13C spin-echo NMR spectroscopy with gated 6Li decoupling: Spectral editing for simple alkyl- and aryllithium compounds - scope and limitations

Eppers, Oswald; Günther, Harald

doi:10.1016/s0040-4039(01)93329-5

Cited by 9 publications

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“…Phenyllithium in Ether . The most reliable method for establishing aggregation, the observation of Li−C J coupling by 13 C or 6/7 Li NMR spectroscopy has not previously been successful for all PhLi aggregates. 4a,b,11a The X-ray crystal structure of the ether-solvated tetramer, observation of 13 C ipso carbon signals upfield of the better established THF-solvated dimer, 4a,b 7 Li quadrupolar relaxation rates,5a and measurements of colligative properties 6b,9a suggested a predominantly tetrameric structure. Eppers and Günther identified PhLi tetramer and dimer on the basis of characteristic multiplet patterns of C 6 H 5 Li/C 6 D 5 Li mixtures by 6 Li NMR (isotopic fingerprint method) 11a…”

Section: Resultsmentioning

confidence: 99%

“…During the time we worked on this problem, a number of additional solution 10a,11-13 and solid state spectroscopic and thermochemical studies of PhLi were reported. The powerful isotopic fingerprint technique was applied to PhLi in several solvents. 11a,b …”

Section: Introductionmentioning

confidence: 99%

“…X-ray crystallographic studies have shown PhLi can be tetrameric (ether solvate), trimeric (mixed aggregate with LiBr), dimeric (TMEDA solvate),3a or monomeric (PMDTA solvate) 3b in the solid state, depending on coordinating ligands and added salts. It is predominantly tetrameric in ether 5a,11a and dimeric in THF, 6a,11a THF/TMEDA,11a and dimethoxymethane 11b solutions. The PMDTA solvate has a monomeric structure in THF solution. 10a,11c Phenyllithium has also been used in several studies of organolithium reactivity. 9b,c, From these studies, PhLi clearly has an unusual richness of structure both in the solid state and in solution.…”

Section: Introductionmentioning

confidence: 99%

“…It is predominantly tetrameric in ether 5a,11a and dimeric in THF, 6a,11a THF/TMEDA,11a and dimethoxymethane 11b solutions. The PMDTA solvate has a monomeric structure in THF solution. 10a,11c Phenyllithium has also been used in several studies of organolithium reactivity. 9b,c, From these studies, PhLi clearly has an unusual richness of structure both in the solid state and in solution.…”

Section: Introductionmentioning

confidence: 99%

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Aggregation and Reactivity of Phenyllithium Solutions

et al. 1998

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Phenyllithium forms a mixture of tetramer and dimer in ether. Complete conversion to dimeric solvates is achieved by the addition of THF, dioxolane, DME, or TMEDA in near stoichiometric amounts. The addition of 2,5-dimethyltetrahydrofuran favors dimer, but tetramer is still detectable at 14 equiv of cosolvent. PMDTA converts PhLi to monomer in ether. In THF, PhLi is a mixture of dimer and monomer. Addition of TMEDA forms a series of complexes, but the dimer/monomer ratio is essentially unaffected. PMDTA and HMPA form monomeric PhLi stoichiometrically. HMTTA and DMPU also result in monomer formation but several equiv are required. 12-Crown-4 shows no spectroscopically detectable complexation of PhLi in THF. All of the cosolvents tested increase the reactivity of PhLi in THF in a test metalation reaction: HMPA and 12-crown-4 show the largest effects, PMDTA is intermediate, and HMTTA and TMEDA result in the least activation. In two selectivity tests, HMPA and 12-crown-4 show a substantially lower selectivity than the other cosolvents. We postulate that a contribution from a highly reactive separated ion pair (SIP) intermediate is responsible for the lower selectivity.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Aggregation and Reactivity of Phenyllithium Solutions

et al. 1998

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“…135 Phase selection for 13 C signals is then achieved with gated 6 Li decoupling, where the intensity of the detected 13 C signal from 13 C-6 Li n systems for an evolution period of τ = 1/2J ( 13 C, 6 Li) is governed by the equation…”

Section: Figure 11mentioning

confidence: 99%

Lithium NMR

G��nther

2007

Encyclopedia of Magnetic Resonance

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The ¹³C Chemical Shift of the ipso Carbon Atom in Phenyllithium

et al. 1995

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Phenyllithium was labeled with 6Li and I3C at the ipso carbon atom, and the tetramer was measured by solid-state NMR. The chemical shift tensor data were obtained by a moment analysis of the spinning side bands and were compared with the results obtained by calculations with the IGLO method. Although no splitting by dipolar spin coupling to 6Li was found the very good agreement between IGLO predictions and experimental results allowed alignment of the tensor axis to the molecular frame and interpretation of the data. The large deshielding of the isotropic chemical shift is mainly due to a decrease of the AE term in GP.Phenyllithium plays a role in organometallic chemistry, both as an important reagent for preparative chemistry ['] and as a model compound for quantum mechanical calculations[*]. Numerous papers were published on its preparation ["], and UV spectra were reportedlb1, ebullioscopic measurements have been performedL71, and several crystal structures of complexes with different ligands forming tetramers, dimers, and monomers are known[s-lOl. Similarly, many NMR studies were reported both in the liquid ["-"] and the solid ~t a t e [ *~-~~I . Astonishingly, however, is the lack of understanding of the 13C-chemical shift data with respect of the @so carbon atom and the C-Li bond. Normally, if in an aliphatic compound H is replaced by Li, the corresponding carbon signal is shifted to lower frequencies. This is usually interpreted in terms of the higher electron density at the anionic carbon atom. On replacement of one hydrogen atom by lithium in benzene, however, in phenyllithium the signal of ipso carbon atom is shifted to higher frequencies (A6 = 58 ppm). Several interpretations have been forwarded for this effect. Grant and Fraenkel[21] were the first to argue that a lower averaged excitation energy would be responsible for a larger op term and therefore yielding the high frequency shift. Seebach and cow~rkers[~~l distinguished on a qualitative basis between a CT and a x electron density at the metalated carbon atom and proposed enhanced r s density vs lowered x density. Similar arguments were put forward by Schleyer and coworker~ [~~].Understanding 13C-chemical shift data on a molecular level is only possible, when the chemical shift tensor and its alignment with respect to the molecular frame are known. This approach has been amply forwarded by the research group of Grant, who succeeded in measuring the chemical shift tensors of many basic molecules, such as acetylene, benzene, ethylene, and o t h e r~ [~~-~~] .We have therefore set up a project to measure the chemical shift tensor of the @so carbon in phenyllithium in order to see whether a congruence with the theoretical predictions by the IGLO method can be obtained. This should lead to a more profound interpretation of the chemical shift in solution. Initially it was hoped that alignment of the principal axis of the chemical shift tensor with respect to the molecular frame would be achievable by an observation of the dipolar 6Li,13C spin coupl...

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13C spin-echo NMR spectroscopy with gated 6Li decoupling: Spectral editing for simple alkyl- and aryllithium compounds - scope and limitations

Cited by 9 publications

References 13 publications

Aggregation and Reactivity of Phenyllithium Solutions

Aggregation and Reactivity of Phenyllithium Solutions

Lithium NMR

The ¹³C Chemical Shift of the ipso Carbon Atom in Phenyllithium

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13C spin-echo NMR spectroscopy with gated 6Li decoupling: Spectral editing for simple alkyl- and aryllithium compounds - scope and limitations

Cited by 9 publications

References 13 publications

Aggregation and Reactivity of Phenyllithium Solutions

Aggregation and Reactivity of Phenyllithium Solutions

Lithium NMR

The 13C Chemical Shift of the ipso Carbon Atom in Phenyllithium

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The ¹³C Chemical Shift of the ipso Carbon Atom in Phenyllithium