Compounds bearing aryl-sulfur and aryl-phosphorus bonds have found numerous applications in drug development, organic materials, polymer science, and homogeneous catalysis. We describe palladium-catalyzed metathesis reactions of both compound classes, each of which proceeds through a reversible arylation manifold. The synthetic power and immediate utility of this approach are demonstrated in several applications that would be challenging to achieve by means of traditional cross-coupling methods. The C(sp)-S bond metathesis protocol was used in the depolymerization of a commercial thermoplastic polymer and in the late-stage derivatization of a drug. The C(sp)-P variant led to the convenient preparation of a variety of phosphorus heterocycles, including a potential chiral ligand and fluorescent organic materials, via a ring-closing transformation.

Transfer hydrogenation, alkene metathesis and alkyne metathesis possess great value to the synthetic chemistry community. One of the key features of these processes is their reversibility which can be attributed to the presence of the same number and type of functional groups in both the reactants and products, making these reactions isofunctional. These classic reactions have recently inspired the development of novel shuttle and metathesis reactions that offer great promise for synthetic chemistry. This review describes and systematically categorizes both recent and older examples of shuttle and metathesis reactions other than transfer hydrogenation and alkene/alkyne metathesis.

In this Perspective, we describe an emerging type of catalysis that enables the catalytic reversible transfer of chemical entities beyond the well-established transfer hydrogenation reactions. Shuttle catalysis facilitates the transfer of small molecules (e.g., CO, HCN) or reactive intermediates between two substrates in an isodesmic process. In many cases, these often safer processes provide unprecedented synthetic flexibility and complement other catalytic bond-forming and bond-breaking reactions

Shuttle catalysis has recently emerged as a powerful new concept that provides a platform for performing both functionalization and defunctionalization reactions. In this concept article, applications of shuttle catalysis as a novel strategy in organic synthesis are discussed. This includes using forward shuttle catalysis reactions for challenging bond-forming processes that avoid the use of hazardous chemicals. Shuttle catalysis also facilitates the transfer of reactive functionality as a route to procure a broad range of compounds using one simple procedure. Reverse shuttle catalysis reactions are also discussed as a method for the valorization of biomass and waste materials. Another area of interest, shuttle-catalysis-assisted reactions, wherein the transfer of a small molecule is utilized in a catalytic cycle, is also described. Possible future directions in this exciting new field are also suggested.

Scheme 2. Experiments to compare relative ease of C-CN oxidative addition/reductive elimination at benzylic and non-benzylic positions. Scheme 3. Development of novel HCN donors for regioselective transfer hydrocyanation. Following further optimization of the reaction conditions, the applicability of this HCN-free, branched-selective transfer hydrocyanation was examined (Scheme 4). A variety of functional groups were well tolerated in the reaction including both electron donating and electron withdrawing moieties. Besides the high branched selectivity of this new reaction, which is complementary to previous HCN transfer protocols, 8d,9a we were also interested to see if it exhibits greater functional group tolerance, given that the current reaction conditions do not require co-catalytic Lewis acid. Indeed, substrates possessing acidic protons (1r-1w) or a Boc-protected amine (1v) were well tolerated. Finally, vinylheteroarenes were also investigated and several common heterocyclic cores (1x-1z) were found to successfully undergo transfer hydrocyanation with high selectivity. Scheme 4. Substrate scope for the regioselective, HCN-free transfer hydrocyanation. a,b a Yields are given for isolated and purified material. Reactions were conducted on a 0.5 mmol scale. b Regioselectivity was determined by 1 H NMR analysis of the crude reaction mixture. c Aldehyde 1k was also isolated in 33% yield.

In this perspective article, we discuss catalytic isodesmic reactions, a group of chemical reactions that proceed through the redistribution of chemical bonds -i. e. all bonds present in the starting materials are reformed in the products. These reactions are usually reversible and provide a complementary approach to the kinetically controlled strategies traditionally employed in chemical synthesis.To emphasize the power of these reactions across the molecular sciences, we present selected applications of these reactions in organic synthesis, chemical biology, biomass valorization, waste treatment, and materials science. We finally speculate that the development of novel catalytic isodesmic reactions beyond the "classics" (alkene/alkyne metathesis and transfer hydrogenation) holds great promise to solve crucial challenges in synthetic chemistry in the years to come.Keywords: isodesmic · metathesis · shuttle · catalysis · reversible "It pays to speculate as widely and wildly as possible; people remember only when you are right"Scheme 11. Preparation of stapled peptides by ring closing metathesis. Scheme 12. Selective ring closing metathesis in the synthesis of cylindrocyclophanes. Scheme 15. Preparation of photonic crystals by ROMP. Reproduced with permission from Wiley-VCH from Ref. 40. Scheme 16. Assembly of shape-persistent macrocycles enabled by alkyne metathesis. Scheme 17. Synthesis of 2-catenanes by alkene metathesis.

A large number of reports describe the formation of the fundamental C-N bond in homogeneous catalysis. Among them, only a few are able to introduce the unprotected amino group, despite the appealing insertion of this key functional group. Recently, a broad range of methods have been reported that enable direct access to the primary amine using either ammonia or other nitrogen sources. In this short review, we illustrate the progress achieved in this field.

A flow chemistry procedure for Sonogashira and Heck cross‐coupling reactions using a low‐level palladium perovskite catalyst (LaFe0.95Pd0.05O3) deposited on cerium oxide is reported. The catalyst was generated in situ at high temperature using a flow platform. The system could be applied to a wide range of functionalised substrates, allowing clean and fast delivery of the products within a few minutes (10–30 min), after scavenging with thiourea polymer (QP‐TU) and sulfonic acid resin (QP‐SA). The use of an in‐line evaporator/solvent switching device allowed us to recover the solvent and transport the material into the following stage. The catalyst could be continuously used for at least 24 h, without any noticeable decrease in the catalytic efficiency. In one example the system was scaled to deliver 10 mmol of product.