Tetrahydrofuran , basicity anions

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

The solvation numbers of ions such as Mg2+, Al3+, and Be2+ may be determined by low temperature PMR techniques as mentioned earlier. The solvation number for small spherical ions may be determined in certain circumstances using a titration technique suggested by Van Geet (15). It is based on the competition by water for the solvation sphere of sodium ions in tetrahydrofuran (THF) measured by Na shifts. The salt must contain a large anion, which is assumed to be unhydrated during the titration otherwise a sum of hydration numbers would be determined. The assumptions made by Van Geet are basically those of the present treatment. His apparent constant is for the reverse of the equilibrium of Equation 21 and can be identified as l/K[P]p, where [P]f is the free THF concentration, effectively constant in the early stages of the titration. 

Cryptands were found to react with metal solutions in basic solvents to generate the alkali metal cryptate and an alkali anion (alkalide), for example (Na[2.2.2])+Na (62, 63). 23Na-NMR measurements of this salt in methylamine, tetrahydrofuran, and ethylamine solutions showed that the Na resonance is shifted strongly upheld from the Na resonance (free or complexed) as shown in Fig. 7. The anion resonates at approximately the same frequency as that calculated for the free... 

The large steric volume of some of the bulky Cp s is, of course, the source of slightly different reactivity patterns in complex formation when compared with normal or pentamethylcyclopentadienyl reaction schemes. For example, many metathetical reactions between the pentaphenylcyclo-pentadienyl anion and transition metal halides fail in etheral solvents [such as tetrahydrofuran (thf) and ether] (40,95), although the same reaction with the tetraphenylcyclopentadienyl anion proceeds without problems (48,96,97). Aside from the high steric volume, a lower base strength hypothesis is invoked for the pentaphenyl-Cp anion to account for the failures (40). The desired decaphenyl- or pentaphenylmetallocenes were eventually obtained using much less basic solvents, such as xylenes (40). 

The Alkylation Reaction. Ether solvents and naphthalene often are employed in the Sternberg alkylation reaction. Inasmuch as these substances are reactive in strongly basic solution, concern has been expressed about their polymerization reactions and about their incorporation into the alkylation product. In view of these potential problems we examined the reaction of potassium with carefully purified tetrahydrofuran at 25 C. The results shown in Figure 1, Curve A, indicate that the reaction is insignificant. On the other hand, the results shown in Figure 1, Curve B, indicate that potassium reacts rapidly with naphthalene to form naphthalene anion radical and naphthalene dianion. The reduction to the dianion is about 80% complete after 4.5 hr. 

The authors opted to install the bromotetrahydropyran A-ring last due to its possible instability under radical, strongly basic, and/or acidic conditions. The D-ring was envisioned to arise from a stereoselective epoxidation followed by cyclization to afford the tetrahydrofuran framework. Key to achieving this plan was accessibility to structure 56 (Scheme 10). This fragment in turn was envisioned to be assembled by coupling the anion derived from 57 with epoxide 58. Compound 58 could presumably be accessed via stereoselective cyclizations from diol 59. 

T, must be taken. Stability decreases with increasing atomic number of the group-IIA element and aryl are more stable than alkyl. Donors exert a stabilizing effect and, therefore, solvents such as tetrahydrofuran (THE) or pyridine (PY) are expedient for preparation and handling them. Diethyl ether is not sufficiently basic to impart adequate stability. The reactivity can be correlated with ionic character in the Pb-group-IIA element bond, the organolead moiety being anionic. 

Cyclic vinyl epoxides are versatile building blocks (Table 11) which have been used in palladium-assisted routes to carbocyclic nucleosides. A formal synthesis of ( )-aris-teromycin101, the carbocyclic analog of adenosine, has been accomplished employing ni-tromethane as the nucleophile which serves as an acyl anion equivalent (Table 11. entry 2). The aldehyde is released by subsequent basic potassium permanganate oxidation. If nitromethane is used diluted in tetrahydrofuran, then a mixture of mono- and bis-alkylated product is formed. Whereas the alkylation of cyclohexenoxide with dimethyl malonate proceeds in a 1,4-crs fashion under palladium(O) catalysis, the 1.2-/ra/i.v-product is formed under basic conditions in the absence of the palladium(O) catalyst. 

Acetoxylation of poly(vinyl chloride) can be carried out under homogeneous conditions. Crown ethers, like 18-crown-6, solubilize potassium acetate in mixtures of benzene, tetrahydrofuran, and methyl alcohol to generate unsolvated, strongly nucleophilic naked acetate anions. These react readily with the polymer under mild conditions. Substitutions of the chlorine atoms on the polymeric backbones by anionic species take place by a Sn2 mechanism. The reactions can also proceed by a Sivl mechanism. That, however, requires formations of cationic centers on the backbones in the rate-determining step and substitutions are in competition with elimination reactions. It is conceivable that anionic species may (depending upon basicity) also facilitate... 

Kawi et al. [4, 60] reported on experiments comparing the iridium carbonyl chemistry in NaY zeolite with that in the more basic NaX zeolite (some of whose basicity should perhaps be attributed to the excess NaOH used in the preparation which was ultimately not washed out). The results showed that the supercages in the NaX zeolite were sufficiently basic to provide an effident medium for the synthesis of anionic iridium carbonyl clusters. When [Ir(CO)2(acac)] in NaX zeolite was treated with CO, it was transformed into [HIr4(CO)n] and then into [Irg(CO)t5]. The anionic carbonyl dusters trapped in the cages were characterized by infrared and EXAFS spectroscopies and could not be extracted from the zeolite by ion exchange with bis(triphenylphosphine)iminium chloride, [PPN][Q], in tetrahydrofuran solution. 

Methyllithium is a strong basic and a powerful nucleophile. Compared to another synthetically important organolithium compound, -butyllithium, methyllithium reacts slowly with tetrahydrofuran at room temperature. An etheral solution of methyllithium is stable for a long time. Most of the reactions involving methyllithium are conducted at low temperatures. Methyllithium is used for deprotonations and as a source of methyl anion. Ketones can be converted to tertiary alcohols using methyllithium as... 

The role of the solvent may be important in anionic-coordinated polymerizations. Firstly it can play a role in the preparation of the initiator. It is known that alcoholysis reactions are solvent dependant. Secondly, it can interfere by its coordination strength on the initiator. In the case of the polymerization of propylene oxide with zinc alcoholates Ishimori etal [27] have shown that a retardation effect was observed when weakly basic solvents such as tetrahydrofuran, diethyl ether or dioxane are used. Triethylamine inhibits the polymerization almost completely. 

The structure of the related complex MeCCo3(CO)9, has been determined by X-ray diffraction. This compound is prepared by the reaction of Co2(CO)s with MeCXa (X = Halogen) in a weakly basic solvent such as ethanol or tetrahydrofuran at the boiling point. A possible reaction sequence for its formation involves the initial disproportionation of the carbonyl to give [ 0(00)4] anions. This reaction commonly occurs in the presence of nitrogen or oxygen bases ... 

---