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

Transition-Metal-Catalyzed C−S, C−Se, and C−Te Bond Formation via Cross-Coupling and Atom-Economic Addition Reactions

Visible-light photocatalysis has evolved over the last decade into a widely used method in organic synthesis. Photocatalytic variants have been reported for many important transformations, such as cross-coupling reactions, α-amino functionalizations, cycloadditions, ATRA reactions, or fluorinations. To help chemists select photocatalytic methods for their synthesis, we compare in this Review classical and photocatalytic procedures for selected classes of reactions and highlight their advantages and limitations. In many cases, the photocatalytic reactions proceed under milder reaction conditions, typically at room temperature, and stoichiometric reagents are replaced by simple oxidants or reductants, such as air, oxygen, or amines. Does visible-light photocatalysis make a difference in organic synthesis? The prospect of shuttling electrons back and forth to substrates and intermediates or to selectively transfer energy through a visible-light-absorbing photocatalyst holds the promise to improve current procedures in radical chemistry and to open up new avenues by accessing reactive species hitherto unknown, especially by merging photocatalysis with organo- or metal catalysis.

Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis?

The palladium-catalyzed coupling of amines with aryl halides or aryl alcohol derivatives, commonly dubbed Buchwald–Hartwig amination, has matured from a synthetic laboratory procedure to a technique that is widely used in natural product synthesis as well as in other fields of academic interest. Furthermore, due to the versatility and reliability of this reaction, researchers in industrial environments have included this methodology in their toolbox as a standard procedure for the synthesis of amine derivatives. Therefore, it is not surprising that first industrial processes up to ton-scale have been performed using this cross-coupling reaction. The authors who are involved in the application of this reaction to industrial processes on this scale give an overview of the recent developments in this field of chemistry, also including fundamental principles, with a special focus on the industrial approach and issues to be considered relevant in scaling-up this transition metal-catalyzed chemistry. This review differs from the already existing excellent academic reviews by focusing on the practical problems arising during implementing the methodology in an industrial environment as well as giving practical hints to this end.

Gang Zheng

Papers

Highly active, air-stable versatile palladium catalysts for the C-C, C-N, and C-S bond formations via cross-coupling reactions of aryl chlorides.

Highly Active, Air-Stable Versatile Palladium Catalysts for the C—C, C—N, and C—S Bond Formations via Cross-Coupling Reactions of Aryl Chlorides.