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

Author(s) Eppers, O.; Günther, H.
Journal Magn Reson Chem
Date Published 2020 Feb
Abstract

Cyclopropyllithium, C H 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 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. Li and 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, 1 •(LiBr) . J( C, 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 Li, Li exchange spectroscopy and analyzed by temperature-dependent 1D Li line-shape calculations. These yielded the equilibrium constants K for the chemical exchange Li (C H ) Br ⇌ 2 Li C H Br. Their temperature dependence leads to van't Hoff parameters of ΔH° = 4.6 kJ/mol, ΔS° = 41.4 J/mol K, and ΔG° = -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 = 32.2 kJ/mol were calculated for the forward reaction Li (C H ) Br → 2 Li C H Br and ΔH = 37.5 kJ/mol, ΔS = -8.4 J/mol K, and ΔG = 40.0 kJ/mol for the reverse reaction 2Li C H Br → Li (C H ) Br .

DOI 10.1002/mrc.4952
ISSN 1097-458X
Citation Magn Reson Chem. 2020;58(2):131138.

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