Beyond Lindlar and without hydrogen: Transfer hydrogenation of internal alkynes catalyzed by a palladium(0) catalyst containing an N‐heterocyclic carbene ligand gives Z alkenes without over‐reduction to alkanes (see scheme). Contrary to most transfer hydrogenations, ketones are not reduced. As such, this is the first catalyst that shows excellent stereo‐ and chemoselectivity for the semihydrogenation of alkynes without the need for hydrogen gas.

Mechanistic questions concerning palladium and norbornene catalyzed aryl-aryl coupling reactions are treated in this paper: how aryl halides react with the intermediate palladacycles, formed by interaction of the two catalysts with an aryl halide, and what is the rational explanation of the "ortho effect" (caused by an ortho substituent in the starting aryl halide), which leads to aryl-aryl coupling with a second molecule of aryl halide rather than to aryl-norbornyl coupling. Two possible pathways have been proposed, one involving aryl halide oxidative addition to the palladacycle, the other passing through a palladium(II) transmetalation, also involving the palladacycle, as previously proposed by Cardenas and Echavarren. Our DFT calculations using M06 show that, in palladium-catalyzed reaction of aryl halides, not containing ortho substituents, and norbornene, the intermediate palladacycle formed has a good probability to undergo transmetalation, energetically favored over the oxidative addition leading to Pd(IV). The unselective sp(2)-sp(2) and sp(2)-sp(3) coupling, experimentally observed in this case, can be explained in the framework of the transmetalation pathway since the energetic difference between aryl attack onto the aryl or norbornyl carbon of the palladacycle intermediate is quite small. On the other hand, according to the experimentally observed "ortho effect", selective aryl-aryl coupling only occurs in the reactions of ortho-substituted metallacycles. The present work offers the first possible rationalization of this finding. These in situ formed palladacycles containing an ortho substituent could more easily undergo oxidative addition of an aryl halide rather than reductive elimination from the transmetalation intermediate as a result of a steric clash in the transition state of the latter. The now energetically accessible Pd(IV) intermediate, featuring a Y-distorted trigonal bipyramidal structure, can account for the reported selective aryl-aryl coupling through a reductive elimination which is easier than aryl-norbornyl coupling. Thus, the steric effect represents the main factor that dictates the energetic convenience of the system to follow the Pd(IV) or the transmetalation pathway. Ortho substituents cause a higher energy transition state for reductive elimination from the transmetalation intermediate than for oxidative addition to the metallacycle palladium(II) and the pathway based on the latter predominates.

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The first C3 -symmetric 44-core-valence-electron triangular palladium clusters, [{(SAr')(PAr3 )Pd}3 ](+) , have been synthesized by activation of the CS bond of isothioureas. Owing to delocalized metal-metal bonding, these stable complexes are the first noble-metal analogues of the π-aromatic cyclopropenyl cation [C3 H3 ](+) , with their all-metal aromaticity involving d-type atomic orbitals.

A simple synthetic method allows the one-pot assembly of C3 -symmetric, 44-core-valence-electron, triangular Pd or Pt clusters and their heterobimetallic mixed Pd/Pt analogues. These mixed metal complexes are the first examples of stable triangular all-metal heteroaromatics. In contrast to traditional heteroaromatic molecules formed combining main-group elements, they actually retain structural and electronic features of their homonuclear analogues.

DedicatedtoProfessor Iwao Ojima on the occasiono fhis 70th birthday As imple catalytic method involving all-metal aromatic frameworks as precatalysts ensures an efficient route to (Z)-alkenes. Aromatic triangular palladium clusters wereu sed to reduce internal alkynesw ithout any trace of the formation of alkane side products. These trinuclear complexes provide ac atalytic system that parallels the activity and selectivity of their best mononuclear peers, and the catalyst likely operates through complementary mechanisms.All-metal aromaticsa re cyclic molecules that present delocalized molecular orbitals similart ot hose of regular aromatics but involve metal atoms in their cores.[1] They can therefore display ag reater variety of bondingm odes and represent an ideal bridge that connects discrete homogeneous complexes with heterogeneous metallic nanoparticles thanks to their delocalized metal-metal bonding.[2] We wanted to gain somei nsight into the consequences of metal aromaticity by studying the catalytic behavior of stable and easily accessible triangular d-orbital-aromatic palladium clusters. [3] We reasoned that semihydrogenation of alkynes would be both ac hallenging and meaningful playground for aromatic palladium clusters.[4] Whereas these reactions are wells tudied in homogenous catalysis, [5] no catalytic methodf or the semireductiono fa lkynes has been reportedb yu sing discrete Pd clusters,a nd examples with other clusters are rare.[6] (Z)-Alkenes can be formed in high yields by using electron-rich Pd 0 complexes bearing suitable N-heterocyclic carbenes or chelating diphosphinesa sl igands. [7] Their efficiency represents ap robing test and therefore at ool to check the potential of metal aromatics. [8] Trinuclear metal-aromatic complexes are interesting platforms for catalytic applications( Scheme 1). Featuring noncoordinating counteranions, the positive charge of these clusters shouldp rovide them with Lewis acid character.A tt he same time, their delocalized HOMO might grant Lewis basic properties to the triangularc ore. Their resonance-enhanced stabilization should increaset heir robustness. From ap ractical point of view,t hese complexes are definitely stable to oxygen and moisture, are formed in highy ields from commercial reagents in one-pot,and feature easily tunable organic fragments. [3] We report the application of am etal-aromatic framework in ac atalytic reaction. Triangular Pd 3 clusters were used to efficiently reduce internal alkynes to (Z)-alkenesu nder transfer-hydrogenation conditions, completely excluding over-reduction to alkanes and preserving other reducible groups, even in the presence of al arge excess amount of the donor.We used 1-phenylpropyne (1a)a sam odel substrate. Its internalt riple bond is conjugated with the aromatic ring, which makesi tt he candidate of choice to evaluatec atalyst efficiency (Table 1). In at ypical experiment, 1a (0.3 mmol) was stirred with aP d 3 catalyst (1 mol %) and triethylammonium formate (5 equiv.) in THF.T he mixture was heatedt or eflux, an...

A method for the synthesis of phenanthridines from benzylamines and aryl iodides which uses a dual palladium-catalyzed process is developed. The domino sequence ends via an intramolecular amination and an oxidative dehydrogenation. No protecting group or prefunctionalization of the amine is required, and the process uses dioxygen as the terminal oxidant.

Highly symmetric [Pd] clusters that present delocalized metal-metal bonds can catalyse the selective semi-reduction of internal alkynes to cis-alkenes. Studies on factors governing the formation of all-metal aromatics enabled the design of an optimised catalytic system that delivers cis-alkenes with almost complete selectivity on a gram scale with very low catalyst loadings (0.03 mol%).