The Aggregation Behavior of Butyllithium in Diethyl Ether in the Presence of LiBr, LiClO4, and Phenyllithium: A Deuterium-Induced Secondary6Li-NMR Isotope-Effect Study

NMR spectrocopy of organolithium compounds, part XXXIV: Cyclopropyllithium and lithium bromide (1:1) in diethylether/tetrahydrofuran-Identification of a fluxional mixed tetramer

Cyclopropyllithium, C₃ H₅ Li (1), was studied in the presence of one equivalent lithium bromide (LiBr) in diethylether (DEE)/tetrahydrofuran (THF) mixtures and in THF as solvents. Increasing the THF concentration in DEE/THF leads in the ⁶ Li NMR spectrum to a main signal (S1) at δ0.85 (rel. to ext. LiBr/THF) and a second resonance (S2) at δ0.26 aside from a minor component at δ0.07. In pure THF, the ratio of these signals was 66: 28:6. ⁶ Li and ¹³ C NMR allowed to identify the main signal as belonging to a mixed dimer, 1•LiBr, and the signal at 0.26 ppm to a fluxional mixed tetramer, 1₂ •(LiBr)₂ . ¹ J(¹³ C,⁶ Li) coupling constants of 11.0 and 9.8 Hz were measured at 168 K for S1 and S2, respectively, and chemical exchange between both signals was detected by 2D ⁶ Li,⁶ Li exchange spectroscopy and analyzed by temperature-dependent 1D ⁶ Li line-shape calculations. These yielded the equilibrium constants K_eq for the chemical exchange Li₄ (C₃ H₅ )₂ Br₂ ⇌ 2 Li₂ C₃ H₅ Br. Their temperature dependence leads to van't Hoff parameters of ΔH° = 4.6 kJ/mol, ΔS° = 41.4 J/mol K, and ΔG°₂₉₈ = -7.8 kJ/mol. From the rate constants k, Eyring parameters of ΔH^* = 42.0 kJ/mol, ΔS^* = 33.0 J/mol K, and ΔG^*₂₉₈ = 32.2 kJ/mol were calculated for the forward reaction Li₄ (C₃ H₅ )₂ Br₂ → 2 Li₂ C₃ H₅ Br and ΔH^* = 37.5 kJ/mol, ΔS^* = -8.4 J/mol K, and ΔG^*₂₃₈ = 40.0 kJ/mol for the reverse reaction 2Li₂ C₃ H₅ Br → Li₄ (C₃ H₅ )₂ Br₂ .

Mechanisms and Beyond: Elucidation of Fluxional Dynamics by Exchange NMR Spectroscopy

Detailed mechanistic information is crucial to our understanding of reaction pathways and selectivity. Dynamic exchange NMR techniques, in particular 2D exchange spectroscopy (EXSY) and its modifications, provide indispensable intricate information on the mechanisms of organic and inorganic reactions and other phenomena, for example, the dynamics of interfacial processes. In this Review, key results from exchange NMR studies of small molecules over the last few decades are systemised and discussed. After a brief introduction to the theory, the key types of dynamic processes are identified and fundamental examples given of intra- and intermolecular reactions, which, in turn, could involve, or not, bond-making and bond-breaking events. Following that logic, internal molecular rotation, intramolecular stereomutation and molecular recognition will first be considered because they do not typically involve bond breaking. Then, rearrangements, substitution-type reactions, cyclisations, additions and other processes affecting chemical bonds will be discussed. Finally, interfacial molecular dynamics and unexpected combinations of different types of fluxional processes will also be highlighted. How exchange NMR spectroscopy helps to identify conformational changes, coordination and molecular recognition processes as well as quantify reaction energy barriers and extract detailed mechanistic information by using reaction rate theory in conjunction with computational techniques will be shown.

NMR studies of dilithiostyrenes: aggregation, NMR parameters, and DFT calculations for (E)-1-Lithio-2-(o-lithiophenyl)-1-trimethylsilylethene

The dilithio compound (E)-1-lithio-2-(o-lithiophenyl)-1-trimethylsilylethene (5) was synthesized from 2-trimethylsilylbenzo-[b]tellurophene (6) with lithium-6 and a detailed analysis of its ¹ H, ⁶ Li, ¹³ C, and ²⁹ Si NMR spectra showed 5 to form a dimer 5₂ in tetrahydrofuran and diethylether, while addition of tetramethylethylenediamine stabilizes a monomer 5₁ . A monomer-dimer equilibrium exists with K at 230 K = 1.25 and ΔG230o = -0.43 kJ mol^-1 . Homonuclear ⁶ Li,⁶ Li coupling of 0.25 ± 0.07 Hz in the dimer was detected by a 1D-⁶ Li,⁶ Li INADEQUATE experiment, and scalar ⁶ Li,¹³ C coupling constants were obtained from ¹³ C satellites in the ⁶ Li spectrum, from ¹³ C multiplet simulation and ⁶ Li,¹³ C-HMQC spectra. In addition, structures and coupling constants of 5₁ and 5₂ were calculated by density functional theory (DFT) methods. It was found that the magnitude of the ⁶ Li,¹³ C spin-spin interactions shows an inverse correlation with the C-Li bond lengths. The intra-aggregate exchange in the dimer, caused by 180° rotation of one monomer unit within the solvent cage, was studied by ⁶ Li DNMR and line shape analysis and yielded ΔG298≠ = 60 ± 3 kJ mol^-1 ; ΔH^≠  = 84 ± 3 kJ mol^-1 ; ΔS^≠  = 80 ± 3 J mol^-1  K^-1 for this process. Copyright © 2015 John Wiley & Sons, Ltd.