For more than 70 years, nitrogen-centered radicals have been recognized as potent synthetic intermediates. This review is a survey designed for use by chemists engaged in target-oriented synthesis. This review summarizes the recent paradigm shift in access to and application of N-centered radicals enabled by visible-light photocatalysis. This shift broadens and streamlines approaches to many small molecules because visible-light photocatalysis conditions are mild. Explicit attention is paid to innovative advances in N−X bonds as radical precursors, where X = Cl, N, S, O, and H. For clarity, key mechanistic data is noted, where available. Synthetic applications and limitations are summarized to illuminate the tremendous utility of photocatalytically generated nitrogen-centered radicals. CONTENTS 1. Introduction 2354 2. Nitrogen-centered Radicals Can Be Generated from Nitrogen−Chlorine Bonds in the Presence of Photoredox Catalysts 2357 3. Nitrogen-centered Radicals Can Be Generated from Nitrogen−Nitrogen Bonds within N-Aminopyridinium Salts in the Presence of Photoredox Catalysts 2359 4. Nitrogen-centered Radicals May Be Generated from Nitrogen−Sulfur Bonds 2363 5. Nitrogen-centered Radicals Can Be Generated from Nitrogen−Oxygen Bonds 2364 5.1. Iminyl Radicals Can Be Generated from O-Aryl and O-Acyl Oxime Analogues 2364 5.1.1.Radicals Prepared from O-Aryl or O-Acyl Oximes Can Add Across Olefins 2364 5.1.2. Radicals Prepared from O-Acyl Oximes Can Cyclize onto Arenes or Vinyl Arenes 2366 5.1.3. Radicals Prepared from O-Acyl Oximes Can Abstract Hydrogen Atoms 2367 5.2. Hydroxyacid-derived Oximes Are Appropriate Precursors to Iminyl Radicals 2368 5.2.1. These Iminyl Radicals Can Serve as Intermediates in Iminofunctionalization Reactions 2368 5.2.2. Hydroxyacid-derived Iminyl Radicals Can React with Chalcogens to Install New Chalcogen−Nitrogen Bonds 2370 5.3. Ketoximes and Aldoximes Can Serve as Precursors to Persistent Iminyl Radical Intermediates 2371 5.4. Iminyl Radicals That Are Generated from Strained Cyclic Oximes Can Engage in Ringopening Cascade Reactions 2374 5.4.1. These Ring-opening Cascades Are Wellestablished Strategies to Affect Distal Carbofunctionalization Reactions Involving Olefins 2374 5.4.2. By Using Hydroxyacid-derived Oxime Substrates, Ring-Opening Cascades Can Result in Direct Atom-or Group-transfer Processes 2375 5.4.3. Ring-opening Cascades Can Trigger Alkene Carbofunctionalization Reactions 2376 5.4.4. Ring-opening Cascades Can Be Intercepted by Transition-metal Mediated Bond-forming Processes 2378 5.4.5. Ring-opening Cascades Can Rely on O-Acyl Cyclic Oxime Substrates to Affect Alkene Carboetherification Reactions 2379
In this review, we discuss quantum tunneling in the temperature range normally used to carry out reactions, ca.-78 °C and higher. Both heavy-and hydrogen-atom tunneling are included. Experimental data and computational results are discussed. Recent studies suggest that tunneling is widespread at typical reaction temperatures.
A nitrosamine photooxidation reaction is shown to generate a peroxy intermediate by experimental physical-organic methods. The irradiation of phenyl and methyl-substituted nitrosamines in the presence of isotopically labeled 18-oxygen revealed that an O atom was trapped from a peroxy intermediate to trimethylphosphite or triphenylphosphine, or by nitrosamine itself, forming two moles of nitramine. The unstable peroxy intermediate can be trapped at low temperature in postphotolyzed solution in the dark. Chemiluminescence was also observed upon thermal decomposition of the peroxy intermediate, that is, when a postphotolysis low-temperature solution is brought up to room temperature. A DFT study provides tentative information for cyclic nitrogen peroxide species on the reaction surface.
Site-selective functionalization of unactivated C(sp 3 )−H centers is challenging because of the ubiquity and strength of alkyl C−H bonds. Herein, we disclose a position-selective C(sp 3 )−C(sp 2 ) cross-coupling reaction. This process engages C(sp 3 )− H bonds and aryl bromides, utilizing catalytic quantities of a photoredox-capable molecule and a nickel precatalyst. Using this technology, selective C−H functionalization arises owing to a 1,6-hydrogen atom transfer (HAT) process that is guided by a pendant alcohol-anchored sulfamate ester. These transformations proceed directly from N−H bonds, in contrast to previous directed, radical-mediated, C−H arylation processes, which have relied on prior oxidation of the reactive nitrogen center in reactions with nucleophilic arenes. Moreover, these conditions promote arylation at secondary centers in good yields with excellent selectivity.
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