Institut de Chimie Moléculaire de Paris : organique, inorganique et biologique

Functionalization via covalent grafting of organic functions allows to tune the redox and acid-base properties, and the solubility of polyoxometalates, to enhance their stability and biological activity and to reduce their toxicity, to facilitate their implementation in extended structures and functional devices. We discuss herein the electronic and binding connections, and the various synthesis methodologies. We emphasize on organonitrogen, organosilyl and organophosphonyl derivatives with special attention to synthesis, characterization and potential applications in catalysis and materials science. We also consider the giant molybdenum oxide-based clusters especially the porous capsule-type clusters (Keplerates) which have high relevance to this context.

Since Ellman's seminal works, over the past ten years tert-butanesulfinimines have proved to be useful chiral amino intermediates for organic synthesis. Through highly stereoselective reactions, amongst which reductions, nucleophilic 1,2-additions and ylide condensations, a broad range of nitrogen-containing compounds has been synthesized. Although the stereoselectivity levels are high in most cases, the sense of the stereoinduction is generally not predictable. The object of this critical review is to present the models proposed to rationalize the stereochemical outcome of the reactions involving tert-butanesulfinimines and to point out an obvious lack of homogeneity amongst them (128 references).

Calculations suggest that complexes of borane with N-heterocyclic carbenes (NHC) have B-H bond dissocation energies more then 20 kcal/mol less than free borane, diborane, borane-THF, and related complexes. Values are in the range of popular radical hydrogen atom donors like tin hydrides (70-80 kcal/mol). The resulting prediction that NHC borane complexes could be used as radical hydrogen atom donors was verified by radical deoxygenations of xanthates by using either AIBN or triethylborane as initiator.

This tutorial review aims at presenting recent contributions dealing with organic chemistry of organophosphorus radicals. The first part briefly lays out the physical organic background of such intermediates. In a second part the use of organophosphorus radicals possessing a P-H bond that can undergo homolytic cleavage as alternative mediators is detailed. The third part is focused on radical additions of phosphorus-centered radicals to unsaturated compounds, an old reaction that is being rejuvenated. Lastly, radical eliminations of phosphorus-centered radical are introduced in the fourth part. Most of the latter are relatively novel reactions, and have never been reviewed previously.

[reaction: see text] A polymer-supported catalyst for Huisgen's [3+2] cycloaddition reaction between azides and alkynes was prepared from copper(I) iodide and Amberlyst A-21. This catalyst was then used in an automated synthesis of 1,4-disubstituted 1,2,3-triazoles giving access to these products in good yields. The catalyst has shown good activity, stability, and recycling capabilities.

For many years, our research group has been interested in the new developments of cobalt-mediated cyclizations. In this article, our recent achievements in the field of inter- and intramolecular [2 + 2 + 2] cyclizations are compiled.

Round and round it goes: Propargylic esters are versatile substrates for Au-based catalysts. However, under typical conditions the starting Au-coordinated propargylic ester 1 is in rapid equilibrium with the gold vinylic carbenoid species 2 and with gold allene species 3. A number of factors dictate which intermediate is lower in energy and which type of products form.

Cyclopentenylidene gold complexes can easily be formed from vinyl allenes through a Nazarov-like mechanism. Such carbenes may transform in four different ways into polycyclic frameworks: electrophilic cyclopropanation, C-H insertion, C-C migration, or proton shift. We have studied the selectivity of these different pathways and used our findings for the expedient preparation of valuable complex molecules. An application to the total synthesis of a natural product, Delta(9(12))-capnellene, is presented. DFT computations were carried out to shed light on the mechanisms.

It's a trap! Both epoxides and aziridines substituted by an aryl ketone can be reduced efficiently using visible-light photoredox catalysts. The radicals generated were trapped by allyl sulfones, and formed α-branched β-hydroxy or amino derivatives with high diastereocontrol (see scheme; dtbbpy=4,4′-di-tert-butyl-2,2′-bipyridine, ppy=2-phenylpyridine). Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Abstract The increasing use of polyoxometalates in the fields of material sciences, catalysis, and biology has raised the interest in chirality of such systems. This review provides a summary of the different strategies followed: i) chirality in solid‐state arrangements, ii) chiral polyoxometalate frameworks, and iii) chiral polyoxometalate–organic hybrids. Through the discussion of selected examples, an outline for future work is drawn. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

We have developed an expedient method for the synthesis of polycyclic compounds from propargyl acetates or vinyl allenes involving up to three Au(I)-catalyzed elemental steps: 3,3-rearrangement, metalla-Nazarov reaction, and electrophilic cyclopropanation. The reaction proceeds under very mild conditions and in short times. The mechanism has been studied by DFT computations.

To understand some experimental data at odds with the computed mechanism of the CpCo(L2)-catalyzed [2 + 2 + 2] cyclotrimerization of ethyne, DFT computations were carried out following the fate of methyl- and hydroxycarbonyl-substituted alkynes to give the corresponding arenes. The key intermediate in all cases is a triplet cobaltacyclopentadiene obtained by oxidative coupling of the corresponding CpCo(bisalkyne) complex and subsequent spin change via a minimum energy crossing point (MECP). From that species, two different catalytic cycles lead to an arene product, depending on the nature of the alkyne and other ligands present: either alkyne ligation to furnish a cobaltacyclopentadiene(alkyne) intermediate or trapping by a sigma-donor ligand to generate a coordinatively saturated cobaltacyclopentadiene(PR3) complex. The former leads to the CpCo-complexed arene product via intramolecular cobalt-assisted [4 + 2] cycloaddition, whereas the latter may, in the case of a reactive dienophile (butynedioic acid), undergo direct intermolecular [4 + 2] cycloaddition to generate a cobaltanorbornene. The bridgehead cobalt atom is then reductively eliminated after another change in spin state from singlet to triplet. The necessary conditions for one or the other mechanistic pathway are elaborated.

Chloride ligands are crucial in the gold-catalyzed cycloisomerization of allenynes to give hydrindienes such as 1, which are formally products of CH activation and formed completely selectively over the usual Alder-ene products (2; see scheme). This effect could be rationalized by a DFT study that sheds new light on the electrophilic metal-catalyzed cycloisomerization of polyunsaturated systems.

Abstract After a brief introduction emphasizing the synthetic relevance of the allylic C–H activation step, evoking the first pioneering stoichiometric studies that sowed the “seeds” of this subject, and analyzing similarities and differences between a “classical” and a “direct” Pd‐catalyzed allylation process, this review outlines some selected examples of palladium‐catalyzed direct allylic functionalization. This old reaction, ignored for many years, is now living a new and exciting era.

Catalytic cornerstone: Lanthanide(III) complexes of a lacunary Dawson-type polyoxometalate catalyze Lewis acid mediated reactions (see figure, TMS=trimethylsilyl). The compounds (NBu4)5H2[α1-Ln(H2O)4P2W17O61] (Ln=Yb, Sm, Eu, La) are much more chemoselective than the lanthanide triflates. Furthermore, the polyoxotungstic framework can play a role, presumably through H-bonding to the substrates.

Organotrifluoroborates have been oxidized by copper(II) salts and Dess–Martin periodinane via radical intermediates, as evidenced by TEMPO spin-trapping experiments. This new method of radical generation is compatible with functionalization and CC bond formation through Giese-type addition reactions (see scheme; DMSO=dimethyl sulfoxide, TEMPO=2,2,6,6-tetramethyl-1-piperidinyloxyl, free radical).

As radical chain cascade precursors, N-acylcyanamides give rise to amide–iminyl radicals which, when appropriately substituted, can finally yield pyrroloquinazolines. The versatility of these new radical acceptors is illustrated by the formation of N-heterocycles with wide structural variation and by the total synthesis of luotonin A.

The use of N-heterocyclic carbene (NHC) as a ligand in the gold(I)-catalyzed cycloisomerization of enyne results in the assembly of a new carbocyclic product.

Katalytischer Eckstein: Lanthanoid(III)-Komplexe eines lakunaren Dawson-Polyoxometallats wirken als Lewis-Säure-Katalysatoren (siehe Bild, TMS=Trimethylsilyl). Die Verbindungen (NBu4)5H2[α1-Ln(H2O)4P2W17O61] (Ln=Yb, Sm, Eu, La) sind deutlich chemoselektiver als die entsprechenden Lanthanoidtriflate. Darüber hinaus kann das Polyoxowolframat-Gerüst über Wasserstoffbrücken zu den Substraten beteiligt sein.

Cobalt cyclopentadienyl complexes incorporating a fumarate and a CO ligand (see picture) efficiently catalyze inter- and intramolecular [2+2+2] cycloadditions of alkynes, nitriles, and/or alkenes to give benzenes, pyridines, or 1,3-cyclohexadienes. Unlike catalysts such as [CpCo(CO)(2)] or [CpCo(C(2)H(4))(2)] (Cp = C(5)H(5)), they are air-stable, easy to handle, compatible with microwave conditions, and do not necessarily require irradiation to be active.