10 research outputs found
Helimeric porphyrinoids: Stereostructure and chiral resolution of meso -tetraarylmorpholinochlorins
The synthesis and chiral resolution of free-base and Ni(II) complexes of a number of derivatives of meso-tetraphenylmorpholinochlorins, with and without direct β-carbon-to-o-phenyl linkages to the flanking phenyl groups, is described. The morpholinochlorins, a class of stable chlorin analogues, were synthesized in two to three steps from meso-tetraphenylporphyrin. The conformations and the relative stereostructures of a variety of free-base and Ni(II) complexes of these morpholinochlorins were elucidated by X-ray diffractometry. Steric and stereoelectronic arguments explain the relative stereoarray of the morpholino-substituents, which differ in the free-base and Ni(II) complexes, and in the monoalkoxy, β-carbon-to-o-phenyl linked morpholinochlorins, and the dialkoxy derivatives. The Ni(II) complexes were all found to be severely ruffled whereas the free-base chromophores are more planar. As a result of the helimeric distortion of their porphyrinoid chromophores, the ruffled macrocycles possess a stable inherent element of chirality. Most significantly, resolution of the racemic mixtures was achieved, both by classical methods via diastereomers and by HPLC on a chiral phase. Full CD spectra were recorded and modeled using quantum-chemical computational methods, permitting, for the first time, an assignment of the absolute configurations of the chromophores. The report expands the range of known pyrrole-modified porphyrins. Beyond this, it introduces large chiral porphyrinoid π-systems that exist in the form of two enantiomeric, stereochemically stable helimers that can be resolved. This forms the basis for possible future applications, for example, in molecular-recognition systems or in materials with chiroptic properties. © 2011 American Chemical Society
An Approach to Mimicking the Sesquiterpene Cyclase Phase by Nickel-Promoted Diene/Alkyne Cooligomerization
Artificially
mimicking the cyclase phase of terpene biosynthesis inspires the invention
of new methodologies, since working with carbogenic frameworks containing
minimal functionality limits the chemist’s toolbox of synthetic
strategies. For example, the construction of terpene skeletons from
five-carbon building blocks would be an exciting pathway to mimic
in the laboratory. Nature oligomerizes, cyclizes, and then oxidizes
γ,γ-dimethylallyl pyrophosphate (DMAPP) and
isopentenyl pyrophosphate (IPP) to all of the known terpenes. Starting
from isoprene, the goal of this work was to mimic Nature’s
approach for rapidly building molecular complexity. In principle,
the controlled oligomerization of isoprene would drastically simplify
the synthesis of terpenes used in the medicine, perfumery, flavor,
and materials industries. This article delineates our extensive efforts
to cooligomerize isoprene or butadiene with alkynes in a controlled
fashion by zerovalent nickel catalysis building off the classic studies
by Wilke and co-workers
An Approach to Mimicking the Sesquiterpene Cyclase Phase by Nickel-Promoted Diene/Alkyne Cooligomerization
Artificially
mimicking the cyclase phase of terpene biosynthesis inspires the invention
of new methodologies, since working with carbogenic frameworks containing
minimal functionality limits the chemist’s toolbox of synthetic
strategies. For example, the construction of terpene skeletons from
five-carbon building blocks would be an exciting pathway to mimic
in the laboratory. Nature oligomerizes, cyclizes, and then oxidizes
γ,γ-dimethylallyl pyrophosphate (DMAPP) and
isopentenyl pyrophosphate (IPP) to all of the known terpenes. Starting
from isoprene, the goal of this work was to mimic Nature’s
approach for rapidly building molecular complexity. In principle,
the controlled oligomerization of isoprene would drastically simplify
the synthesis of terpenes used in the medicine, perfumery, flavor,
and materials industries. This article delineates our extensive efforts
to cooligomerize isoprene or butadiene with alkynes in a controlled
fashion by zerovalent nickel catalysis building off the classic studies
by Wilke and co-workers
An Approach to Mimicking the Sesquiterpene Cyclase Phase by Nickel-Promoted Diene/Alkyne Cooligomerization
Artificially
mimicking the cyclase phase of terpene biosynthesis inspires the invention
of new methodologies, since working with carbogenic frameworks containing
minimal functionality limits the chemist’s toolbox of synthetic
strategies. For example, the construction of terpene skeletons from
five-carbon building blocks would be an exciting pathway to mimic
in the laboratory. Nature oligomerizes, cyclizes, and then oxidizes
γ,γ-dimethylallyl pyrophosphate (DMAPP) and
isopentenyl pyrophosphate (IPP) to all of the known terpenes. Starting
from isoprene, the goal of this work was to mimic Nature’s
approach for rapidly building molecular complexity. In principle,
the controlled oligomerization of isoprene would drastically simplify
the synthesis of terpenes used in the medicine, perfumery, flavor,
and materials industries. This article delineates our extensive efforts
to cooligomerize isoprene or butadiene with alkynes in a controlled
fashion by zerovalent nickel catalysis building off the classic studies
by Wilke and co-workers
An Approach to Mimicking the Sesquiterpene Cyclase Phase by Nickel-Promoted Diene/Alkyne Cooligomerization
Artificially
mimicking the cyclase phase of terpene biosynthesis inspires the invention
of new methodologies, since working with carbogenic frameworks containing
minimal functionality limits the chemist’s toolbox of synthetic
strategies. For example, the construction of terpene skeletons from
five-carbon building blocks would be an exciting pathway to mimic
in the laboratory. Nature oligomerizes, cyclizes, and then oxidizes
γ,γ-dimethylallyl pyrophosphate (DMAPP) and
isopentenyl pyrophosphate (IPP) to all of the known terpenes. Starting
from isoprene, the goal of this work was to mimic Nature’s
approach for rapidly building molecular complexity. In principle,
the controlled oligomerization of isoprene would drastically simplify
the synthesis of terpenes used in the medicine, perfumery, flavor,
and materials industries. This article delineates our extensive efforts
to cooligomerize isoprene or butadiene with alkynes in a controlled
fashion by zerovalent nickel catalysis building off the classic studies
by Wilke and co-workers
Mechanism, Reactivity, and Selectivity of Nickel-Catalyzed [4 + 4 + 2] Cycloadditions of Dienes and Alkynes
Density
functional theory (DFT) calculations with B3LYP and M06
functionals elucidated the reactivities of alkynes and <i>Z</i>/<i>E</i> selectivity of cyclodecatriene products in the
Ni-catalyzed [4 + 4 + 2] cycloadditions of dienes and alkynes. The
Ni-mediated oxidative cyclization of butadienes determines the <i>Z</i>/<i>E</i> selectivity. Only the oxidative cyclization
of one <i>s-cis</i> to one <i>s-trans</i> butadiene
is facile and exergonic, leading to the observed 1<i>Z</i>,4<i>Z</i>,8<i>E</i>-cyclodecatriene product.
The same step with two <i>s-cis</i> or <i>s-trans</i> butadienes is either kinetically or thermodynamically unfavorable,
and the 1<i>Z</i>,4<i>E</i>,8<i>E</i>- and 1<i>Z</i>,4<i>Z</i>,8<i>Z</i>-cyclodecatriene isomers are not observed in experiments. In addition,
the competition between the desired cooligomerization and [2 + 2 +
2] cycloadditions of alkynes depends on the coordination of alkynes.
With either electron-deficient alkynes or alkynes with free hydroxyl
groups, the coordination of alkynes is stronger than that of dienes,
and alkyne trimerization prevails. With alkyl-substituted alkynes,
the generation of alkyne-coordinated nickel complex is much less favorable,
and the [4 + 4 + 2] cycloaddition occurs
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Science with the Cherenkov Telescope Array
The Cherenkov Telescope Array, CTA, will be the major global observatory for
very high energy gamma-ray astronomy over the next decade and beyond. The
scientific potential of CTA is extremely broad: from understanding the role of
relativistic cosmic particles to the search for dark matter. CTA is an explorer
of the extreme universe, probing environments from the immediate neighbourhood
of black holes to cosmic voids on the largest scales. Covering a huge range in
photon energy from 20 GeV to 300 TeV, CTA will improve on all aspects of
performance with respect to current instruments.
The observatory will operate arrays on sites in both hemispheres to provide
full sky coverage and will hence maximize the potential for the rarest
phenomena such as very nearby supernovae, gamma-ray bursts or gravitational
wave transients. With 99 telescopes on the southern site and 19 telescopes on
the northern site, flexible operation will be possible, with sub-arrays
available for specific tasks. CTA will have important synergies with many of
the new generation of major astronomical and astroparticle observatories.
Multi-wavelength and multi-messenger approaches combining CTA data with those
from other instruments will lead to a deeper understanding of the broad-band
non-thermal properties of target sources.
The CTA Observatory will be operated as an open, proposal-driven observatory,
with all data available on a public archive after a pre-defined proprietary
period. Scientists from institutions worldwide have combined together to form
the CTA Consortium. This Consortium has prepared a proposal for a Core
Programme of highly motivated observations. The programme, encompassing
approximately 40% of the available observing time over the first ten years of
CTA operation, is made up of individual Key Science Projects (KSPs), which are
presented in this document
Science with the Cherenkov Telescope Array
The Cherenkov Telescope Array, CTA, will be the major global observatory for very high energy gamma-ray astronomy over the next decade and beyond. The scientific potential of CTA is extremely broad: from understanding the role of relativistic cosmic particles to the search for dark matter. CTA is an explorer of the extreme universe, probing environments from the immediate neighbourhood of black holes to cosmic voids on the largest scales. Covering a huge range in photon energy from 20 GeV to 300 TeV, CTA will improve on all aspects of performance with respect to current instruments. The observatory will operate arrays on sites in both hemispheres to provide full sky coverage and will hence maximize the potential for the rarest phenomena such as very nearby supernovae, gamma-ray bursts or gravitational wave transients. With 99 telescopes on the southern site and 19 telescopes on the northern site, flexible operation will be possible, with sub-arrays available for specific tasks. CTA will have important synergies with many of the new generation of major astronomical and astroparticle observatories. Multi-wavelength and multi-messenger approaches combining CTA data with those from other instruments will lead to a deeper understanding of the broad-band non-thermal properties of target sources. The CTA Observatory will be operated as an open, proposal-driven observatory, with all data available on a public archive after a pre-defined proprietary period. Scientists from institutions worldwide have combined together to form the CTA Consortium. This Consortium has prepared a proposal for a Core Programme of highly motivated observations. The programme, encompassing approximately 40% of the available observing time over the first ten years of CTA operation, is made up of individual Key Science Projects (KSPs), which are presented in this document