Chemical shifts butyllithium

The perfluorinated tetracyclic 1,4-dithiocin structure (262) was assigned to a product obtained in low yield from the reaction of l,2-dichloro-3,3,4 4-tetrafluorocyclobutene with butyllithium and sulfur chloride (71TL2871). The product is a stable high melting solid. The 19F NMR chemical shifts very closely resemble those of dodecafluorotricyclobutabenzene, suggesting a ring current in (262). 

Some other reaction courses were also seen. For example, 3a, R=octyl with butyllithium gave no significant yield of dodecane. The 31P chemical shift of an important component suggested an ylide, perhaps derived from butyltriphenylphosphonium from attack at P rather than C. Ethyl magnesium bromide with 3a, R=octyl gave mostly 1-bromooctane, but with butylmagnesium chloride, about 20% yield of dodecane was observed with no 1-chlorooctane. 

Figure 3b. The Li-7 NMR (34.8 MHz) of n-butyllithium in n-hexane. With the spectrum width equal to 4000 Hz, the single sharp peak comes at 1.6 ppm downfield from LiCIOt (aq). In the context of this chapter, a positive value of chemical shift...

The result with allyllithium 23 a differs only insignificantly from that of an earlier report (10.5 kcal/mol)25>. Complexation of 23 a with TMEDA does not influence the rate of exchange. Hexamethylphosphoric triamide (HMPT), 15-crown-5 ether and [2.1.1]cryptand in tetrahydrofuran (THF) led to rapid decomposition of 23a. Addition of n-butyllithium had essentially no effect on the barrier. Since the 13C NMR chemical shifts of 23 a are independent of the solvent, it is assumed that 23 a exists as a contact ion pair or higher aggregate in the NMR experiments. (The other alkali metals should also form contact ion pairs with the allyl anion because of their well-known tendency to form contact ion pairs even more readily than the lithium cation 26)). 

The Li chemical shift for benzyllithium is solvent dependent with reference to internal butyllithium, it has the values 1.06 (THF), 1.47 (Et20), and 2.07 (C H ). This upfield shift, from tetrahydrofuran to benzene, goes with a reduction in the one-bond scalar coupling, from 132 to 116 Hz this was interpreted as "due to a substantial increase in anion-cation interaction with transfer of electron density from the benzyl moiety to lithium the greater the s-character of the C-H orbitals, the more p-character in the C-Li orbital" (101). 

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