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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]

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 S N 2- to S N 1-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 Sb V catalyst is used to demonstrate that this unorthodox polyether cascade cyclization mechanism occurs., (© 2020 Wiley-VCH GmbH.)

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 k auto /k cat >10 4  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., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)