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

Abstract

Cyclopropyllithium, C 3 H 5 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, 1 2 •(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 lineshape calculations. These yielded the equilibrium constants K eq for the chemical exchange Li 4 (C 3 H 5 ) 2 Br 2 Ð 2 Li 2 C 3 H 5 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 Li 4 (C 3 H 5 ) 2 Br 2 ! 2 Li 2 C 3 H 5 Br and ΔH * = 37.5 kJ/mol, ΔS * = −8.4 J/mol K, and ΔG * 238 = 40.0 kJ/mol for the reverse reaction 2Li 2 C 3 H 5 Br ! Li 4 (C 3 H 5 ) 2 Br 2 . K E Y W O R D SLi exchange dynamic NMR, NMR lithium-6 carbon-13 cyclopropyllithium aggregate