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Current
Organic Chemistry
ISSN: 1385-2728
Current Organic
Chemistry
Volume 10, Number 9, June 2006
Contents
Organometallic Chemistry, Coordination Chemistry and
Catalysis
Guest Editor: Julio Perez
Editorial Pp.
921
Zinc (II) Bromide: An Overview of its Solution Chemistry and
Recent Applications in Synthesis Pp. 923-936
Robert A. Gossage
[Abstract]
Synthesis of Aryl- and Heteroaryl-Substituted Cyclopentadienes
and Indenes and their Use in Transition Metal Chemistry
Pp.937-953
Markus Enders and Robert W. Baker
[Abstract]
Recent Advances in Chromium Catalysts for Olefin Polymerization
Pp. 955-963
Kevin M. Smith
[Abstract]
Low-Valent Titanium-Mediated Reductive Coupling of
Carbonyl Compounds Pp. 965-980
Folami T. Ladipo
[Abstract]
Stereoselective Aza-Diels-Alder Reactions Pp. 981-1005
Gerald B. Rowland, Emily B. Rowland, Qiang Zhang and Jon C.
Antilla
[Abstract]
Palladium(II)-Catalysed Rearrangement Reactions Pp.
1007 1020
Kate N. Fanning, Andrew G. Jamieson and Andrew Sutherland
[Abstract]
Recent Advances in Chemical Vapor Deposition Pp.
1021-1033
Bradley D. Fahlman
[Abstract]
Major Advances in the Hydrolysis of Peptides and Proteins
by Metal Ions and Complexes Pp. 1035-1049
Kathryn B. Grant and Miki Kassai
[Abstract]
Abstracts
[Back to top]
Editorial
This second issue of Current Organic Chemistry on
the topic Organometallic Chemistry, Coordination Chemistry
and Catalysis consists of eight reviews that I am sure
will attract the interest of the readers.
Robert Gossage (Acadia University, Canada)
reviewed the applications of ZnBr2
in organic synthesis. The Lewis acid activity of this reagent
is related with its structure in solution (where it is always
complexed). An overview of the synthesis/availability, solid
state structure, solubility and solution speciation of ZnBr2
in different solvents precedes a description of the reactions
that it mediates: formation of C-C and C-O bonds, ring (including
heterocycle) formation, ring-opening of epoxides and aziridines,
bromine transfer reagent, and selective removal of leaving
or protecting groups.
The paper by Markus Enders* (Universität
Heidelberg, Germany) and Robert W. Baker*
(University of Sydney, Australia) deals with the synthesis
of aryl- and heteroaryl-substituted cyclopentadienes and indenes
and their use in transition metal chemistry, mainly in olefin
polymerization catalysis. The selective introduction of aryl
substituents allows the design of a well-defined geometric
environment around π-coordinated
metals, which steric and electronic environment can be controlled
further through the presence of an additional coordinating
functionality within the aryl substituent and by freezing
the rotation about the aryl-Cp C–C-bond.
Kevin M. Smith (University of Prince Edward
Island, Canada) focuses on recent advances in Cr(III) Cp,
diketiminato, salicylaldiminato and other well-defined single
component catalysts for olefin polymerization. The review
(87 references, 38 from the 2004-05 period) highlights the
difficulties in the study of these paramagnetic species, and
the usefulness of high throughput screening strategies for
rapid evaluation of structure-activity relationships, and
of DFT studies both for rationalizing the observed reactivity
of known catalysts and for suggesting new potential catalytic
systems.
Folami T. Ladipo (University of Kentucky,
USA) reviews the low valent Ti-mediated reductive coupling
of carbonyl compounds. The paper describes the range and selectivity
of the Ti compounds that mediate pinacol and McMurry couplings,
two powerful methods for the creation of C-C bonds that employ
a low valent Ti reagent usually prepared by an in situ
reduction of a Ti(IV) or Ti(III) precursor. The nature of
the precursor, reductant, additives, conditions, etc. are
discussed. Chemo- and stereolectivity deserve special attention.
With regard to mechanistic information, the nature of the
active species is discussed in the context of well characterized
Ti compounds.
Gerald B. Rowland, Emily B. Rowland, Qiang Zhang and
Jon C. Antilla* (University of Mississippi and University
of South Florida, USA) wrote on the stereoselective aza-Diels-Alder
reaction, a powerful method to prepare nitrogen-containing
natural products. The review addresses both diastereo- and
enantioselective versions. Recent advances include the incorporation
of more and more types of imines as useful substrates, the
application of lanthanide-based Lewis acid catalysts, catalysts
that are effective with low loadings, metal-free organic catalysts,
catalysts that do not require anhydrized solvents or an inert
atmosphere, and greater selectivities.
Kate N. Fanning, Andrew G. Jamieson and Andrew Sutherland*
(University of Glasgow, UK) reviewed the area of Pd (II)-catalyzed
Cope, Claisen and aza-Claisen [3,3] sigmatropic rearrangements.
Whereas non-catalyzed versions of these reactions, useful
for the synthesis of biologically and medicinally important
products, have been known for a long time, the employment
of Pd(II) catalysis notably lowers the reaction temperatures
and generally improves selectivity. Mechanism, stereochemical
outcome (a high degree of chirality transfer results from
a highly ordered transition state) and synthetic applications
are considered.
Bradley D. Fahlman (Central Michigan University,
USA) has written a review of recent advances in chemical vapor
deposition. CVD is the method of choice for thin film growth,
allowing control of the properties of the resulting film,
and is also useful to generate fiber-optic preforms, carbon
nanotubes, etc. Metal β-diketonato,
β-ketoiminato,
β-diketiminato,
amidinato, guanidinato, triazenido, azolato, allylimido complexes,
and other precursors (such as ternary sulfides and heterometallic
alkoxides) of stoichiometrically complex materials, are discussed.
Kathryn B. Grant* and Miki Kassai (Georgia
State University, USA) wrote on major advances in the hydrolysis
of peptides and proteins using metal complexes. Under mild
temperature and pH conditions, several metal complexes are
able to promote the hydrolysis of the extremely stable amide
bond of peptides and proteins. Such reagents hold promise
for protein sequencing, the study of solution structure of
proteins, generation of semi-synthetic proteins, therapeutic
agents targeting disease-related proteins, etc. Metal complexes
are more selective peptide-cleaving reagents than most endopeptidases,
and, unlike other chemicals, cleave peptide bonds under nondenaturing
conditions.
I appreciated very much the generosity of the several anonymous
reviewers; thanking them is not a point of courtesy: their
assistance has been a substantial contribution to the final
result.
Julio Perez
Universidad de Oviedo,
Spain
[Back to top]
Zinc (II) Bromide: An Overview of its Solution
Chemistry and Recent Applications in Synthesis
Robert A. Gossage
An overview of the use of Zinc bromide (ZnBr2)
in organic synthesis and other applications is presented.
Some details on the structure of the inorganic material and
its ligated adducts is also detailed. This aspect is presented
in a context that attempts to relate the observed coordination
chemistry with the activity of the title compound in Lewis
acid-mediated processes.
[Back to top]
Synthesis of Aryl- and Heteroaryl-Substituted Cyclopentadienes
and Indenes and their Use in Transition Metal Chemistry
Markus Enders and Robert W. Baker
This article summarizes the synthesis of cyclopentadienes,
indenes, cyclopentadienyl ligands and indenyl ligands which
have a directly bonded aromatic or heteroaromatic substituent
(HCpAr, HIndAr, CpAr, IndAr)
a. Generally applicable procedures that allow the specific
introduction of a variety of aromatic substituents are selected.
Section 2.1 describes ring closure reactions of precursors
with aromatic substituents which directly lead to cyclopentadienes
or indenes. These routes may be used for the synthesis of
chiral and enantiopure ligands. In all other synthetic pathways,
the formation of the C–C-single bond between the five-membered
ring and the aromatic cycle is the essential step. Nucleophilic
attack of alkali cyclopentadienides (M-Cp, M = Li, Na, K)
on aliphatic halides is an often used reaction which leads
to a wide variety of Cp-derivatives with alkyl or functionalised
alkyl chains. However, nucleophilic attack on an aromatic
ring by cyclopentadienides or indenides is limited to a few
activated aromatics such as perfluorobenzene, naphthyl sulfoxides
or naphthol sulfonate esters (section 2.2). In order to use
the aromatic reagent as the nucleophile, it has to be treated
with a Cp+-synthon. However Cp+-derivatives
do not exist as isolable species and therefore cyclopentenones
are mostly used as Cp+-synthons. b As
many cyclopentenone derivatives are available, a large variety
of Cp-derivatives can be obtained in this way (section 2.3).
Difficulties arise when the 1,4-addition dominates or when
the cyclopentenone is deprotonated by the nucleophile. In
addition an initially formed HCpAr-derivative may
polymerise under the acidic dehydration condition used. Such
problems can be avoided by the use of cobaltocenium salts
as complex stabilized Cp+ moieties. Nucleophiles
can be added, leading to η4-coordinated
cyclopentadienes, which may be liberated by oxidation and
used in further synthesis (section 2.4). Transition metal
catalysed cross-coupling reactions leading to C–C-bond
formation between a Cp-ligand and an aromatic substituent
have also been described. In such reactions aryl halogenides
(ArX, Ar = aryl or heteroaryl, X = halogen) can be coupled
with H-Cp, M-Cp, coordinated Cp-ligands or ring metallated
Cp-complexes (section 2.5). The polarity of the cross-coupling
reaction may also be reversed, as exemplified by the Suzuki
coupling of 2-bromoindene with aryl boronic acids.
Due to the rigidity of aryl- or heteroaryl-Cp-ligands their
metal complexes show in some cases significant differences
compared to derivatives with more flexible substituents. Selected
examples for this special reactivity are given in section
3.
[Back to top]
Recent Advances in Chromium Catalysts for Olefin Polymerization
Kevin M. Smith
Research interest in non-metallocene olefin polymerization
catalysts has led to advances in the synthesis of well-defined
paramagnetic organometallic complexes of first-row transition
metals, including chromium. Single component polymerization
catalysts have been prepared using cationic Cr(III) alkyl
complexes with substituted cylopentadienyl or β-diketiminato
ancillary ligands. High throughput screening strategies have
been developed to assist in the rapid evaluation of structure-activity
relationships. Density functional theoretical studies have
also proved useful both for rationalizing the observed reactivity
of known catalysts and for suggesting new potential catalytic
systems.
[Back to top]
Low-Valent Titanium-Mediated Reductive Coupling of
Carbonyl Compounds
Folami T. Ladipo
This account describes recent organic synthesis applications
and developments in mechanistic understanding of low-valent
titanium-mediated reductive coupling of carbonyl compounds.
Pinacol and McMurry coupling reactions are among the most
powerful methods for constructing carbon-carbon bonds, and
have served as the key step in the synthesis of various natural
and synthetic products. The range and selectivity of low-valent
titanium reagents that mediate pinacol and McMurry reactions
are discussed. The nature of their active species is discussed
in conjunction with relevant chemistry of well-characterized
low-valent titanium complexes, all of which serve to inform
current understanding of the reaction mechanisms.
[Back to top]
Stereoselective Aza-Diels-Alder Reactions
Gerald B. Rowland, Emily B. Rowland, Qiang Zhang and Jon C.
Antilla
The Diels-Alder reaction is one of the most useful and
often studied transformations in organic chemistry. The reaction
has been employed, often as the key step, in the total synthesis
of a wide range of biologically-active natural products. The
utility of the reaction is based upon its ability to form
six-membered cyclic systems with up to four contiguous stereocenters
as well as the reactions ability to incorporate heteroatoms
into the cycloadduct. The use of imines as the dienophile
has been studied in great detail over the last fifteen years.
The development of stereoselective variants of the reaction
has been at the forefront of these studies. The use of chiral
auxiliaries and chiral catalyst to control the stereochemistry
of the product has seen much success over this period of time.
The purpose of this review is to give a detailed discussion
of the recent advances in Diels-Alder methodology involving
a carbon-nitrogen double bond. The review is divided into
two major sections. The first discusses the development of
Diastereoselective aza-Diels-Alder reactions, while the second
part discusses Enantioselective variants of the reaction.
[Back to top]
Palladium(II)-Catalysed Rearrangement Reactions
Kate N. Fanning, Andrew G. Jamieson and Andrew Sutherland
The synthesis of new carbon-carbon and carbon-heteroatom
bonds in a stereochemically well defined manner still presents
a challenge in modern synthetic methodology. An important
class of reactions, which can carry out the aforementioned
transformations, are the [3,3]-sigmatropic rearrangements.
These rearrangements have traditionally been carried out thermally
at high temperatures. However, the discovery that these transformations
can be accelerated by Pd(II) at ambient temperature has led
to more widespread use of these reactions for the synthesis
of biologically active and pharmaceutically important molecules.
This review presents an overview of the use of palladium(II)
catalysis for the acceleration of three of the most commonly
used rearrangements in organic chemistry, namely, the Cope,
Claisen and aza-Claisen rearrangements. In particular, the
mechanism of catalysis, stereochemical outcome and synthetic
application of these reactions will be discussed.
[Back to top]
Recent Advances in Chemical Vapor Deposition
Bradley D. Fahlman
Chemical vapor deposition (CVD) represents the method-of-choice
for thin film growth, as well as nanomaterials such as carbon
nanotubes. Distinct advantages of this technique include low
cost, and strict control over the resultant film stoichiometry,
morphology, and thickness. The most critical component of
a CVD process is the precursor that is employed. Hence, there
has been a continuing interest in the development of new CVD
precursors that would be useful for a range of thin film applications.
This review will highlight recent advances in CVD precursor
design, including both single-source and co-reactant precursor
systems. Whereas the earliest examples of CVD precursors included
metal β-diketonates,
this review will feature novel ligand systems that have already
been used, or have been proposed for future MOCVD applications.
In addition, we will describe new methodologies that have
improved the deposition process through innovations in precursor
delivery and decomposition within the CVD system. These latter
developments have extended the scope of CVD applications to
include precursors that are not particularly volatile, and
have allowed for a reduction in the level of impurities in
the growing film.
[Back to top]
Major Advances in the Hydrolysis of Peptides and Proteins
by Metal Ions and Complexes
Kathryn B. Grant and Miki Kassai
Metal ions and complexes that hydrolyze peptides and proteins
have become increasingly important in recent years. These
reagents have shown great promise for use in a variety of
applications including protein sequencing and proteomics.
When metal-assisted hydrolytic cleavage is accomplished under
nondenaturing conditions of temperature and pH, their use
can be extended to include the study of protein function and
solution structure, the generation of semi-synthetic proteins,
the proteolytic cleavage of bioengineered fusion proteins,
and therapeutics. Yet, because of the extreme stability of
the peptide amide bond, hydrolytically active metals are limited
in number and there is now great interest in the development
of new, more efficient reagents. In this review, we provide
a description of relevant, early work with metal ions and
complexes that have been used to hydrolyze unactivated peptide
amide bonds in peptides and proteins. More importantly, we
present an overview of recent contributions that have been
made toward the development of synthetic metalloproteases
that catalyze hydrolysis under near physiological conditions
of temperature and pH.
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