ChemCatChem

Golden metamorphose! The cover picture shows a linear unsaturated molecule, which in the presence of a gold catalyst rearranges to a complicated branched product, an isomerization involving an oxygen atom transfer. This resembles the fascinating metamorphose of a caterpillar to a butterfly. In their Full Paper on p. 1200 ff., A. S. K. Hashmi, K. Graf et al. report that this conversion proceeds in a highly diastereoselective manner. A complex cascade involving activation by π coordination of gold, a [3,3]-sigmatropic rearrangement, and an electrocyclic ring opening is suggested as the mechanism of this conversion.

Nature's first green is gold The cover picture shows the gold(I)-catalyzed cycloisomerization of O-tethered 1,6-enynes, which proceeds via fused tricyclic compounds with a dihydropyran moiety. As described in their Communication on p. 1096 ff., L. Fensterbank et al. continue this ring-expansion with a two-step sequence for the transformation of the cyclized compounds into valuable ketomacrolactones, with a motif that echoes natural products. And all this in the golden surroundings of Paris!

Palladium double act he cover picture shows different forms of palladium catalyst present in the Suzuki–Miyaura reaction mixture. In their Full Paper on p. 1152 ff., A. M. Trzeciak et al. describe the use of palladium complexes of the formula [Pd(NHC)₂X₂] (NHCN-heterocyclic carbene) as catalyst precursors to form Pd⁰NHC complexes and Pd⁰ nanoparticles. All of these species are involved in the catalytic cycle leading to 2-methylbiphenyl as a reaction product.

Rhodium-catalyzed polymerization goes heavyweightThe cover picture shows a neutral rhodium(I) N-heterocyclic carbene hydrosilylation catalyst immobilized on the surface of MCM-41. In their Full Paper on p. 1133 ff., F. J. Fernández-Alvarez, L. A. Oro et al. describe how these rhodium-containing materials catalyze the preparation of poly(silyl ether)s by hydrosilylation. These results show that the immobilized catalysts provide polymers with higher average molecular weight than those obtained by using the corresponding homogenous systems, which could be attributed to a confinement effect exerted by the channels of MCM-41.

Catalysis à la carte: Based on the International Symposium on Homogeneous Catalysis (ISHC), this Special Issue has a particular focus on homogeneous catalysis. Organometallic, supramolecular, bioinspired, coordination, and organocatalysis can all be found in this great collection of articles.

From all natural to bioactive: This review shows that organocatalytic methods based on the use of nucleophilic phosphorus catalysts have been successfully applied as key steps in the formal or total synthesis of complex natural products. These methods also afford suitable tools for the constitution of either focused or diversity-oriented-synthesis inspired libraries of small molecules for biological screening.

Ligand breakthrough: Using a well-designed chiral cyclopentadienyl rhodium complex as the catalyst, an efficient asymmetric CH bond functionalization/cyclization was performed at RT in the presence of a catalytic amount of dibenzoylperoxide (DBPO) leading to tetrahydroiso-quinolinone derivatives with moderate to good yields and very good enantiomeric ratios.

From pots and pans to hydrometalation: Hydrometalation reactions using boron and zirconium hydrides transform alkenes into useful alkylmetal nucleophiles. These nucleophiles can be employed in the copper-catalyzed 1,4-additions to α,β-unsaturated carbonyl compounds or allylic substitutions. Important advances have been achieved recently, which enabled also enantioselective versions of these transformations.

On the way to a green carbon cycle: Ding and co-workers developed the highly efficient catalyst 1 for the hydrogenation of cyclic carbonates to diols and methanol (TON up to 87 000) by modification of the industrially applied omega process.

A big promotion: One reason for the irreproducibility that is sometimes found in hydrogenation reactions of CO catalysed by [Ru₃(CO)₁₂] in molten [PBu₄]Br is the possible presence of [HPBu₃]Br, which acts as a promoter by providing a source of HBr.

Rhodium teaches toleration: The diolefin-amine–Rh^I complex [Rh(OTf)(trop₂NH)(PPh₃)] (see scheme) is an effective pre-catalyst for the amidation of a wide range of aldehydes with primary amines. The corresponding amides are obtained in good-to-excellent yields under very mild reactions conditions and a variety of functional groups are tolerated.

Golden (I): A supramolecular ligand–template strategy is applied to gold(I) catalysis. The supramolecular ligands, formed from a phosphoramidite backbone and two templates, display different regioselectivity in the hydroalkoxylation of allenes compared to non-supramolecular analogues. Based on these findings, the catalytic activity of gold(I) complexes could be tunable at a supramolecular level.

The old switcheroo! The ring-opening polymerization activity towards meso-lactide significantly differs when cerium complexes with two bis(phenolate) ligands (OSSO type) change their oxidation state from +4 to +3. This switch can be performed in situ by chemical reduction or oxidation of the catalyst. The redox switching is possible up to three times in a row. PLA=polylactide.

Do it with a tridentate ligand: The use of readily available tridentate nitrogen donor ligands gives highly active and selective Fe catalysts in the epoxidation of a series of alkenes.

Goldenyne:O-tethered 1,6-enynes that contain a strained ring at the 3 position can cycloisomerize upon cationic gold(I) catalysis through a ring-expansion process. A two-step sequence allows the transformation of the cyclized compounds into ketomacrolactones, which are reminiscent of natural products.

No carboxylato ligand, no activation: A series of bis(carboxylato)ruthenium(II) complexes are prepared. An internal stereospecific CH bond cleavage reaction occurs to give a ruthenalactone through prior dissociation of a PMe₃ ligand. Here, a coordinated carboxylato ligand acts as an internal base, and a typical proton acceptor such as a proton sponge does not assist the reaction.

Resourceful resorcinarene: Imidazolium salts based on a rigid resorcinarene platform are used as catalyst precursors in Suzuki–Miyaura cross-coupling reactions, fostering higher catalytic activity than precursor salts devoid of a cavity. The Pd centres of the catalysts obtained from the precursors are found to locate preferentially outside the cavity and the presence of a bulky resorcinarenyl substituent seems to favor the reductive elimination step.

Proceed with cation: New Ru^II catalysts for aqueous-phase HCOOH decomposition into H₂ and CO₂ that use a series of oligocationic, ammoniomethyl-substituted triarylphosphine ligands are developed. Excellent activities are achieved, that is, TOFs of 1950 h⁻¹ and TONs of over 10 000.

Getting stuck to rhodium: A new family of rhodium N-heterocyclic carbene (NHC) species is described. The chlorido derivative was immobilized on the inorganic support MCM-41. Both the homo- and heterogeneous catalysts are effective for the copolymerization of terephthalaldehyde and 1,1,3,3,5,5-hexamethyltrisiloxane, which affords the corresponding poly(silyl ether).

Back to the future! State-of-the-art group IV metal polymerization catalysts are applied to intramolecular CN bond-forming reactions. Cationic species show remarkable catalytic activities for the production of N-containing heterocycles from secondary amino alkenes. Additional studies on the amido precursors highlight the role of metal coordination environment on the final catalytic performance.

A robust cross-coupling system: [Pd(NHC)₂X₂] complexes are synthesized and evaluated in aryl–aryl cross-coupling catalysis. All the obtained complexes demonstrate very high activity in the Suzuki–Miyaura reaction performed in ethylene glycol under mild conditions. Mechanistic investigations reveal an operationally homogeneous reaction pathway, as evidenced by using ESI-MS, TEM, and mercury poisoning experiments.

Controlled length polystyrene-supported PPh₃ ligands are made by atom transfer radical polymerization, involving direct copolymerization of styrene and 4-diphenylphosphinostyrene (SDPP). Controlled M_n and low dispersity are obtained with low SDPP content (≤25 %). The performance of these polymeric ligands in rhodium-catalyzed hydroformylation of 1-octene is evaluated as a function of polymer length, degree of P incorporation, and P/Rh ratio.

Di(e)imine another day: Pd–dmso complexes with α-diimines are synthesized, characterized, and applied to ethylene/methyl acrylate cooligomerization. The complex with a new nonsymmetric α-diimine and dmso was found to be highly productive and showed a long catalyst lifetime. In the graphic, cat=[Pd(CH₃)(L)(Ar-BIAN)][PF₆].

Finding the ligand: The covalent modification of sterol carrier protein type 2 like domain with nitrogen ligands affords artificial metalloenzymes for the copper-catalyzed Diels–Alder reaction (see picture). A distinct influence of the location of the ligand within the host on the catalytic performance was observed. An ee of 25 % was found for one ligand–protein construct. The results provide a suitable framework for the development of more selective systems.

Constrained chiral center: The first general, efficient and highly enantioselective α-hydroxyamination reactions of carbonyl compounds are described, securing a practical synthetic access to conformationally constrained α-amino acid derivatives exhibiting a quaternary chiral center at the α-position.

Old is gold! The gold(I)-catalyzed reaction of allyl 2-en-4-ynyl ethers represents a new mechanistic pathway leading to 1,3,6-trien-4-yl ketones under mild reaction conditions (see scheme). Remarkable diastereoselectivity combined with perfect regioselectivity is the key feature of the reaction cascade.

Homo and hetero—genius! A stepwise catalytic reduction of ketone 1 to alcohol 2 and subsequently to aryl(imidazo[1,2-b]pyridazinyl)methane 3 is described, which provides synthetically useful chemoselectivity at acceptably low catalyst loadings.