Synthesis of Unsubstituted and 4,4′-Substituted Oligobipyridines as Ligand Strands for Helicate Self-Assembly

A general strategy for the synthesis of oligobipyridine ligands 2 – 5 containing from two to five 2,2′-bipyridine subunits, for helical metal complexes is described (sec Scheme). Both the unsubstituted parent strands ( a series) as well as their derivatives bearing fester or amide functions in the 4,4′-positions of the bipyridine moieties ( b – d series) have been obtained.     

Sur les plus grands facteurs premiers d'un entier

Let and, for each integern such that (n)k, denote byP _k (n) itsk ^th largest prime factor. Further, given a set of primesQ of positive density <1 satisfying a certain regularity condition, defineP(n, Q), as the largest prime divisor ofn belonging toQ, assuming thatP(n,Q)=+ if no such prime factor exists. We provide estimates of , fork2, and of . We also study the median value of the functionP(n,Q) and that of the functionP _k (n) for eachk1.     

Silica-Supported Tantalum Catalysts for Asymmetric Epoxidations of Allyl Alcohols

Tantalum good, titanium bad: This appears to be the case for silica-supported catalysts for the asymmetric epoxidation of allyl alcohols. Complexes such as [SiO-Ta(OEt)(4)] were prepared from silica and [Ta(=CHCMe(3))(CH(2)CMe(3))(3)], and in the presence of a tartrate and an alkyl hydroperoxide, these surface tantalum compounds lead to efficient and convenient catalysts for the asymmetric epoxidation of 2-propen-1-ol (R=H) and trans-2-hexen-1-ol (R=nPr; see reaction).     

Cyclodextrin chemistry. Selective modification of all primary hydroxyl groups of α- and β-cyclodextrins

Two efficient methods are described for the selective modification of all six primary hydroxyl groups of α-cyclodextrin (α-CD, 1 1 ). One, using an indirect strategy, involves protection of all 18 hydroxyl functions as benzoate esters, followed by selective deprotection of the six primary alcohol groups. The other, using a direct strategy, involves selective activation of the primary hydroxyl groups via a bulky triphenylphosphonium salt, which is then substituted by azide anion as the reaction proceeds. A number of modified α-cyclodextrin derivatives have been prepared and fully characterized, among which are: the useful intermediate α-cyclodextrin-dodeca (2, 3) benzoate ( 3 ); hexakis (6-amino-6-deoxy)-α-cyclodextrin hexahydrochloride ( 7 ); hexakis (6-amino-6-deoxy)-dodeca (2, 3)-O-methyl-α-cyclodextrin hexahydrochloride ( 9 ), hexa (6)-O-methyl-α-cyclodextrin ( 13 ). The direct substitution is shown to be even more efficient for β-cyclodextrin ( 16 ), giving the heptakis (6-azido-6-deoxy)-β-CD-tetradeca (2, 3)acetate ( 17 ), while the indirect strategy fails. The compounds are characterized by extensive use of ¹³C- and ¹H-NMR. spectroscopy. The steric and statistical problems of selective polysubstitution reactions for the cyclodextrins are discussed, and possible reasons for the observed differences in reactivity between α- and β-cyclodextrins are examined. The dodecabenzoate 3 presents a very marked solvent effect on physical properties (IR. and NMR. spectra, optical rotation); the effects observed may be ascribed to an unusually strong intramolecular network of hydrogen bonds which severely distorts the α-cyclodextrin ring and lowers the symmetry from six-fold to three-fold.     

Cryptates Sphériques. Synthèse et Complexes d'Inclusion de Ligands Macrotricycliques Sphériques

Spherical Cryptates. Synthesis and Inclusion-Complexes of Spherical Macrotricyclic Ligands A general strategy for the synthesis of spherical macrotricyclic ligands has been developed. Four spherical cryptands, SC - 24 , SC - 25 , SC - 26 and SC - 27 have been obtained by this route. The synthesis and cation-complexing properties of these compounds are described in detail. Stability constants and cation exchange rates of the spherical cryptates obtained with alkali and alkaline-earth cations have been determined. Highly stable complexes are formed by SC - 24 ; the Rb⁺ and Cs⁺ cryptates of SC - 24 are the most stable complexes of these cations known to date. The size of the intramolecular cavity affects the complexation selectivity. The cation exchange rates are very slow, and the corresponding free energies of activation are even larger than, for macrobicyclic cryptates of similar stability. Both the high complex stabilities and the high activation energies required for cation exchange indicate a marked ‘spherical cryptate effect’ resulting from the highly connected nature of the molecular architecture of spherical macrotricyclic ligands.     

CRYPTATES XIX. Ligands polyaza-polyoxa-macrobicycliques: Synthèse et complexes métalliques

Polyaza-polyoxa macrobicyclic ligands: its synthesis and metal complexes. The synthesis of the polyaza-polyoxa macrobicyclic ligands 1–4 is described. They form complexes with a variety of metal cations, transition metal cations as well as alkali and alkaline-earth cations. These complexes may be formulated as cation inclusion complexes, cryptates, in which the cation is contained in the intramolecular cavity. The properties of the complexes are described. An especially interesting feature is that these ligands, polymines of macrobicyclic topology, provide a means of trapping transition metal cations inside a molecular cavity; thus they impose coordination geometries and may modify the spectral and redox properties of the cations.     

Accroissement de la reactivite des phosphorines en tant que dienes et philodienes par complexation du phosphore

The 2-Phenyl-4,5-dimethylphosphorin P-W(CO)₅ complex reacts easily as a dienophile with 2,3-dimethylbutadiene through its 1,6-positions and as a diene with N-phenylmaleimide, dimethyl acetylenedicarboxylate and cyclopentadiene, through its 1,4-positions.     

Anion-receptor molecules: Macrocyclic and macrobicyclic effects on anion binding by polyammonium receptor molecules

The stability constants for anion binding by the acyclic hexaamine 1 , its macrocyclic analogue 2 , and the bicyclic compound 3 in their protonated forms are reported. Compound 3 forms stable and selective complexes with halide ions, the stability sequence being I^? > Br^? > Cl^?. Compound 2 forms more stable complexes with sulfate, oxalate, and malonate dianions than its acyclic analogue 1 and shows a better selectivity pattern. Compound 3 forms stronger complexes with oxalate^2? than 2 and shows a remarkably high binding selectivity between oxalate^2? and malonate^2?. The comparison of the ability of 1–3 to complex anions demonstrates the macrocyclic and macrobicyclic effects on anion binding stability and selectivity.     

Ab initio calculations of all-trans octatetraene and eight isoelectronic conjugated chains: Semiempirical heats of formation and stabilization energies

In the framework of our study of the changes of the electric polarizability upon substituting heteroatomic linkages in conjugated hydrocarbon backbones, we have been led to examine the relative stability of eight compounds isoelectronic to the all-trans octatetraene using the Cox and Pilcher concept of stabilization energy (SE ), SE = ΔH_a – ΣN_ABE_AB, where ΔH_a is the heat of atomization of the species under consideration and the E_AB's are bond-energy terms. Full geometry optimizations at the 4-31G level have been performed to obtain the equilibrium geometries needed to deduce reliable semiempirical heats of formation from appropriate isodesmic processes. Some of the compounds containing the ? CH?N? and ? N?N ? groups have electric polarizabilities and stabilization energies comparable to octatetrene.     

Channel-Type Molecular Structures. Part 2. Synthesis of Bouquet-Shaped Molecules Based on a β-Cyclodextrin Core

A new series of channel-type molecules presenting the features of potential transmembrane structures is described. They result from the grafting of amphiphilic side chains on a β-cyclodextrin derivative 3 that constitutes the organizing core. They belong to the ‘bouquet’ family (B). Compounds bearing poly(oxyethylene) side chains, B 16 and 17 , and their polymethylene analogues B 18 and 19 , were synthesized. The properties investigated emphasize the suitability of such molecules to be incorporated into lipid bilayer membranes.