NMR spectroscopy of organolithium compounds

H. Gunther, D. Moskau, P. Bast, D. Schmalz, Modem NMR Spectroscopy of Organolithium Compounds, Angew. Chem. Int. Ed. Engl. 1987, 26, 1212—1220. 

W. Bauer, P. von Rague Schleyer, Recent Results in NMR Spectroscopy of Organolithium Compounds, in Advances in Carbanion Chemistry (V Snieckus, Ed.), 1992, 1, JAI Press, Greenwich, CT. 

The main advances in analysis of organolithium compounds are related to their structural characterization by instrumental methods. These rely heavily on NMR spectroscopy and, when possible, on crystallographic methods, although other spectroscopic and physicochemical techniques are occasionally employed. A modern approach to the solution of complex analytical problems involves, in addition to the evidence afforded by these experimental techniques, consideration of quantum mechanical calculations for certain structures. The results of such calculations support or deny hypothetical assumptions on structural features of a molecule or possible results of a synthetic path. The following two examples illustrate these proceedings. 

Part of the NMR spectroscopy techniques mentioned in Section IV.B can be used for quantitative determination of organolithium compounds in solution. 

Bauer, W. (1995). NMR of organolithium compounds general aspects and application of two-dimensional heteronuclear Overhauser effect spectroscopy (HOESY). In Lithium Chemistry, ed. Sapse. A.-M., and Schleyer, P. V. R., Wiley-lnterscience, New York, 125-172. 

W. Bauer, NMR of Organolithium Compounds General Aspects and Application of Two-Dimensional Heteronuclear Overhauser Effect Spectroscopy. In Lithium Chemistry (Eds. A.-M. Sapse and P.V.R. Schleyer), Wiley, New York, 1995, pp. 125-172. 

For more general treatment of NMR spectroscopy for the identification of organolithium compounds, see ... 

The structure of the organolithium compound 89 in solution, obtained on metaUation of a cyclic aldonitrone (88), according to equation 29, seems to have the resonant structure (89), as determined by C, N and Li NMR spectroscopy. Based on the spectroscopic evidence and ab initio calculations (MP2/6-31 + G ) on a simplified model compound (H... 

A study of the state of association of the functionalized organolithium compounds 204a-d was carried out by multinuclear ( H, Li, Li, C, N and P) NMR spectroscopy, using Li- and N-enriched species. Spectral evidence, supported in part by XRD crystallographic evidence, points to compounds 204a-c being dimerically associated in etheric solutions in three different forms (205-207). The interconversion among these three... 

Organolithium compounds of structure 275 can been applied as transfer agents for transition metal ions, for example, as shown in equation 54 for scandium(III) with tetrahedral coordination (276). The structure of these complexes, elucidated by XRD crystallography, shows the transition metals forming part of an anionic entity, paired to solvated lithium cations. Further structural information can be obtained from H, and "B NMR spectroscopies. ... 

A planar arrangement (297) for a cluster of four Li atoms, consisting of two equilateral triangles, is found by XRD for the solid complex 298. Each Li atom is coordinated to methylene groups of both types (from n-BuLi and the metallated carbosilane) and to N and O atoms of the substituent in the carbosilane. Further characterization of the solid 298 can be made by Li and CP/MAS NMR spectroscopies, and in solution by H, Li, and Si NMR spectroscopies. The combination of both organolithium compounds in 298 is found to form a more effective reagent than each of them alone °. 

Compound 388 is an acylating agent for electron-deficient alkenes, in a Michael addition process. It is formed by treating molybdenum hexacarbonyl with an organolithium compound, followed by quenching the intermediate 387 with boron trifluoride (equation 104). The structure of 388 (R = Ph) can be elucidated by NMR spectroscopy. Other examples of enantioselective and diastereoselective Michael-type additions involving lithium-containing intermediates in the presence of chiral additives can be found elsewhere in the literature . 

The dimeric complex 74 reacts with phenylacetylene or ferrocenylacetylene to yield the tetrameric complexes 75a and 75b, respectively, according to equation 26. These complexes are stable in CDCI3 solution in the absence of air and can be characterized by H and NMR spectroscopies. The low solubility of 75a in unreactive organic solvents precludes detailed studies of the solution structure in reactive solvents it decomposes to a dimeric complex, 76, according to equation 27 3. j jjg association behavior of these complexes resembles that of analogous organolithium compounds - 303... 

The tetraalkyltins show little tendency to increase their coordination with an external ligand. The exception is the organic nucleophile from organolithium compounds which gives, reversibly, the 5-coordinate tin anion which can be observed by NMR spectroscopy, and which leads to the exchange of alkyl groups (equation 5-6). These reactions are discussed in Sections 5.3.5 and 22.1. An NMR study of the product from treating tribu-tyl(hydroxymethyl)tin with butyllithium implied that it had the 6-coordinate structure 5-2.25... 

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