There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

J. Appl. Cryst.の発刊に際して

N-Heterocyclic carbenes have become universal ligands in organometallic and inorganic coordination chemistry. They not only bind to any transition metal, be it in low or high oxidation states, but also to main group elements such as beryllium, sulfur, and iodine. Because of their specific coordination chemistry, N-heterocyclic carbenes both stabilize and activate metal centers in quite different key catalytic steps of organic syntheses, for example, C-H activation, C-C, C-H, C-O, and C-N bond formation. There is now ample evidence that in the new generation of organometallic catalysts the established ligand class of organophosphanes will be supplemented and, in part, replaced by N-heterocyclic carbenes. Over the past few years, this chemistry has been the field of vivid scientific competition, and yielded previously unexpected successes in key areas of homogeneous catalysis. From the work in numerous academic laboratories and in industry, a revolutionary turning point in oraganometallic catalysis is emerging.

http://aether.cmi.ua.ac.be/artikels/Artikels%20Gitte/Artikels/Reviews/NHC-%20liganden%20Herrmann.pdf

N-heterocyclic carbenes: a new concept in organometallic catalysis.

Hydrogen Storage in Metal–Organic Frameworks

The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of β-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

The catalytic addition of an organic amine R2N H bond to alkenes or alkynes (hydroamination) to give nitrogen containing molecules is of great interest for academic and industrial research. Since today most amines are made in multistep syntheses, hydroamination would offer the most attractive alternative synthetic route. It was shown, that hydroamination can be catalyzed by transition metals (dand f-block) and alkali metals.[1] Depending on the catalytic system either an activation of the C C multiple bond or the N H bond takes place. Alkene or alkyne activation is usually accomplished with late-transition metals through coordination of the C C multiple bond to the metal center. In contrast, the amino function can be activated by alkali or early transition metals, which generate an amido species, or by N H oxidative addition to electron-rich transition metals. Depending on the nature of the catalysts and the substrates either Markovnikov or anti-Markovnikov products are obtained [Eq. (1)]. The scope of catalytic hydroamination has been reviewed recently.[1–6] Today, the vast number of enantioselective syntheses of molecules bearing an amine functionality use classical stoichiometric reactions with chiral auxiliaries or utilize enantiomerically pure starting materials.[7] As a result of the increasing interest in the hydroamination reaction some research groups have started to investigate the enantioselective R2N H addition to C C double bonds. In early transition-metal chemistry Marks et al. implemented C1-symmetric organolanthanide ansa-metallocene catalysts of the type [Me2Si(hC5Me4)(h-C5H3R*)LnE(SiMe3)2] (1)[8–10] (R*= ( )-menthyl, (þ)-neomenthyl, ( )-phenylmenthyl; Ln=La, Nd, Sm, Y, Lu; E=CH, N) and [Me2Si(hOHF)(h-C5H3R*)LnN(SiMe3)2] (2)[11] (OHF= octahydrofluorenyl; R*= ( )menthyl; Ln= Sm, Y, Lu) in the enan-

Enantioselective catalytic hydroamination of alkenes.

A pentanuclear dysprosium hydroxy cluster of composition [Dy5(μ4-OH)(μ3-OH)4(μ-η2-Ph2acac)4(η2-Ph2acac)6] (1; Ph2acac = dibenzoylmethanide) was prepared starting from [DyCl3·6H2O] and dibenzoylmethane. Both static (dc) and dynamic (ac) magnetic properties of 1 have been studied. Below 3 K, the appearance of slow relaxation of the magnetization typical for single-molecule magnets is seen, even if no hysteresis effects on the M vs H data are observed above 1.8 K.

Pentanuclear Dysprosium Hydroxy Cluster Showing Single-Molecule-Magnet Behavior

The homoleptic bis(trimethylsilyl)amides of Group 3 metals and lanthanides of the general type [Ln{N(SiMe3)2}3] (1) (Ln=Y, lanthanide) represent a new class of Tishchenko precatalysts and, to a limited extent, precatalysts for the hydroamination/cyclization of aminoalkynes. It is shown that 1 is the most active catalyst for the Tishchenko reaction. This contribution presents investigations on the scope of the reaction, substrate selectivity, lanthanide-ion size-effect, and kinetic/mechanistic aspects of the Tishchenko reaction catalyzed by 1. The turnover frequency is increased by the use of large-center metals and electron-withdrawing substrates. The reaction rate is second order with respect to the substrate. While donor atoms, such as nitrogen, oxygen, or sulfur, on the substrate decrease the turnover frequency, 1 shows a tolerance for a large number of functional groups. For the hydroamination/cyclization of aminoalkynes, 1 is less active than the well-known metallocene catalysts. On the other hand, 1 is much more readily accessible (one-step synthesis or commercially available), than the metallocenes and might therefore be an attractive alternative catalyst.

Peter W. Roesky

Papers

Enantioselective catalytic hydroamination of alkenes.

Pentanuclear Dysprosium Hydroxy Cluster Showing Single-Molecule-Magnet Behavior

Homoleptic Lanthanide Amides as Homogeneous Catalysts for Alkyne Hydroamination and the Tishchenko Reaction

Aminotroponiminate Complexes of the Heavy Alkaline Earth and the Divalent Lanthanide Metals as Catalysts for the Hydroamination/Cyclization Reaction

Intramolecular Hydroamination of Functionalized Alkenes and Alkynes with a Homogenous Zinc Catalyst