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 Keq 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 .

The solution structure and the aggregation behavior of (E)-2-lithio-1-(2-lithiophenyl)-1-phenylpent-1-ene (1) and (Z)-2-lithio-1-(2-lithiophenyl)ethene (2) were investigated by one- and two-dimensional H-1-,C-13-, and Li-6-NMR spectroscopy. In Et(2)O, both systems form dimers which show homonuclear scalar Li-6,6Li spin-spin coupling. In the case of 2, extensive Li-6,H-1 coupling is observed. In tetrahydrofuran and in the presence of 2 mol of N,N,N',N'-tetramethylethylenediamine (tmeda), the dimeric structure of 1 coexists with a monomer. The activation parameters for intra-aggregate exchange in the dimers of 1 and 2 (1 (Et(2)O): Delta H-not equal = 62.6 +/- 13.9 kJ/mol, Delta S-not equal = 5.8 +/- 14.0 J/mol K, Delta G(not equal) (263) = 61.1 kJ/mol; 2 (dimethoxyethane): Delta H-not equal = 36.9 +/- 6.5 kJ/mol, Delta S-not equal = -61 +/- 25 J/mol K, Delta G(not equal) (263) = 54.0 kJ/mol) and the thermodynamic parameters for the dimer-monomer equilibrium for 1 (Delta H-degrees = 26.7 +/- 5.5 kJ/mol, Delta S-degrees = 63 +/- 27 J/mol K), where the monomer is favored at low temperature, were determined by dynamic NMR studies.

The fine-structure of COSY signals from spin-1 AX systems has been calculated within the framework of the product operator formalism and simulated numerical density matrix calculations with the program SMART. The theoretical predictions have been verified experimentally for the deuteron AX system of 3,3'-norcamphor, where the homonuclear H-2, H-2 coupling is 0.35 Hz. By comparing the H-2, H-2 COSY experiment with the 2D-H-2, H-2 TOCSY experiment for azulene-d(g), it was shown that magnetisation transfer is more efficiently detected by the TOCSY sequence. Finally, several Li-7, Li-7 COSY experiments for an organolithium compound are reported for the first time.

The aggregation behaviour of butyllithium (BuLi), phenyllithium (PhLi), and lithium diisopropylamide (LDA) in dimethoxy- and diethoxymethane (methylal and ethylal, respectively) has been studied by NMR spectroscopy using the isotopic fingerprint method and 13C,6Li as well as 15N,6Li spin-spin coupling constants. In both solvents, LDA exists as a dimer, while BuLi forms a tetramer. PhLi forms a dimer in methylal, whereas two major aggregates exist in ethylal. Due to solvent viscosity at lower temperatures, their structure could not be determined.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Vicinal deuterium-induced Li-6-NMR isotope shifts have been found for (D5)phenyllithium. These shifts are to low field, and their magnitude varies from 7 to 19 ppb with different solvents and donor molecules. On the basis of the isotopic fingerprints observed in the Li-6-NMR spectra of (C6H5Li/C6DLi)-Li-6Li-6 mixtures, the aggregation behaviour of phenyllithium under different solvent conditions and in the presence of amines as donor ligands (TMEDA, PMDTA) was studied. Tetramers and dimers are observed in Et2O, dimers as well as momomers in Et2O/TMEDA and in THF, and momomers in THF/PMDTA. The hitherto unidentified species in Et2O/TMEDA was shown to be a monomer.

The 1D- and 2D-Li-6,Li-6-INADEQUATE experiments are described as new tools for the detection of scalar coupled nonequivalent Li-6 nuclei in organolithium clusters. Practical applications of these sequences are demonstrated for the Li-6-NMR spectra of (E)-1-lithio-2-(2-lithiophenyl)-1-phenylhex-1-ene (1) and (E)-2-lithio-1-2-lithiophenyl)-1-phenylpent-1-ene (2), where signals due to dimers and monomers can be distinguished. The performance of the 2D-Li-6,Li-6-INADEQUATE and the Li-6,Li-6-COSY-45-LR experiment are compared. The Li-6 chemical shifts of 1 and 2 are discussed.

A homonuclear Li-6, Li-6 spin-spin coupling has been directly measured in the case of the dimer of (E)-2-lithio-1-phenyl-1-o-lithiophenylpent-1-ene. This constitutes the first direct observation of such a parameter. The well known INADEQUATE experiment in its one- and two-dimensional versions is applied for the first time to a spin system of spin-1 nuclei. It is demonstrated that the experiment is very sensitive for the detection of Li-6, Li-6 spin-spin interactions, and thus provides a promising alternative to the COSY experiment and an additional tool for structural studies on organolithium compounds in solution.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

AbstractDeuterium‐induced isotope effects on 6Li chemical shifts, transmitted over two bonds, 2Δ(6Li)(2H/1H), have been measured for the first time for methyllithium and the system methyllithium/lithium iodide. The observed shifts are to low field and range from 15 to 20 ppb per CD3 group. The multiplicity and intensity distribution of the 6Li signals resulting for mixtures of CH3‐ and CD3‐containing samples in the limit of slow inter‐ as well as intra‐aggregate exchange yield information about the size of the various clusters present in diethylether solution. It is expected that these isotope shifts can facilitate structural investigations of organolithium compounds. In some cases, the results of such measurements are expected to be less ambiguous then the conclusions based on multiplets that arise from scalar 13C,6Li spin‐spin coupling.

J-modulation of 13C NMR signals by 1J(13C,6Li) has been used to characterize the multiplicity of NMR signals of lithiated carbons in phenyl- and n-butyllithium.