Solvation of Organolithium Compounds

In the alkyllithium initiated polymerizations of vinyl monomers, Lewis bases such as ethers and amines alter the kinetics, stereochemistry, and monomer reactivity ratios for copolymerization. In general, the magnitude of these effects has been directly or indirectly attributed to the extent or nature of the interaction of the Lewis base with the organolithium initiator or with the organolithium chain end of the growing polymer. Unfortunately, all of these observed effects are kinetic in nature, and therefore the observed effects of solvent represent a composite effect on the transition-state versus the ground state as shown below in Eq. (6), where 5 represents the differential 

Several years ago 80 83), a systematic investigation of the energetics of interaction of Lewis bases with organolithium compounds was undertaken. The enthalpies of 

The reliability of the calorimetric results for these reactive organometallic compounds was further substantiated by the observations that the results were all quite reproducible ( 0.1 kcal/mole) and that the results obtained do not depend on (1) the source or method of purification of the base or the solvent (2) the source or method of purification of the alkyllithium and (3) the sodium content of the lithium metal used to prepare the alkyllithiums. Furthermore, the calorimetric equipment was regularly calibrated with internationally accepted standards for calorimetry. 

8 All enthalpies obtained at 25° by addition of 0.050 ml or less into 195 ml of 0.04 M or 0.08 M alkyllithium (base/Li atom ratio S 0.08) b The numbers in parenthesis represent the [Base]/[Li atom] ratio for each measurement c 0.04 M d 0.08 M e Cyclohexane solvent r Hexane solvent 8 Benzene solvent 

Anionic Polymerizations of Non-Polar Monomers Involving Lithium 

However, for hexameric alkyllithiums in hydrocarbon solution, bases can coordinate to form either a solvated hexamer (Eq. (9))  

The numbers in parenthesis represent the [Base]/[Li atom] ratio 

---

Studies of lithium ion solvation of organolithium compounds are important for a thorough understanding of the behavior of these complex reagents. The chiral lithium amide... 

The crystal structures of many organolithium compounds have been determined.44 Phenyllithium has been crystallized as an ether solvate. The structure is tetrameric with lithium and carbon atoms at alternating corners of a highly distorted cube. The lithium atoms form a tetrahedron and the carbons are associated with the faces of the tetrahedron. Each carbon is 2.33 A from the three neighboring lithium atoms and an ether molecule is coordinated to each lithium atom. Figures 7.2a and b show, respectively, the Li-C cluster and the complete array of atoms, except for hydrogen 45 Section 6.2 of Part A provides additional information on the structure of organolithium compounds. 

It has been found that the Li quadrupole parameters x( Li) and /]( Li) are sensitive probes of solid state structures of organolithium compounds, for example with respect to aggregate size, solvation, ion pair structure and the X-Li-X structural angle. These results will be discussed in the following sections. 

The study of the interactions between organic compounds and aUtali-metal cations, in the gas phase, is related to many topics such as ion solvation, catalysis and molecular recognition. Furthermore, mass spectrometry has been used for the analyses of organolithium compounds and supramolecular assemblies that contain lithium cations. Alkali cationization is an important ionization technique, implemented for the analyses of a wide range of organic compounds. Finally, gas-phase studies are also useful for the quantitative determination of lithium cation affinity. The interaction between lithium cation and organic substances is thus related to different aspects of gas-phase chemistry and mass spectrometry. 

Brandt, P. Haeffner, F. A DFT-derived model predicts solvation-dependent configurational stability of organolithium compounds a case study of a chiral a-thioallyllithium compound. /. Am. Chem. Soc. 2003, 325, 48-A9. 

The difference between the findings of Beinert et al. and Foss et al. is presumably at least in part a reflection of the ability of benzene to provide some degree of solvation to organolithium compounds, in contrast to the extreme inertness of cyclohexane. A similar explanation can be provided" for the successful preparation of O.IM l,4-dilithio-l,l,4,4-tetraphenylbutane from 1,1-diphenylethyIene and lithium metal in benzene after only 48 hours reaction time. When cyclohexane is employed as solvent for this reduction, it is necessary to add substantial amounts of a promoter, such as anisole. 

An alternative to the use of a dipolar aprotic solvent is to use a nonpolar medium and a cation solvating additive. The use of beta-diamines to solvate and enhance the reactivity of organolithium compounds is well known and documented [15]. Polyethylene glycol derived bases were known to be self-solvating as early as 1963 [16]. 

Organolithium compounds can add to a, (3-unsaturated ketones by either 1,2- or 1,4-addition. The most synthetically important version of the 1,4-addition involves organocopper intermediates, and is discussed in Chap 8. However, 1,4-addition is observed under some conditions even in the absence of copper catalysts. Highly reactive organolithium reagents usually react by 1,2-addition, but the addition of small amounts of HMPA has been found to favor 1,4-addition. This is attributed to solvation of the lithium ion, which attenuates its Lewis acid character toward the carbonyl oxygen.111... 

Organolithium compounds of structure 275 can been applied as transfer agents for transition metal ions, for example, as shown in equation 54 for scandium(III) with tetrahedral coordination (276). The structure of these complexes, elucidated by XRD crystallography, shows the transition metals forming part of an anionic entity, paired to solvated lithium cations. Further structural information can be obtained from H, and "B NMR spectroscopies. ... 

The crystal structures of many organolithium compounds have been determined.35 Phenylhthium has been crystallized as an ether solvate. The structure is tetrametic, with lithium and carbon atoms at alternating comers of a strongly distorted cube. Each carbon is... 

The principal structures into which organolithium compounds assemble are unsolvated octahedral hexamers 6, and cubic tetramers, and solvated cubic tetramers 7, bridged dimers 8 and monomers, 9. In common among the solvated species, lithium is always tetracoor-dinate so that the dissociating direction is exothermic with negative entropy change due to the increase in coordination of lithium to a ligand ether or a tertiary amine. 

Among unsolvated organolithium compounds only the alkyllithiums are soluble in noncoordinating solvents such as alkanes and arenes. Their states of aggregation depend on the structure close to lithium. Thus primary, tertiary and secondary alkyllithiums, all unsolvated, assemble into respectively hexamers, tetramers and equilibrium mixtures of hexamers and tetramers. Most organolithium compounds dissolve in and coordinate with donor compounds such as ethers and tertiary amines. The actual structures depend critically on the nature of the donor. Thus, diethyl ether solvates tend to be mainly cubic tetramers (with some dimers) while THF favors mixtures of monomers and dimers. Tertiary vicinal diamines such as TMEDA and 1,2-di-Af-piperidinoethane, DPE, favor bidentated coordinated dimers. Finally, in the presence of triamines such as pentamethyl-triethylenediamine PMDTA and l,4,7-trimethyl-l,4,7-triazacyclononane TMTAN, many organolithium compounds form tridentately complexed monomers. 

We have shown how organolithium compounds adopt a variety of structures which differ in state of aggregation and degree of solvation. These species interconvert rapidly at equilibrium by different mechanisms, such as intermolecular C—Li exchange ligand transfer and dissociation-recombination processes as well as first-order reorganizations such as inversion and rotation. Dynamics of many of these processes have been determined by our methods of NMR line shape analysis. 

---