This critical review examines transition metal-catalyzed decarboxylative couplings that have emerged within recent years as a powerful strategy to form carbon-carbon or carbon-heteroatom bonds starting from carboxylic acids. In these reactions, C-C bonds to carboxylate groups are cleaved, and in their place, new carbon-carbon bonds are formed. Decarboxylative cross-couplings constitute advantageous alternatives to traditional cross-coupling or addition reactions involving preformed organometallic reagents. Decarboxylative reaction variants are also known for Heck reactions, direct arylation processes, and carbon-heteroatom bond forming reactions.

COMMUNICATIONSon tantalum or zirconium were used as molecular precursors. In these cases, however, alkyl groups are detached during the grafting process as a consequence of prot~Iysis.'~] In our case, all alkyl groups first remain at the metal since the acidic surface silanols attack the nitridomolybdenum moiety, whereas only in the second step an I-elimination occurs, yielding the corresponding alkylidene species. Further work in this area is in progress.

A new strategy for the regiospecific construction of unsymmetrical biaryls is presented, in which easily available salts of carboxylic acids are decarboxylated in situ to give arylmetal species that serve as the nucleophilic component in a catalytic cross-coupling reaction with aryl halides. The catalyst system consists of a copper phenanthroline complex that mediates the extrusion of CO2 from aromatic carboxylates to generate arylcopper species, and a palladium complex that catalyzes the cross-coupling of these intermediates with aryl halides. This bimetallic system allows the direct coupling of various aryl, heteroaryl, or vinyl carboxylic acids with aryl or heteroaryl iodides, bromides, or chlorides at 160 degrees C in the presence of a mild base such as potassium carbonate. The present scope and potential economic impact of the reaction are demonstrated by the synthesis of 42 biaryls, some of which are of substantial industrial relevance. Remaining challenges and future perspectives of the new transformation are discussed.

N-Heterocyclic “carbene” ligands derived from imidazole are alternatives for the well-established phosphines in organometallic catalysis. In contrast to phosphines, they do not easily dissociate from transition metals (e.g., palladium, rhodium) so they seem suited for chiral modifications. This publication reports on the synthesis and coordination chemistry of novel bidentate ligands with both imidazoline-2-ylidene and oxazoline moieties. The synthesis follows straightforward routes, using N-functionalized imidazolium salts and 2-amino alcohols. The crystal structures of palladium and rhodium complexes are presented. With rhodium, the ligands show a chelating coordination mode.

The mechanism of the cross-coupling of phenylboronic acid with acetic anhydride, a viable model of the widely used Suzuki reaction, has been studied by DFT calculations at the BP86/6-31G level of theory. Two alternative catalytic cycles have been investigated, one starting from a neutral Pd(0)L(2) complex, the other from an anionic "Jutand-type" [Pd(0)L(2)X](-) species. The reaction profiles are in good agreement with the experimental findings, as both pathways require only moderate activation energies. Both pathways are dominated by cis-configured square-planar palladium(II)diphosphine intermediates. Despite careful investigations, we did not find in this model reaction any evidence for five-coordinate palladium(II) intermediates, which are commonly believed to cause the profound effects of counterions in palladium-catalyzed transformations. Instead, our calculations suggest that the higher catalytic activity of anionic complexes, such as [Pd(PMe(3))(2)OAc](-), may arise from their stronger ability to coordinate to carbon electrophiles. The transmetalation sequence is the same for both catalytic cycles, involving the dissociation of one phosphine ligand from the palladium. In the decisive transition state, in which the phenyl group is transferred from boron to palladium, the acetate base is found to be in a bridging coordination between these two atoms.

Density functional calculations on the title reaction are reported using the gradientcorrected BP86 functional with a standard basis (LANL2DZ) and a larger basis of triple-ζ quality (EXT). Several reaction pathways for oxidative addition of aryl halides to Pd(0) species have been explored, particularly for the reaction of phenyl iodide with Pd(PMe 3 ) 2 OAc -. We confirm that three-coordinate anionic Pd(0) species as proposed by Amatore and Jutand are stable intermediates and can serve as starting points for catalytic reactions. However, we did not find any evidence for the existence of the proposed five-coordinate Pd(II) complexes. Instead, stable four-coordinate intermediates were found, in which the aryl halides coordinate linearly to the palladium via the halide atom, with no significant energy barrier. With these adducts as a starting point, two energetically feasible pathways for the actual C-I cleavage reactions have been identified, which both lead to cis-configured Pd(II) complexes. The subsequent cis-trans isomerization requires significantly more activation than all preceding steps during the oxidative addition. The density functional calculations provide a plausible mechanism for the title reaction that is consistent with the available experimental facts. Scheme 2. Alternative Pathways for the Oxidative Addition Reaction Oxidative Addition of Aryl Halides to Pd(0) Complexes

The power of two metals: A Pd/Cu catalyst system mediates the in situ formation of acyl nucleophiles by decarboxylation of readily accessible and stable salts of α‐oxocarboxylic acids and their cross‐coupling with aryl or heteroaryl bromides to give ketones. The reaction may be used in the presence of many functional groups and provides good yields.

A silver-based catalyst system has been discovered that effectively promotes the protodecarboxylation of various carboxylic acids at temperatures of 80-120 degrees C--more than 50 degrees C below those of the best known copper catalysts.