Potassium counterion

Typical initiators for living anionic polymerization of siloxanes include conventional organoalkali compounds and lithium siloxanolates22). Initiators containing lithium counterions are preferable to sodium or potassium counterions due to the lower catalytic activity of lithium in siloxane redistribution reactions. Living anionic polymeriza-... 

Potassium counterion and inorganic impurities ammonium, potassium, sodium, trimethylammonium hydroxide (TMAH), lithium... 

The potassium salt of the 2,2 -dipyridyl acetylene anion-radical represents another important example. In this case, the spin and charge are localized in the framework of N-C-C=C-C-N fragment. The atomic charge on each nitrogen atom is -0.447, that is, close to unity in total. The energy of this ion pair is minimal when the potassium counterion is located midway between the two rather close nitrogen lone pairs. Such a structure is consistent with the fact that the ESR spectrum of this species is almost insensitive to temperature. It means that the counterion does not hop between two remote sites of the anion-radical (Choua et al. 1999). 

When a tetraalkylammonium cation is used as a counterion in solvents of high polarity, such as AN or DME, the alkyl groups of the cation hinder the mutual approach of species with different charges. Ion pairs with the potassium cation are stable. This follows from a comparison of the polarographic behavior of the three isomeric dinitrobenzenes in the same solvent (DMF) using tetraethylammonium or potassium perchlorate as the carrier electrolyte (Todres 1970). The halfwave potentials corresponding to the conversion of p- and m-dinitrobenzenes into anion-radicals are independent of whether tetraethylammonium or potassium counterions are employed. The anion-radical is formed from o-dinitrobenzene at a potential that is less negative by almost 100 mV when... 

Thus kp for lithium counterion is 1/300 of kp for potassium counterion. The low reactivity and association of lithium alkoxide was reported in the anionic polymerization of epoxides.We have found that two fold increase of the lithium initiator concentration has led to a decrease of the kp nearly to one half. This indicates that the kinetic order with respect to the initiator would be near to zero, suggesting a very high degree of association of the active species. Thus the propagation reaction appears to proceed in practice through a very minor fraction of monomeric active species in case of lithium catalyst. 

These values are shown in Table I. It is to be noted that kcyclic oligomers, while kp>x increases with the increase of x, and kp 2 being very low. Thus the high yield of the dimer at equilibrium is due to concentration by its low reactivity in polymerization. The value of kpjX and kd,x in case of potassium counterion is roughly calculated to be 1000 times larger than the value in Table I. 

Gawlcy and co-workers2 report that KDA is superior to LDA and to n-BuLi for deprotonation of dimethylhydrazones (7, 126-130) or of oxime ethers. The reaction is generally complete in THF at —78° in 15 minutes or less. The potassium counterion does not interfere with cuprate formation or conjugate addition. 

Further studies were directed to examine different SCBs and the effect of different counterions. Potassium counterions provide improved efficiency as compared to lithium or sodium counterions. The most efficient system in terms of formation of carbanions was achieved with diphenylsilacyclobutane in combination with potassium tert-butoxide and diphenylethylene <2004MI856>. Di-block copolymers from ethylene oxide and methyl methacrylate (or styrene) were synthesized by this method with 85% efficiency (Scheme 14) <2004MI856>. 

In a recent paper, Soumillion and co-workers [49] were able to identify CIP and SSIPin the P-naphtholate anion/alkali cation/tetrahydrofuran system. They found out that with lithium, a CIP is formed whereas with sodium/crown ether, a SSIP results. Using uncomplexed sodium or potassium counterion, mixtures of CIPs and SSIP s were detected. All their conclusions are based on spectral shifts in the transient absorption and emission spectra which were gained using laser flash spectroscopy. 

Phosphonoacetate cycHzation. Intramolecular cyclization of keto phosphonates can be used for construction of macrocyclic a, -unsaturated lactones. Stork s laboratory found that use of lithium isopropoxide or lithium hexamethyidisilazide in THF containing 1% HMPT minimized formation of cyclic dilactones. Use of sodium or potassium counterions was much less satisfactory. An example is shown in equation (I). 

A systematic study of the reductive alkylation of acetophenones revealed that the desired transformation (Scheme 30) required a careful selection of reagents and conditions. The best results were obtained from reduction by potassium in ammonia at -78 °C, with t-butyl alcohol as the proton source. Exchange of the potassium counterion of the enolate (152 M = K) for lithium then ensured regioselective alkylation at C-1 to give (153) in 80-90% yields (Scheme 30). Metals other than potassium as the reductant led to undesirable side reactions with the carbonyl group, which included simple reduction to the methylcar-binol and ethylbenzene (lithium or sodium), while the absence of a proton source or presence of a strong... 

The potassium counterion plays a major role in the latter cases (Table 23, compare entries 4 and 6 with 5 and 7). The possibility of a preferred (Z)-enolate has been suggested to explain the enantioselectivity of rearrangements carried out with KH-18-crown-6 (Table 23, compare entries 4 with 8 and 6 with 9). 

In this work, optical rotation in dilute solutions of kappa carrageenans is used to interpret the activity coefficient of potassium counterions and to demonstrate a conformational transition associated with an increase of the charge parameter corresponding to a dimerization. 

In a study on the electrophilic azide transfer to chiral enolates, Evans found that the use of potassium bis(trimethylsilyl)amide was crucial for this process. The KN(TMS)2 played a dual role in the reaction as a base, it was used for the stereoselective generation of the (Z)-enolate (1). Reaction of this enolate with trisyl azide gave an intermediate triazene species (2) (eq 4). The potassium counterion from the KN(TMS)2 used for enolate formation was important for the decomposition of the triazene to the desired azide. Use of other hindered bases such as Lithium Hexamethyldisilazide allowed preparation of the intermediate triazene however, the lithium ion did not catalyze the decomposition of the triazene to the azide.This methodology has been utilized in the synthesis of cyclic tripeptides. 

Polar solvents increase formations of ether groups. Nonpolar solvents, used with lithium, magnesium, or aluminum counterions, yield products that are high in ketones. " The same solvents, used with sodium or potassium counterions, form polymers with predominately polyester units. ... 

Again, good separations are obtained with ion-exclusion phases in the potassium form. The underlying retention mechanism is a combination of adsorption and weak interactions between the oxygen-containing functional groups of the solutes and the potassium counterions of the sulfonate groups at the stationary phase. 

The stereoselective Rauhut-Currier reaction catalysed by a cysteine derivative has been explored computationally with DFT (M06-2X). Both methanethiol and a chiral cysteine derivative have been studied as nucleophiles. The complete reaction pathway involves rate-determining elimination of the thiol catalyst from the Michael addition product and the stereochemistry has been found experimentally to be extremely sensitive to the reaction conditions, such as the number of water equivalents and the effect of potassium counterion. 

E. Clementi and G. Corongiu, /. Biol. Phys., 11,33 (1983). Structure of Aggr tes of Witer and Lithium, Sodium or Potassium Counterions with Nucleic Acid in Solution. 

---