Basicity lithium cations

Product 34 predominates in the polar aprotic solvent (acetonitrile), while in the polar protic solvent (methanol) products 35 are formed preferentially. The different products are caused by the relative rate of deprotonation against desilylation of the aminium radical, that is in turn governed by the action of enone anion radical in acetonitrile as opposed to that of nucleophilic attack by methanol. In an aprotic, less silophilic solvent (acetonitrile), where the enone anion radical should be a strong base, the proton transfer is favoured and leads to the formation of product 34. In aprotic solvents or when a lithium cation is present, the enone anion radical basicity is reduced by hydrogen bonding or coordination by lithium cation, and the major product is the desilylated 35 (Scheme 4). 

The ability of the crown ether ligand, 12-crown-4, to separate the lithium cation from the organic moiety, thus stabilizing the SSIP structure, was also observed in the study dealing with cyclopentadienyllithium systems . As described in Section II.C.3, SSIP structures of these compounds are characterized by low x( Li) values. However, it is important to reahze that the variation found for x( Li) is basically caused by the local symmetry around the lithium cation and other highly symmetric situations will also lead to small x( Li) values. Examples are the sandwich compounds mentioned in Section n.C.3. It is thus necessary to consider also / Li and C chemical shift data in order to classify a certain complex as SSIP or CIP. 

The same group studied the lithium cation basicities of a series of compounds of the general formula R R R PO, i.e. phosphine oxides, phosphinates, phosphonates and phosphates, by using Fourier Transform Ion Cyclotron Resonance (FTTCR) mass spectrometry. A summary of their results is shown in Figure 4. The effect of methyl substitution on LCA as well as the correlation between LCA and PA was also investigated by Taft, Yanez and coworkers on a series of methyldiazoles with an FTICR mass spectrometer. They showed that methyl substituent effects on Li binding energies are practically additive. 

The most comprehensive study of lithium cation basicities for organic bases was conducted by Taft, Gal and coworkers who investigated the effect of molecular structure on the gas-phase cation and proton basicities. Taft s LCA scale was revised and extended, and the lithium cation basicity scale now includes over 200 compounds. In the same work the correlations between gas-phase basicities toward lithium cation (LCB) and proton (GB) were examined. Good correlations are obtained provided that separate lines are drawn for homogeneous families and the differences in slopes are traced back to the different sensitivities to structural effects. Large deviations are explained by either a different attachment center for Li+ and H+ or a chelation effect toward Li+. Figure 5 describes three types of interactions that involve chelation of a lithium cation. 

The thermodynamic parameters characterizing the basicity of the tetrazole ring with respect to lithium cation in the gas phase have been obtained experimentally (ion cyclotron resonance) and by calculations G2, G2(MP2), B3LYP/6-31+G )) <2000PCA2824, 2004JCI1727, 2004PCA4812>. 

Ion pair separation can also be facilitated by utilizing salt effects [2,6,16]. The basic principle is exemplary illustrated in Sch. 4 for a special salt effect induced by the addition of e.g., lithium perchlorate (generally in a polar solvent like acetonitrile). Applying this procedure, the primarily formed radical ion pairs (either as contact or solvent separate ion pairs) are subsequently replaced by the formation of a new and tight ion pair between the acceptor radical anion (A -) and lithium cation (Li+). PET reactions often proceed solely under these condition, e.g., when using ketones as PET-sensitizers [16]. 

Interestingly, no correlation could be observed from their monomer ion-pair acidities (pAT0 in THF) and the second-order rate constant for the monomer in their reaction with m-chlorobenzyl bromide (Table 2, right), a linear relationship occurs when the corresponding cesium salts are alkylated with methyl tosylate. On the other hand according to the authors, this accounts for the fact that the lithium cation is as important as the basicity of the enolate. 

Determination of lithium cation basicity from molecular structure./. Chem. Inf. Comput. Sci., 44, 1727-1736. 

Tamm, K., Fara, D.C., Katritzky, A.R., Burk, P. and Karelson, M. (2004) A quantitative structure-property relationship study of lithium cation basicities. /. Phys. Chem. A, 108, 4812-4818. 

In the lithiation of fluoroanisoles (15) and (16) and their derivatives, butyllithium exclusively deprotonates the less acidic protons from the position ortho to the alkoxyl group. On the other hand, deprotonation takes place at the more acidic site, i.e. the ortho-position next to the fluorine substituent on reaction of the substrates with super bases, such as BuLi— t-BuOK or BuLi—N,N,N/,N//,N//-pentamethyldiethylenetriamine, in which lithium cation is stabilized by chelation in the combined base-system (Scheme 3.5) [ 14]. The lithium cation interacts preferentially with the more Lewis basic alkoxyl oxygen to locate butyllithium close to the position ortho to the alkoxyl group, enhancing kinetic deprotonation (see 17 in Scheme 3.5). 

Revised and expanded scale of gas-phase lithium cation basicities. An experimental and theoretical study ... 

Takano et al. used the Ireland-Claisen rearrangement of an allyl lactate in the total synthesis of calcitriol lactone (Scheme 4.76) [74]. The rearrangement proceeded with a 6.7 1 diastereoselectivity via O-silylation of the intermediate li-che-lated Z-enolate. Takano noted that the corresponding benzyl ether gave significantly lower de (70%) than the PMB ether. This is presumably due to the PMB ether s greater Lewis basicity and hence its greater propensity to coordinate a lithium cation. 

When the content of CajfPO ) in the NCPE is increased to 20% the ionic conductivity of the NCPE decreases. This decrease in the ionic conductivity can also be attributed to the change in the crystallinity of PEO in the nanocomposite polymer electrolytes (Capuglia et al., 1999). According to Scrosati and co-workers (Scrosati et al., 2001), the Lewis acid groups of the added inert filler may compete with the Lewis acid lithium cations for the formation of complexes with the PEO chains as well as the anions of the added lithium salt. In the present study, the filler nano CajfPO lj, which has a basic center can react with the Lewis acid centers of the polymer chain and these interactions lead to the reduction in the crystallinity of the polymer host. Nevertheless, the result provides LL conducting pathways at the filler surface and enhances ioiuc transport. 

Burk P, Koppel lA, Koppel I, Kurg R, Gal J-F, Maria P-C, Heneros M, Notario R, Abboud J-LM, Anvia F, Taft RW. Revised and expanded scale of gas-phase lithium cation basicities. An experimental and theoretical study. J Phys Chem A. 2000 104 2824-33. 

Obviously, only components of mixtures that are converted to ions can be separated in the IMS dimension and detected by MS. Simnltaneous ionization of compounds, such as basic and acidic lipids, is critical and is generally unsuccessful using ESI. The synergy of TSA (homebuilt instrument) using MILD conditions for ionization provided us with the ability to ionize and gas-phase separate fatty acids in the positive mode by facile adduction of lithium cation(s).< Figure 9.6 shows a typical example obtained on a MALDI-IMS-MS instrument (SYNAPT) for M being oleic... 

In order to overcome these stability problems a wide-range of alternative dopants have been studied in an effort to reproduce the transport properties of LISICON whilst eliminating the problems of aging. The introduction of trivalent cations into the basic lithium germanate structure can cause an adjustment in the lithium concentration, but in this case it is possible to introduce additional, interstitial, lithium cations or vacancies on the lithium position. Which of these types of doping occurs depends on the nature of the substitution that occurs. This can be most clearly illustrated by looking at the introduction of aluminium cations. These can enter the structure in place of and so introduce an interstitial lithium cation ... 

The development of mass spectrometric techniques has led to the constmction of not only the well-known GB and PA scales, but also the lithium cation basicity scale (Taft et al., 1990 Burk, Koppel, Gal et al., 2000) [93] and many metal cation basicity and affinity scales in the gas phase. A selection of the most informative scales is presented in Chapter 6. 

Table 6.3 Lithium cation affinity (LiCA) and basicity (LiCB) scales (kj mol j-... 

As in the case of the lithium cation, there are roughly two major sources of sodium cation affinities and basicities ... 

---