Although fire is now rarely used in synthetic chemistry, it was not until Robert Bunsen invented the burner in 1855 that the energy from this heat source could be applied to a reaction vessel in a focused manner. The Bunsen burner was later superseded by the isomantle, oil bath, or hot plate as a source for applying heat to a chemical reaction. In the past few years, heating and driving chemical reactions by microwave energy has been an increasingly popular theme in the scientific community. This nonclassical heating technique is slowly moving from a laboratory curiosity to an established technique that is heavily used in both academia and industry. The efficiency of "microwave flash heating" in dramatically reducing reaction times (from days and hours to minutes and seconds) is just one of the many advantages. This Review highlights recent applications of controlled microwave heating in modern organic synthesis, and discusses some of the underlying phenomena and issues involved.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.200400655

Controlled microwave heating in modern organic synthesis.

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

A wide array of forms of palladium has been utilized as precatalysts for Heck and Suzuki coupling reactions over the last 15 years. Historically, nearly every form of palladium used has been described as the active catalytic species. However, recent research has begun to shed light on the in situ transformations that many palladium precatalysts undergo during and before the catalytic reaction, and there are now many suggestions in the literature that narrow the scope of types of palladium that may be considered true “catalysts” in these coupling reactions. In this work, for each type of precatalyst, the recent literature is summarized and the type(s) of palladium that are proposed to be truly active are enumerated. All forms of palladium, including discrete soluble palladium complexes, solid-supported metal ligand complexes, supported palladium nano- and macroparticles, soluble palladium nanoparticles, soluble ligand-free palladium, and palladium-exchanged oxides are considered and reviewed here. A considerable focus is placed on solid precatalysts and on evidence for and against catalysis by solid surfaces vs. soluble species when starting with various precatalysts. The review closes with a critical overview of various control experiments or tests that have been used by authors to assess the homogeneity or heterogeneity of catalyst systems.

On the Nature of the Active Species in Palladium Catalyzed Mizoroki–Heck and Suzuki–Miyaura Couplings – Homogeneous or Heterogeneous Catalysis, A Critical Review

Large-scale applications of transition metal-catalyzed couplings for the synthesis of pharmaceuticals.

Nickel-Catalyzed Cross-Couplings Involving Carbon-Oxygen Bonds Brad M. Rosen, Kyle W. Quasdorf, Daniella A. Wilson, Na Zhang, Ana-Maria Resmerita, Neil K. Garg,* and Virgil Percec* Roy & Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, United States Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States

/pdf/nickel-catalyzed-cross-couplings-involving-carbon-oxygen-1zs88el3fl.pdf

Nickel-Catalyzed Cross-Couplings Involving Carbon-Oxygen Bonds

When complexed by selected ligands in either the BIPHEP or the SEGPHOS series, CuH is an extremely reactive catalyst capable of effecting asymmetric hydrosilylations of aromatic ketones at temperatures between −50 and −78 °C. Inexpensive silanes serve as stoichiometric sources of hydride. Substrate-to-ligand ratios exceeding 100,000:1 have been documented. The level of induction is usually in the >90% ee category. The nature of the reagent has been investigated using spectroscopic and chemical means, although its composition remains unclear.

Asymmetric Hydrosilylation of Aryl Ketones Catalyzed by Copper Hydride Complexed by Nonracemic Biphenyl Bis-phosphine Ligands

Results from aromatic aminations and Kumada couplings, together with spectroscopic analyses (TEM, EDX, ICP-AES, React-IR), reveal that catalysis using nickel-on-charcoal (Ni/C) is most likely of a homogeneous rather than heterogeneous nature. In the course of a reaction with Ni/C, nickel bleed from the support was calculated to be as high as 78%. However, the existence of an equilibrium for this homogeneous species between nickel located inside vs outside the pore system of charcoal strongly favors the former, thus leaving only traces of metal detectable in solution. This accounts for virtually complete recovery of nickel on the charcoal following filtration of a reaction mixture and allows for recycling of the catalyst. TEM and EDX data were used to explain different reactivity profiles of Ni/C, which depended upon the method of reduction used to convert Ni(II)/C to Ni(0) as well as the level of nickel loading on the support.

Will Chrisman

Papers

Asymmetric Hydrosilylation of Aryl Ketones Catalyzed by Copper Hydride Complexed by Nonracemic Biphenyl Bis-phosphine Ligands

On the nature of the 'heterogeneous' catalyst: nickel-on-charcoal.

Ligand-accelerated, copper-catalyzed asymmetric hydrosilylations of aryl ketones.

Copper Hydride-Catalyzed Tandem 1,4-Reduction/Alkylation Reactions

Hydrosilylation of aldehydes and ketones catalyzed by [Ph3P(CuH)]6