33089-36-0Relevant articles and documents
Effect of Crown Ethers on the Selectivity of Electrophilic Aromatic Nitration
Masci, Bernardo
, p. 4081 - 4087 (1985)
Relative rates, isomer distributions, and partial rate factors have been determined for the nitration reactions of benzene, toluene, m-xylene, anisole, and mesitylene with tetrabutylammonium nitrate and trifluoroacetic anhydride in homogeneous CH2Cl2 solutions.The observed selectivity values have been compared with those obtained when 12-crown-4, 15-crown-5, 18-crown-6, 21-crown-7, or 24-crown-8 was added to the nitrating mixture.Small effects, if any, were observed with the smaller ligands, but marked variations in both substrate and positional selectivity appeared in the presence of 18-crown-6, 21-crown-7, and 24-crown-8.Such effects were found to depend on crown ether concentration and to vanish when KClO4 was added.A reaction scheme is proposed where both uncomplexed and complexed nitronium ion contribute to variable extents to the overall reaction.The selectivity of the crown ether associated electrophile relative to the unassociated one seems to result from the balance of two different effects: an increase in the sensitivity to the electronic effects of the substituents and a larger crowding in the transition state.The largest effects on selectivity were observed with 21-crown-7 which causes ortho-para-directing groups to act as essentially para-directing groups.Data for toluene nitration in the presence of 21-crown-7 fit very well the Brown selectivity relationship; the selectivity factor spanning a remarkably wide range on changing the crown ether concentration.
Crown Cation Complex Effects. 21. Spectral Evidence Bearing on the Interaction between Arenediazonium Cations and 21-Crown-7 in Nonpolar Solutions
Beadle, James R.,Khanna, Raj K.,Gokel, George W.
, p. 1242 - 1246 (1983)
15-Crown-5 does not complex arenediazonium tetrafluoroborates, but 21-crown-7 is known to complex them more strongly than does 18-crown-6.Despite this, the infrared and ultraviolet band shifts for complex vs. noncomplexed forms are far smaller for the former than for the latter.It is suggested that instaed of the crown completely and tightly surrounding the diazonio function, the crown collars (nearly encircling) the diazonio group and then uses the remaining donor atom(s) either to solvate the terminal nitrogen atom or interact as a base with the ?-acidic aromatic ring, providing additional stability.Since the mode by which 18-crown-6 and 21-crown-7 solvate the diazonium ion in each case differs, the spectral manifestations of this interaction differ.
The macrobicyclic cryptate effect in the gas phase
Chen, Qizhu,Cannell, Kevin,Nicoll, Jeremy,Dearden, David V.
, p. 6335 - 6344 (2007/10/03)
The alkali cation (Li+, Na+, K+, Rb+, and Cs+) binding properties of cryptands [2.1.1], [2.2.1], and [2.2.2] were investigated under solvent-free, gas-phase conditions using Fourier transform ion cyclotron resonance mass spectrometry. The alkali cations serve as size probes for the cryptand cavities. All three cryptands readily form 1:1 alkali cation complexes. Ligand-metal (2:1) complexes of [2.1.1] with K+, Rb+, and Cs+, and of [2.2.1] with Rb+ and Cs+ were observed, but no 2:1 complexes of [2.2.2] were seen, consistent with formation of 'inclusive' rather than 'exclusive' complexes when the binding cavity of the ligand is large enough to accommodate the metal cation. Kinetics for 2:1 ligand-metal complexation, as well as molecular mechanics calculations and cation transfer equilibrium constant measurements, lead to estimates of the radii of the cation binding cavities of the cryptands under gas-phase conditions: [2.1.1], 1.25 ?; [2.2.1], 1.50 ?; [2.2.2], 1.65 ?. Cation transfer equilibrium studies comparing cryptands with crown ethers having the same number of donor atoms reveal that the cryptands have higher affinities than crowns for cations small enough to enter the cavity of the cryptand, while the crowns have the higher affinity for cations too large to enter the cryptand cavity. The results are interpreted in terms of the macrobicyclic cryptate effect: for cations small enough to fit inside the cryptand, the three-dimensional preorganization of the ligand leads to stronger binding than is possible for a floppier, pseudo-two-dimensional crown ether. The loss of binding by one ether oxygen which occurs as metal size increases for a given cryptand is worth approximately 25 kJ mol-1, and accounts for the higher cation affinities of the crowns for the larger metals. The Li+ affinity of 1,10-diaza-18-crown-6 is ~1 kJ mol-1 higher than that of 18-crown-6, while the latter has lower affinity than the former for all of the larger alkali cations (about 7 kJ mol-1 lower for Na+, and about 15 kJ mol-1 lower for K+, Rb+, and Cs+). The equilibrium measurements also allow the determination of relative free energies of cation binding for a number of crown ethers and cryptands. Molecular mechanics modeling with the AMBER force field is generally consistent with the experiments.
Macrocyclic chemistry in the gas phase: intrinsic cation affinities and complexation rates for alkali metal cation complexes of crown ethers and glymes
Chu, In-Hou,Zhang, Hong,Dearden, David V.
, p. 5736 - 5744 (2007/10/02)
Reactions of 12-crown-4, 15-crown-5, 18-crown-6, and 21-crown-7, as well as the acyclic analogs triglyme, tetraglyme, and penta(ethylene glycol), with Li+, Na+, K+, Rb+, and Cs+, are observed and characterized using Fourier transform ion cyclotron resonance mass spectrometry (FTICR/MS) and tandem quadrupole mass spectrometry in the gas phase to obtain information on intrinsic host-guest interactions in the absence of the complicating effects of solvation. Radiatively stabilized attachment of the cations to the ligands is a rapid process, with rates in some cases a factor of 2 or more times the Langevin collision rate. The attachment efficiencies increase linearly with cation charge density, suggesting that attachment involves charge-induced rearrangement of the ligands to adopt favorable binding conformations. Attachment is more efficient, and more strongly dependent on charge density, for the cyclic ligands than for their acyclic counterparts. Metal-ligand undergo reaction with a second ligand to form 1:2 metal-ligand complexes, or "sandwiches". The efficiencies of crown sandwich formation are strongly dependent on the ratio of cation radius to binding cavity radius; when the ratio is than one, the efficiencies are too low to measure, but they become measurable at a ratio of 1:1 and increase by about 4 orders of magnitude as the ratio incrrases to about 1.25:1, At higher ratio values, efficiencies fall off slowly, probably due to decreasing cation density. The relative cation affinities of the various ligands are compared both collision-induced dissociation "kinetic" methods, with the tandem quadrupole, and using "bracketing" cation reactions in the FTICR. The tandem quadrupole results are in some cases dependent on the means of producing the 1;2 metal-ligand complexes, and in some cases they do not agree with the FTICR results. The two methods are compared and reasons for the discrepancies are discussed. We favor the FTICR results, which indicate that proton and alkali cation affinities increase with an increase in the number of oxygen donor atoms in the crowns. Equilibria observed in metal exchange reactions between 18-crown-6 and 21-crown-7 were found to always lie on the side of the cation bound to the larger ligand, but K+ has the smallest equilibrium constant of any of the alkali metals, reflecting the excellent size match between K+ and 18-crown-6.
