Oxidative aromatization of olefins with dioxygen catalyzed by palladium trifluoroacetate

Molecular oxygen can replace sacrificial olefins as the hydrogen acceptor in the palladium trifluoroacetate catalyzed dehydrogenation of cyclohexene and related cyclic olefins into aromatics. One of the major drawbacks of the homogeneous system is the tendency of the palladium trifluoroacetate to precipitate as palladium(0) at elevated temperatures. The use of better ligands affords catalysts that can operate at higher temperatures, although they are less reactive than palladium trifluoroacetate

Kinetic resolution of racemic {alpha}-olefins with ansa-zirconocene polymerization catalysts: Enantiomorphic site vs. chain end control

Copolymerization of racemic {alpha}-olefins with ethylene and propylene was carried out in the presence of enantiopure C1-symmetric ansa metallocene, {1,2-(SiMe2)2({eta}5-C5H-3,5-(CHMe2)2)({eta}5-C5H3)}ZrCl2 to probe the effect of the polymer chain end on enantioselection for the R- or S-{alpha}-olefin during the kinetic resolution by polymerization catalysis. Copolymerizations with ethylene revealed that the polymer chain end is an important factor in the enantioselection of the reaction and that for homopolymerization, chain end control generally works cooperatively with enantiomorphic site control. Results from propylene copolymerizations suggested that chain end control arising from a methyl group at the beta carbon along the main chain can drastically affect selectivity, but its importance as a stereo-directing element depends on the identity of the olefin

Synthesis of several bisabolane sesquiterpenoids from xanthorrhizol isolated from C. Xanthorrhiza

Xanthorrhizol was isolated from the essential oil of fresh rhizomes of C. xanthorrhiza in 20.2% yield by fractionation using vacuum liquid chromatography. Several bisabolane-type sesquiterpenoids have been synthesised from this xanthorrhizol. Both diastereomers of 10,11-dihydro-10,11-dihydroxyxanthorrhizols, sesquiterpenoids isolated from the Mexican medicinal plant, Iostephane heterophylla, have been prepared in three steps from xanthorrhizol via Sharpless asymmetric dihydroxylation as the key steps. Fremy’s salt oxidation of xanthorrhizol gave curcuhydroquinone in 60% yield, which was successfully reduced with sodium dithionite to curcuhydroquinone in 100% yield. Sequential acetylation and Sharpless asymmetric dihydroxylation on curcuhydroquinone led to the diacetate derivative of helibisabonol A. Cleavage of the diacetate esters by reduction with lithium borohydride furnished helibisabonol A, an allelopathic agent isolated from Helianthus annuus (sunflowers). The unexpected difficulty in deprotection of helibisabonol A diacetate was due to acidic, basic and air-sensitive natures of helibisabonol A. An allylic alcohol derivative of O-methylxanthorrhizol, (3S,6R)-(3- methoxy-4-methylphenyl)-2-methylhept-1-en-3-ol, has been synthesised from xanthorrhizol in five steps via Sharpless asymmetric dihydroxylation as the key steps. Sharpless asymmetric dihydroxylation in all syntheses gave excellent enantioselectivity (ee > 98%). The enantiomeric excess and the absolute configuration of the diol was determined by the modified Mosher’s method

Bio-ethylene production: alternatives for green chemicals and polymers

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Radical-Mediated Anti-Markovnikov Hydrophosphonation of Olefins

The radical-mediated addition of triphenylphosphonium tetrafluoroborate to olefins (hydrophosphonation) is reported. Both standard radical initiators and photochemical conditions are effective, up to the gram scale. The phosphonium salts are shown to serve as Z-selective Wittig olefination reagents, even without purification

Bi- and tri-dentate imino-based iron and cobalt pre-catalysts for ethylene oligo-/polymerization

Recent progress on the use of iron and cobalt complex pre-catalysts for ethylene reactivity is reviewed. The review is organized in terms of the denticity of the chelate ligands employed, with particular reference to the influence of the ligand frameworks and their substituents on the catalytic performance for ethylene oligomerization/polymerization catalysis. The majority of the systems bear tri-dentate ligation at the iron/cobalt centre, though it is clear that bi-dentate iron/cobalt complex pre-catalysts have also attracted significant attention. Such systems produce in most cases highly linear products ranging from oligomeric α-olefins to high molecular weight polyethylene, and as such are promising candidates for both academic and industrial considerations

Z-Selective Homodimerization of Terminal Olefins with a Ruthenium Metathesis Catalyst

The cross-metathesis of terminal olefins using a novel ruthenium catalyst results in excellent selectivity for the Z-olefin homodimer. The reaction was found to tolerate a large number of functional groups, solvents, and temperatures while maintaining excellent Z-selectivity, even at high reaction conversions