Counterion, alkali metal

The more hindered (37c) is to be preferred the PB is less susceptible to Michael addition and (37c) as well as (37cH) are less nucleophilic than those of the lower esters (see Sect. 14.8.5 for an example). In the absence of side reactions these PBs are, upon workup, converted into the dihydro derivatives that can be reoxidized back to the PBs by bromine or by anodic oxidation [68, 87, 88]. The base strength of (38) can be modified either by substitution [89] or by complexation with alkali metal counterions [86, 89]. 

In 1972, when the main thrust for these then-new materials was the development of efficient polymer membranes for chlor-alkali cells, the F. I. du Pont Co. reported much information, mostly in the form of product literature, in the form of empirical equations regarding the water mass and volume uptake of sulfonate form Nafion as a function of FW (in the range 1000—1400), some alkali metal counterion... 

Both theory and experiment point to an almost perpendicular orientation of the two butadiene H2C=C(t-Bu) moieties (see Scheme 3.53). On passing from the neutral molecule to its anion-radical, this orthogonal orientation should flatten because the LUMO of 1,3-butadiene is bonding between C-2 and C-3. Therefore, C2-C3 bond should be considerably strengthened after the anion-radical formation. The anion-radical will acquire the cisoidal conformation. This conformation places two bulky tert-butyl substituents on one side of the molecule, so that the alkali metal counterion (M+) can approach the anion-radical from the other side. In this case, the cation will detain spin density in the localized part of the molecular skeleton. A direct transfer of the spin population from the SOMO of the anion-radical into the alkali cation has been proven (Gerson et al. 1998). 

It will be seen that there is an almost equal distribution of the charge between a and Y positions in THF for the heavier alkali metal counter-ions. If we suppose that increased charge produces an increased reactivity at a given position, then more vinyl unsaturation will be produced in THF than in hydrocarbon solvents and the highest vinyl content with heavier alkali metal counterions. The order in THF is however reversed, i.e. the highest, vinyl structures are produced by lithium catalysis (17) although microstructure determinations in this solvent normally apply to reactions with an appreciable free anion contribution and hence cannot be simply interpreted. In dioxane (18) and diethylether... 

There is evidence in the case of anionic exchange resins (7,28) that an increase in temperature (to 130°C.) causes a reversal of selectivity for the alkali metal counterion the higher temperature increases thermal motion and perhaps partially dehydrates the counterion. Demonstration... 

A significant rate enhancement for the C02 insertion process was noted in the presence of alkali metal counterions (Table I), even in the highly coordinating THF solvent. This rate acceleration was not, however, catalytic in alkali metal counterion, since the once formed carboxylate was observed to form a tight ion pair (76, 77) via its uncoordinated oxygen atom with the alkali metal ion, as evinced by infrared spectroscopy in the v(C02) region. That is, the counterion was consumed during the carbon dioxide insertion reaction. 

Carbon dioxide insertion into the W-C bond of CH3W(CO)j was not retarded by excess carbon monoxide. In other words, as Fig. 10 illustrates, the C02 insertion process does not involve a coordinatively unsaturated intermediate. This observation could only be made when an alkali metal counterion was present, since the rate of C02 insertion was much faster than that of CO insertion under this condition. On the other hand, [PNP][CH3W(CO)5] undergoes CO insertion (80) at a much faster rate than carbon dioxide insertion. Both processes exhibited similar metal (W > Cr) and R(CH3 > C6H5) dependences. 

In summary, the anions of group 14 metalloles (C4E rings) have either localized nonaromatic or delocalized aromatic structures, depending of the metal E, Si vs Ge and Sn, and on the substituents, methyl vs phenyl group. As pointed out by West and coworkers198, structural studies of metallole anions and dianions of this kind with different substituents will be of value. Additionally, it should be noted that experimentally observed structures in the solid state for silole and germole dianions are greatly influenced by the nature of the alkali metal counterion. 

Alkali metal counterion has been found to control the enolate protonation stereoselectivity.12 This remarkable phenomenon has been reported for lithium and potassium enolates of a norborneol derivative. 

Thermodynamic constants have been determined for the disproportionation equilibria of diazafluorene ketyls, e.g., 112 and its 1,8-diaza isomer, in the presence of each alkali-metal counterion.368... 

Tetrahydrofuran is a useful solvent for such reactions. This fairly polar solvent (dielectric constant = 7.6 at room temperature) promotes transfer of the 3 electron from the sodium to the aromatic compound and stabilizes the resultant complex. The stability of such complexes depends on the solvent, alkali metal counterion, and the nature of the aromatic compound. 

The thieno[2,3-Z)]thiophene-2,5-semidione (129) has been observed, together with several substituted derivatives, both symmetrical and unsym-metrical. These radicals form ion-pairs with alkali-metal counterions in... 

In 1998, Baker and Kirby conducted a 31P NMR investigation of electron exchange in the two-electron reduced heteropoly blue complex of [(P2Wi706i)2Th]18 (which contains an equilibrium mixture of oxidized, two-electron- and four-electron-reduced species) as a function of alkali metal counterion, concentration, and temperature. They interpreted their data in terms of Equation (8) in which the more strongly pairing alkali metals (M in Equation (8) = K+, Rb+, and Cs+ but not Li+) form an ion bridge between the two defect HPA units in the syn isomer. This interaction stabilizes the syn isomer and drives an apparent syn-anti equilibrium, Equation (8), to the left.118 The change in chemical shifts and other features of the 31P NMR spectra of these Th sandwich POM complexes as a function of the counterion defined a qualitative method to estimate the association of monocations with POM polyanions ... 

Initiation by electron transfer is based on the ability of the alkali metals to supply electrons to the double bonds. This yields an anion radical and a positively charged, alkali-metal counterion. Initiation may be effected (a) by direct attack of the monomer on the alkali metal, or (b) by attack on the metal through an intermediate compound such as naphthalene. Both result in bifunctional initiation, that is, formation of species with two carbanionic ends. 

Nafion does not yield typical high resolution spectra for nuclei (19F and 13C) in the fluorocarbon phase at interesting temperatures (< 90°C), even in the presence of water of swelling. In contrast, one can observe NMR signals of alkali metal counterions and water of swelling, even in the presence of small amounts of water. Thus it is possible to study aqueous regions of Nafion directly and to obtain information on the polymer structure indirectly. 

These results are interesting in themselves, but they are perhaps most significant in demonstrating that the generation of carbenes by deprotonation with strong anionic bases carrying alkali metal counterions will produce complexed species. Carbenes which dimerise like 8, 9, and 11 (R = Me, Et, /-Pr) can only be observed in solution, and it is an open question how much the observed properties are influenced by the complexation. This problem will be discussed further in sections 2.3 and 2.4, but we have been concerned for some time with... 

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