Crown ethers: sensors for ions and molecular scaffolds for materials and biological models.

Publisher Summary This chapter discusses the hydrogen bonding, which has been recognized as the single most important intermolecular interaction, The chapter presents the hypothesis that there are three kinds of hydrogen bond: weak, strong, and very strong. These are determined by the shape of the potential energy well and the respective positions of hydrogen and deuterium within the well, which can be used to provide information about the well that applies in a particular example. The role of hydrogen bonding in catalysis has been discussed, although mainly in terms of the salicylate ion as a leaving group. With its strong intramolecular hydrogen bond playing an essential part in the reaction mechanism, it seems likely that this will herald other systems where the role of a strong hydrogen bond may serve as the key step in a catalytic process.

Hydrogen Bonding and Chemical Reactivity

Superacids, defined as acids with a Hammett acidity function H0 ≤ −12, are useful materials, but a need exists for new, designable solid state systems. Here, we report superacidity in a sulfated metal–organic framework (MOF) obtained by treating the microcrystalline form of MOF-808 [MOF-808-P: Zr6O5(OH)3(BTC)2(HCOO)5(H2O)2, BTC = 1,3,5-benzenetricarboxylate] with aqueous sulfuric acid to generate its sulfated analogue, MOF-808-2.5SO4 [Zr6O5(OH)3(BTC)2(SO4)2.5(H2O)2.5]. This material has a Hammett acidity function H0 ≤ −14.5 and is thus identified as a superacid, providing the first evidence for superacidity in MOFs. The superacidity is attributed to the presence of zirconium-bound sulfate groups structurally characterized using single-crystal X-ray diffraction analysis.

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Superacidity in sulfated metal-organic framework-808.

1,1-Dihalo-1-alkenes are valuable synthetic tools in organic
chemistry and serve as interesting synthetic intermediates in a variety of non-metal-assisted chemical transformations. Moreover, the vinyl dihalide functionality is an attractive and versatile bidentate electrophile for organometallic chemistry.

Synthesis and Metal-Catalyzed Reactions of gem-Dihalovinyl Systems

We evaluated the o-nitrobenzyl platform for designing photolabile protecting groups with red-shifted absorption that could be photolyzed upon one- and two-photon excitation. Several synthetic pathways to build different conjugated o-nitrobenzyl backbones, as well as to vary the benzylic position, are reported. Relative to the reference 4,5-dimethoxy-2-nitrobenzyl group, several o-nitrobenzyl derivatives exhibit a large and red-shifted one-photon absorption within the near-UV range. Uncaging after one-photon excitation was studied by measuring UV-visible absorption and steady-state fluorescence emission on model caged ethers and esters. In the whole series investigated, the caged substrates were released cleanly upon photolysis. Quantum yields of uncaging after one-photon absorption lie within the 0.1-1 % range. We observed that these drop as the maximum wavelength absorption of the o-nitrobenzyl protecting group is increased. A new method based on fluorescence correlation spectroscopy (FCS) after two-photon excitation was used to measure the action uncaging cross section for two-photon excitation. The series of o-nitrobenzyl caged fluorescent coumarins investigated exhibit values within the 0.1-0.01 Goeppert-Mayer (GM) range. Such results are in line with the low quantum yields of uncaging associated with cross-sections of 1-50 GM for two-photon absorption. Although the cross-sections for one- and two-photon absorption of o-nitrobenzyl photolabile protecting groups can be readily improved, we emphasize the difficulty in enlarging the corresponding action uncaging cross-sections in view of the observed trend of their quantum yield of uncaging.

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o‐Nitrobenzyl Photolabile Protecting Groups with Red‐Shifted Absorption: Syntheses and Uncaging Cross‐Sections for One‐ and Two‐Photon Excitation

The reaction of o-bromo- and p-chloro-nitrobenzenes and of 1,4-dinitrobenzene with sodium borohydride in a dilute solution in dimethyl sulphoxide solution leads to some nitrobenzene. When tritiated borohydride is employed, aromatic hydrogen exchange occurs to a comparable extent.Under similar conditions 1-X-2,4-dinitrobenzenes mostly undergo either displacement of the 2-nitro-group by hydrogen or ring reduction. With these compounds it is possible to observe the n.m.r. spectra of the unstable cyclohexadienyl anions (hydride Meisenheimer adducts), formed by hydride addition to the 3- and/or the 5-position, which decompose into the final products.For 2,4-dinitroanisole it is shown by an isotope-labelling experiment that the displacement of the 2-nitro-group is accompanied by intramolecular hydrogen migration from the methylene group of the Meisenheimer adduct (at the 3-position) to the 2-position. This mechanism is the nucleophilic analogue of the mechanism of ipso-attack with migration, recognised as one of the reaction paths in electrophilic aromatic substitution. The term ‘vicinal attack’ is proposed for this general class of aromatic substitution reactions in which the entering group first attacks and attaches itself to a ring position adjacent to the position that it occupies in the reaction product and from which the leaving group is displaced.

Sodium borohydride as a reagent for nucleophilic aromatic substitution by hydrogen: the role of hydride Meisenheimer adducts as reaction intermediates

The rates of acid-catalysed hydrolyses of some new macrocyclic ether acetals, analogous in structure to crown ethers, are found to be markedly depressed by some metal ions; the behaviour is consistent with selective cation-binding and a resulting lowering of the basicity of the acetal oxygen atoms.

Crown ether acetals: detection of cation binding by kinetic measurements

Rate measurements are reported for the reduction of acetone pivalaldehyde, and benzaldehyde by sodium and tetramethylammonium tetrahydridoborates in dimethyl–sulphoxide–water systems, some containing a low concentration of sodium hydroxide to impede the hydrolysis of tetrahydridoborate. The reactions obey second-order kinetics. The rate constants decrease as the water content of the solvent is reduced but, with the most reactive substrate (benzaldehyde), the reduction is still detectable even in a solvent not containing any added water. Evidence has been obtained that the reduction product of benzaldehyde in dimethyl sulphoxide solution is sodium tetrakisbenzyloxyborate, which is readily hydrolysed to benzyl alcohol. A condensation product of benzaldehyde and dimethyl sulphoxide (2-methylsulphinyl-1-phenylethanol) has been isolated as a by-product. Attempts to trap borane during the reduction gave negative results.

Reduction of carbonyl compounds by sodium borohydride (tetrahydridoborate) in water, dimethyl sulphoxide, and their mixtures as solvents: products and kinetics

Rate measurements are reported for the formation of a Meisenheimer adduct in solutions of 1,3,5-trinitrobenzene and trichloroacetic acid or sodium trichloroacetate at very low concentrations (<10–4M). Study of the rate constants and final absorbances of the solutions shows that the rate-limiting step is in all cases the unusually rapid unimolecular breakdown of the trichloroacetate anion (Ea83 kJ mol–1; ΔS‡ 11 J mol–1 K–1). The rate constants of the reactions of the trichloromethyl anion thus formed with trinitrobenzene or hydrogen ions are nearly equal. From the effect of added hydrochloric acid on the reaction rate a value of the hiterto unreported dissociation constant of trichloroacetic in DMSO (2.9 × 10–4 mol l–1) has been deduced. Approximate deuterium isotope effects are derived for the dissociation constant and for some of the rate constants. Preliminary measurements show that the analogous breakdown of tribromoacetic acid in DMSO is a considerably faster reaction.

The decarboxylation of trichloroacetic acid and the reactions of the trichloromethyl anion with 1,3,5-trinitrobenzene and with hydrogen ions: kinetic measurements in dimethyl sulphoxide solution

Solutions of trichloro- and tribromo-acetic acid in dimethyl sulphoxide in the presence of carbonyl compounds react at room temperature to give insoluble products corresponding to addition of H–CX3 across the carbonyl double bond.

The decomposition of trihalogenoacetic acids in dimethyl sulphoxide: a mild route from carbonyl compounds to trihalogenomethylethanol and trihalogenomethyl ketones

Victor Gold

Papers

Sodium borohydride as a reagent for nucleophilic aromatic substitution by hydrogen: the role of hydride Meisenheimer adducts as reaction intermediates

Crown ether acetals: detection of cation binding by kinetic measurements

Reduction of carbonyl compounds by sodium borohydride (tetrahydridoborate) in water, dimethyl sulphoxide, and their mixtures as solvents: products and kinetics

The decarboxylation of trichloroacetic acid and the reactions of the trichloromethyl anion with 1,3,5-trinitrobenzene and with hydrogen ions: kinetic measurements in dimethyl sulphoxide solution

The decomposition of trihalogenoacetic acids in dimethyl sulphoxide: a mild route from carbonyl compounds to trihalogenomethylethanol and trihalogenomethyl ketones