Relativity and the chemistry of UF6: A molecular Dirac—Hartree—Fock—CI study

The electronic structure and bonding of UF₆ and UF₆⁻ are studied within a relativistic framework using the MOLFDIR program package. A stronger bonding but more ionic molecule is found if one compares the relativistic with the nonrelativistic results. The first peak in the photoelectron spectrum of Karlsson et al. is assigned to the 12γ_8u component of the 4t_1u orbital, in agreement with other theoretical and experimental results. Good agreement is found between the experimental and theoretical 5f spectrum UF₆⁻. Some properties, like the dissociation energy and electron affinity, are calculated and the necessity of a fully relativistic framework is shown. The Breit interaction has an effect on the core spinors and the spin-orbit splitting of these spinors but the influence on the valence spectrum is negligible. © 1996 John Wiley & Sons, Inc.     

Maleic anhydride homopolymerization during melt functionalization of isotactic polypropylene

The homopolymerization of maleic anhydride was attempted at 190°C, during the melt-functionalization of polypropylene, either with or without organic peroxide using a Brabender plastograph. The free radical homopolymerization of pure maleic anhydride was also attempted either with or without organic peroxide, at 190°C, in vacuum-sealed glass vials. In all cases, free low molecular weight maleic anhydride oligomers were observed by low molecular weight size exclusion chromatography (SEC). This maleic anhydride homopolymerization tends to prove that the ceiling temperature of poly(maleic anhydride) probably lies above the previously published value of 160°C for these specific experimental conditions. © 1996 John Wiley & Sons, Inc.     

4-Cyano-2-oxo-1,2,4-oxadiazolo[2,3-a]quinoxaline 5-N-oxides. New synthetic method and reaction with alcohols. Potential cytotoxic activity

Several quinoxaline 1,4-di-N-oxides have been shown to be efficient and selective cytotoxins for hypoxic cells. We present now a series of 4-cyano-2-oxo-1,2,4-oxadiazolo[2,3-a]quinoxaline 5-N-oxides 2a-2k . They were prepared starting from 3-amino-2-quinoxalinecarbonitrile 1,4-di-N-oxides 1a-1k and 2-chloroethyl isocyanate in dry dioxane at 100–110°. A reaction mechanism is proposed. The treatment of 1a with phenyl isocyanate afforded 2a . Reaction of 2c with silica gel yielded 1c . Compounds 2a-2g were heated in the presence of ethanol and 2-propanol giving the corresponding carbamates 3a-3g and 4a-4g . Compound 2d was already obtained by heating a mixture of 1d and ethyl chloroformiate. Compound 2b was prepared when the carbamate 3b was heated at 150°. Quinoxalines were tested as cytotoxic agents both in oxic and hypoxic cells. The most interesting compounds were 3g and 4g .     

Nonlinear optical polymers. Second harmonic generation in corona-poled thin films

The synthesis and second harmonic coefficients, d_3,1 and d_3,3 as well as the related susceptibilities χ⁽²⁾_zzz of five series of (NLO-dye methacrylate)-(methyl methacrylate) copolymers were investigated. The NLO-chromophores bound covalently to the polymer backbone were 5-(2,2-dicyanovinyl)-or 4-(2-cyano-2-methoxycarbonyl)vinyl-1-piperidino-2-thiophene (P₁ and P₂), 4-nitro-4′-alkoxy-stilbene (P₃), 4-nitro-3′-methoxy-4′-alkoxystilbene (P₄) and 4-nitro-4′-alkoxy-α-cyano stilbene (P₅). The second order nonlinear optical properties of corona-poled aligned thin polymer films, using a needle electrode in order to induce noncentrosymmetry, were evaluated. Nonlinear susceptibilities, χ⁽²⁾_zzz, of the films were derived from the analysis of full-angle Maker fringe patterns at 1064 nm, χ⁽²⁾_zzz values as high as 1.98×10⁻⁷ esu for P₂ copolymers and of 1.19×10⁻⁷ esu for P₃ copolymers could be achieved.     

Mechanisms of fire retardance in glass fibre polymer composites

It is shown that the introduction of glass fibres in the poly(butylene terephthalate) (PBT) matrix, increases its flammability. The mechanism of action of a typical fire retardant for PBT which is a mixture of a brominated organic compound and antimony trioxide, seems to involve both the condensed and the gas phase.     

Polyacrylamide–graft–poly(ethylene oxide)

Water-soluble graft copolymers were synthesized by copolymerization of acrylamide with mono-methoxy-poly(ethylene oxide)-methacrylates (Me-PEO-MA). The Me-PEO-MA macromers were synthesized by a catalytic esterification of methacrylic acid with mono-methoxy-poly(ethylene glycol)s with different molecular weights. The graft copolymers obtained were characterized by ¹H-NMR spectroscopy, gel permeation chromatography (GPC) and Ubbelohde viscosimetry. The rheological behaviour of aqueous polymer solutions, which are expected to show hydrophobic association at elevated temperatures, was studied with a cone and plate-rheometer.     

Investigations of a highly crowded phosphino-substituted biphenyl: A precursor for a λ3, λ5-diphosphaphenanthrene

The synthesis of 2,2′-bis(bis(dimethylamino)-phosphino)-3,3′5,5′-tetra-tert-butylbiphennyl ( 5 ) is described. It was extensively studied by ¹H, ¹³C, and ³¹P NMR spectroscopy. Furthermore, the X-ray analysis of 5 is reported. Crystals of 5 are tetragonal, space group P¯42₁c, a = b = 24.770 (3) Å, c = 12.658 (4) Å, Z = 8. The surprising reaction of 5 with proton acids leading to the formation of various phosphorus containing five- and six-membered ring compounds is discussed. On reaction of one of the six-membered ring compounds ( 9 ) with magnesium in THF, a λ³, λ⁵-diphosphaphenanthrene ( 19 ) was obtained.     

Unraveling the reactivity of a cationic iminoborane: avenues to unusual boron cations

A cationic terminal iminoborane [Mes*N B ← IPr₂Me₂][AlBr₄] (3⁺[AlBr₄]⁻) (Mes* = 2,4,6-tri-tert-butylphenyl and IPr₂Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) has been synthesized and characterized. The employment of an aryl group and N-heterocyclic carbene (NHC) ligand enables 3⁺[AlBr₄]⁻ to exhibit both B-centered Lewis acidity and BN multiple bond reactivities, thus allowing for the construction of tri-coordinate boron cations 5⁺–12⁺. More importantly, initial reactions involving coordination, addition, and [2 + 3] cycloadditions have been observed for the cationic iminoborane, demonstrating the potential to build numerous organoboron species via several synthetic routes.

An NHC-stabilized aryliminoboryl cation exhibits both boron-centered Lewis acidity and multiple bond reactivity and could be utilized as an effective synthon for unusual cationic boron species.     

SuFEx-enabled,chemoselective synthesis of triflates,triflamides and triflimidates

Sulfur(vi) Fluoride Exchange (SuFEx) chemistry has emerged as a next-generation click reaction, designed to assemble functional molecules quickly and modularly. Here, we report the ex situ generation of trifluoromethanesulfonyl fluoride (CF₃SO₂F) gas in a two chamber system, and its use as a new SuFEx handle to efficiently synthesize triflates and triflamides. This broadly tolerated protocol lends itself to peptide modification or to telescoping into coupling reactions. Moreover, redesigning the S^VI–F connector with a S O → S NR replacement furnished the analogous triflimidoyl fluorides as SuFEx electrophiles, which were engaged in the synthesis of rarely reported triflimidate esters. Notably, experiments showed H₂O to be the key towards achieving chemoselective trifluoromethanesulfonation of phenols vs. amine groups, a phenomenon best explained—using ab initio metadynamics simulations—by a hydrogen bonded termolecular transition state for the CF₃SO₂F triflylation of amines.

Triflyl fluoride gas (CF₃SO₂F) and its aza analogues are reported as new SuFEx activators. These S^VI–F reagents react efficiently with a variety of nucleophiles, yet the presence of water grants complete chemoselectivity to phenols.