Bases Potassium carbonate-18-Crown

The solubilization of sodium or potassium carbonate into apolar solvents such as benzene or acetonitrile with the aid of 18-crown-6 [3] generates a powerful base that has been used for a variety of preparative transformations (Fedorynski et al., 1978). Mechanistic studies have not been reported thus far. 

Compound 8 can be reacted with substituted iodoarenes using copper metal and potassium carbonate <1980CB358> or potassium hydroxide <1994MCL(242)127> as a base. The reaction proceeds at 170-180°C and gives good yields. Addition of crown ether 18-Cr-6 allows a lower temperature to be used <2000JCD2105>. 

Hexaza 18-crown-6 151 (7.8%) and 152 (8.0%) have also been synthesized by treatment of 2,6-fc(chloromethyl)pyridine with jym-dimethylethylenediamine or piperazine in dimethylformamide, utilizing potassium carbonate as the base. The cobalt(II) and copper(II) complexes 153 of 151 have been made and the single crystal X-ray structure determination of each shows that in both cases the metal ion is octahedrally coordinated by the six nitrogen atoms of the ligand. In the case of... 

Z)-OL, -Unsaturated esters,l Wittig-Homer reactions generally show a preference for formation of (E)-alkenes. Thus (E)-a,p-unsaturated esters are obtained preferentially on reaction of aldehydes with trimethyl phosphonoacetate under usual conditions (potassium f-butoxide). Use of a highly dissociated base can favor (Z)-selectivity. The most effective base for this purpose is potassium hexamethyldisilazide, KN[Si(CH3)3]2, in combination with 18-crown-6, although even potassium carbonate with the crown ether is fairly effective. The (Z)-selectivity can be further enhanced by use of 1 as the phos-phonoester. Under these conditions, (Z)-unsaturated esters can be prepared from aliphatic and aromatic aldehydes with Z/E ratios as high as 50 1. The method is also useful for transformation of unsaturated aldehydes to (E,Z)-dienoates and (E,E,Z)-trienoates. 

A typical preparation of the polycarbonates is shown for polymer III in Scheme I. 1,4-Benzenedimethanol is activated by reaction with two equivalents of p-nitrophenyl chloroformate in pyridine and the resulting symmetrical dicarbonate is then used in a polycondensation with an equimolar amount of 2-cyclohexen-l,4-diol in a solid-liquid phase-transfer catalyzed reaction with 18-crown-6 as catalyst and solid anhydrous potassium carbonate as base. Alternately, the same polymer can be prepared by condensation of bis(4-nitrophenyl)-2-cyclohexen-1,4-ylene dicarbonate [12] with 1,4-benzenedimethanol or through a variety of similar polycondensations using diol biscar bonylimidazolides [13,14]. 

The yield of the tetraaza-18-crown-6 product was 40% with potassium carbonate but only 10% when sodium carbonate was used. The amount of 9-crown-3 by-product was greater when sodium carbonate was used. The same reaction using diethylene glycol ditosylate instead of 2-chloroethyl ether with cesium carbonate as the base was reported recently, but with only a 20% yield (Craig et al., 1989). 

Kulstad and Malmsten reported that the diaza-crowns could be prepared by reacting a bis primary amine-containing ether and a diiodo ether with sodium or potassium carbonate as the base (method E-4) (Kulstad and Malmsten, 1979, 1980a). The yield was 17% for the preparation of diaza-15-crown-... 

TDA-1, like Aliquat 336 a cheap and perhaps less toxic substitute for crown ethers, can be used to assist in the formation of phenacyl esters (104) from phenacyl bromides when potassium carbonate is employed as the base.100 An electrochemical procedure for the alkylation of acids by alkyl halides or tosylates gives variable but often excellent yields and could be useful in special... 

Phase transfer catalyzed reactions in which ylides are formed from allylic and ben-zylic phosphonium ions on cross-linked polystyrenes in heterogeneous mixtures, such as aqueous NaOH and dichloromethane or solid potassium carbonate and THF, are particularly easy to perform. Ketones fail to react under phase transfer catalysis conditions. A phase transfer catalyst is not needed with soluble phosphonium ion polymers. The cations of the successful catalysts, cetyltrimethylammonium bromide and tetra-n-butylammonium iodide, are excluded from the cross-linked phosphonium ion polymers by electrostatic repulsion. Their catalytic action must involve transfer of hydroxide ion to the polymer surface rather than transport of the anionic base into the polymer. Dicyclohexyl-18-crown-6 ether was used as the catalyst for ylide formation with solid potassium carbonate in refluxing THF. Potassium carbonate is insoluble in THF. Earlier work on other solid-solid-liquid phase transfer catalyzed reactions indicated that a trace of water in the THF is necessary (40). so the active base for ylide formation is likely hydrated, even though no water is included deliberately in the reaction mixture. 

We have found that instead of NaOH, anhydrous sodium or potassium carbonates can be used for the generation of anions, even from rather weak CH acids. In these cases the system contains organic reactants in liquid phase, the catalyst (TAA salt or crown ether) dissolved in a nonpolar solvent and anhydrous alkali carbonate in solid phase. Here the acid-base equilibrium occurs on the surface of the solid phase, subsequently the carbanions are transformed from this adsorbed state into the organic phase in form of TAA salts. 

It has recently been shown that crown ether catalyzed Wittig reactions give product distributions typical of the so-called salt-free reaction [6]. Using potassium r-butoxide or potassium carbonate as base, the yields of olefin were high and the isomer ratio was quite solvent dependent. The condensation of ethylidenetriphenyl-phosphine with benzaldehyde, for example, produced over 90% of i3-methylstyrene in either tetrahydrofuran or dichloromethane, but the olefin was 85% cis in the former case and 78% trans in the latter [5]. 

Anhydrous sodium or potassium carbonates have been shown to act as efficient strong bases in solid-organic liquid two-phase systems in the presence of crown ethers this by-passes the requirement for concentrated aqueous hydroxide solutions in the equivalent liquid-liquid techniques. Among the reactions possible with this new method are the alkylation of active methylene compounds, the Williamson ether synthesis, and the Darzens reaction. 

Dibromocyclopropanes [e.g. (4)] can be prepared from alkenes and bromoform using the very simple and inexpensive base anhydrous potassium carbonate in the presence of 18-crown-6 at 140 °C. The two sulphur ylides (5) and (6) are useful intermediates for the synthesis of bis-methoxycarbonylcyclopropanes (7) and silylcyclopropanes (8) respectively. 

The chemistry can be illustrated with the case of adenine isodideoxynucleoside. The key precursor for the coupling reaction was compound 14 (Scheme 7), which can be synthesized in excellent yields (17%) from D-xylose. Condensation of 14 with adenine stereospecifically and regiospecifically was carried out by reaction with this base in the presence of potassium carbonate and 18-crown-6 in DMF. Deprotection of the resulting product with sodium methoxide in methanol gave the target isodideoxynucleoside 15 in 55% yield (for the last two steps). The structure of 15 was confirmed by its UV spectrum (260 nm, e 14,000), H and C NMR data (single compound and absence of astereoisomer), and optical rotation ([a]p = -26.6°). TTiis magnitude of levorotation is the... 

EXPLOSION and FIRE CONCERNS nonflammable NFPA rating (not rated) explosive reaction with crown ethers or potassium hydroxide violent reaction with lithium, sodium-potassium alloy, acetone, or bases incompatible with metals, caustic alkali, and strong oxidants decomposition emits highly toxic gases and vapors (such as hydrogen bromide and bromine) use dry chemical, carbon dioxide, water spray, fog or foam for firefighting purposes. 

---