Potassium complexes crown ethers

On treatment with a potassium fluoridc-crown ether complex, alkyl 1,2,2,2-tetrachloroethyl carbonates are cleaved at the carbonyl group-oxygen bond to give high yields of alkyl fluoroformates [ 5] (equation 25)... 

Notwithstanding these examples, the preference for A-coordination is not absolute. 5 -bound NCS ligands are found bound to potassium in crown ether complexes. " " For example, the 3, 5 -difluoro-4 -hydroxybenzyl-armed monoaza-15-crown-5 ether forms F-bridged polymer-like... 

Sodiuim bexakis(formato)molybdate, 1235 Sodium complexes crown ethers, 37 Sodium molybdate, 1230 Sodium peroxoborate, 101 Sodium/potassium ATPase vanadate inhibition, 567 Sodium pyroantimonate, 265 Sodium tetrahydroborate, 101... 

Lithium alkoxides play an important role in the oligomeric cyclization of dinitriles in the synthesis of phthalocyanines. The combination of potassium alkoxide-crown ether complex in a hydrocarbon solvent gives a very powerful catalyst for production of vinyl ethers from alcohols and acetylene (Eq. 7.9). ... 

Leigh D A, Moody A E, Wade F A, King T A, West D and Bahra G S 1995 Second harmonic generation from Langmuir-Blodgett films of fullerene-aza-crown ethers and their potassium ion complexes Langmuir 11 2334-6... 

The metal-ion complexmg properties of crown ethers are clearly evident m their effects on the solubility and reactivity of ionic compounds m nonpolar media Potassium fluoride (KF) is ionic and practically insoluble m benzene alone but dissolves m it when 18 crown 6 is present This happens because of the electron distribution of 18 crown 6 as shown m Figure 16 2a The electrostatic potential surface consists of essentially two regions an electron rich interior associated with the oxygens and a hydrocarbon like exterior associated with the CH2 groups When KF is added to a solution of 18 crown 6 m benzene potassium ion (K ) interacts with the oxygens of the crown ether to form a Lewis acid Lewis base complex As can be seen m the space filling model of this... 

The macrocychc hexaimine stmcture of Figure 19a forms a homodinuclear cryptate with Cu(I) (122), whereas crown ether boron receptors (Fig. 19b) have been appHed for the simultaneous and selective recognition of complementary cation—anion species such as potassium and fluoride (123) or ammonium and alkoxide ions (124) to yield a heterodinuclear complex (120). 

The crown ethers and cryptates are able to complex the alkaU metals very strongly (38). AppHcations of these agents depend on the appreciable solubihty of the chelates in a wide range of solvents and the increase in activity of the co-anion in nonaqueous systems. For example, potassium hydroxide or permanganate can be solubiHzed in benzene [71 -43-2] hy dicyclohexano-[18]-crown-6 [16069-36-6]. In nonpolar solvents the anions are neither extensively solvated nor strongly paired with the complexed cation, and they behave as naked or bare anions with enhanced activity. Small amounts of the macrocycHc compounds can serve as phase-transfer agents, and they may be more effective than tetrabutylammonium ion for the purpose. The cost of these macrocycHc agents limits industrial use. 

Rate differences observed between the same bromophenylcarbene (241) when prepared by two different routes, diazirine photolysis and the reaction of benzylidene dibromide with potassium r-butoxide, vanish when a crown ether is added to the basic solution in the latter experiment. In this case the complexing potassium bromide is taken over by the crown ether, and selectivity towards alkenes reaches the values of the photolytic runs (74JA5632). 

In the absence of die polyether, potassium fluoride is insoluble in benzene and unreactive toward alkyl halides. Similar enhancement of solubility and reactivity of other salts is observed in the presence of crown ethers The solubility and reactivity enhancement result because the ionic compound is dissociated to a tightly complexed cation and a naked anion. Figure 4.13 shows the tight coordination that can be achieved with a typical crown ether. The complexed cation, because it is surrounded by the nonpolar crown ether, has high solubility in the nonpolar media. To maintain electroneutrality, the anion is also transported into the solvent. The cation is shielded from interaction with the anion as a... 

Frensch and Vdgtle have recently appended three crown ether units to the cyclo-triveratrylene unit . Note that Hyatt had previously prepared the open-chained relatives of this structure (see Sect. 7.3 and Eq. 7.6). Whereas Hyatt prepared the cyclo-triveratrylene skeleton and then appended polyethyleneoxy arms to it, Frensch and Vogtle conducted the condensation reaction (formaldehyde/HCl) on the preformed benzocrown. Thus benzo-15-crown-5 was converted into the corresponding tris-crown (IS) (mp 203.5—205.5°) in 4% yield. The yield was somewhat higher for the condensation of benzo-18-crown-6, but in both cases, yield ranges were observed. These species formed 1 3 (ligand/salt) complexes with sodium and potassium ions. 

Figure 4.11 Molecular structures of typical crown-ether complexes with alkali metal cations (a) sodium-water-benzo-I5-crown-5 showing pentagonal-pyramidal coordination of Na by 6 oxygen atoms (b) 18-crown-6-potassium-ethyl acetoacetate enolate showing unsymmelrical coordination of K by 8 oxygen atoms and (c) the RbNCS ion pair coordinated by dibenzo-I8-crown-6 to give seven-fold coordination about Rb.

Perhaps because of inadequate or non-existent back-bonding (p. 923), the only neutral, binary carbonyl so far reported is Ti(CO)g which has been produced by condensation of titanium metal vapour with CO in a matrix of inert gases at 10-15 K, and identified spectroscopically. By contrast, if MCI4 (M = Ti, Zr) in dimethoxy-ethane is reduced with potassium naphthalenide in the presence of a crown ether (to complex the K+) under an atmosphere of CO, [M(CO)g] salts are produced. These not only involve the metals in the exceptionally low formal oxidation state of —2 but are thermally stable up to 200 and 130°C respectively. However, the majority of their carbonyl compounds are stabilized by n-bonded ligands, usually cyclopentadienyl, as in [M(/j5-C5H5)2(CO)2] (Fig. 21.8). 

The highest enantioselectivities in the base-catalyzed Michael additions have so far been obtained using achiral bases complexed to chiral crown ethers. The addition of methyl 2,3-dihydro-l-oxo-1//-indene-2-carboxylate (1) to 3-buten-2-one using 4 mol% of a [l,T-binaphthalcnc]-2,2 -diol derived optically active crown ether 3 in combination with potassium AY/-butoxide as the base illustrates this successful method 259 260 It is assumed that the actual Michael donor is the potassium enolate complex of 1 and crown ether 3. 

Normally, persulfate (41) can only be used to initiate polymerization in aqueous or part aqueous (emulsion) media because it has poor solubility in most organic solvents and monomers. However, it has been reported that polymerizations in organic solvent may be initiated by crown ether complexes of potassium persulfate.234 237 Quaternary ammonium persulfates can also serve as useful initiators in organic media. 4 The rates of decomposition of both the crown ether complexes and the quaternary ammonium salts appear dramatically... 

In this way hosts 141-145 with both a cr-bonded Lewis acidic boron atom for complexation of anions and a conventional multidentate ligand for cations are generated. Complexation experiments of the 21-membered crown [6] boronate 142 with different potassium salts KX (X = F, Cl, Br, I, SCN, CN, OMe) indicate that there is a high specificity for the incorporation of KF, whereby F is bound covalently to the boron atom and is complexed by the crown ether (146, Fig. 39). An X-ray study has shown that the complexation of KF is heterotopic, i.e., both ions are complexed inside the same host. Some of the salts can only be bound in a monotopic way (KI and KSCN) [237]. 

Figure 3.57. Crown ethers a) hexaoxaoctadecane (18-crown-6) b) dibenzo-18-crown-6 c) complex of dicyclohexyl-18-crown-6 with potassium salt.

K2Hg(CN)4 and the crown ether dibenzo-18-crown-6 in aqueous ethanol yield the crystalline [K(C2oH2406)]2[I Ig(CN)4]-2I I2C) the structure contains two independent potassium-crown complexes and a distorted tetrahedral Hg(CN)4 entity (rav(l Ig—C) 218.4, rav(C—N) 112.3 pm) which coordinates to one of the complexed K ions through one of the CN ligands (slight lengthening of the C—N bond).116... 

Lehn 242 243) has described a solid phase model of a K+ channel based on the crown ether 37. The crystal structure of this inclusion complex reveals stacking of the crown ethers into vertical columns, empirical formula [2 37,2 K, 3 H20]2+, linked by water and potassium ions. The counter ions, empirical formula [K, 3 Br, 4 H20]2, comprise a polymeric chain running parallel to the columns. 

This type of chemistry can also be used for measurement of cations using a selective ligand (crown ether) linked to the chromophore. The colour change on complexation of potassium is from red to yellow. 

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