Carbon-lithium coupling constants

The carbon-lithium coupling constants themselves are, for many examples, simply related to the states of aggregation n (equations 54a and 54b)11. 

To provide support for this assignment, we examined the IR and NMR spectra of several species derived from the lithium enolate 4 (Scheme 20.5, Table 20.1). The C chemical shift at C2, along with the one-bond C2 carbon-hydrogen coupling constant (Jch) provides a distinction between the oxygen-metallated structure and the two carbon-metallated structures, whereas the C chemical... 

With increasing temperature above 185 K and with increasing concentration of the n-butyllithium, the authors reported progressive averaging of the 13C—6Li coupling constant of dimers as well as of the resonances of dimer with tetramer. A line shape analysis of the 13C NMR of lithium bound carbon, using our PI method, best took account of the interconversion of tetramers with dimers via a degenerate process (equation 75),... 

Using low-temperature 13C NMR spectra, Reich and coworkers found that phenyl-lithium-6Li (18) in diethyl ether-<7 0 consists of an equilibrium between dimers and tetramers25. The spectra of these species were well resolved and identified by their one bond 13C—6Li coupling constants and multiplicities of their 13C resonances for lithium bound carbon, 7.6 Hz and 1 2 3 2 1 for dimers and 5.1 Hz and 1 3 6 7 6 3 1 for tetramers. On increasing the temperature above 170 K, the coupling constants and shifts between the species progressively average out. Line shape analysis provides the... 

The second dynamic process involves C—Li exchange. Around and below 230 K, most of the internally solvated allylic lithium compounds exhibit one bond spin coupling between 13C and 7Li (/ = 3/2) and to 6Li (/ = 1). The 13C NMR of lithium bound carbon consists of equally spaced equal multiplets, an equal triplet for coupling to 6Li and an equal quartet for coupling to 7Li. The separation between adjacent lines are the coupling constants. 

An extensive collection of both experimental and theoretical evidence suggests that the most accurate description of the ylide is one in which an easily pyramidalized carbanion is stabilized by an adjacent tetrahedral phosphonium center (25-34). These conclusions are supported by NMR studies and X-ray crystal structure determinations. Thus, increased electron density at the a-carbon of nonstabilized ylides is consistent with the upheld chemical shift in the NMR spectrum by comparison with the parent phosphonium salts (Table 3, entries 1-5) (26). However, the chemical shift by itself is not a reliable indicator of ylide structure. This is most clearly seen in some of the conjugated ylides, and also in entry 3, which differs from entry 1 only by the presence of lithium bromide. Both the lithium-free (1) and the lithium-containing ylides (3) have the same chemical shift, but they differ dramatically in the coupling constant. In entry 3,... 

The Sign of the Lithium-Carbon Nuclear Spin Coupling Constant in Methyl-lithium Tetramer, W. McFarlane and D. S. Rycroft, J. Organometal. Chem., 64, 303 (1974). 

Lithium covalently bound to carbon may be observed by Li NMR in lithium alkyls. In such molecules Li has a small chemical shift range (- ll ppm). Tables of chemical shifts and coupling constants are to be found in the review by Gunther. 

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