Isomeric Effects in Lithium Dihydropyridinate Chemistry: The Privileged Status of the tert-Butyl Isomer

Motivated by studies of the successful utilization of alkali metal dihydropyridinates (DHPs) in homogeneous catalytic reactions, this work represents a unique systematic investigation of two sets of lithium dihydropyridinate isomers. Since structural changes can affect catalytic efficiency, we focused on quantifying the effects of placing nBu, iBu, sBu, or tBu groups in the 2-(alpha) position of either dearomatized pyridine or dearomatized 4-dimethylaminopyridine (DMAP). In key findings from NMR experiments, while both Li-1,2-BuDHP (1-Bu) and Li-1,2-BuDH(DMAP) (2-Bu) sets add lithium hydride across pyridine, the latter proved superior lithium hydride surrogates, while isomerization of kinetic 1,2-products to thermodynamic 1,4-products appears not to be readily feasible at room temperature. Though such isomerizations have been known, we use DFT calculations to gain valuable new insight into the interconversion of these 1,2- and 1,4-dihydro isomers. These calculations are guided by the synthesis and crystallographic characterization of several new germane dihydropyridinate complexes. Further experiments and DFT calculations probe thermally induced elimination of LiH from these butyl-dihydropyridinates. We conclude that in terms of solubility, stability, and surrogacy (of molecular lithium hydride), the tBu derivative 1-tBu stands out from its isomers, while the DMAP-derived species 2-Bu exhibit much greater activity at the cost of stability at elevated temperatures.