Correlation between the 6 Li, 15 N Coupling Constant and the Coordination Number at Lithium

The 6 Li, 15 N coupling constants of lithium amide dimers and their mixed complexes with n ‐butyllithium, formed from five different chiral amines derived from ( S )‐[ 15 N]phenylalanine, were determined in diethyl ether (Et 2 O), tetrahydrofuran (THF) and toluene. Results of NMR spectroscopy studies of these complexes show a clear difference in 6 Li, 15 N coupling constants between di‐, tri‐ and tetracoordinated lithium atoms. The lithium amide dimers with a chelating ether group exhibit 6 Li, 15 N coupling constants of ∼3.8 and ∼5.5 Hz for the tetracoordinated and tricoordinated lithium atoms, respectively. The lithium amide dimers with a chelating thioether group show distinctly larger 6 Li, 15 N coupling constants of ∼4.4 Hz for the tetracoordinated lithium atoms, and the tricoordinated lithium atoms have smaller 6 Li, 15 N coupling constants, ∼4.9 Hz, than their ether analogues. In diethyl ether and tetrahydrofuran, mixed dimeric complexes between the lithium amides and n ‐butyllithium are formed. The tetracoordinated lithium atoms of these complexes have 6 Li, 15 N coupling constants of ∼4.0 Hz, and the 6 Li, 15 N coupling constants of the tricoordinated lithium atoms differ somewhat, depending on whether the chelating group is an ether or a thioether; ∼5.1 and ∼4.6 Hz, respectively. In toluene, mixed trimeric complexes are formed from two lithium amide moieties and one n ‐butyllithium. In these trimers, two lithium atoms are tricoordinated with 6 Li, 15 N coupling constants of ∼4.6 Hz and one lithium is dicoordinated with 6 Li, 15 N coupling constants of ∼6.5 Hz.