Synthesis and characterization of ordered polyurethanes from nonsymmetric diisocyanate and ethylene glycol

An ordered polyurethane with a head‐to‐head (H‐H) or tail‐to‐tail (T‐T) content over 95% was prepared by polyaddition reaction of a nonsymmetric monomer, p‐isocyanatobenzyl isocyanate (1) with a symmetric monomer, ethylene glycol (2). The model reactions were studied in detail to demonstrate the feasibility of polymer formation. The polymerization was conducted in THF in the presence of triethylamine (TEA) at 0 °C by slow addition of a half amount of 2 to 1, followed by removing THF and then adding the rest of 2 in DMF at once at 30 °C in the presence of dibutyltin dilaurate (DBTL). The microstructure of the polymer obtained was investigated by ¹³C NMR spectroscopy, and it was found that the polymer had the expected structural regularity. The constitutional regularity of polymers influenced their thermal properties. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2106–2114, 2000     

D3h‐Symmetric Porphyrin‐Based Rigid Macrocyclic Ligands for Multicofacial Multinuclear Complexes in a One‐Nanometer‐Sized Cavity

The one‐step synthesis of D_3h‐symmetric cyclic porphyrin trimers 1 composed of three 2,2′‐[4,4′‐bis(methoxycarbonyl)]bipyridyl moieties and three porphyrinatozinc moieties was achieved from a nickel‐mediated reductive coupling of meso‐5,15‐bis(6‐chloro‐4‐methoxycarbonylpyrid‐2‐yl)porphyrinatozinc. Although cyclic trimers 1 were obtained as a mixture that included other cyclic and acyclic porphyrin oligomers, an extremely specific separation was observed only for cyclic trimers 1 when using columns of silica gel modified with pyrenylethyl, cyanopropyl, and other groups. Structural analysis of cyclic trimers 1 was carried out by means of NMR spectroscopy and X‐ray crystallography. Treatment of an η³‐allylpalladium complex with a cyclic trimer gave a tris(palladium) complex containing three η³‐allylpalladium groups inside the space, which indicated that the bipyridyl moieties inside the ring could work as bidentate metalloligands.     

Highly Efficient Dehydrogenative Coupling of Hydrosilanes with Amines or Amides Using Supported Gold Nanoparticles

Hydroxyapatite‐supported gold nanoparticles (Au/HAP) can act as a highly active and reusable catalyst for the coupling of hydrosilanes with amines under mild conditions. Various silylamines can be selectively obtained from diverse combinations of equimolar amounts of hydrosilanes with amines including less reactive bulky hydrosilanes. This study also highlights the applicability of Au/HAP to the selective synthesis of silylamides through the coupling of hydrosilanes with amides, demonstrating the first example of an efficient heterogeneous catalyst. Moreover, Au/HAP shows high reusability and applicability for gram‐scale synthesis.     

Rhodium‐Catalyzed Allylation of Aldehydes with Homoallylic Alcohols by Retroallylation and Isomerization to Saturated Ketones with Conventional or Microwave Heating

The treatment of an aldehyde with a tertiary homoallylic alcohol at 100–250 °C in the presence of cesium carbonate and a rhodium catalyst leads to allyl transfer from the homoallylic alcohol to the aldehyde. The process includes Rh‐mediated retroallylation to form an allyl rhodium species as the key intermediate. The homoallylic alcohol formed initially through allyl transfer is converted under the reaction conditions into the corresponding saturated ketone when bulky ligands are used. Microwave heating at 250 °C accelerates the reaction significantly.     

Nickel-catalyzed borylative ring-opening reaction of vinylcyclopropanes with bis(pinacolato)diboron yielding allylic boronates

Vinylcyclopropanes bearing one or two electron-withdrawing groups on the cyclopropane ring undergo nickel-catalyzed borylative ring opening with bis(pinacolato)diboron to yield allylic boronates. The reaction proceeded with high E selectivity.     

A new aliphatic ester of hydroxysalicylic acid from fermented Carica papaya L. preparation with a potential hair growth stimulating activity

Abstract

An aliphatic ester of hydroxysalicylic acid (6), reported for the first time from a natural source in addition to five known compounds were isolated from the fermented Carica papaya L. preparation, a commercialized functional food. The known compounds were identified as 5-hydroxymethylfurfuraldehyde (1), trans-caffeic acid (2), butyl 4-hydroxybenzoate (butylparaben) (3), lycopene (4), benzyl isothiocyanate (5). Compounds 1 and 3 were reported for the first time from Papaya fruits through this study. The new compound showed a moderate antioxidant activity and a potent hair growth stimulating activity in vitro.     

Direct and quantitative monitoring of catalytic organic reactions under heterogeneous conditions using direct analysis in real time mass spectrometry

This new method overcomes problems of conventional analytical methodologies such as light scattering and sampling reproducibility issues. We used this method for mechanistic studies of catalytic reactions under heterogeneous conditions. Direct-type hydroxymethylation reactions and Mukaiyama-type hydroxymethylation reactions both catalyzed by a scandium–bipyridine ligand complex under micellar conditions were employed as examples of heterogeneous reactions. For direct-type hydroxymethylation reactions, initial reaction rate assays revealed first-order dependency on both substrate and catalyst. On the other hand, Mukaiyama-type hydroxymethylation reactions showed first-order rate dependency on substrate, zero-order on catalyst and saturation kinetics on formaldehyde.

A direct and quantitative method for monitoring heterogeneous organic reactions has been developed by using direct analysis in real time mass spectrometry (DART-MS) with an isotope-labeled reaction product as an internal standard.     

Cobalt-mediated cross-coupling reactions of primary and secondary alkyl halides with 1-(trimethylsilyl)ethenyl- and 2-trimethylsilylethynylmagnesium reagents

[reaction: see text] This paper describes cobalt-mediated cross-coupling reactions of alkyl halides with 1-(trimethylsilyl)ethenylmagnesium bromide and 2-(trimethylsilyl)ethynylmagnesium bromide, respectively. The cobalt system allows for employing secondary as well as primary alkyl halides as the substrates. The reactions offer facile formations of alkyl-alkenyl and alkyl-alkynyl bonds. The reaction mechanism would include single-electron transfer from a cobalt complex to alkyl halide to generate the corresponding alkyl radical. The cobalt system thus enables sequential radical cyclization/alkenylation and cyclization/alkynylation reactions of 6-halo-1-hexene derivatives.     

Nickel-catalyzed allylation of allyl carbonates with homoallyl alcohols via retro-allylation providing 1,5-hexadienes

A highly efficient and mild method for the synthesis of 1,5-hexadienes, nickel-catalyzed reactions of Boc-protected allyl alcohols with homoallyl alcohols, has been developed. Nickel-mediated retro-allylation allows for the use of homoallyl alcohols as allylmetal equivalents in the synthesis of 1,5-hexadienes.     

Crystal structure of ethene-/α-olefin copolymers with various long comonomers (C8–C26)

Ethene-/α-olefin copolymers having a wide range of comonomers between 8 and 26 carbon atoms in length were characterized by wide angle X-ray diffraction (WAXD). It was found that the crystallinities for the shorter comonomers (C₈, C₁₂) matched fairly well with the crystallinities from DSC and volumetric measurements, while for the longer comonomers (C₁₈, C₂₆) distinct differences between the three methods were found. This was explained by the presence of an additional crystalline phase of the side chains slightly deforming the amorphous peak.