Current Organic Chemistry - Volume 10, Issue 9, 2006

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, ringopening 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....

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 acidmediated processes.

This article summarizes the synthesis of cyclopentadienes, indenes, cyclopentadienyl ligands and indenyl ligands which have a directly bonded aromatic or heteroaromatic substituent (HCpAr, HIndAr, Cp Ar, 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. β 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.

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.

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.

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.

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.

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.

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 semisynthetic 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.