1H and 7Li NMR observations of exchange processes in alkyllithium-ate complexes

Abstract

The kinetics of 7Li exchange between LiR and LiAlR 4, where R = CH 3, C 2H 5 and CH 2Si(CH 3) 3, have been observed in diethyl ether solutions. In agreement with earlier conclusions, the exchange is found to be rate-determined by dissociation of alkyllithium tetramer, with rate constant k 1. Line shape analysis of the NMR spectra over a wide range of temperature yields an Arrhenius activation energy of 12.4 ± 1.5 kcal/mole (99% confidence level) and pre-exponential term of 1.43 x 10 13 sec t-1 for R = CH 3; 11.3 ± 1.7 kcal/mole and 2.12 x 10 12 sec −1 for R = C 2H 5. For R = Me 3SiCH 2, 7Li exchange is rapid on the NMR time scale (i.e., k 1>∼10 3 sec −1) at −90°C. It thus appears that dissociation of the alkyllithium tetramer is promoted by increased electron release and/or increased steric requirement of the alkyl group. Alkyl group exchange is slow at room temperature in mixtures of Me 3SiCH 2LiLiAl(CH 2SiMe 3) 4 and Me 3SiCH 2LiLiCr(CH 2SiMe 3) 4. 7Li exchange is rapid at −95°C in mixtures of Me 3SiCH 2Li and Me 3SiCH 2Cu at various ratios. In the 1H spectrum distinct resonances corresponding to a 1 1 Li/Cu species and Me 3SiCH 2Li are seen for Li/Cu ratios > 1, at −50°C and below. The NMR results show that the only significant mixed species in the solutions is the 1 1 , probably a dimeric species (Me 3siCH 2) 4Li 2Cu 2.