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Aryl-aryl bond formation one century after the discovery of the Ullmann reaction.

Interest in catalysis by metal nanoparticles (NPs) is increasing dramatically, as reflected by the large number of publications in the last five years. This field, "semi-heterogeneous catalysis", is at the frontier between homogeneous and heterogeneous catalysis, and progress has been made in the efficiency and selectivity of reactions and recovery and recyclability of the catalytic materials. Usually NP catalysts are prepared from a metal salt, a reducing agent, and a stabilizer and are supported on an oxide, charcoal, or a zeolite. Besides the polymers and oxides that used to be employed as standard, innovative stabilizers, media, and supports have appeared, such as dendrimers, specific ligands, ionic liquids, surfactants, membranes, carbon nanotubes, and a variety of oxides. Ligand-free procedures have provided remarkable results with extremely low metal loading. The Review presents the recent developments and the use of NP catalysis in organic synthesis, for example, in hydrogenation and C--C coupling reactions, and the heterogeneous oxidation of CO on gold NPs.

Nanoparticles as Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis

Ruthenium(II)-Catalyzed C–H Bond Activation and Functionalization

A number of studies were conducted to demonstrate C-H activation for the construction of C-B bonds. Investigations revealed that the conversion of C-H bonds to C-B bonds was both thermodynamically and kinetically favorable. The reaction at a primary C-H bond of methane or a higher alkene B 2(OR)4 formed an alkylboronate ester R' -B(OR)2 and the accompanying borane H-B(OR2. The ester and the borane were formed on the basis of calculated bond energies for methylboronates and dioaborolanes. The rates of key steps along the reaction pathway for the conversion of a C-H bond in an alkane or arene to the C-B bond in an alkyl or arylboronate ester were favorable. These studies also highlighted the accessible barriers for C-H bond cleavage and B-C bond formation during the borylation of alkanes and arenes.

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C−H Activation for the Construction of C−B Bonds

The ability of platinum and gold catalysts to effect powerful atom-economic transformations has led to a marked increase in their utilization. The quite remarkable correlation of their catalytic behavior with the available structural data, coordination chemistry, and organometallic reactivity patterns, including relativistic effects, allows the underlying principles of catalytic carbophilic activation by π acids to be formulated. The spectrum of reactivity extends beyond their utility as catalytic and benign alternatives to conventional stoichiometric π acids. The resulting reactivity profile allows this entire field of catalysis to be rationalized, and brings together the apparently disparate electrophilic metal carbene and nonclassical carbocation explanations. The advances in coupling, cycloisomerization, and structural reorganization—from the design of new transformations to the improvement to known reactions—are highlighted in this Review. The application of platinum- and gold-catalyzed transformations in natural product synthesis is also discussed.

Catalytic Carbophilic Activation: Catalysis by Platinum and Gold π Acids

A novel combination of Pd(OAc)2/pyridine/MS3A catalyzes the aerobic oxidation in toluene of a variety of primary and secondary alcohols into the corresponding aldehydes and ketones in high yields. Various substituents and protecting groups are compatible with this oxidation. The ca. 2:3 ratio of O2 uptake to product yield is observed, whereas in the absence of MS3A, the ratio is ca. 1:1, suggesting the in situ formation of H2O2 and its decomposition by MS3A into water and oxygen. A catalytic cycle including the formation of a Pd(II)-alcoholate followed by β-elimination of a Pd(II)H species and a carbonyl compound and then the formation of a Pd(II)OOH species is proposed.

Palladium(II)-Catalyzed Oxidation of Alcohols to Aldehydes and Ketones by Molecular Oxygen.

A heterogenized Pd catalyst, Pd(II)−hydrotalcite (palladium(II) acetate−pyridine complex supported by hydrotalcite) catalyzes the aerobic oxidation in toluene of a variety of primary and secondary alcohols into the corresponding aldehydes and ketones in high yields using atmospheric pressure of air as a sole oxidant under mild conditions. This catalyst is also effective for the oxidation of allylic alcohols, especially such as geraniol and nerol, without any isomerization of an alkenic part. The catalyst can be easily prepared from all commercially available reagents and reused several times.

Pd(II)−Hydrotalcite-Catalyzed Oxidation of Alcohols to Aldehydes and Ketones Using Atmospheric Pressure of Air

A novel enantioselective C−C bond cleavage has been achieved using palladium catalysts and chiral N,P-bidentate ligands in the asymmetric arylation, vinylation, and allenylation of tert-cyclobutanols. In these reactions, the enantioselective β-carbon elimination of Pd(II) alcoholate formed in situ is the key step. Treatment of tert-cyclobutanols with arylating reagents in toluene in the presence of Pd(OAc)2, a chiral ferrocene-containing N,P-bidentate ligand, and Cs2CO3 affords optically active γ-arylated ketones in excellent yields with high enantioselectivity (up to 95% ee). When vinylating reagents are used in place of arylating ones, the asymmetric vinylation also proceeds to afford optically active γ-vinylated ketones in high yields with good to high enantioselectivity. When propargylic acetates are used, which are known to generate (σ-allenyl)palladium complexes with Pd(0) species, asymmetric allenylation occurs to afford optically active γ-allenylated ketones in moderate to good yields with moderat...

Palladium-catalyzed asymmetric arylation, vinylation, and allenylation of tert-cyclobutanols via enantioselective C-C bond cleavage.

Pd(OAc)2-catalyzed oxidation of alcohols to aldehydes and ketones by molecular oxygen

Novel Ruthenium‐ and Platinum‐Catalyzed Sequential Reactions: Synthesis of Tri‐ and Tetrasubstituted Furans and Pyrroles from Propargylic Alcohols and Ketones

Sakae Uemura

Papers

Palladium(II)-Catalyzed Oxidation of Alcohols to Aldehydes and Ketones by Molecular Oxygen.

Pd(II)−Hydrotalcite-Catalyzed Oxidation of Alcohols to Aldehydes and Ketones Using Atmospheric Pressure of Air

Palladium-catalyzed asymmetric arylation, vinylation, and allenylation of tert-cyclobutanols via enantioselective C-C bond cleavage.

Pd(OAc)2-catalyzed oxidation of alcohols to aldehydes and ketones by molecular oxygen

Novel Ruthenium‐ and Platinum‐Catalyzed Sequential Reactions: Synthesis of Tri‐ and Tetrasubstituted Furans and Pyrroles from Propargylic Alcohols and Ketones