HMPA. These triple ions, which were not observed for the model compounds, were doubly chelated and diastereomeric. Chelation caused a decrease in the electrophilicity of the lithium atom, possibly because of the steric effects of the coordination. Chelation also caused large changes in the configurational stability of the lithium reagents, stabilizing it by 1.5--6.8 kcal/mol. The chelated organolithium reagents showed a decrease in the racemization barrier with increasing ion separation, in contrast to the model compounds, consistent with an inversion mechanism dominated by the decoordination of the chelating group from the lithium as opposed to a rotation barrier. The reactivity of chelated lithium reagents was found to be two orders of magnitude slower than that of the model compounds.;II. The equilibrium isotope effect has been used successfully to study many types of equilibrium, mostly for carbocations. The requirements for a system to be studied by this technique are the following: (1) the compounds must be in equilibrium; (2) the two equilibrating compounds must show a change in hybridization and (3) the two equilibrating species must show different Nuclear Magnetic Resonance (NMR) shifts. We have used this technique to show that bis(3,5-bis(trifluoromethyl)phenylthio)methyllithium is equilibrating contact and separated ions in THF. We have also applied the equilibrium isotope effect to silyl-substituted allenyl-propargyllithium. reagents and found a range of equilibrating structures. The lithium reagent 1-lithio-1-phenyl-2-butyne is equilibrating, partially bridged allenyl-propargyllithium reagents in a range of solvents. The silyl-substituted lithium reagents are localized, equilibrating allenyl-propargyllithium reagents tetrahydrofuran and methyl ether, and equilibrating, partially bridged allenyl-propargyllithium reagents in 2,5-dimethyltetrahydrofuran.