DYNAMICS OF SOLVATED LI+ WITHIN EXO,EXO-[1,3-BIS(TRIMETHYLSILYL)ALLYL]LITHIUM N,N,N',N'-TETRAMETHYLETHYLENEDIAMINE COMPLEX

The title compound 4TMEDA, prepared from 3-(trimethylsilyl)propene by a series of metalation (sec-butyllithium), silylation, and remetalation reactions, respectively, 1 → 2 → 3 → 4, is revealed via 13C and proton NMR, including a Saunders deuterium perturbation experiment, to be electronically symmetrical and exist in the exo-exo configuration. It is proposed that a small 13C NMR shift (0.48 ppm) between C1 and C3 of allyl in 4-TMEDA and a large shift between the two(CH3)2N groups (4.65ppm) are due to dissymmetry of solvation about lithium (one TMEDA and one diethyl ether) within the tight monomeric ion pair. Above 150 K there is signal averaging of the C, with C3 resonances and of the peaks for (CH3)2N in bound TMEDA. NMR line-shape analysis shows, most likely, all these spectral changes to be due to the same dynamic process, the rotation of the solvatedLi+ moiety with respect to the allylic loop. This rate of rotation is 300 s-1 at 160 K with ∆H* = 7.7 kcal. © 1990, American Chemical Society. All rights reserved.