NMR studies of dilithiostyrenes: aggregation, NMR parameters, and DFT calculations for...

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.