| Using a low-temperature, multinuclear NMR technique that we developed for studying the solution structures of organolithium reagents, we searched for a correlation between the ratio of contact (CIP) and solvent-separated (SIP) ion pairs in solution and the ratio of 1,2 and 1,4 addition produced for a variety of sulfur-stabilized organolithium reagents in their additions to enones. The ratio of contact and separated ion pairs was manipulated by changes in the solvent (generally through the addition of HMPA), by changes in temperature, and by alteration of the electronic properties of the lithium reagents. Curtin-Hammett limitations prevented us from proving a correlation. However, our results are consistent with the CIP/SIP distribution being an important factor, but it cannot be the only one. Changes in diastereomeric product ratios as well as a rate reduction that was observed for one system upon addition of HMPA suggest that lithium catalysis also affects the product distribution. Our research demonstrates that nearly any stabilized lithium reagent can be induced to cleanly add either 1,4 or 1,2 with proper manipulation of solvent and temperature.;We studied tris(trimethylsilyl)methyllithium and a variety of lithium reagents possessing internal pyridyl nitrogen chelation and proved through $sp{13}$C labeling that they form triple ions (TI). The former exists as a mixture of CIP, SIP, and TI in THF. The CIP/SIP and monomer/triple ion interconversion barriers were exceptionally large (9.7 and 16.6 kcal/mol respectively). We report the first measurement of the relative reactivities of a monomer and a triple ion. The triple ion does not react directly with Mel, but must first dissociate into reactive monomers. A disagreement between the kinetic results and the extrapolated coalescence thermodynamic data for the dissociation of the triple ion is attributed to solvent molecule loss at higher temperatures. Pyridyl-chelated lithium reagents show a propensity to form triple ions in THF/HMPA medium, and better stabilization leads to more triple ion formation and with less HMPA. We present the first evidence that a Crypt (2.1.1) -complexed lithium can affect a solution property of its counterion. |
