Cross-couplings and related reactions are a class of highly efficient synthetic protocols that are generally promoted by molecular Pd species as catalysts. However, catalysts based on more or less highly dispersed Pd metal have been also employed for this purpose, and their use, which was largely limited to the Heck reaction until the turn of the century, has been extended in recent years to most reactions of this class. This review provides a critical overview on these recent applications of Pd metal catalysts. Particular attention is devoted to the discussion of the mechanistic pathways that have been proposed to explain the catalytic role of Pd metal. Furthermore, the most outstanding Pd metal based catalytic systems that have emerged are illustrated, together with the development of novel approaches to boost the reactivity of Pd metal. A section summarizing the current industrial applications of Pd metal catalyzed reactions of this kind concludes the review.

NHC-Au-X (NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, X- = BArF-, BF4-, SbF6-, OTf-, NTf2-, ClO4-, OTs-, TFA(-)) catalysts were tested in the hydration of allcynes. A complete rationalization of the counterion effect enabled us to develop a highly efficient methodology under solvent-, silver-, and acid-free conditions. Thus, it was possible to use room or mild (60 degrees C) temperature and to reduce the catalyst loading up to 0.01 mol % with respect to the substrate, leading to high TON (10(4)) and TOF (10(3) h(-1)) values. The favorable catalytic conditions allowed us to reach very low E factor (0.03-0.06) and high EMY (94-97) values. Finally, the absence of solvent permits easy separation of the liquid product from solid catalyst and ionic additives by distillation, giving products with high purity that are uncontaminated by metals. This opens the way to catalyst recycling (up to four times) without loss of activity. The overall catalytic and kinetic evidence, supported by computational results, confirms that the anion plays a crucial role in all steps of the reaction mechanism: pre-equilibrium, nucleophilic attack, and protodeauration. As a matter of fact, only the two complexes bearing OTf- and NTf2- counterions showed catalytic activity; all others are completely inactive. Protodeauration is the rate-determining step under these aprotic and apolar conditions, and in our calculations, the first anion-mediated proton transfer takes place easily in one step, leading to a gold enol complex. Different pathways have been computationally explored for the conversion of gold enol to ketone product by modeling different experimental conditions

Terdentate ruthenium(II) complexes of general formula RuX(CNN)(dppb) (X ) chloride, hydride, alkoxide; dppb ) Ph 2 P(CH 2 ) 4 PPh 2 ), where CNN is a deprotonated 2-aminomethyl-6-arylpyridine ligand, have been prepared. The orthometalated derivative RuCl(b)(dppb) (1) has been obtained by reaction of RuCl 2 (PPh 3 )(dppb) with N, N-dimethyl-2-aminomethyl-6-(4-methylphenyl)pyridine (Hb) in 2-propanol and in the presence of triethylamine by elimination of PPh 3 and HCl. Similarly, RuCl(a)(dppb) (2) and the chiral analogue RuCl(c)(dppb) (3), containing primary amine ligands, have been isolated starting from 2-aminomethyl-6-(4-methylphenyl)pyridine (Ha) and (R)-2,2-dimethyl-1-(6-phenylpyridin-2-yl)propylamine (Hc), respectively. The synthesis of the functionalized pyridines Ha-Hc is here described, whereas the crystal structure of 3 has been determined through an X-ray diffraction experiment. Treatment of 1-3 with sodium or potassium isopropoxide gives the corresponding hydrides RuH(b)(dppb) (4), RuH(a)(dppb) (5), and RuH(c)(dppb) (6) from the ruthenium isopropoxide complexes, via a β-H elimination process. Studies in solution show that the isopropoxides bearing a NH donor group are in equilibrium with the corresponding hydrides (5 and 6). Reaction of 5 with benzophenone leads to the alkoxide Ru(OCHPh 2 )(a)(dppb) (7), which has been proven to interact with benzhydrol in C 6 D 6 , leading to the adduct 7‚(HOCHPh 2 ), the alkoxide ligand, and the alcohol being in rapid exchange. Complexes 2 and 3 display a remarkable high catalytic activity for the transfer hydrogenation of ketones to alcohol in 2-propanol using a very small amount of catalyst. With the chiral complex 3 (0.005 mol %) methyl-aryl ketones can be quickly reduced (TOF ranging from 5.4 × 10 5 to 1.4 × 10 6 h -1 ) with an enatiomeric excess up to 88%.

Herein, we synthesized and characterized through NMR and X-ray techniques a new set of [(NHC)-Au-X] complexes (NHC = 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene), differing in the counterion X (X = OMs , NO3 , ClO4 , 2,2,3,3,4,4,5,5,6,6,7,7,7-tridecafluoroheptanoate (PFHp )). All of these complexes, together with those already known having NTf2 and phthalimide (ptm ) as counterions, were tested as catalysts in the methoxylation of 3-hexyne. The results were analyzed together with those obtained previously. The values of activation parameters (Delta H-? and Delta S-?) for different anions are also reported. The overall catalytic and kinetic evidence, together with an extensive computational work, confirm the general mechanistic picture given recently in which the anion plays an active role in all steps of the reaction mechanism: pre-equilibrium, nucleophilic attack, and protodeauration. Medium-coordinating anions (OMs , OTs ) containing a highly symmetric anchoring group give the best catalytic performances. This is due to the following reasons: (a) the pre-equilibrium is shifted toward the outer sphere ion pair, (b) their characteristic basicity promotes the nucleophilic attack, and (c) the possible paths leading to the deactivation of the catalyst are inhibited. These highly symmetric tridentate anions destabilize the unreactive tricoordinated gold species, which instead may be formed by anions with a planar anchoring group, such as PFHp and TFA . A general trend between coordinating ability and catalytic performances in the alkoxylation of alkynes may be established only when the geometric features of the anion are taken into account. The role of the anion has been also investigated in connection with the nature of the nucleophile. In particular, when the alcohol is a poor nucleophile, a large difference in reactivity is observed, while the use of suitably functionalized alcohols, which may contribute to polarizing the -OH bond through intramolecular interactions, flattens the anion effect

Complexes of the general formula [MXH 2 (dppp) 2 ] n+ (M ) Ru, Os; X ) H, Cl, CO; dppp ) 1,3-bis-(diphenylphosphino)propane) have been prepared and characterized, and the effect of the donor/acceptor properties of X on their structure and acidity has been studied. The five-coordinate complexes [MCl(dppp) 2 ] + (M ) Ru (1a), Os (1b)) react with H 2 gas in CH 2 Cl 2 to give the complexes [MCl(η 2 -H 2 )(dppp) 2 ] + (M ) Ru (2a), Os (2b)) containing elongated dihydrogen ligands. The molecular structure of 2b has been determined by X-ray crystallography (monoclinic, space group P2 1 /n with a ) 13.314 (7) Å, b ) 18.63(2) Å, c ) 23.20(2) Å, ) 94.58(6)°, and Z ) 4). Chlorohydride [OsH(Cl)(dppp) 2 ] (3b) reacts with H 2 gas in the presence of Na[BPh 4 ] forming [OsH 3 (dppp) 2 ] + (4b). Protonation of [OsH 2 (dppp) 2 ] (5b) with HBF 4 ‚Et 2 O also gives 4b. A combination of X-ray crystallography (monoclinic, space group P2 1 /n with a ) 13.392(3) Å, b ) 25.306(7) Å, c ) 21.247(7) Å, ) 91.15(2)°, and Z ) 4) and 1 H and 31 P NMR studies indicate that 4b is a classical trihydride. Hydridocarbonyls [MH(CO)(dppp) 2 ] + (M ) Ru (6a), Os (6b)) are protonated by F 3 CSO 3 H in CD 2 Cl 2 to yield [M(CO)(η 2 -H 2 )(dppp) 2 ] 2+ (M ) Ru (7a), Os (7b)), which were characterized in solution. 7a is stable only at low temperature. Compound 7b is a highly acidic dihydrogen complex with an estimated pK a of -6.