Structures of the Lithium Salts of Aromatic Secondary Amines in Weakly Polar Aprotic Solvents

Lithium indolide (lithioindoline) in THF is shown, by vapor pressure barometry, to be a dimer, and its temperature-dependent, concentration-independent¹³C chemical shifts are interpreted in terms of the equilibrium Li₂A₂(THF)₂⇆ Li₂A₂(THF)₄(A = amide anion). Lithium 1,2,3,4-tetrahydroquinolide exhibits the same behavior. Lithium 2-methylindolide, 2-methyl-1,2,3,4-tetrahydroquinolide, and A-methyl-, N-n-butyl-, and N-isopropylanilides form the monomer LiA(THF)₃as well as the two dimers. Monomer-dimer exchange is slow on the¹³C NMR time scale at low (<-80 °C) temperatures. Lithium N-(2-methoxyethyl)anilide forms only Li₂A₂(THF)₂. Lithium N-tert-butylanilide exists as LiA(THF)₂and LiA(THF)₃depending on temperature. All the above salts form only Li₂A₂(Et₂0)₂in diethyl ether. Both the indolide and N-tert-butylanilide salts exhibit slow E-Z isomerism on the¹³C NMR time scale at -110 °C.⁶Li,¹⁵N spin-spin couplings of 3.8 and 7.5 Hz are observed for Li₂(PhNCH₃)₂(Et₂0)₂and Li(PhNPr¹)(THF)₃, respectively, in the corresponding solutions below -80 °C. Rotation of the phenyl group in N-methyl-,N-n-butyl-, N-(2-methoxyethyl)-, and N-isopropyl- but not N-tert-butylanilides is slow on the¹³C NMR time scale below -50 °C. Rotation of the tert-butyl group in LiA(THF)₃(A = N-tert-butylanilide) is comparable with the¹³C NMR time scale at -100 °C.