|Thesis Title||I. Addition of Sulfur Stabilized Lithium Regents to Epoxides. II. Rapid Injection NMR spectroscopy. III. Structure of Chelated Aryllithium Reagents.|
Aaron carried out an extensive kinetic study of the reaction of epoxides with sulfur-substituted organolithium reagents. He found that the effect of HMPA was very complex. With some lithium reagents addition of HMPA caused large rate increases, with others large rate decreases. Aaron traced the origin of these effects to the ease of ion pair separation. Large rate increases were observed for hard-to-separate ion pairs like 2-lithio-1,3-dithiane, rate decreases were observed for those which were already partly or completely separated in THF solution.
In the course of the research on the structure and reactivity of organolithium reagents, there were a number of cases where several isomeric structures or aggregates were present. To address the issue of which of these was actually the reactive species in solution Aaron built the first generation of rapid-injection NMR devices (RINMR) which allowed the injection of samples into organolithium solutions (or vice-versa) and the measurement of NMR spectra within a second or two at temperatures down to -140 deg C. Most of his efforts were aimed at investigating the chemistry of the unusual lithium reagent (Me3Si)3CLi, first studied by Bill Sikorski.
Aaron also made major contributions to working out the details of the solution structure of several chelated aryllithium reagents, most notably he characterized the open dimer and pentacoordinate monomer of PMDTA-complexed 2-methoxymethylphenyllithium, which was first prepared and studied by Wayne Goldenberg.
Although he did not get to run many experiments on it, Aaron also constructed the rapid injection NMR (RINMR) apparatus, which was improved by Marty Bevan and then very effectively used by Amanda Jones , Kris Kolonko and Kristin Plessel to study previously inaccessible reaction rates of organolithium reagents under non-Curtin-Hammett conditions.