Breaking the <i>tert</i> -Butyllithium Contact Ion Pair: A Gateway to Alternate Selectivity in Lithiation Reactions
Michael P. Crockett, Jeanette Piña, Achyut Ranjan Gogoi, Rémy F. Lalisse, Andrew Nguyen, Osvaldo Gutiérrez, Andy A. Thomas
Abstract
The effects of Lewis basic phosphoramides on the aggregate structure of t- BuLi have been investigated in detail by NMR and DFT methods. It was determined that hexamethylphosphoramide (HMPA) can shift the equilibrium of t- BuLi to include the triple ion pair ( t -Bu–Li– t -Bu) − /HMPA 4 Li + which serves as a reservoir for the highly reactive separated ion pair t -Bu – /HMPA 4 Li + . Because the Li-atom’s valences are saturated in this ion pair, the Lewis acidity is significantly decreased; in turn, the basicity is maximized which allowed for the typical directing effects within oxygen heterocycles to be overridden and for remote sp 3 C–H bonds to be deprotonated. Furthermore, these newly accessed lithium aggregation states were leveraged to develop a simple γ-lithiation and capture protocol of chromane heterocycles with a variety of alkyl halide electrophiles in good yields.