18 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
Iron-Catalyzed [2 + 2 + 2] Cycloaddition Reactions of Diynes with Oxyphosphaethynes To Construct 2‑Phosphaphenol Derivatives
Iron-catalyzed
[2 + 2 + 2] cycloaddition reactions of diynes with siloxyphosphaethynes
have been developed to give 2-phosphaphenol derivatives. The use of
electronically neutral siloxyphosphaethynes generated in situ by the
reaction of anionic phosphaethynolate with silyl triflates is key
to achieving the reactions
Cooperative Catalytic Reactions Using Organocatalysts and Transition Metal Catalysts: Propargylic Allylation of Propargylic Alcohols with α,β-Unsaturated Aldehydes
Propargylic allylation of propargylic alcohols with α,β-unsaturated
aldehydes in the presence of a thiolate-bridged diruthenium complex
and a secondary amine as cocatalysts has been found to give the corresponding
propargylic allylated products, where the γ-propargylation occurs
selectively at the α,β-unsaturated aldehydes, in high
yields as a mixture of two diastereoisomers
Visible-Light-Mediated Utilization of α-Aminoalkyl Radicals: Addition to Electron-Deficient Alkenes Using Photoredox Catalysts
Synthetic use of α-aminoalkyl radicals
formed by
single electron oxidation of amines is quite limited. Here we demonstrate
addition of α-aminoalkyl radicals to electron-deficient
alkenes by visible-light-mediated electron transfer using transition metal polypyridyl complexes as photocatalysts, via a sequential redox pathway
Copper-Catalyzed Enantioselective Propargylic Etherification of Propargylic Esters with Alcohols
Enantioselective
propargylic etherification of propargylic esters
with not only aliphatic alcohols but also phenols in the presence
of a catalytic amount of copper-Pybox complex gives the corresponding
propargylic ethers in good to high yields with a high to excellent
enantioselectivity (up to 99% ee). The result described here provides
the first successful example of enantioselective propargylic etherificatio
Hydroboration of Alkynes Catalyzed by Pyrrolide-Based PNP Pincer–Iron Complexes
To
utilize iron complexes as catalysts, the application of a well-designed
ligand is critical to control the reactivity of the iron center. Recently,
our group has succeeded in the synthesis of iron complexes bearing
a pyrrolide-based PNP pincer ligand and their application to the catalytic
transformation of dinitrogen into ammonia under mild reaction conditions.
As an extensive study, we report the iron-catalyzed hydroboration
of alkynes with pinacolborane, where the corresponding <i>E</i>-isomers are obtained selectively
Synthesis, Structure, and Reactivity of Group VI Metal Complexes Bearing Group IV Metallocenyldiphosphine Moieties and a Pentamethylcyclopentadienyl Ligand
A series of group VI metal complexes
bearing a group IV metallocenyldiphosphine
moiety and a pentamethylcyclopentadienyl ligand have been prepared
and structurally characterized. Reactions of trinuclear complexes
with KC<sub>8</sub> give the corresponding neutral chloride-bridged
complexes. Further reaction of the resulting chloride-bridged complex
with NaBAr<sup>F</sup><sub>4</sub> gives the corresponding monocationic
chloride-bridged complex. On the other hand, reactions of the neutral
chloride-bridged complexes with <i>n</i>-BuLi give the corresponding
chloride-bridged alkyl complexes, where the group IV metallocene(III)
moiety
and the coordinatively unsaturated group VI metal(II) center are in
close proximity. Spectroscopic and crystallographic studies of the
reaction of a chloride-bridged alkyl complex with CO or a protic acid
indicate that further disproportionation takes place at the alkyl
zirconocene(III) moiety
Ruthenium- and Copper-Catalyzed Enantioselective Propargylic Alkylation of Propargylic Alcohols with β-Keto Phosphonates
The enantioselective propargylic alkylation of propargylic
alcohols
with β-keto phosphonates in the presence of a thiolate-bridged
diruthenium complex and a copper complex as cocatalysts gives the
corresponding propargylic alkylated products in excellent yields with
high diastereo- and enantioselectivities (up to 97% ee)
Synthesis and Reactivity of Hybrid Phosphido- and Hydrosulfido-Bridged Diruthenium Complexes: Transformations into Diruthenium and Tetraruthenium Complexes Bridged by Phosphido and Sulfido Ligands
The reaction of the monophosphido-bridged diruthenium(III)
complex
[Cp*RuCl(μ-PMe<sub>2</sub>)(μ-Cl)RuClCp*] with sodium
hydrogen sulfide affords the hybrid phosphido- and hydrosulfido-bridged
diruthenium(III) complex [Cp*RuCl(μ-PMe<sub>2</sub>)(μ-SH)RuClCp*].
The hydrosulfido-bridged diruthenium(III) complex can be further converted
into the corresponding sulfido-bridged multinuclear ruthenium(III)
complex via deprotonation of the hydrosulfido ligand. The hydrosulfido-bridged
diruthenium(III) complex also reacts with bases to afford a coordinatively
unsaturated diruthenium(III) complex, where insertion of terminal
alkynes further occurs to form phosphido-bridged diruthenium(III)
complexes bearing ruthenathiacyclobutene moieties
Synthesis, Structure, and Reactivity of Group VI Metal Complexes Bearing Group IV Metallocenyldiphosphine Moieties and a Pentamethylcyclopentadienyl Ligand
A series of group VI metal complexes
bearing a group IV metallocenyldiphosphine
moiety and a pentamethylcyclopentadienyl ligand have been prepared
and structurally characterized. Reactions of trinuclear complexes
with KC<sub>8</sub> give the corresponding neutral chloride-bridged
complexes. Further reaction of the resulting chloride-bridged complex
with NaBAr<sup>F</sup><sub>4</sub> gives the corresponding monocationic
chloride-bridged complex. On the other hand, reactions of the neutral
chloride-bridged complexes with <i>n</i>-BuLi give the corresponding
chloride-bridged alkyl complexes, where the group IV metallocene(III)
moiety
and the coordinatively unsaturated group VI metal(II) center are in
close proximity. Spectroscopic and crystallographic studies of the
reaction of a chloride-bridged alkyl complex with CO or a protic acid
indicate that further disproportionation takes place at the alkyl
zirconocene(III) moiety