Lithium neutral pairs

In the preceding chapter the formation of the less stable of two isomeric acids on rapid neutralization of an organometallic compound was explained in terms of a free ion or ion pair as the reagent. The compounds involved were the salts of comparatively strong acids, however, and the media were highly polar. The ion or ion pair would be less likely to be the reagent in the case of the salt of a very weak acid, such as an alkyl lithium compound, in a non-polar solvent. Salts of unsaturated but weak acids might exist in either of two partly covalent forms this possibility provides an alternative explanation for the formation of the less stable of the two isomeric acids on neutralization. Presumably the bulk of the covalent or partly covalent... 

The reduction process of polycycles by lithium metal converts the neutral atoms to anions. The electron transfer is best achieved in ethereal solvents. This enables the stabilization of the lithium cation by coordination to the oxygen atoms of the solvent. The hydrocarbon anion and the cation are linked together by electrostatic forces in which the solvent molecules are also involved, therefore the ion-solvation equilibrium should be considered8. The limiting cases in this equilibrium are free ions and contact ion-pairs (CIP), and in between there are several forms of solvent separated ion-pairs (SSIP)9. In reality, anionic species of aromatic hydrocarbons in ethereal solvents exist between CIP and SSIP. Four major factors influence the ion-solvation equilibrium of lithium-reduced 7T-conjugated hydrocarbons, as observed by H and 7Li NMR spectroscopies8,10. 

The reduction of 40 with lithium metal affords a pair of diastereomeric tetraanions (404, Figure 12). In contrast to the neutral compound the internal rotation of the dibenzo-COT unit in 404 is slowed at low temperatures, due to steric hindrance caused by the solvation shells of the ion-pairs. At ambient temperatures, dynamic behavior is observed and only broad and averaged signals can be seen. This is attributed to the rotational process observed in 40. It should be noted that this dynamic behavior is not observed in the potassium salt of 404 because this cation favors the formation of CIPs. 

If solvent-separated ion pairs or free ions were present, they should produce similar polymer microstructure to that obtained from contact ion pairs since propagation will involve only the allyl anion. There is no evidence for anything other than contact ion pairs in 1 1 lithium complexes with chelating diamines or triamines in hydrocarbon solvents. Only by using excess diamine or the more powerful chelating tetramines can we test the idea. As mentioned previously these are capable of producing some separated ion pairs when the anion is a sufficiently weak nucleophile to be displaced from the lithium by a neutral tertiary amine. With a benzyllithium tetramine complex, both contact and separated ion pair structures were observed spectroscopically. Since allyl and benzyl anions have rather similar charge delocalization, it is reasonable to expect that a tetramine complex of polybutadienyllithium would have similar proportions of contact and separated ion pairs. 

The only reasonable assumption is that charge density has been transferred from the anion to the lithium atom in the contact ion pair. The three-center mechanism provides a convenient mechanism for this exchange. In agreement with this, the charge on the lithium atom in (NH3)2Li fluorenyl is calculated by the CNDO technique to be neutral or even slightly negative, —0.05e- compared with the value expected for the isolated disolvated cation of 0.35. 

Preliminary results for Li salts are shown in Table XII.It is seen that for the neutral radical TTBP the Li enhancement is negative, whereas for the radical anion DBSQ it is positive. Both these radicals have a similar electronic distribution, so the different effects of the radicals may be due to the stronger interaction between the positively charged lithium ion and the negative DBSQ ion in forming a transient ion pair, and thus allowing a transfer of some unpaired electron spin density. Although for WBPC the unpaired electron density resides on... 

Other methods, among which thermolysis or photolysis of tetrazene [59], photolysis of nitrosoamines in acidic solution [60], photolysis of nitrosoamides in neutral medium [61], anodic oxidation of lithium amides [62], tributylstannane-mediated homolysis of O-benzoyl hydroxamic derivatives [63, 64], and spontaneous homolysis of a transient hydroxamic acid sulfinate ester [65] could have specific advantages. The redox reaction of hydroxylamine with titanium trichloride in aqueous acidic solution results in the formation of the simplest protonated aminyl radical [66] similarly, oxaziridines react with various metals, notably iron and copper, to generate a nitrogen-centered radical/oxygen-centered anion pair [67, 68]. The development of thiocarbazone derivatives by Zard [5, 69] has provided complementary useful method able to sustain, under favorable conditions, a chain reaction where stannyl radicals act simply as initiators and allow transfer of a sulfur-containing... 

A salt MX dissolves in a polymer matrix as it does in a liquid electrolyte to ionize and produce cations M+, anions X, and neutral ion pairs [MX]°. The neutral ion pair can combine further with a cation or an anion to form a triple ion [M2X]+ or [MX2]". The formation of neutral ion pairs [MX]° leads to a decrease in the concentration of carriers. The movement of the triple ion [M2X]+ or [MX2] is sluggish due to its size, and the ionic conductivity decreases due to the existence of these triple ions. Functional oxygen-containing groups in the chain segments have electron lone pairs, and the lithium ion has unoccupied orbitals (2s), so that the Li+ ion can form coordination structures with oxygen... 

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