Amine-Chelated Aryllithium ReagentsStructure and Dynamics

Abstract

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 (6)Li and (15)N, 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 degrees 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, DeltaG(++)(-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, DeltaDeltaG > or = 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 (DeltaDeltaG(++) > or = 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.

Solid-State Structures and Racemization of Lithiated Schllkopf's Bis[Lactim Ether]

“…Despite the fact that there are always two signals apparent for the CH 2 O H-atoms closest to the Li-center, it could also be possible that the tridentate PMDTA ligand does not permanently coordinate to the Li-atom with all of its three N-atoms. This phenomenon has been observed both in the solid state [8] and in solution [9] of organolithium complexes. However, 7 Li-NMR spectra indicated only one type of chemical environment with respect to Li, even at À 808.…”

Simulation of DNMR spectra using propagator formalism and Monte Carlo method

“…In fact the Hamiltonian, the density matrix and the precession operator blocks according to the coherence levels and the size of the largest block is only n n=2 . This size still means an exponentially scaling memory requirement but is significantly smaller than the memory needed for the conventional calculations [8][9][10][11][12][13][14][15][16][17] using density matrix blocks. The RAM requirement of the program is independent of the number of exchanging sites.…”

“…The most widespread simulation programs (DNMR5 [8][9][10][11], MEXICO [12][13][14], WinDNMR [15][16], Bruker's TOPSPIN DNMR module [17]) are based on the calculation of transitions using the average density matrix. The most important limitation of this method is the huge computer memory requirement even for simple spin systems.…”

Carbanion generation through tin-lithium exchange of vinyl-, aryl-, and hetarylstannanes

“…This was illustrated in chelation studies of the aryllithiums in Figure 1, which were synthesized by tin-lithium exchange from the corresponding trimethylstannyl compounds. 62,63 The bromide byproducts from lithium-halogen exchange of the aryl bromides would have interfered with the intended spectroscopic studies. Halide-free aryllithium reagents were also obtained for the selenium and sulfur derivatives shown in Equation 36 and Equation 37.…”