Current Organic Chemistry - Volume 11, Issue 17, 2007

The present issue, Number 17, is the second part of Volume 11, corresponding to the 2007 Organometallic Chemistry, Coordination Chemistry and Catalysis thematic issue of Current Organic Chemistry. The need to publish two separate numbers due to the large overall length reflects that Curr. Org. Chem. is increasing its attractiveness as a venue for publication of midsize reviews by some of the lead authors in the field. The generous effort of the many anonymous reviewers, to whom I wish to express my sincere appreciation, made possible to publish a larger number of contributions while maintaining a high standard of quality. This issue comprises five review papers dealing with synthesis, application of new ligands, catalysis and molecular recognition by coordination and organometallic complexes. Pamela J. Shapiro (University of Idaho, USA) contributed the paper titled The Elusive Chemistry of Bent-sandwich Chromocene. Bis(cyclopentadienyl)chromium was first synthesized by pioneers such as Fischer and Wilkinson in the nineteen fifties; however, due to the low stability, the reactivity studies of chromocenes have developed at a very slow pace compared with those of metallocenes of other early metals, or with half-sandwich chromium complexes. The review, with an emphasis on ansa-chromocenes, an area to which the author's group has made significant contributions, covers the efforts of several groups to synthesize chromocenes of sufficient stability to permit an exploration of their chemistry. The reactions studied include oxidation, addition of Lewis bases, in some cases with concomitant ring-slippage, protonation to afford a chromocenium hydride, and ring borylation. Hon Man Lee,* Chun-Chin Lee, and Pi-Yun Cheng (National Changhua University of Education, Taiwan) wrote the paper Recent Development of Functionalized N-heterocyclic Carbene Ligands: Coordination Chemistry and Catalytic Applications. Due to their strongly σ donor character, that affords highly electron-rich metal centers, their high steric profile, and the thermal stability of their metal complexes, N-heterocyclic carbene ligands have become, after the seminal work by Arduengo, who showed that the free carbenes were indeed stable enough to be isolable, some of the most widely employed ligands in transition metal coordination chemistry and, in particular, in catalysis. As a result, the topic have been reviewed many times. In this paper, with 152 references, Lee and co-workers focus on recent advances in the chemistry of polydentate ligands that combine N-heterocyclic carbenes with classical donor groups such as phosphines, N donors (pyridine, oxazoline, pyrazole, imine, amine, amido), and O- or S-donors. Literature is covered up to August 2006. David Necas (Charles University, Czech republic) amd Martin Kotora* (Charles University and Academy of Sciences, Czech republic) have reviewed the field of rhodium-catalyzed C-C bond cleavage reactions. High selectivity, atom economy and mild conditions are attractive features of these reactions. The authors look first at the C-C cleavage in carbocyclic compounds: cyclopropanes (including alkylidenecyclopropanes and vinylcyclopropanes), cyclobutanes, cyclobutanones, vinylcyclobutanones, cyclobutenones, and non-strained cyclic ketones, and then discuss acyclic substrates such as allyl malonates, alkylaromatic phosphines, and unsaturated nitriles. Finally, decarbonylation of aldehydes and acid derivatives are considered. Susanne Striegler (Auburn University, USA) wrote Binuclear metal complexes in molecular recognition and catalysis. The review highlights recent developments in several areas: (a) selective recognition and hydrolysis of phosphate monoester, mainly using dizinc (II) complexes (as found in the alkaline phosphatase enzyme), (b) the recognition and hydrolysis of double stranded DNA with dinuclear complexes of Zn(II), Ru(II), Cu(II), etc., (c) recognition of DNA with Pt(II) complexes showing antitumor activity, (d) metal complexes as components of self-assembled structures that display allosterism in their host-guest behavior, and (e) dicopper complexes as carbohydrate recognition agents......

Until recently the chemistry of bent-sandwich chromocene eluded examination due to the propensity of dicyclopentadienylchromium toward cyclopentadienyl ring loss and thermodynamic barriers to coordinating two electron donating ligands such as CO. The use of modified cyclopentadienyl ligands such as acyclic pentadienyl ligands and linked dicyclopentadienyl ligands, by changing the electronic demands of the metal, have allowed access to neutral and cationic bent-sandwich chromocene derivatives with interesting structures, redox properties, and ligand coordination/substitution behavior. This review article provides an overview of the synthesis, magnetic properties, and reactivity of Cp2Cr (Cp = C5H5) and related compounds, the chemistry of open chromocene compounds containing acyclic pentadienyl ligands, and a more detailed discussion of the chemistry of ansa-chromocene complexes, which has been the research focus of the author.

N-heterocyclic carbenes are extremely versatile ligands in homogeneous catalysis and coordination chemistry. A concurrent focus in this field is the construction of functionalized N-heterocyclic carbenes containing classical donor groups. The interest stems from the fact that multidentate ligand scaffolds can be finely constructed to fulfill specific purposes, such as providing extra-stability for metal complexes, possessing hemilability for effective generation of vacant coordination site, and allowing fine tuning of electronic and steric effects for inducing chemo-, stereo-, and enantioselectivities in catalytic reactions. There are numerous works on such multidentate ligands reported in the literature, in particular, those containing phosphorus, oxygen, and nitrogen functionalities. In this review, we will summarize the current development on functionalized N-heterocyclic carbenes in combination with classical donors of group 15 and 16 elements, illustrating typical preparations of the ligand precursors and their transition metal complexes. The coordination chemistry as well as catalytic applications of these ligands will be presented.

Metal complexes that hydrolyze DNA are important, due to their potential use as therapeutic agents, tools in elucidating oligonucleotide structure, and models for naturally-occurring metalloprotein nucleases. Therefore, this review focuses on recent advances in the hydrolysis of DNA and phosphate diester model compounds by non-lanthanide metal complexes.

Binuclear metal complexes provide an intriguing class of compounds considering both their role in molecular recognition and in catalysis. This review highlights recent developments for the selective recognition of phosphate monoester dianions as diagnostic agents for DNA bulged sequences. The molecular recognition, hydrolysis and oxidation of double-stranded DNA by dinuclear metal complexes are highlighed focusing on recent developments that may lead to new antitumor agents. Additionally, investigations on the allosterism generated by weak host-guest interactions between metalcontaining hosts and metal-containing guests are discussed, and the use of dinuclear copper(II) complexes as tool for selective carbohydrate recognition is emphasized.

Rhodium complexes have found a wide application in the catalytic C-C bond activation of various substrates depending on the ligand environment. The reaction proceeds with strained carbocycles (cyclopropanes, cyclobutanes and their derivatives), with non-strained C-C bonds activated by carbonyl or other functionalities, or it can result in decarbonylation (aldehydes, acyl halides etc.). Rhodium catalyzed C-C bond cleavage reactions were also used as key steps in syntheses of several natural compounds or biologically active molecules.