Phenyllithium aggregates

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. 

Wen and Grutzner used, among other NMR parameters, the QSC of the lithium enolate of acetaldehyde to deduce that it exists as tetramers of different solvation in THF and THF/n-hexane solvent systems . However, the most thorough study of Li QSC and the most interesting in the present context was reported by Jackman and coworkers in 1987167 -pjjg effects on the QSC values of both aggregation and solvation in a number of organolithium systems was studied in this paper, i.e. different arylamides, phenolates, enolates, substituted phenyllithium complexes and lithium phenylacetylide. 

Part, but not all, of the effect of these solvents is due to deaggregation of the organolithium. The importance of aggregation in determining reactivity is illustrated by methyllithium, which, as a monomer, should be more basic than phenyllithium by about 10 / Ka units. However, a 0.01 M solution of MeLi in THF is only three times more reactive than PhLi, and at 0.5 M concentration in THF PhLi is more reactive than MeLi.13... 

In THF, the alkyllithium compounds are aggregated [157] and the situation is reminiscent of the conditions in hydrocarbon solutions. At high concentrations, the association number (i. e. the number of molecules in the aggregate) decreases. This anomaly is explained by the existence of aggregate—solvent complexes, for example (MeLi)4 8THF Benzyllithium and its polymeric analogue polystyryllithium are not associated. Phenyllithium is mostly present as a dimer or monomer. Both forms are in equilibrium and are solvated. Only the monomeric form of the initiator is active. In practice, benzyllithium reacts only in the form of an ion pair. The fraction of the free benzyl anion must be very small [151c]. 

Figure 5 Deuterium-induced isotopic fingerprints in Li NMR spectra of partially deuteriated organolithium aggregates (a) phenyllithium monomer (THF/penta-methyldiethylenetriamine — 122°Q (b) phenyllithium dimer (Et20/tetramethylethyl-enediamine -111°C) (c) methylhthium tetramer (R=CHj EtjO, — 92 Q (d) fluxional phenyllithium tetramer (Et20, — 102 C). The measu H/ H isotope shifts for 5( Li) are 19.2, 10.4, 15.6, and 7.0 ppb, respectively. All systems were Li labelled and 50% of the organic ligands were perdeuterated v C Li) = 58.88 MHz...

Figure 13 Typical spin multiplets of the lithiated carbon from simple alkyllithium aggregates showing C, Li coupling (a) phenyllithium monomer (—100°C) (b) n-butyllithium dimer (—100°C) (c) /ert-butyllithium tetramer (—88°C) the coupling constants are 14.8, 7.9, and S.4Hz, respectively...

An interesting mixed tetrameric complex containing three equivalents of phenyllithium and one equiv-alent of lithium bromide, i.e. [(PhLi)3-LiBr3Et20], depicted as (116), has been characterized." In this mixed aggregate the lithium atom diagonally opposite Ae bromide in the tetramer rem s unsolvated by an ether molecule. Recall that the cyclopropyllithium lithium bromide-diethyl ether complex (44) with... 

When a methylene group spacer is inserted between the ortho heteroatom and the carbanionic center, the coordination geometry of the anionic center is no longer restricted to be planar for intramolecular chelation to occur. Hence, o-(dimethylaminomethyl)phenyllithium (126) crystallizes from an ether/hex-ane solution as the internal-chelated tetramer (127). This structure is analogous to tetrameric phenylli-thium (119). When an additional dimethylaminomethyl group is substituted at the ortho position as in 2,3,5,6-tetrakis(dimethylaminomethyl)phenyllithium, the aggregate crystallizes as the dimer (128). Hie lithium atoms in both (127) and (128) are coordinated to four other nonlithium atoms this coordination can only be achieved by dimerization and tetramerization respectively. 

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. 

C,C) and V(C,H) couplings in vinyllithium were reported by Kamienska-Trela et A very small value for the former was measured, i.e. 35.8 Hz. This should be compared with the /(C,C) through the double bond in (Et3Sn)2C=CEt(BEt2), of 30.6 Hz. Values of /( C, Li) and /( N, Li) couplings were measured as a probe of the aggregation behaviour of butyllithium, phenyllithium and lithium diisopropylamide. ... 

Subtle structural differences have been observed in some dimers of lithiated 1,3-bis[(dimethylamino)alkyl]benzenes [14]. Direct lithiation of l,3-bis[(dimethylami-no)ethyl]henzens with H-butyllithium produced a symmetric dimer, in which the coordination sphere of lithium was saturated by coordination of the chelating amino substituents. An analogous lithiation reaction performed between 1,3-bis[(dimethylamino)/ ro 7y/]benzene and n-butyllithium yielded a mixed dimeric aggregate comprising the expected phenyllithium derivative and nBuLi in a 1 1 molar ratio. In the molecular structure four lithium ions and four bridging carbon atoms form a ladder-type framework. 

Treatment of (7 )-cinnamaldehyde (70) with an equivalent of phenyllithium in THF yields the enol (71), as expected. However, a number of other products are formed with an excess, including dihydrochalcone (72), the result of a tandem addition )3-alkylation. Observed concentration effects and variation of product distribution with reaction conditions, together with computational results, point towards a mechanism in which dimeric PhLi attacks the aldehyde without prior de-aggregation. ... 

Reich HJ, Green DP, Medina MA, Goldenberg WS, Gudmimdsson BO, Dykstra RR, Phillips NH (1998) Aggregation and reactivity of phenyllithium solutions. J Am Chtan Soc 120 7201-7210... 

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. 

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