Phenyllithium aggregation state

The intermediacy of cyclohexyne or cyclopentyne in the decomposition of, respectively, 2-fluoro- or 2-chloro-l-cyclohexenyllithium and 2-bromo-l-cyclopentenyllithium is hypothetical. The adducts obtained upon treatment with phenyllithium and lithium benzenethiolate may result from an addition/elimination rather than from an elimination/addition sequence. The reaction order, a kinetic gold standard, is not the same in DEE and THF. Moreover, whatever the number, it is treacherous as long as the aggregation state of the organolithiums is unknown. 

In the solid state NMR study, uncomplexed phenyllithium, assumed to be a tetramer, as well as the TMEDA complexed dimer and the PMDTA complexed monomer were investigated. Both Li and Li isotopes were used in the preparations. The C spectra of the complexes are presented in Figure 12. It is evident that the substitution of Li with Li has profound effects on the Unewidths, especially of the ipso-carbon at ca 180 ppm in the aggregated uncomplexed system (Figure 12a and 12b, respectively). This is in accordance with the previously mentioned study of methyllithium. However, even the other positions are affected by the dipolar couplings to the four quadrupolar lithium cations, but to a lesser extent due to the larger C-Li distances. 

Wittig l56) and Waack 157) showed ebullioscopically and osmometrically, respectively, that phenyllithium is a dimer in etheral solvents. Thonnes and Weiss, 58) found a TMEDA complexed phenyllithium dimer in the solid state, and calculations performed by Schleyer et al. 159) similarly showed the dimer to be the most stable species. The 13C nmr spectrum of phenyl-6Li in THF shows a quintuplet at —118 °C which also reveals a dimeric aggregate l49,160). Thus experimental investigations of the structure in solution and in the solid state as well as a theoretical study (corresponding to the situation in the gas-phase) lead remarkably to the same result a phenyllithium dimer structure seems to be the most stable one. 

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