Electron-transfer oxidation of dioxenes. An example of radical cation disproportionation

Tris(p-bromophenyl ammoniumyl) tetrafluoroborate induces an easy electron-transfer process on dioxenes leading quantitatively to the corresponding α-diketones. A mechanism involving the disproportionation of the intermediate radical cations is discussed.     

Recognition of alkali metal halide contact ion pairs by uranyl-salophen receptors bearing aromatic sidearms. The role of cation-pi interactions

Hard anions have long been known to bind strongly to the uranium of uranyl-salophen complexes. Upon functionalization of the salophen framework with one or two benzyloxy substituents, efficient ditopic receptors for alkali metal ions are obtained. The solid-state structures of complexes formed by the two-armed receptor 1 with CsF and with the chlorides of K+, Rb+, and Cs+ reported here reveal the existence of dimeric supramolecular assemblies in which two receptor units assemble into capsules fully enclosing (MX)2 ion quartets. In addition to the strong coordinative binding of the anion to the uranyl center and to electrostatic cation-anion interactions, stabilizing interactions arise from coordination of each cation to six oxygens, three from each receptor, and most importantly, to two aromatic sidearms belonging to different receptors. There are marked differences in organization at the supramolecular level in the CsCl complex of the one-armed receptor 3, in that four uranyl-salophen units instead of two are assembled in a capsule-like arrangement housing a (CsCl)2 ion quartet. However, both receptors achieve the common goal of having each metal cation in close contact with carbon atoms of two aromatic rings. 1H NMR data provide strong evidence that cation-pi(arene) interactions with the sidearms participate in binding also in solution.     

Ethylene-hindered amine copolymer: A self-stabilized polyolefine

Monomers having a norbornene as polymerizable group and a 2,2,6,6-tetramethyl piperidine as stabilizing moiety were prepared. This monomer was successfully copolymerized with ethylene using VAc₃/AlEt₂Cl as the catalytic system. Copolymers exhibited better stability under accelerated aging than polyethylene stabilized by classical blending with commercial hindered amines. Any attempt made to obtain copolymers using a high activity TiCl₄/MgCl₂ catalyst was unsuccessful. © 1994 John Wiley & Sons, Inc.     

Dramatically enhanced carbon acidity of the nitrobenzyl fragment in a nickel(II) scorpionate complex

The -CH(2)- group of the 2-nitrobenzyl pendant arm of the scorpionate complex I deprotonates in basic aqueous solution (pK(a) = 10.6), due to the coordination of the nitronate group to the nickel(II) center. Metal coordination enhances 2-nitrobenzene acidity by 10 orders of magnitude. [reaction: see text]     

Transition metal complexes with bidentate ligands spanning trans-positions. VI. The preparation of some diaryl- and dialkyl derivatives of 2,11-bis (phosphinomethyl)benzo [c]phenanthrene

The preparation of the ditertiary phosphines 2,11-bis (di-m-tolylphosphinomethyl)benzo [c]phenanthrene ( 1b ), 2,11-bis (di-p-anisylphosphinomethyl)benzo-[c]phenanthrene ( 1c ), 2,11-bis (di-m-trifluoromethylphenylphosphinomethyl) benzo-[c]phenanthrene ( 1d ), 2,11-bis (dicyclohexylphosphinomethyl)benzo [c]phenanthrene ( 1e ) and 2,11-bis [di-(t-butyl)phosphinomethyl]benzo [c]phenanthrene ( 1f ), by a combination of synthetic routes is described.     

RuO4-catalyzed oxidative polycyclization of squalene. Determination of the configuration of the penta-tetrahydrofuranyl diol product

The configuration of the penta-tetrahydrofuranyl diol (penta-THF) product obtained by a single-step, RuO₄-catalyzed oxidative polycyclization of squalene, has been determined as cis-threo-cis-threo-trans-threo-trans-threo-trans. The cis-cis-trans-trans-trans sequence for the five contiguous THF rings has been established through extensive 2D-NMR spectroscopic studies carried out both on the intact molecule and on some of its derivatives, including the oxidative cleavage products obtained by degradation of the penta-THF with PCC/AcOH. Four different chemical approaches were devised to determine each of the four threo relationships within each carbon pair connecting adjacent THF rings in the molecule. To this aim, studies have been carried out either on some intermediates of the process leading to penta-THF, obtained by stopping the oxidation of squalene prior to completion, or on a degradation product of the penta-THF, obtained from the latter through a bidirectional double oxidative degradation with PCC.     

Tuning porphyrin/DNA supramolecular assemblies by competitive binding

Addition of an intercalating but nonaggregating porphyrin, AuT4, to a preformed adduct between DNA and a strongly aggregating porphyrin, t-H2Pagg, permits control of the extent of aggregation of the latter. The size and the intensity of the ICD and RLS signals depend linearly on the concentration of the intercalating dye and decrease markedly on increasing the concentration of the available binding sites on the biopolymer template.     

Why, on interaction of urea-based receptors with fluoride, beautiful colors develop

[reaction: see text] Urea-based receptors, containing electron-withdrawing chromogenic substituents, in a DMSO solution, in the presence of varying excess of fluoride, do not form H-bond complexes, but undergo stepwise deprotonation of the two N-H fragments, an event which is signaled by the development of vivid colors. Double deprotonation is also observed in the presence of hydroxide. Less basic anions (CH3COO-, H2PO4-) induce deprotonation of only one N-H.     

Synthesis of 4-aryl-1H-1,2,3-triazoles through TBAF-catalyzed [3 + 2] cycloaddition of 2-aryl-1-nitroethenes with TMSN3 under solvent-free conditions

TBAF-catalyzed [3 + 2] cycloaddition reactions of 2-aryl-1-cyano- or 2-aryl-1-carbethoxy-1-nitroethenes 1 with TMSN3 under SFC allow the corresponding 4-aryl-5-cyano- or 4-aryl-5-carbethoxy-1H-1,2,3-triazoles 2 to be prepared under mild reaction conditions and with good to excellent yields (70-90%). The proposed protocol does not require dried glassware or inert atmosphere.     

Stereoselective synthesis and optical resolution of chiral calix[4]arenes with mixed ligating functionalities

Exhaustive alkylation of syn-proximal bis[(2-pyridylmethyl)oxy]calix[4]arene 1 with t-butyl bromoacetate or 2-(chloromethyl)quinoline hydrochloride in the presence of Cs₂CO₃ affords selectively a new type of inherently chiral calix[4]arenes 2a,b in the partial cone conformation. (±) 2b has been resolved into its optical antipodes by an HPLC method.