55 research outputs found
Gravitational fermion creation during an anisotropic phase of cosmological expansion
The free Dirac equation is solved in a Bianchi Type I space-time, which
represents a homogeneous but anisotropic universe, to show the creation of
fermionic particles. It is found that unlike in the isotropic case, massless
fermion production is possible. An estimate of the energy density of massless
particles created during an early anisotropic phase of cosmological expansion
is shown to cause substantial back-reaction on the gravitational field. The
potential relevance to dark matter particle production, primordial
magnetogenesis, and early universe cosmology is discussed briefly.Comment: Accepted for publication in Physical Review
Copper-Catalyzed Hydroboration of Carbon Dioxide
A copper/N-heterocyclic
carbene catalyzed hydroboration of carbon
dioxide has been developed to give a formic acid derivative selectively
under mild conditions. Investigations directed toward understanding
the catalytic cycle of this process have been carried out, and the
hydroboration product can be directly used as a formylation reagent
for various amines
Synthesis of Functional Polyolefins Using Cationic Bisphosphine Monoxide–Palladium Complexes
The copolymerization of ethylene with polar vinyl monomers,
such
as vinyl acetate, acrylonitrile, vinyl ethers, and allyl monomers,
was accomplished using cationic palladium complexes ligated by a bisphosphine
monoxide (BPMO). The copolymers formed by these catalysts have highly
linear microstructures and a random distribution of polar functional
groups throughout the polymer chain. Our data demonstrate that cationic
palladium complexes can exhibit good activity for polymerizations
of polar monomers, in contrast to cationic α-diimine palladium
complexes (Brookhart-type) that are not applicable to industrially
relevant polar monomers beyond acrylates. Additionally, the studies
reported here point out that phosphine-sulfonate ligated palladium
complexes are no longer the singular family of catalysts that can
promote the reaction of ethylene with many polar vinyl monomers to
form linear functional polyolefins
Copolymerization of Propylene and Polar Monomers Using Pd/IzQO Catalysts
Palladium catalysts bearing imidazoÂ[1,5-<i>a</i>]Âquinolin-9-olate-1-ylidene
(IzQO) ligands polymerize α-olefins while incorporating polar
monomers. The steric environment provided by N-heterocyclic-carbene
(NHC) enables regioselective insertion of α-olefins and polar
monomers, yielding polypropylene, propylene/allyl carboxylate copolymers,
and propylene/methyl acrylate copolymer. Known polymerization catalysts
bearing NHC-based ligands decompose rapidly, whereas the present catalyst
is durable because of structural confinement, wherein the NHC-plane
is coplanar to the metal square plane. The present catalyst system
enables facile access to a new class of functionalized polyolefins
and helps conceive a new fundamental principle for designing NHC-based
ligands
Ruthenium Catalyzed Hydrogenation of Aldehyde with Synthesis Gas
The hydrogenation of aldehyde utilizing
synthesis gas as a dihydrogen
source was examined with various ruthenium catalysts, among which
Ru–cyclopentadienone complexes (Shvo-type catalysts) exhibited
higher activity than others. DFT calculations proved that the exchange
of coordinated carbon monoxide by dihydrogen is relatively preferable
in Shvo-type catalysts compared to others, which is a pre-equilibrium
for the generation of the hydrogenation-active species
Synthesis of Functional Polyolefins Using Cationic Bisphosphine Monoxide–Palladium Complexes
The copolymerization of ethylene with polar vinyl monomers,
such
as vinyl acetate, acrylonitrile, vinyl ethers, and allyl monomers,
was accomplished using cationic palladium complexes ligated by a bisphosphine
monoxide (BPMO). The copolymers formed by these catalysts have highly
linear microstructures and a random distribution of polar functional
groups throughout the polymer chain. Our data demonstrate that cationic
palladium complexes can exhibit good activity for polymerizations
of polar monomers, in contrast to cationic α-diimine palladium
complexes (Brookhart-type) that are not applicable to industrially
relevant polar monomers beyond acrylates. Additionally, the studies
reported here point out that phosphine-sulfonate ligated palladium
complexes are no longer the singular family of catalysts that can
promote the reaction of ethylene with many polar vinyl monomers to
form linear functional polyolefins
Synthesis of Functional Polyolefins Using Cationic Bisphosphine Monoxide–Palladium Complexes
The copolymerization of ethylene with polar vinyl monomers,
such
as vinyl acetate, acrylonitrile, vinyl ethers, and allyl monomers,
was accomplished using cationic palladium complexes ligated by a bisphosphine
monoxide (BPMO). The copolymers formed by these catalysts have highly
linear microstructures and a random distribution of polar functional
groups throughout the polymer chain. Our data demonstrate that cationic
palladium complexes can exhibit good activity for polymerizations
of polar monomers, in contrast to cationic α-diimine palladium
complexes (Brookhart-type) that are not applicable to industrially
relevant polar monomers beyond acrylates. Additionally, the studies
reported here point out that phosphine-sulfonate ligated palladium
complexes are no longer the singular family of catalysts that can
promote the reaction of ethylene with many polar vinyl monomers to
form linear functional polyolefins
Acceptorless Dehydrogenation of C–C Single Bonds Adjacent to Functional Groups by Metal–Ligand Cooperation
Unprecedented direct acceptorless
dehydrogenation of C–C
single bonds adjacent to functional groups to form α,β-unsaturated
compounds has been accomplished by using a new class of group 9 metal
complexes. Metal–ligand cooperation operated by the hydroxycyclopentadienyl
ligand was proposed to play a major role in the catalytic transformation
Dehydrogenation of Dimethylamine–Borane Catalyzed by Half-Sandwich Ir and Rh Complexes: Mechanism and the Role of Cp* Noninnocence
Half-sandwich Cp*Rh<sup>III</sup> complexes (Cp* = η<sup>5</sup>-1,2,3,4,5-pentamethylcyclopentadienyl)
supported by 2,2′-bipyridine
or 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine
catalyze dehydrogenation of dimethylamine–borane (Me<sub>2</sub>NH·BH<sub>3</sub>) to produce H<sub>2</sub> and dimethylamino–borane
dimer (Me<sub>2</sub>Nî—¸BH<sub>2</sub>)<sub>2</sub> with turnovers
of 2200. The Ir<sup>III</sup> analogues, on the other hand, display
dramatically poorer catalytic activity. Mechanistic inferences drawn
from stoichiometric reactions and DFT calculations suggest noninnocent
involvement of the Cp* moiety as a proton shuttle
Intermolecular Three-Component Arylsilylation of Alkynes under Palladium/Copper Cooperative Catalysis
An intermolecular
three-component arylsilylation of alkynes has
been developed under mild palladium/copper cooperative catalysis.
The reaction proceeds through <i>syn</i>-addition of an
aryl group and a silyl group across the carbon–carbon triple
bond of an alkyne. This represents the first transition-metal-catalyzed
fully intermolecular arylsilylation of alkynes, and transformations
of the resulting products have also been demonstrated
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