Organolithium aggregation

The use of aliphatic solvents causes profound changes in the observed kinetic behavior for the alkyllithium initiation reactions with styrene, butadiene, and isoprenc. i.e.. Ihe inverse correspondence between the reaction order dependence for alkyllithium and degree of organolithium aggregation is generally not observed. Also, initial rales of initiation in aliphatic solvents are several orders of magnitude less lhan those observed, under equivalent conditions, in aromatic solvents. Furthermore, pronounced induction periods are observed in aliphatic hydrocarbon solvents,... 

Figure 2 Schematic representation of organolithium aggregates (a) dimer (b) tetramer (c) hexamer shaded circles represent lithium atoms, circled carbons organic ligands...

Figure 4 Statistical distribution of deuterated and non-deuterated ligands in simple organolithium aggregates (a) monomer (b) dimer (c) static tetramer (d) fluxional tetramer...

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...

These combined findings, taken in concert with those of O Driscoll show that even in aromatic solvents the inverse relationship between organolithium aggregation state and reaction order of the initiation step is not always observed. Excess monomer (relative to initiator) can influence the initiation event possibly through 7t-complexation with the organolithium aggregates. Consistent with this... 

In aliphatic solvents the inverse correspondence between reaction order dependence for alkyllithium and degree of organolithium aggregation is not observed (49). In addition, the rates of initiation in aliphatic solvents are several orders of magnitude less than in aromatic solvents. Most reaction orders for alkyllithium initiators in aliphatic solvents are close to imity. These results suggest that in aliphatic solvents the initiation process may involve the direct addition of monomer with aggregated organolithium species (eq. 26) to form a cross-associated species. 

The role of organolithium aggregates and mixed aggregates in organolithium mechanisms has been reviewed. The synthetic utility of the regio- and stereo-selective lithiation of aziridines has been highlighted. " The latest developments on the direct metallation (mainly lithiation chemistry) of various 1,2,3-triazoles, 1,2,4-triazoles, tetrazoles, and 1,3,4-oxadiazoles have been presented. ... 

An alternative interpretation of these fractional kinetic orders in alkyl-lithium concentration as proposed by Wakefield [19] is that the rate-determining step involves coordination of a DPE molecule to one face of the polyhedral organolithium aggregate. As suggested previously [8], incomplete or stepwise dissociation equilibria such as those shown in Scheme 3 would be expected to require less energy as predicted by theoretical calculations [32]. It is important to note that Brown and coworkers [34, 35] have reported that dissociation energies for tetramer-dimer equilibria are 46.1 kj/mol and lOOkJ/mol for methyl-lithium in ether [34] and ferf-butyllithium in cyclopentane [35], respectively. 

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