| Current
Organic Chemistry
ISSN: 1385-2728
Current Organic Chemistry
Volume 12, Number 16, November 2008
Contents
Organometallic Chemistry, Coordination
Chemistry and Catalysis thematic
Guest Editor: Julio Perez
Editorial Pp. 1340
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Recent Advances in Iron Catalysis in Organic Synthesis
Pp. 1341-1369
Eike B. Bauer
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Recent Developments of Chiral Diaminocarbene-Metal
Complexes for Asymmetric Catalysis Pp. 1370-1387
David R. Snead, Hwimin Seo and Sukwon
Hong
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Reactions of Early Transition Metal – Carbon
Bonds With N-Heterocycles Pp. 1388-1405
Paula L. Diaconescu
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Titanium and Zirconium Mediated or Catalyzed Umpolung
Reactions Pp. 1406-1430
Hairong Guan
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Abstracts
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Editorial
The issue number 16 of Curr. Org. Chem. 2008 (volume
12) features four reviews. Their subjects are iron catalysis
in organic synthesis, chiral diaminocarbene metal complexes
for asymmetric catalysis, the reactivity of early transition
metal M-C bonds with N-heterocycles, and the Ti- or Zr-mediated
or catalyzed umpolung reactions.
Eike B. Bauer (University of Missouri at Saint Louis, USA)
contributed the paper Recent Advances in Iron Catalysis
in Organic Synthesis, which deals with developments in
the field of homogeneous catalysis with iron compounds since
2004, avoiding overlap with previous reviews. C-C forming
reactions (including cyclizations, cyanohidrin formation,
and allylation reactions), formation of C-O and C-N bonds,
reactions involving sulfur, polymerizations, redox reactions,
and biomimetic oxidations are included. The catalysts range
from simple mixtures of FeCl3
and ligands such as amines to well-characterized complexes,
including porphirine complexes and organometallic well-characterized
complexes. Ligands used in combination with iron sources include
phosphines and N-heterocyclic carbenes as well as polymer-inmobilized
ligands. For all these transformations, advantages of iron-based
catalysts over more widely used systems are low price and
low toxicity.
David R. Snead, Hwimin Seo and Sukwon Hong* (University of
Florida, USA) wrote
Recent Developments of Chiral Diaminocarbene-Metal Complexes
for Asymmetric Catalysis. N-heterocyclic carbenes are
the type of ligands deserving more attention in recent times,
due to their desirable properties in catalysis. This paper
reviews, with special attention to the most recent literature
not covered in previous reviews, asymmetric reactions catalyzed
by transitions metal complexes of chiral monodentate or bidentate
NHC (and some isolated example of non-cyclic analog) ligands.
These are classified by metal (Ru, Rh, Ir, Pd, Cu and Ni)
and, within each metal, reactions type. Some of these reactions
show excellent enantioselectivities.
Paula L. Diaconescu (University of California at Los Angeles,
USA) is the author of Reactions of Early Transition Metal-
Carbon Bonds with N-Heterocycles, a review divided into
two sections: (a) reactions of early transition metal (groups
3-5 and f-block metals) complexes featuring M-C σ
bonds leading to ortho-metalation or insertion to
yield functionalized heterocycles, and (b) heterocycle ring-opening
via reaction with metal complexes containing metal-element
multiple bonds. The interest in these processes stems from
the search of methods that allow heterocycle functionalization,
and from the difficulty of cleaving the C-N bonds of nitrogen
heterocycles, a process that takes place during industrially-relevant,
heterogeneously-catalyzed hydrodenitrification (HDN), required
to avoid catalyst poisoning in subsequent industrial steps
and generation of toxic nitrogen oxides.
Hairong Guan (University of Cincinnati, USA) is the author
of Titanium and Zirconium Mediated or Catalyzed Umpolung
Reactions, centered on polarity reversal of carbonyl
and imine derivatives. Acyl anions, a-amino carbanion or a-hydroxy
carbanion equivalents, homoenolates and homoenolate equivalents
are the most useful organometallic reagents in C-C bond forming
reactions. Due to the high electropositive character of Ti
and Zr, the carbanionic character of the metal-bonded carbon
overcomes the electronegativity of N or O atoms, achieving
the polarity reversal. The selectivity (chemo-, regio- and
stereo-) is highlighted as an advantage of some reactions.
I am sure that, thanks to the great job of the authors and
the help of the anonymous reviewers, these reviews will be
of interest to the broad readership of Curr. Org. Chem.
Julio Perez
Oviedo, Spain
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Recent Advances in Iron Catalysis in Organic Synthesis
Eike B. Bauer
Transition metal complexes play a vital role as catalysts
in organic syntheses, enhancing selectivity and rates of many
transformations. Typically iron is not the first choice when
it comes to development of novel catalyst systems. However,
iron has a number of advantages to other transition metals
typically used in catalysis: iron is cheap, non-toxic, environmentally
friendly and abundant. Accordingly, an increasing number of
publications describe novel applications of iron catalysts
in organic synthesis. The review article covers recent developments
in the field since 2004. Iron catalysis in carbon-carbon bond
formations, cyclizations, carbon-heteroatom bond formation,
oxidations, biomimetic oxidations, reductions and polymerizations
is summarized. The article demonstrates the versatility of
iron in organic synthesis and the vigorous research activities
in the area.
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Recent Developments of Chiral Diaminocarbene-Metal Complexes
for Asymmetric Catalysis
David R. Snead, Hwimin Seo and Sukwon Hong
N-heterocyclic carbenes (NHCs) serve as highly
proficient ligands in many transition metal catalyzed reactions.
As such, significant efforts have been devoted to the extension
of NHCs into the field of asymmetric catalysis. Development
was slow at first but is now rapidly accelerating, evidenced
by the increase in number of transformations displaying excellent
enantioselectivities. Chronicled here are the attempts at
asymmetric catalysis with NHC ligands, and the results are
categorized by the metal effecting catalysis as well as the
type of reaction. Ruthenium, rhodium, iridium, palladium,
and copper complexed with NHCs have all demonstrated excellent
enantioselectivities in catalysis, and fundamental reactions
such as olefin metathesis, alkene hydrogenation, kinetic resolution
of secondary alcohols, and conjugate addition have been carried
out with high selectivity.
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Reactions of Early Transition Metal – Carbon Bonds With
N-Heterocycles
Paula L. Diaconescu
Heterocycles are ubiquitous in biological compounds and methods
to functionalize them have been of long standing interest.
Reactions of early transition metal-carbon bonds with heterocycles
are important in two major areas: (1) activation and functionalization
of heterocycles and (2) ring-opening of aromatic heterocycles.
In the first area, metal-carbon σ
bonds determine ortho-metalation and insertion of unsaturated
substrates to yield functionalized heterocycles. In the latter
area, metal-element multiple bonds become important and provide
the driving force for ring-opening reactions. This review
will focus on reactions of nitrogen-containing heterocycles.
Early transition metals are defined as elements belonging
to groups III-V, the lanthanides, and the actinides; the reactivity
of complexes featuring the metal in its highest oxidation
state (d0fn
electron count) and a carbon-based ligand will be described.
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Titanium and Zirconium Mediated or Catalyzed Umpolung Reactions
Hairong Guan
This review presents current strategies of using titanium
and zirconium reagents to achieve umpolung of carbonyl and
imine functionalities. The reactions of organometallic compounds,
including those that are equivalent to acyl anions, α-amino
carbanions, α-hydroxy
carbanions, and homoenolates, are among the most powerful
methods to form carbon-carbon bonds. The remarkable chemoselectivity,
regioselectivity, and stereoselectivity observed in many cases
once more demonstrate great synthetic application of organotitanium
and organozirconium compounds.
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