Amine-chelated aryllithium reagents-structure and dynamics

Multinuclear NMR studies of five-membered-ring amine chelated aryllithium r eagents 2-lithio-N,N-dimethylbenzylamine (1), the diethylamine and diisopro pylamino analogues (2, 3), and the o-methoxy analogue (4), isotopically enr iched in Li-6 and N-15, have provided a detailed picture of the solution st ructures in ethereal solvents (usually in mixtures of THF and dimethyl ethe r, ether, and 2,5-dimethyltetrahydrofuran). The effect of cosolvents such a s TMEDA, PMDTA, and HMPA has also been determined. All compounds are strong ly chelated, and the chelation is not disrupted by these cosolvents. Reagen ts 1, 2, and 3 are dimeric in solvents containing a large fraction of THF. Below - 120 degreesC, 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 eit her by simple ring rotation, which interconverts R and C, or by amine decoo rdination (probably associative, DeltaG(-93)(double dagger) = 8.5 kcal/mol) , which can interconvert all of the isomers. The dimers of 1 are thermodyna mically more stable than those of model systems such as phenyllithium, o-to lyllithium, or 2-isoamylphenyllithium (5, Delta DeltaG greater than or equa l to 3.3 kcal/mol). They are not detectably deaggregated by TMEDA or PMDTA, although HMPA causes partial deaggregation. The dimers are also more robus t kinetically with rates of interaggregate exchange, measured by DNMR line shape analysis of the C-Li signal, orders of magnitude smaller than those o f models (Delta DeltaG(double dagger) greater than or equal to 4.4 kcal/mol ). Similarly, the mixed dimer of I and phenyllithium, 13, is kinetically mo re stable than the phenyllithium dimer by >2.2 kcal/mol. X-ray crystal stru ctures of the TMEDA solvate of 1-A and the THF solvate of 3-B showed them t o 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 I equiv of HMPA. Excess HMPA leads to the formati on of ca. 15% of a triple ion (4-T) in which both nitrogens appear to be ch elated to the central lithium.