Since the discovery of organic azides by Peter Griess more than 140 years ago, numerous syntheses of these energy-rich molecules have been developed. In more recent times in particular, completely new perspectives have been developed for their use in peptide chemistry, combinatorial chemistry, and heterocyclic synthesis. Organic azides have assumed an important position at the interface between chemistry, biology, medicine, and materials science. In this Review, the fundamental characteristics of azide chemistry and current developments are presented. The focus will be placed on cycloadditions (Huisgen reaction), aza ylide chemistry, and the synthesis of heterocycles. Further reactions such as the aza-Wittig reaction, the Sundberg rearrangement, the Staudinger ligation, the Boyer and Boyer-Aube rearrangements, the Curtius rearrangement, the Schmidt rearrangement, and the Hemetsberger rearrangement bear witness to the versatility of modern azide chemistry.

Organic Azides: An Exploding Diversity of a Unique Class of Compounds

Catalytic transformation of ubiquitous C–H bonds into valuable C–N bonds offers an efficient synthetic approach to construct N-functionalized molecules. Over the last few decades, transition metal catalysis has been repeatedly proven to be a powerful tool for the direct conversion of cheap hydrocarbons to synthetically versatile amino-containing compounds. This Review comprehensively highlights recent advances in intra- and intermolecular C–H amination reactions utilizing late transition metal-based catalysts. Initial discovery, mechanistic study, and additional applications were categorized on the basis of the mechanistic scaffolds and types of reactions. Reactivity and selectivity of novel systems are discussed in three sections, with each being defined by a proposed working mode.

Transition Metal-Catalyzed C–H Amination: Scope, Mechanism, and Applications

Catalytic C-H amination: recent progress and future directions.

This communication describes the Pd(OAc)2-catalyzed intermolecular amidation reactions of unactivated sp2 and sp3 C−H bonds using primary amides and potassium persulfate. The substrates containing a pendent oxime or pyridine group were amidated with excellent chemo- and regioselectivities. It is noteworthy that reactive C−X bonds were well-tolerated and a variety of primary amides can be effective nucleophiles for the Pd-catalyzed C−H amidation reactions. For the reaction of unactivated sp3 C−H bonds, β-amidation of 1° sp3 C−H bonds versus 2° C−H bonds is preferred. The catalytic reaction is initiated by chelation-assisted cyclopalladation involving C−H bond activation. Preliminary mechanistic study suggested that the persulfate oxidation of primary amides should generate reactive nitrene species, which then reacted with the cyclopalladated complex.

Intermolecular Amidation of Unactivated sp2 and sp3 C−H Bonds via Palladium-Catalyzed Cascade C−H Activation/Nitrene Insertion

Metalloporphyrins are a class of versatile catalysts with the capability to functionalize saturated C–H bonds via several well-defined atom/group transfer processes, including oxene, nitrene, and carbene C–H insertions. The corresponding hydroxylation, amination, and alkylation reactions provide direct approaches for the catalytic conversion of abundant hydrocarbons into value-added functional molecules through C–O, C–N, and C–C bond formations, respectively. This tutorial review describes metalloporphyrin-based catalytic systems for the functionalization of different types of sp3 C–H bonds, both inter- and intramolecularly, including challenging primary C–H bonds. Additional features of metalloporphyrin-catalyzed C–H functionalization include unusual selectivities and high turnover numbers.

Catalytic C–H functionalization by metalloporphyrins: recent developments and future directions

Co I I -porphyrin complexes catalyze the reaction of aromatic azides (ArN 3 ) with hydrocarbons that contain a benzylic group (ArR 1 R 2 CH) to give the corresponding amines (ArR 1 R 2 C-N-HAr). When at least one of the R substituents is hydrogen, the catalytic reaction proceeds further to give the imine ArRC=NAr in good yields. The reaction mechanism has been investigated. The reaction proceeds through a reversible coordination of the arylazide to the Co I I - porphyrin complex. This unstable adduct can either react with the hydrocarbon in the rate-determining step or decompose by a unimolecular mechanism to afford a putative "nitrene" complex, which reacts with more azide, but not with the hydrocarbon, to afford the byproduct diaryldiazene. The kinetics of the catalytic reaction have been investigated for a range of azides and substituted toluenes. Arylazides with electron-withdrawing substituents react at a faster rate and a good correlation is found between the log(k) and the Taft parameters. On the other hand, an excellent correlation between the logarithm of the rate for substituted toluenes relative to that of toluene and a radical parameter (σ . J J ) alone was found, with no significant contribution by polar parameters. An explanation has been proposed for this anomalous effect and for the very high isotopic effect (k H /k D = 14) found.

Enrica Mangioni

Papers

Amination of benzylic C-H bonds by arylazides catalyzed by CoII-porphyrin complexes: a synthetic and mechanistic study.