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Pnictogen‐bonding catalysis and supramolecular σ‐hole catalysis in general is currently being introduced as the non‐covalent counterpart of covalent Lewis acid catalysis. With access to anti‐Baldwin cyclizations identified as unique characteristic, pnictogen‐bonding catalysis appeared promising to elucidate one of the hidden enigmas of brevetoxin‐type epoxide opening polyether cascade cyclizations, that is the cyclization of certain trans epoxides into cis‐fused rings. In principle, a shift from SN2‐ to SN1‐type mechanisms could suffice to rationalize this inversion of configuration. However, the same inversion could be explained by a completely different mechanism: Ring opening with C−C bond cleavage into a branched hydroxy‐5‐enal and the corresponding cyclic hemiacetal, followed by cascade cyclization under conformational control, including stereoselective C−C bond formation. In this report, a pnictogen‐bonding supramolecular SbV catalyst is used to demonstrate that this unorthodox polyether cascade cyclization mechanism occurs. [ABSTRACT FROM AUTHOR]

Anion‐π catalysis functions by stabilizing anionic transition states on aromatic π surfaces, thus providing a new approach to molecular transformation. The delocalized nature of anion–π interactions suggests that they serve best in stabilizing long‐distance charge displacements. Aiming therefore for an anionic cascade reaction that is as charismatic as the steroid cyclization is for conventional cation‐π biocatalysis, reported here is the anion‐π‐catalyzed epoxide‐opening ether cyclizations of oligomers. Only on π‐acidic aromatic surfaces having a positive quadrupole moment, such as hexafluorobenzene to naphthalenediimides, do these polyether cascade cyclizations proceed with exceptionally high autocatalysis (rate enhancements kauto/kcat >104 m−1). This distinctive characteristic adds complexity to reaction mechanisms (Goldilocks‐type substrate concentration dependence, entropy‐centered substrate destabilization) and opens intriguing perspectives for future developments. [ABSTRACT FROM AUTHOR]

The concept of anion–π catalysis focuses on the stabilization of anionic transition states on aromatic π surfaces. Recently, we demonstrated the occurrence of epoxide‐opening ether cyclizations on aromatic π surfaces. Although the reaction proceeded through unconventional mechanisms, the obtained products are the same as those from conventional Brønsted acid catalysis, and in agreement with the Baldwin selectivity rules. Different mechanisms, however, should ultimately lead to new products, a promise anion–π catalysis has been reluctant to live up to. Herein, we report non‐trivial reactions that work with anion–π catalysis, but not with Brønsted acids, under comparable conditions. Namely, we show that the anion–π templated autocatalysis and epoxide opening with alcoholate–π interactions can provide access to unconventional ring chemistry. For smaller rings, anion–π catalysis affords anti‐Baldwin oxolanes, 2‐oxabicyclo[3.3.0]octanes, and the expansion of Baldwin oxetanes by methyl migration. For larger rings, anion–π templated autocatalysis is thought to alleviate the entropic penalty of folding to enable disfavored anti‐Baldwin cyclizations into oxepanes and oxocanes. [ABSTRACT FROM AUTHOR]

The metal-free reaction between diazo compounds and boronic acids has been established in recent years as a powerful C(sp3)–C bond-forming reaction. This account covers the recent advances in this area. First, the various synthetic applications of reactions with N-sulfonylhydrazones as a convenient source of diazo compounds is discussed. These transformations can be regarded as reductive couplings of carbonyl compounds. Also covered is the incorporation of other mild sources of diazo compounds in these reactions: diazotization of amines and oxidation of hydrazones. Finally, the development of sequential and cascade processes is presented. 1 Introduction 2 Early Work: Reactions Between Alkylboranes and Diazo Compounds or N-Sulfonylhydrazones 2.1 Reactions Between Alkylboranes and Diazo Compounds 2.2 Reactions Between Alkylboranes and N-Sulfonylhydrazones 3 Reactions of N-Sulfonylhydrazones and Diazo Compounds with Aryl and Alkylboronic Acids 3.1 Reactions of Arylboroxines with Diazo Compounds 3.2 Reductive Couplings of N-Sulfonylhydrazones with Aryl- and Alkylboronic Acids 3.3 Three-Component Reactions Between α-Halotosylhydrazones, Boronic Acids and Indoles 4 Reactions of N-Tosylhydrazones with Alkenylboronic Acids 5 Synthesis of Allenes by Reactions with Alkynyl N-Nosylhydrazones 6 Reactions with Diazo Compounds Generated by Diazotization of Primary Amines 7 Reactions with Diazo Compounds Generated by Oxidation of Hydrazones 8 Reactions with Trimethylsilyldiazomethane 9 Cascade Cyclization Reactions with γ- and δ-Cyano-N-tosylhydrazones 10 Summary and Outlook [ABSTRACT FROM AUTHOR]

A new method for the synthesis of polysubstituted conjugated dienes is described, through the palladium-catalyzed cross-coupling between N-tosylhydrazones and alkenyl bromides. The reaction proceeds efficiently when a combination of a highly substituted bromoalkene and a hydrazone derived from a ketone are employed, pointing to the convenience of a sterically encumbered environment. This unprecedented process allows for the stereocontrolled preparation of highly substituted dienes and polyenes. [ABSTRACT FROM AUTHOR]

Two different Pd-catalyzed cascade reactions between o-iodo-N-alkenylanilines and tosylhydrazones are described. The outcome of the cascade processes is determined by the substitution on the N-alkenyl fragment. The reactions with N-tosyl-N-ethylene-o-iodoanilines lead to indoles through a sequence that involves the sequential migratory insertions of a carbene ligand and a C–C double bond, featuring a 5-exo-trig cyclization. The reactions with N-alkyl-N-alkenyl-o-iodoanilines provide 1,4-dihydroquinolines through a cascade reaction that includes a formal 6-endo-trig cyclization. In both cases the benzofused heterocycles are built through the formation of two C–C bonds on the hydrazonic carbon atom. [ABSTRACT FROM AUTHOR]

The Pd-catalyzed reaction between o-iodoallylbenzene and tosylhydrazones gives rise to indene derivatives through a process that involves a carbene migratory insertion followed by an intramolecular carbopalladation, with the formation of two C–C bonds on the same carbon atom. The same strategy has also been applied for the synthesis of 2,3-disubstituted benzofurans and indenones by selecting the appropriate o-substituted iodoarene. [ABSTRACT FROM AUTHOR]

Frontispiece: Pnictogen-Bonding Catalysis: An Interactive Tool to Uncover Unorthodox Mechanisms in Polyether Cascade Cyclizations Keywords: cascade reactions; epoxide-opening polyether cyclizations; pnictogen-bonding catalysis; polyether natural products; -hole catalysis EN cascade reactions epoxide-opening polyether cyclizations pnictogen-bonding catalysis polyether natural products -hole catalysis 1 1 1 12/03/20 20201201 NES 201201 B Epoxide-opening polyether cascade cyclizations b fascinate since the Nakanishi hypothesis was put forward to explain the biosynthesis of brevetoxin B. By now, violations of the Baldwin rules are as common as speeding on the highway, McDonald elegantly mastered the inversion of directionality, but how can I trans i epoxides cyclize into I cis-fused i rings? Cascade reactions, epoxide-opening polyether cyclizations, pnictogen-bonding catalysis, polyether natural products, -hole catalysis. [Extracted from the article]

An abstract on ring closure reactions from the synthesis of palladium-catalyzed polysubstituted indoles and dihydroquinolines is presented.

An abstract of the article "The Pd-Catalyzed Synthesis of Benzofused Carbo-and Heterocycles Through Carbene Migratory Insertion/Carbopalladation Cascades with Tosylhydrazones"by Joe Smith is presented.