- Competition between Catalysts and Inhibitors in Crown Ether Formation and Other Reactions of Associated Substrates
-
Competition between non-covalent catalysts and/or inhibitors helps quantitative investigation in crown ether formation, and promises to be a useful tool when dealing with associated rective substrates in general.
- Vitali, Chiara Antonini,Masci, Bernardo
-
-
Read Online
- Model System for the Template Effect of Alkali and Alkaline-Earth Metal Ions on the Formation of Benzo-18-crown-6 in MeOH Solution
-
Alkali and alkaline-earth metal ions exert a remarkable catalytic effect (template effect) on the cyclizatiion of o-hydroxyphenyl 3,6,9,12-tetraoxa-14-bromotetradecyl ether (ArOH) to benzo-18-crown-6 (B18C6) in MeOH solution.This phenomenon is interpreted as resulting from two opposing factors, a rate-enhancing factor due to an increased proximity of chain ends in the cation-associated precursor ArO- and a rate-retarding factor due to interaction of the nucleophilic end of ArO- with the metal ion.Combination of previous equilibrium data with kinetic measurements from the present work provides a complete set of equilibrium constants for association of seven cations Na+, K+, Rb+, and Cs+ and Ca2+, Sr2+, and Ba2+ with the four ligands ArOH, ArO-, B18C6, and T, the latter being the transition state for cyclization.Following our earlier suggestion, according to which the proximity effect is viewed as a sort of macrocyclic effect, i. e., the one in which the cyclic ligand is T and the acyclic ligand is ArO-, we now present an extrathermodynamic treatment indicating that the magnitude of the above macrocyclic effect is well represented by the KB18C6/KArOH ratio.This ratio provides an estimate for the upper limit of the template effect.The actually observed values are lower by a factor of ca. 4 because of reduction of nucleophilicity due to cation association.
- Ercolani, Gianfranco,Mandolini, Luigi,Masci, Bernardo
-
-
Read Online
- Stable supramolecular dimer of self-complementary benzo-18-crown-6 with a pendant protonated amino arm
-
The novel 4′-(ammoniummethylene)benzo-18-crown-6 cation was synthesized as a hexafluorophosphate salt and found to exist as a cyclophane-like dimer in the solid state, gas phase and in acetonitrile solution.
- Kryatova, Olga P.,Kryatov, Sergey V.,Staples, Richard J.,Rybak-Akimova, Elena V.
-
-
Read Online
- Template Effects. 3. The Quantiative Determination of the Catalytic Effects of Alkali and Alkaline Earth Cations in the Formation of Benzo-18-crown-6 in Methanol Solution
-
Cyclization of the conjugate base of o-hydroxyphenyl 3,6,9,12-tetraoxa-14-bromotetradecyl ether to benzo-18-crown-6 in methanol solution has been found to be strongly and specifically accelerated by added alkali and alkaline earth bromides.The observed accelerations range from 13.2 for Cs+ to a spectacular 540 for Sr2+.The reported data refer to conditions in which the rate is independent of cation concentration, thus providing for the first time a quantitative determination of the template effect of added cations in the formation of a macrocyclic polydentate ligand.The different catalytic abilities as observed for the different cations are tentatively discussed in terms of a combination between proximity of the reactive ends in the bifunctional precursor complexed around the metal ion, and chemical factors arising from the extent of interaction of the cation with the nucleophilic oxide ion.
- Ercolani, Gianfranco,Mandolini, Luigi,Masci, Bernardo
-
-
Read Online
- Catalytic Reductions Without External Hydrogen Gas: Broad Scope Hydrogenations with Tetrahydroxydiboron and a Tertiary Amine
-
Facile reduction of aryl halides with a combination of 5% Pd/C, B2(OH)4, and 4-methylmorpholine is reported. Aryl bromides, iodides, and chlorides were efficiently reduced. Aryl dihalides containing two different halogen atoms underwent selective reduction: I over Br and Cl, and Br over Cl. Beyond these, aryl triflates were efficiently reduced. This combination was broadly general, effectuating reductions of benzylic halides and ethers, alkenes, alkynes, aldehydes, and azides, as well as for N-Cbz deprotection. A cyano group was unaffected, but a nitro group and a ketone underwent reduction to a low extent. When B2(OD)4 was used for aryl halide reduction, a significant amount of deuteriation occurred. However, H atom incorporation competed and increased in slower reactions. 4-Methylmorpholine was identified as a possible source of H atoms in this, but a combination of only 4-methylmorpholine and Pd/C did not result in reduction. Hydrogen gas has been observed to form with this reagent combination. Experiments aimed at understanding the chemistry led to the proposal of a plausible mechanism and to the identification of N,N-bis(methyl-d3)pyridin-4-amine (DMAP-d6) and B2(OD)4 as an effective combination for full aromatic deuteriation. (Figure presented.).
- Korvinson, Kirill A.,Akula, Hari K.,Malinchak, Casina T.,Sebastian, Dellamol,Wei, Wei,Khandaker, Tashrique A.,Andrzejewska, Magdalena R.,Zajc, Barbara,Lakshman, Mahesh K.
-
supporting information
p. 166 - 176
(2020/01/02)
-
- A practical synthesis of benzocrown ethers under phase-transfer catalysis conditions
-
Convenient and effective procedures for the synthesis of benzo-12-crown-4, benzo-15-crown-5, benzo-18-crown-6, benzo-21-crown-7 and benzo-24-crown-8 under phase-transfer catalysis conditions have been developed.
- Bogaschenko, Tatyana,Basok, Stepan,Kulygina, Catherine,Lyapunov, Alexander,Lukyanenko, Nikolay
-
p. 2266 - 2270
(2007/10/03)
-
- Synthesis of formyl derivatives of benzocrown ethers containing N, S, and O heteroatoms in the macrocycle
-
A method for the synthesis of 4'-bromobenzodithia-15(18)-crown-5(6) and 4'-bromobenzodiaza-15(18)-crown-5(6) by condensation of 3,4-di(2'-haloethoxy)bromobenzene with polyoxaalkanes containing terminal SH or NHMe groups was suggested.The method for the synthesis of formyl derivatives of benzocrowns containing N, S, and O heteroatoms in the macrocycle based on the metallation of appropriate bromo derivatives with BunLi followed by treatment of the resulting organolithium intermediates with DMF was developed.Oximes and semicarbazones of benzaldehydes containing a crown ether fragment were obtained, and their transformation into the original aldehydes by treatment with KNO2 in an acid medium was studied. - Key words: 4'-bromobenzocrowns, synthesis, metallation; 4'-formylbenzocrown ethers; semicarbazones and oximes of 4'-formylbenzocrown ethers.
- Gromov, S. P.,Fedorova, O. A.,Vedernikov, A. I.,Samoshin, V. V.,Zefirov, N. S.,Alfimov, M. V.
-
p. 116 - 123
(2007/10/02)
-
- SYNTHESIS AND ALKALI METAL CATION COMPLEXATION BY BENZOCROWN ETHERS
-
New or improved synthetic routes to several benzocrown ethers and alkali metal picrate extraction data for eight benzocrown ethers with varying sizes are reported.
- Czech, Bronislaw P.,Czech, Anna,Knudsen, Brian E.,Bartsch, Richard A.
-
p. 717 - 722
(2007/10/02)
-
- Synthesis of 4'-Vinylbenzo-3n-crown-n Ethers (4<=n<=10)
-
4'-Vinylbenzo-3n-crown-n ethers (4=n=10) are prepared by a "Cs-templated" cyclization reaction of catechol with dimesylates of oligoethylene glycols, acetylation of the resultant benzo-3n-crown-n ethers (4=n=10), reduction of the acyl group, and dehydration of the resultant alcohols with pyridinium tosylate.
- Talma, Auke G.,Vossen, Huib van,Sudhoelter, Ernst J. R.,Eerden, Johan van,Reinhoudt, David N.
-
p. 680 - 683
(2007/10/02)
-
- The Complexation of the Diquat Dication by Dibenzo-3n-crown-n Ethers
-
Spectrophotometric investigations of equimolar mixtures of diquat bis(hexafluorophosphate) (2) and a range of dibenzo-3n-crown-n ethers in acetonitrile reveal the existence of charge-transfer absorption bands at ca. λmax 400 nm.These absorptions are attributable to intermolecular ?-? charge transfer between the electron-rich catechol units of the dibenzo-crown ethers and the electron-deficient bipyridinium ring system of the diquat dication.The qualitative conclusion from these experiments, that the most stable 1:1 complex is formed between dibenzo-30-crown-10 (14) and diquat bis(hexafluorophosphate) (2), led to the isolation from dichloromethane methanol-n-heptane of red crystals of 2 suitable for X-ray crystallography.Although the crystal structure analysis revealed that there are two independent sets of 1:1 complexes (I and II) in the unit cell, the gross structural features of the two complexes are very similar.In addition to the paralell alignment of their three aromatic rings to accommodate the stabilising intermolecular ?-? charge-transfer interaction, there is probably some further host-guest stabilisation to be gained on account of favourable electrostatic interactions between the phenolic oxygen atoms in the host and the nitrogen atoms in the pyridinium rings of the guest.Moreover, there is some evidence for weak C-H...O hydrogen bonding involving principally H-6 and H-6' on the bipyridinium ring system of the guest and certain -CH2OCH2- oxygen atoms in the host.As evidenced by 1H n.m.r. spectroscopy in CD3COCD3, these non-covalent bonding interactions are probably responsible for the formation of stable and ordered 1:1 complexes with similar gross structural features in solution, at least in the cases where dibenzo-30-crown-10 (14), dibenzo-33-crown-11 (15), and dibenzo-36-crown-12 (16) are the hosts.Further evidence for the 1:1 stoicheiometry of these solution complexes, as well as for the complex involving dibenzo-27-crown-9 (13), has come from equilibrium constant measurements for the association between the dibenzo-3n-crown-n (n = 9-12) hosts (13)-(16) and diquat bis(hexafluorophosphate) (2) in acetone.A quantitative treatment of the charge-transfer absorption bands at 400 nm, which affords Ka values of 410, 17500, 10800, and 2000 M-1 for n = 9, 10, 11, and 12, respectively, provides convincing quantitative evidence for (a) 1:1 stoicheiometry and (b) the relative stabilities of the 1:1 complexes in solution.In the case of dibenzo-24-crown-8 (11), a complex of 2:1 (guest-host) stoicheiometry is believed to be formed in acetone with a Ka value of 385000 M-2, as shown by a successfull quantitative treatment of the charge-transfer absorption data by an independent method.
- Colquhoun, Howard M.,Goodings, Eric P.,Maud, John M.,Stoddart, J. Fraser,Wolstenholme, John B.,Williams, David J.
-
p. 607 - 624
(2007/10/02)
-
- Template Effects. 4. Ion Pairing of Aryloxide Ions with Alkali Cations in 99percent Me2SO: Influence on the Rate of Formation of Benzo-18-crown-6 and of Other Williamson-Type Reactions
-
The effect of alkali metal ions on the rate of formation of benzo-18-crown-6 in 99percent Me2SO by cyclization of the conjugate base of o-hydroxyphenyl 3,6,9,12-tetraoxa-14-bromotetradecyl ether has been quantitatively accounted for according to a scheme involving separate contributions from free (ki) and cation-paired (kip) phenoxide ion.The study has included several additional intra- and intermolecular alkylations of phenoxide ions as reference reactions to provide a set of 25 equilibrium constants for the association of five different phenoxides with the five alkali cations.Both Coulombic interaction and coordination with the neutral oxygen donors are important in determining ion pair stability, but the order in all cases is dominated by Coulombic interaction.This suggests contact interaction in the phenoxide-cation pairs, which is also consistent with evidence from the UV spectra.Whereas the rate of formation of B18C6 is depressed by Li+ (kip/ki + (kip/ki = 100).In contrast, in the reference reactions the ion pairs with the alkali are either negligibly reactive or much less reactive than the free anions.The association constants of the alkali cations with B18C6 have been determined under the same conditions.A comparative analysis of the extent of interaction of the cations with the reactant, transition state, and reaction product of the crown ether forming reaction shows that the transition state binds cations more strongly than the reactant or reaction product and reveals that cation interaction with both the negative charge and the neutral donors bear significant contribution to the stability of the ion pair transition state.A rationale for the template effect is presented in terms of proximity effects and chemical effects arising from interaction of the cation with the nucleophilic site of the reactant.
- Illuminati, Gabriello,Mandolini, Luigi,Masci, Bernardo
-
p. 555 - 563
(2007/10/02)
-
- Association of Alkali and Alkaline Earth Cations with Benzo-18-crown-6 and Its Neutral and Negatively Charged Acyclic Analogues in Methanol Solution
-
The effect of added alkali (Li, Na, K, Rb, and Cs) and alkaline earth (Ca, Sr, and Ba) metal bromides on the acid dissociation of o-hydroxyphenyl 3,6,9,12-tetraoxatridecyl ether (ArOH) in MeOH at 25 deg C has been investigated spectrophotometrically over a wide range of salt concentration.Except for the Li ion, all the investigated salts increased significantly the apparent acidity of ArOH in a way that could be accounted for by the formation of 1:1 addition compounds of the metal cations with both ArOH and its conjugate base ArO-.A proper analysis of the experimental data yielded the equilibrium constants for the formation of the associated species.Complexation of the given cations by benzo-18-crown-6 (C) was also studied spectrophotometrically under the same conditions.The set of equilibrium constants here reported constitute a proper basis for a systematic discussion of the ligation properties of sexadentate ligands with oxygen donors belonging to different structural types.The cyclic ligand C binds cations more strongly than does its open-chain analogue ArOH.The magnitude of the macrocyclic effect is found to be mainly determined by the size of the cation, rather than by its charge.On the other hand, the advantage of the anionic ligand ArO- over ArOH is highly sensitive to the charge of the cation with little or no dependence upon its size.This suggests that direct contact between the oxide ion and the cation is lacking in the ArO-Mz+ species.
- Ercolani, Gianfranco,Mandolini, Luigi,Masci, Bernardo
-
p. 7484 - 7489
(2007/10/02)
-
- Ring-Closure Reactions. 19. Kinetics of Formation of Benzo-Crown Ethers by Intramolecular Nucleophilic Substitution. A Comparison between Poly(oxyethylene) and Polymethylene Chains
-
Kinetic data have been obtained for the formation of 9-, 12-, 15-, 21-, 30-, and 48-membered benzo-crown ethers by intramolecular Williamson synthesis in Me2SO-water (99:1, v/v).Comparison with earlier data for the formation of catechol polymethylene ethers provides an insight into the influence on the ease of ring closure of the replacement of methylene groups by oxygen atoms.Little or no effect occurs at ring size 9.In the next higher homologues the ease of cyclization of the poly(oxyethylene) chains is some 3 times as great as that of the polymethylene chains of similar length, but in the limit of the very long chains the two series behave much in the same way.Interestingly, literature data related to the fast, encounter-controlled reactions between end groups linked by poly(oxyethylene) and polymethylene chains reveal much the same features of the slow, activation-controlled reactions.The general picture fits in with well-established conformational arguments based on the expected strain relief due to replacement of trasannular CH...HC interactions with CH...O or O...O interactions.The oxygen atom effect turns out to be a sensitive tool for the detection of even weak strain energies in ring compounds.
- Illuminati, Gabriello,Mandolini, Luigi,Masci, Bernardo
-
p. 4142 - 4145
(2007/10/02)
-
