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Lithium cryptates

From the temperature dependence of the rate of decomplexation /c i = 1/t the activation parameters and AS can finally be determined. Figure 5 displays some temperature-dependent Na spectra for complexation between Na and cryptand C222. (64) Free enthalpies and entropies of activation for complexation between Li" and cryptands C211 and C221 as well as between Na" and cryptands C222 are in Table IV. While for the lithium cryptates the enthalpies of... [Pg.144]

On the other hand, the products under conditions 2 and 4 in Table I show considerable discrepancy from what would be considered a normal distribution of equilibrium products. Although the data were taken at a point when there seemed to be little change occurring in the system, it can still be argued that equilibrium was not achieved in these examples. Later work with lithium cryptate, one of the deviants in Table I, indicates agreement in the distributions of D4, D5, and Dg reported by earlier workers (26). Unfortunately, no data were given for Wp in that publication. [Pg.75]

The anionic polymerization of cyclosiloxanes was examined In benzene and toluene with lithium cryptates as counterions. Only one type of active species Is observed In the case of LI + [211] thus, the kinetics of the propagation and of the by-product cyclosiloxanes formation can be studied In detail for the first time. The reactivity of cryptated sllanolate Ion pairs toward the ring opening of D3 Is greatly enhanced compared to that of other systems. [Pg.23]

Table IV. Molecular Weight Distribution of PDMS Obtained from D3 using Lithium Cryptates as Counterions... Table IV. Molecular Weight Distribution of PDMS Obtained from D3 using Lithium Cryptates as Counterions...
Cl4H25BrMg02, Phenylbis(diethyl ether)magnesium bromide, 29, 532 Cl4H28lLiN204, Lithium cryptate, 39B, 517... [Pg.354]

The ROP of functionalized cyclotetrasiloxanes has been also examined, and some examples are listed in Table 3.5. Here, it is necessary to use highly efficient initiating systems (e.g. lithium cryptates or superbases), as the reactivity of the cychc tetramers is much lower than that of the corresponding trimers. Copolymers produced in this manner have a less regular structure than those produced by the anionic ROP of cyclotrisiloxanes. [Pg.80]

The effect of cryptands on the reduction of ketones and aldehydes by metal hydrides has also been studied by Loupy et al. (1976). Their results showed that, whereas cryptating the lithium cation in LiAlH4 completely inhibited the reduction of isobutyraldehyde, it merely reduced the rate of reduction of aromatic aldehydes and ketones. The authors rationalized the difference between the results obtained with aliphatic and aromatic compounds in terms of frontier orbital theory, which gave the following reactivity sequence Li+-co-ordinated aliphatic C=0 x Li+-co-ordinated aromatic C=0 > non-co-ordinated aromatic C=0 > non-co-ordinated aliphatic C=0. By increasing the reaction time, Loupy and Seyden-Penne (1978) showed that cyclohexenone [197] was reduced by LiAlH4 and LiBH4, even in the presence of [2.1.1]-cryptand, albeit much more slowly. In diethyl ether in the absence of... [Pg.359]

In the complexation reaction cryptand must compete with solvent molecules for the cations in solution. Thus solvents such as methanol with low dielectric constant and solvating power offer a preferrable reaction environment but we have achieved quantitative yields in water. The main problem encountered in syntheses of cryptates has been the presence of other cations such as Na and KT competing for the cryptand. Care is taken to minimize the concentration of competing cations of size similar to the cation intended for complexation by using lithium salts for buffering solutions.-... [Pg.201]

Diaza[12]coronand-4 (21) was condensed with diethylene glycol bismesylate 22 in the presence of butyllithium. Precipitation, occuring during the reaction course, afforded the proton cryptate 24 H+ c= [1.1.1] in 40% yield. It should be noted that [1.1.1] was obtained only in 10% yield via the high-dilution method 23). Lithium promoted cyclization was excluded (as an alternative mechanism) by an additional experiment in which KH served as a base instead of BuLi. Identical yield was achieved, indicating that intramolecular hydrogen bonding was responsible of the cyclization. [Pg.188]

The particular case of lithium acetylacetonate (acac), a canonic example of /I-diketone enolate, was also examined early. It was shown that its chelated (Z,Z) conformation was almost exclusive in methanol at — 60 °C and that dimers were probably formed in which one of the two lithium cations would be chelated by the two acac anions272. A somewhat similar dimer, obtained from the lithium enolate of ethyl acetoacetate complexed by a 2.1.1 cryptate, was characterized in one of the first 7Li NMR studies of enolates (Scheme 65)273. Note that the structure of the three ft -diketone mono- and dilithium enolates displayed in Scheme 64 has been studied, despite their poor solubility, in both THF-dg and DMSO-d6 by 13C NMR260. The data obtained for the monoenolates are consistent with rapidly equilibrating dimers, while the dimers of dienolates seem to form slowly on the NMR time scale. [Pg.569]

SCHEME 65. NMR spectrum of lithium acetylacetonate ( II, 60 MHz, top) and of ethyl acetoac-etate lithium enolate (7Li with LiC104 in water as external reference, 35 MHz, bottom) 2.1.1 cryptate = 4,7,13,18-tetraoxa-l J0-diazabicyclo[8.5.5]eicosane272,273... [Pg.570]

Kinetic parameters for lithium and sodium cryptate [I ] decomplexation (61,64)... [Pg.144]

Values of n in equation 9 can vary from 1 for cryptate-coordinated lithium silanolates (25, 27) and R4NOSi= (35) to 4 for LiOSi=. The latter result was reported to be dependent on dilutions (36). In the absence of polar solvents, n = 2 is commonly reported for potassium silanolate initiators (37, 38). The fractional order is attributed to strongly associated ion pairs at the chain ends. These ion pairs must dissociate to provide a low concentration of unassociated ion pairs prior to propagation. In the case of potassium silanolate, this dissociation is pictured as the initiation of the polymerization (37), as shown by equations 10 and 11. [Pg.77]

For all systems, the lighter isotope Li is enriched in the organic phase compared with Li. Obviously, this also proved true for the Chinese work where the obtained e-value of 30 x 10" is comparable with the results of Jepson and Cairns. On the whole, a significant dependence of the isotopic separation on the polyether as well as on the anion of the lithium salt was found. This can be understood by means of the different interaction of the cryptated cation with the anion in chloroform. According to Jepson and Cairns, the enrichment of Li in the organic phase is attributed to a more stable Li polyether complex compared with the Li compound. [Pg.106]

The work of Boileau38 with lithium catalysts and highly specific cation complexing compounds is particularly significant. The author describes the use of the macrobicyclic ligand or cryptate which forms multicontact complexes with the metal counter-ion of the... [Pg.1299]

The anionic polymerization of cyclosiloxanes is a complex process. For the alkali metal silanolate catalysts the weight of experimental evidence supports a mechanism based on growth from the metal silanolate ion pair. The ion pair is in dynamic equilibrium with ion-pair dimers which, for the smaller alkali metal ions like lithium and sodium, are themselves in dynamic equilibrium with ion-pair dimer aggregates. The fractional order in catalyst which is observed is a direct result of the equilibria between ion pairs, ion-pair dimers and ion-pair dimer aggregates. Polar solvents break down the aggregates and increase the concentration of ion-pair dimers and hence the concentration of ion pairs. Species like crown ethers and the [2.1.1] cryptate which form strong complexes with the metal cation increase the dissociation of ion-pair dimers into ion pairs. In the case of the lithium [2.1.1] cryptate dissociation into ion pairs is complete and the order in catalyst is unity. [Pg.1302]

Pierre and Handel have studied the effect of [2.1.1]-cryptate on the lithium aluminum hydride reduction of cyclohexanone in diglyme [16]. The [2.1.1]-cryptate strongly complexes lithium ion and if sufficient cryptate is used to sequester all of the lithium ion, no reduction occurs. Apparently, lithium ion is needed as an electrophilic catalyst for the reduction to occur (see Eq. 12.8). Consistent with this interpretation is the observation that even in the presence of cryptate, reduction will occur if an excess of lithium iodide is also present. The relatively low reactivity of tetrabutyl-ammonium borohydride in benzene solution may also reflect this property, at least in part [9]. Likewise, the jS-hydroxyethyl quaternary ammonium ions may be better catalysts than non-oxygenated quaternary ions because the hydroxyl may hydrogen bond to carbonyl and provide electrophilic catalysis [5]. Similar, though less dramatic results, have been observed in the reduction of aromatic aldehydes and ketones by lithium aluminum hydride in the presence of [2.1.1]-cryptate [17]. [Pg.220]

The presence of a cation is also important in the lithium aluminum hydride reduction of cyclohexanone [12, 13]. Lithium ion is apparently required as an electrophilic catalyst for the reduction of cyclohexanone and if an appropriately sized cryptate is added to the reducing medium, a retardation of the reduction is observed. Addition of [2.1.1]-cryptate to the reduction system in quantities sufficient to complex all of the available lithium ion leads to a total absence of reduction. An excess of lithium iodide, however, restores reactivity. This reaction is discussed in Sect. 12.5. [Pg.244]

Figure 9-4 The binding of a cation by a polycyclic ether (cryptand) to form a complex (cryptate). The system shown selectively binds the potassium ion, with a binding constant of K = 10 °. The order of selectivity is > Rb+ > Na" > Cs > Li. The binding constant for lithium is about 100. Thus, the total range within the series of alkali metals spans eight orders of magnitude. Figure 9-4 The binding of a cation by a polycyclic ether (cryptand) to form a complex (cryptate). The system shown selectively binds the potassium ion, with a binding constant of K = 10 °. The order of selectivity is > Rb+ > Na" > Cs > Li. The binding constant for lithium is about 100. Thus, the total range within the series of alkali metals spans eight orders of magnitude.

See other pages where Lithium cryptates is mentioned: [Pg.303]    [Pg.24]    [Pg.34]    [Pg.1299]    [Pg.1300]    [Pg.28]    [Pg.93]    [Pg.69]    [Pg.303]    [Pg.24]    [Pg.34]    [Pg.1299]    [Pg.1300]    [Pg.28]    [Pg.93]    [Pg.69]    [Pg.47]    [Pg.102]    [Pg.180]    [Pg.1015]    [Pg.103]    [Pg.188]    [Pg.20]    [Pg.180]    [Pg.131]    [Pg.47]    [Pg.508]    [Pg.512]    [Pg.508]    [Pg.512]    [Pg.509]    [Pg.509]    [Pg.16]    [Pg.1300]    [Pg.47]    [Pg.301]    [Pg.428]    [Pg.508]    [Pg.512]   
See also in sourсe #XX -- [ Pg.303 ]

See also in sourсe #XX -- [ Pg.6 , Pg.8 , Pg.9 ]




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