Counterions, tetrahydrofuran

Cationic ring-opening polymerization is the only polymerization mechanism available to tetrahydrofuran (5,6,8). The propagating species is a tertiary oxonium ion associated with a negatively charged counterion ... 

The nucleophilic addition of Grignard reagents to a-epoxy ketones 44 proceeds with remarkably high diastereoselectivity70. The chelation-controlled reaction products are obtained in ratios >99 1 when tetrahydrofuran or tetrahydrofuran/hexamethylphosphoric triamide is used as reaction solvent. The increased diastereoselectivity in the presence of hexamethylphos-phoric triamide is unusual as it is known from addition reactions to a-alkoxy aldehydes that co-solvents with chelating ability compete with the substrate for the nucleophile counterion, thus reducing the proportion of the chelation-controlled reaction product (vide infra). 

These equilibria give directly only acidity differences between RH and R H and can vary with solvent and counterion. The corresponding — log f values have been converted to pK scales by choosing one compound as the standard and referring others to it. The standard chosen for tetrahydrofuran (THF) solutions is fluorene and it is assigned a pK of 22.9, its value in the DMSO scale (statistically corrected per hydrogen for fluorene the measured pK is 22.6)3,4. 

Complex C (Scheme 21) seems to shows stable penta-coordination for apparently very different reasons. The compound is cationic, but the counterion is a non-coordinating tetraarylborate. Interestingly, neither the dichloromethane solvent nor the diethylether present in the reaction mixture seem to coordinate to the open site in solution. The compound was crystallized from a tetrahydrofuran/pentane mixture as the tetrahy-drofuran (THF) solvate, but in the crystal structure, the THF is remote from the open site at platinum. The open site is shielded somewhat by the methyl groups of the protonated TpMe2 ligand, but it does not appear completely inaccessible. A reasonable explanation for the... 

A thorough study on the ion-exchange mechanism and the effect of distinct counterions in this PO mode was recently presented by Gyimesi-Forras et al. [41]. A large variety of distinct acid additives to methanol, acetonitrile, and tetrahydrofuran (Table 1.1) (without any base added) was investigated in view of the stoichiometric displacement model and their effect on the enantiomer separation of 2-methoxy-2-(l-naphthyl)propionic acid. The stoichiometric displacement model (Equation 1.1) was obeyed also in the PO mode, as revealed by linear plots of log k vs. acid concentration. The slopes and intercepts along with the concentration ranges used with the distinct competitor acids are summarized in Table 1.1. 

The propagation rate constant and the polymerization rate for anionic polymerization are dramatically affected by the nature of both the solvent and the counterion. Thus the data in Table 5-10 show the pronounced effect of solvent in the polymerization of styrene by sodium naphthalene (3 x 1CT3 M) at 25°C. The apparent propagation rate constant is increased by 2 and 3 orders of magnitude in tetrahydrofuran and 1,2-dimethoxyethane, respectively, compared to the rate constants in benzene and dioxane. The polymerization is much faster in the more polar solvents. That the dielectric constant is not a quantitative measure of solvating power is shown by the higher rate in 1,2-dimethoxyethane (DME) compared to tetrahydrofuran (THF). The faster rate in DME may be due to a specific solvation effect arising from the presence of two ether functions in the same molecule. 

There are few studies of the effect of temperature on monomer reactivity ratios [Morton, 1983]. For styrene-1,3-butadiene copolymerization by r-butyllithium in rc-hexane, there is negligible change in r values with temperature with r — 0.03, r2 = 13.3 at 0°C and n = 0.04, r% = 11.8 at 50°C. There is, however, a signihcant effect of temperature for copolymerization in tetrahydrofuran with r — 11.0, r2 = 0.04 at —78°C and r — 4.00, r2 = 0.30 at 25° C. The difference between copolymerization in polar and nonpolar solvents is attributed to preferential complexing of propagating centers and counterion by 1,3-butadiene as described previously. The change in r values in polar solvent is attributed to the same phenomenon. The extent of solvation decreases with increasing temperature, and this results in... 

Similarly, very useful yields and diastereoselectivities were observed on alkylation of (5R)-2,3,5,6,-tetrahydro-5-phenyl-A-(ter/-butoxycarbonyl)-4/f-l,4-oxazin-2-one (7)92, The latter is available from (A)-phenylglycinol and phenyl 2-bromoacetate with subsequent protection of the amino group. The influence of the base, the counterion and the solvent was studied for this example. Sodium hexamethyldisilazanide in tetrahydrofuran/dimethoxyethane turned out to be by far the best conditions for deprotonation. Also, it seems to be essential that the base is added to the chilled solution of 7 and not vice versa. 

A review is given on the kinetics of the anionic polymerization of methyl methacrylate and tert.-butyl methacrylate in tetrahydrofuran and 1,2-dimethoxy-ethane, including major results of the author s laboratory. The Arrhenius plots for the propagation reaction+are linear and independent of the counterion (i.e. Na, Cs). The results are discussed assuming the active centre to be a contact ion pair with an enolate-like anion the counterion thus exhibiting little influence on the reactivity of the carbanion. 

A most thorough investigation of the tetrahydrofuran system was recently reported by Dreyfuss and Dreyfuss (18). They initiated the polymerization bv the decomposition of benzenediazonium hexafluorophos-phate (PhNj, PFg), which provides a Ph+ ion and an extremely stable PFg counterion. It seems that the stability of the counterion is the reason for the simplicity of the system which is... 

Low-lying vacant orbitals of alkali metal cations can, consequently, accept an unpaired electron density even if it is delocalized over an extended 77-system of carbon chains. The anion radical of 1,4-diphenylbutadiene can exist in j-trans and in j-cis forms. The relative amounts of these geometrical isomers appear to depend highly on the counterion/sol-vent system. As counterions, Li+ and K+ were studied tetrahydrofurane, 2-methyltetrahy-drofurane, and dimethoxyethane were employed as solvents (Schenk et al. 1991). Interaction between the anion radical and the cation contributes to a stabilization of the s-cis arrangement if the cation is close to the carbons C-l and C-4. This stabilization is most pronounced in an ion pair with a tight interaction of the countercation and the ir-sys-tem, and indeed, the. v-cis form of the anion radical is only observed under those experimental conditions that favor tight ion pairing. 

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. 

The only other Cp structure of potassium known is the one of the base-free (Me3Si)H4C5K (139) which consists of parallel, one-dimensional zig-zag strands (as in 14). Analogous to many CpTl and Cpln structures (see above) the potassium sits equidistantly between two Cp rings with an electrostatic interaction between the counterions. The tetrahydrofuran coordination surely plays a role in the monomerization of the structure of the title compound (although it is not clear to what extent). The K-Cp distances in both complexes, however, are essentially identical. The preparation and crystal structure of MejCsK(pyridine)2, which has a linear zig-zag chain structure, has been described recently (179b). 

Polymerization of 5-membered cyclic ether, tetrahydrofuran with SbF6 counterion, is the best example of such a system. 

Both oxonium and carboxonium active species are relatively strong electrophiles (stronger than in the case of tetrahydrofuran). Thus, to avoid the termination by interaction with counterion, the stable counterions of low nucleophilicity are required. It has been shown that only the most stable complex anions SbF6 and AsF6 provide the living active species, whereas BF4, SbCl6, and even PF6 anions cause termination [98],... 

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 nature of the counterion affects the rate of growth. The results obtained in tetrahydrofuran solution indicate that kp decreases along the series Li+, Na+, K+, and Cs+ (Table II). This was an unexpected result. Actually, work carried out in another laboratory (4) shows a reverse trend in dioxane. Obviously, the nature of the solvation shell must be of great importance. The discussion of this topic should be postponed, however, until more information is gathered. 

Table II. Effect of Counterion on the Rate of Anionic Homopolymerization of Styrene in Tetrahydrofuran at 25 C.

The spectrophotometric technique determines K whenever the fraction of free ions is very low. The concentration of the free ions may be reduced to an insignificant level by the addition of some readily dissociated salt sharing a common cation with the investigated radical anion. On the other hand, the potentiometric technique yields K, and its value can be used to calculate K if the necessary dissociation constants are known. These constants may be derived from conductometric data (5). For an anthracene and pyrene pair incorporating Na+ as the counterion and tetrahydrofuran (THF) as the solvent, the ratio of equation 8 is only 1.6, which is equivalent to 10 millivolts (mV). However, for an anthracene and naphthalene pair, the ratio is 30.3, which is equivalent to 90 mV. 

The main feature of the reactivity of alkylpyridines is deprotonation of the alkyl group at the carbon adjacent to the ring. Measurements of side-chain-exchange in methanolic sodium methoxide, 4 2 3, 1800 130 1, and of pK values in tetrahydrofuran each have the 7-isomer more acidic than the a-isomer, both being much more acidic than the / -isomer, though the actual carbanion produced in competitive situations can depend on both the counterion and the solvent. Alkyllithiums selectively deprotonate an a-methyl where amide bases produce the... 

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