STRUCTURE-REACTIVITY PRINCIPLES OF SODIUM HEXAMETHYLDISILAZIDE
Woltornist, Ryan Alexander
As part of ongoing efforts to pique the synthetic organic chemistry community's interest in organosodium chemistry by probing structure-reactivity-selectivity relationships, the Collum group has put considerable effort to remove multiple stigmas associated with sodium diisopropylamide (NaDA) and its utility. By contrast, sodium hexamethyldisilazide (NaHMDS) is arguably the pre-eminent organosodium reagent in both academic and industrial laboratories, finding applications requiring nuanced control of regio-, stereo-, and chemoselectivity. To this end, we have studied a variety of NaHMDS–solvent combinations to provoke potential consumers to think beyond the standard solvents. Ground state solution structures of NaHMDS in synthetically relevant solvents are an essential prerequisite for accurate mechanistic models to be proposed. Accordingly, the method of continuous variations (MCV) was utilized to accurately determine the aggregation state. In addition to MCV, 15N–29Si coupling constants proved invaluable not only for the characterization of aggregates, but also to determine the primary-shell solvation effects of synthetically relevant solvents such as tetrahydrofuran (THF) and N,N,N’,N’-tetramethylethylenediamine (TMEDA). Monitoring coupling constant versus solvent concentration in the high-temperature, rapid-exchange limit afforded the solvation number based on unweighted least-squares fit derived from the equilibrium. Finally, simple solvent titration methods as well as a thorough investigation using density functional theory (DFT) computations allowed for the full characterization of this important sodium amide. Rate and mechanistic studies of NaHMDS-mediated enolizations of simple ketones were subsequently investigated. Our previous effort to understand NaHMDS ground state structures allowed for the correlation of solution structures of NaHMDS with the observed reactivities and selectivities for these enolizations. Highly solvent-dependent E-Z selectivities ranging from 20:1 E/Z for Et3N/toluene to 1:90 E/Z for THF serve as benchmarks to correlate selectivity with mechanism. Detailed rate studies using synthetically relevant solvents revealed 9 unique mechanisms covering dimer-, monomer-, ion-pair-, and free-ion-based pathways. DFT computations mostly corroborate the experimental results. However, even when strict isodesmic comparisons are made, ionized structures seem to deviate substantially from experiment.
mechanism; physical organic; sodium amides; sodium hexamethyldisilazide; solution structure
Lambert, Tristan H.; Fors, Brett P.
Chemistry and Chemical Biology
Ph. D., Chemistry and Chemical Biology
Doctor of Philosophy
dissertation or thesis