Counterions reactions

It was previously noted that enolates of a-allyloxy ketones were capable of either 2,3- or 3,3-rear-rangement, depending upon counterion, reaction conditions and substituents. Ester enolates show a greater propensity for the 2,3-pathway, as illustrated by the geraniol-derived alkoxyacetate (198) which afforded hydroxy ester (200) as the sole product upon treatment with LDA in THF at -78 to 0 C. The 3,3-product (201) could be obtained by addition of TBDMS-Cl and HMPA to the enolate at -78 °C and thermal rearrangement of the TBS enol ether (199b Scheme 14). ... 

Cationic Methods. Cationic methods have also found utility in preparation of block polymers however, generally speaking these methods suffer from a variety of synthesis problems including monomer-chain end equilibria, facile chain end termination or transfer with certain counteranions, and difficult molecular weight control. Most examples of living cationic systems include the use of an oxonium cation with a carefully selected counterion (Reaction 8). 

Eor solubility reasons, a dual solvent system, i.e., THF/pyridine has to be used. Through variations in the metal counterion, reaction temperature, and the aldehyde used, it was shown that, contrary to Trans-1,3-dithiane 1,3-dioxide, best reactivities were obtained at low temperatures (—78 °C) with a lithium ion. Even at that temperature with a sodium ion, as demonstrated by scrambling experiments with aldehydes, equilibration begins to occur with a slow reverse reaction. Upon heating, the equilibration erodes the good initial kinetic ratio and diminishes the yield. To completely circumvent this, the authors used 2.4 equiv of LiHMDS at —78°C. This results in the formation of a bis-sulfinyl... 

Charge density Valence of counterions Reaction constant Volume fraction of solvent Volume fraction of PE... 

It turned out that the dodecylsulfate surfactants Co(DS)i Ni(DS)2, Cu(DS)2 and Zn(DS)2 containing catalytically active counterions are extremely potent catalysts for the Diels-Alder reaction between 5.1 and 5.2 (see Scheme 5.1). The physical properties of these micelles have been described in the literature and a small number of catalytic studies have been reported. The influence of Cu(DS)2 micelles on the kinetics of quenching of a photoexcited species has been investigated. Interestingly, Kobayashi recently employed surfactants in scandium triflate catalysed aldol reactions". Robinson et al. have demonshuted that the interaction between metal ions and ligand at the surface of dodecylsulfate micelles can be extremely efficient. ... 

Further evidence for an increased efficiency of complexation in the presence of micellar aggregates with bivalent metal counterions is presented in Table 5.4. The apparent rate constants of the reaction of 5.1c with 5.2 in the presence of micelles of Co(DS)2, Ni(DS)2, Cu(DS)2 and Zn(DS)2 are compared to the rate constants for the corresponding bivalent metal ion - dienophile complexes in the absence of micelles. The latter data are not dependent on the efficiency of the formation of the catalyst - dienophile complex whereas possible incomplete binding will certainly be reflected in the former. The good correlations between 1 and and the absence of a correlation between and... 

The large sulfur atom is a preferred reaction site in synthetic intermediates to introduce chirality into a carbon compound. Thermal equilibrations of chiral sulfoxides are slow, and parbanions with lithium or sodium as counterions on a chiral carbon atom adjacent to a sulfoxide group maintain their chirality. The benzylic proton of chiral sulfoxides is removed stereoselectively by strong bases. The largest groups prefer the anti conformation, e.g. phenyl and oxygen in the first example, phenyl and rert-butyl in the second. Deprotonation occurs at the methylene group on the least hindered site adjacent to the unshared electron pair of the sulfur atom (R.R. Fraser, 1972 F. Montanari, 1975). 

The ketone is added to a large excess of a strong base at low temperature, usually LDA in THF at -78 °C. The more acidic and less sterically hindered proton is removed in a kineti-cally controlled reaction. The equilibrium with a thermodynamically more stable enolate (generally the one which is more stabilized by substituents) is only reached very slowly (H.O. House, 1977), and the kinetic enolates may be trapped and isolated as silyl enol ethers (J.K. Rasmussen, 1977 H.O. House, 1969). If, on the other hand, a weak acid is added to the solution, e.g. an excess of the non-ionized ketone or a non-nucleophilic alcohol such as cert-butanol, then the tautomeric enolate is preferentially formed (stabilized mostly by hyperconjugation effects). The rate of approach to equilibrium is particularly slow with lithium as the counterion and much faster with potassium or sodium. 

The reaction medium plays a very important role in all ionic polymerizations. Likewise, the nature of the ionic partner to the active center-called the counterion or gegenion-has a large effect also. This is true because the nature of the counterion, the polarity of the solvent, and the possibility of specific solvent-ion interactions determines the average distance of separation between the ions in solution. It is not difficult to visualize a whole spectrum of possibilities, from completely separated ions to an ion pair of partially solvated ions to an ion pair of unsolvated ions. The distance between the centers of the ions is different in... 

Charge-Transfer Salts. Most charge-transfer salts can be prepared by direct mixing of donors and acceptors in solution. Semiconducting salts of TCNQ have been prepared with a variety of both organic and inorganic counterions. Simple salts of the type TCNQ can be obtained by direct reaction of a metal such as copper or silver with TCNQ in solution. Solutions of metal iodides can be used in place of the metals, and precipitation of the TCNQ salt occur direcdy (24). 

Unsaturation Value. The reaction temperature, catalyst concentration, and type of counterion of the alkoxide affect the degree of unsaturation. The tendency for rearrangement of PO to aHyl alcohol is greatest with lithium hydroxide and decreases in the following order (100) Li+ >... 

A protonic acid derived from a suitable or desired anion would seem to be an ideal initiator, especially if the desired end product is a poly(tetramethylene oxide) glycol. There are, however, a number of drawbacks. The protonated THF, ie, the secondary oxonium ion, is less reactive than the propagating tertiary oxonium ion. This results in a slow initiation process. Also, in the case of several of the readily available acids, eg, CF SO H, FSO H, HCIO4, and H2SO4, there is an ion—ester equiUbrium with the counterion, which further reduces the concentration of the much more reactive ionic species. The reaction is illustrated for CF SO counterion as follows ... 

The rate of ion propagation, is independent of the counterion and has been found to be about 46 X 10 in all cases for CF SO", AsF, SbF, SbCFg, PF g, and BF/ counterions. Conditions were the same for all counterions, ie, 8.0 M of monomer in CCI4 solvent and 25°C polymerization temperature. With less stable counterions such as SbCF and BF at most temperatures, the influence of transfer and termination reactions must be taken into account (71). 

Displacement of a tertiary amine from a quaternary (eq. lb) iavolves the attack of a nucleophile on the a-carbon of a quaternary and usually competes with the Hoffman elimination (173). The counterion greatiy iafluences the course of this reaction. For example, the reaction of propyltrimethylammonium ion with hydroxide ion yields 19% methanol and 81% propylene, whereas the reaction with phenoxide ion yields 65% methoxybenzene and 15% propylene (174). 

Polyelectrolyte complex membranes are phase-inversion membranes where polymeric anions and cations react during the gelation. The reaction is suppressed before gelation by incorporating low molecular weight electrolytes or counterions in the solvent system. Both neutral and charged membranes are formed in this manner (14,15). These membranes have not been exploited commercially because of then lack of resistance to chemicals. 

Winstein suggested that two intermediates preceding the dissociated caibocation were required to reconcile data on kinetics, salt effects, and stereochemistry of solvolysis reactions. The process of ionization initially generates a caibocation and counterion in proximity to each other. This species is called an intimate ion pair (or contact ion pair). This species can proceed to a solvent-separated ion pair, in which one or more solvent molecules have inserted between the caibocation and the leaving group but in which the ions have not diffused apart. The free caibocation is formed by diffusion away from the anion, which is called dissociation. 

---