Amine-Chelated Aryllithium ReagentsStructure and Dynamics

Multinuclear NMR studies of five-membered-ring amine chelated aryllithium reagents 2-lithio-N,N-dimethylbenzylamine (1), the diethylamine and diisopropylamino analogues (2, 3), and the o-methoxy analogue
(4), isotopically enriched in 6Li and 15N, have provided a detailed picture of the solution structures in ethereal
solvents (usually in mixtures of THF and dimethyl ether, ether, and 2,5-dimethyltetrahydrofuran). The effect
of cosolvents such as TMEDA, PMDTA, and HMPA has also been determined. All compounds are strongly
chelated, and the chelation is not disrupted by these cosolvents. Reagents 1, 2, and 3 are dimeric in solvents
containing a large fraction of THF. Below −120 °C, three chelation isomers of the dimers are detectable by
NMR spectroscopy:  one (A) with both nitrogens coordinated to one lithium of the dimer, and two (B and C)
in which each lithium bears one chelating group. Dynamic NMR studies have provided rates and activation
energies for the interconversion of the 1-A, 1-B, and 1-C isomers. They interconvert either by simple ring
rotation, which interconverts B and C, or by amine decoordination (probably associative, ΔG⧧-93 = 8.5 kcal/mol), which can interconvert all of the isomers. The dimers of 1 are thermodynamically more stable than
those of model systems such as phenyllithium, o-tolyllithium, or 2-isoamylphenyllithium (5, ΔΔG ≥ 3.3 kcal/mol). They are not detectably deaggregated by TMEDA or PMDTA, although HMPA causes partial
deaggregation. The dimers are also more robust kinetically with rates of interaggregate exchange, measured
by DNMR line shape analysis of the C−Li signal, orders of magnitude smaller than those of models (ΔΔG⧧
≥ 4.4 kcal/mol). Similarly, the mixed dimer of 1 and phenyllithium, 13, is kinetically more stable than the
phenyllithium dimer by >2.2 kcal/mol. X-ray crystal structures of the TMEDA solvate of 1-A and the THF
solvate of 3-B showed them to be dimeric and chelated in the solid state as well. Compound 4, which has a
methoxy group ortho to the C−Li group, differs from the others in being only partially dimeric in THF,
presumably for steric reasons. This compound is fully deaggregated by 1 equiv of HMPA. Excess HMPA
leads to the formation of ca. 15% of a triple ion (4-T) in which both nitrogens appear to be chelated to the
central lithium.