Current Organic Chemistry - Volume 16, Issue 1, 2012

Full text available

Current Organic Chemistry has completed 15 years of its existence and it has become an important journal, indispensable in every institution where research and teaching are being carried on in organic chemistry. It has achieved a very respectable Impact Factor and is often the first choice for authors wishing to publish reviews in a cutting edge field of organic chemistry. I would like to thank the Regional Editor, Executive Editors and Editorial Board Members of Current Organic Chemistry for their constant guidance and support. I would also like to thank Mr. Qasit Malik, Ms. Donia Akhter and Mr. Asad Ali in Bentham Science Publishers for their assistance in the publication process.

Nature's capability of conducting chemical transformations has motivated scientists globally to mimic such processes. One of the fundamental core areas behind this is metal centered catalysis, since many of the known enzymes are in fact metalloenzymes. In this context, the significant enzymatic role of molybdenum in biochemical reactions, such as nitrogen fixation, induced the use of molybdenum complexes as biomimetic catalysts in organic transformations. Within this field, several experts have developed many works dealing with the exploitation of the capabilities of Mo-based catalytic systems, both homo- and heterogeneous, and ranging from oxidation to metathesis to C-X bond formation catalysis, taking advantage of the variety of oxidation states available for this metal. Furthermore, Mo-based catalytic systems have high importance in both academia (the early works of Sharpless and Mimoun) and in industry (the Halcon and ARCO processes) and have been (and still are) the aim of many works every year. In this special issue, our goal was to provide an overview, as broad as possible, about recent developments on the use of molybdenum catalysts. The paper by Szymanska-Buzar deals mostly with reactions of Mo carbonyl complexes under photo-irradiation conditions to promote the addition of Group IV compounds to olefins. This is an important reaction for the synthesis of intermediates in organic syntheses. The second paper, by Kuhn, focus on recent developments of the use of cyclopentadienyl complexes in chiral and achiral epoxidation reactions depending on the choice of auxiliary ligands. On the other hand, the paper by Fernandes reviews the important and unexpected performance of high valent dioxo-molybdenum complexes in reduction and in C-X bond formation reactions, made possible by a special association of reagents. The paper by Calhorda aims at a mechanistic overview of oxo-molybdenum complexes catalyzed oxidation reactions of olefins and sulfur substrates, based on computational results. In a leap, the role of molybdenum catalysts in heterogeneous catalysis is also reviewed. First, the paper by Maurya describes the application of functional polymers to the immobilization of molybdenum catalysts in order to perform oxidation and polymerization reactions. Last but not least, the paper by Nunes targets the use of molybdenum catalysts immobilized in mesoporous materials in two major catalytic applications - oxidation and metathesis reactions. While not covering every aspect of molybdenum chemistry, these reviews provide a wide-ranging illustration of several areas which will make this issue attractive, namely in what concerns organic transformations. Finally, as a guest editor for this special issue, I am very grateful for the valuable and excellent contributions from my colleagues, and I am also highly indebted to other colleagues, acting as referees, for their expert comments. A final word of acknowledgment is also due to Ms. Donia Akhter (Assistant Manager of the Publication Department, Current Organic Chemistry) for her assistance in the organization of this issue. Thank you all!

This is a review of the published papers that focus on the reactivity of carbonyl complexes of molybdenum in low oxidation states (usually zero or two) and possible applications of molybdenum complexes as precursors of catalysts in different reactions of unsaturated organic substrates such as alkenes and alkynes. The following reactions are analyzed: metathesis, metathesis polymerization, free radical reactions of alkenes, hydrogenation, hydrosilylation, hydrogermylation, hydrostannation, hydroarylation, asymmetric allylic alkylation, and epoxidation. In addition to the interesting practical aspects of these catalytic reactions, they provide additional insights into molybdenum chemistry.

Applications of η5-cyclopentadienyl molybdenum complexes as catalysts for achiral and chiral epoxidation of olefins are reviewed. The discussion is categorized according to the different complex types which arise from the variety of ancillary ligands coordinated to the (η5-C5R5)Mo moiety. The yields and turnover frequencies (TOFs) achieved under standard conditions with these different catalysts have been considered and thus, an assessment of the dependence of catalytic activity on the ligand environment is presented.

The purpose of this review is to demonstrate that high valent oxo-molybdenum complexes are excellent catalysts for C-X bond forming reactions, including carbon-carbon, carbon-heteroatom (C-N, C-O, C-S, C-P, C-Br, and C-I), and carbon-hydrogen bonds.

Mo(VI) complexes MoO2X2L2 (X=halide or Me, L neutral ligand) behave as catalysts for olefin epoxidation in the presence of t-butylhydroperoxide (TBHP). The active species results from OH activation of TBHP, which protonates one oxo group and leads to a seven coordinate complex, with a new OOR ligand. It was found that several Mo(II) complexes Cp'Mo(CO)3X (Cp'=C5R5, Cp* or C5H5, Cp) acted as precursors for the same reactions and the resulting Cp'MoO2X could also oxidize sulfides and sulfoxides, both with TBHP and H2O2 as oxidants. A review of the reaction mechanisms proposed for these reactions, by us and some other authors, and based on computational studies is given in this work. More than one active species can be found for the CpMoO(OH)( η1-OOR)X intermediate, opening several competitive pathways. They differ by the O-H···O hydrogen bond formed between OH and one oxygen of the OOR ligand. This complex can also further react with oxidant to afford a peroxide complex CpMoO(η2-O2)X, which can also promote oxidation reactions. The activation energies depend on hydrogen bond assistance, so that the Cl and Me derivatives of CpMoO2X behave differently (the peroxide complex has only been found active with Me), and on the steric constraints, more obvious when comparing Cp with Cp*. The preferred mechanism will thus depend on the specific substituents, but energy barriers are comparable.

Procedures for covalent bonding of ligands to polymer support and their reactions with molybdenum precursors to obtain polymer-supported molybdenum complexes are outlined. These supported molybdenum complexes are used as catalysts for organic transformations such as oxidation of alkenes, phenols, allyl alcohol, ethylbenzene and benzoin, and for polymerization reaction. Higher activity, recyclability and high turn over numbers of most supported catalysts are confirmed in many cases which offer real prospect for technological developments.

In most of the chemical and pharmaceutical industrial processes at least one catalytic step is essential during their synthesis. Catalysts speed up chemical reactions but can be recovered unchanged at the end of the reaction. In this respect, heterogeneous catalysts surpass their homogeneous congeners by offering better recyclability as well as an easier separation from the reaction medium. Increasing research in recent years has led to new catalysts capable of directing a given reaction towards a specific product as well as allowing reactions to be carried out at lower temperatures and pressures with higher selectivity towards the desired product. Due to their relevance in biological processes, Mo-catalyzed reactions have attracted considerable interest. In this context, the present work aims at reviewing some of the most recent advances on heterogenized Mo-species in mesoporous materials. In this way, two of the most relevant reactions using olefins as substrates- epoxidation and metathesis where Mo is a major player are assessed. The former is relevant because epoxides are important intermediaries in industry, while the latter is quite significant since without it many organic transformations would simply be impossible.

The pseudotrisaccharide allosamidin 1 is a potent family-18 chitinase inhibitor, and it demonstrates biological activities against insects, fungi, and the Plasmodium falciparum life cycle. Compound 1 contains two N-acetylhexosamine residues with the unusual D-allo-configuration and a novel aminocyclitol (i.e. allosamizoline 2), joined by two β-(1→4) glycosidic bonds. Many traditional syntheses of compounds 1-2 and their analogues have been reported. Herein, recent development for the synthesis of compounds 1-2 and their analogues was reviewed. Donohoe and Rosa's approach to allosamizoline 2 involves a key-step ring-closing metathesis (RCM) to form the cyclopentene core followed by halocyclization to form the oxazoline unit. Rojas et al. reported that rhodium-catalyzed oxidative cyclization of glucal 3-carbamates led to oxazolidinone-protected mannosamine derivatives. Huang et al. described the solid-phase synthesis of allosamidin 1 and its analogues. The compound 1 and its analogues were obtained by iterative glycosylation reactions, catalytic hydrogenation, acetylation, and deacetylation, respectively. Withers and his co-workers' research shows that chitobiose and chitotriose thiazolines exhibit chitinase inhibition activity. In a word, the goal is to investigate the novel methods for the synthesis of allosamidin and its analogues, which is convenient for the discovery of allosamidin analogues with high activity.

A series of highly-functionalized 2'-hydroxychalcones have been synthesized using a microwave-assisted Claisen-Schmidt condensation. Conversion of these 2'-hydroxychalcones to their corresponding flavanones was then performed utilizing protic ionic liquids (pIL) and microwave irradiation. This methodology drastically reduces reaction time to 15 minutes compared to typical thermal methods (24 hrs) and is tolerant to a broad range of functional groups. Several chalcones reported bear four and five substituents - a degree of substitution rarely reported in the literature.