Iron/Photosensitizer-Catalyzed Directed C–H Activation Triggered by the Formation of an Iron Metallacycle

Abstract

C−H functionalization reactions catalyzed by iron, the most abundant transition metallic element in the Earth's crust, are one of the ideal synthetic methods. However, there are a limited number of strategies for iron-catalyzed directed C−H activation reactions when compared to reactions catalyzed by other first-row transition metals. Here, we report the iron/photosensitizer-catalyzed C−H alkenylation of amide derivatives via a σ-CAM (σ-complex-assisted metathesis) promoted by the in situ generation of an iron metallacycle. Mechanistic experiments suggest that the C−H bond cleavage would proceed via the σ-CAM from a metallacycle generated in situ rather than via the oxidative addition to a low-valent iron species.

Rhodium(I)‐Catalyzed Asymmetric Hydroarylative Cyclization of 1,6‐Diynes to Access Atropisomerically Labile Chiral Dienes

“…The mechamism of this system has been ambiguous in previous studies because two distinct pathways have been proposed for the same hydroaylation of 1,6‐diynes with acetophenone. Tanaka and Matsunaga proposed a pathway of oxidative cyclization of diynes, [21a,c] while Shibata indicated that arene C−H oxidative addition may be operative [21b] . The same ambiguity also exists in C−H activation‐hydroarylation reactions of 1,6‐enynes [28] .…”

Modern Organometallic C−H Functionalizations with Earth‐Abundant Iron Catalysts: An Update

“…Very recently, Matsunaga and co-workers reported the first example of the iron-catalyzed CÀ H alkenylation of benzamides with 1,6-diynes using an iron/photosensitizer hybrid catalyst (Scheme 31). [79] The catalytic system was developed after their recent discovery on an iron/photosensitizer catalyzed [2 + 2 + 2] cycloaddition reaction via an iron metallacycle intermediate. [80] The reaction requires an interplay of several reagents, including FeI 2 , dppp [1,3-bis(diphenylphosphino)-propane] ligand, 4CzPN as the organic photosensitizer, triethylamine, and AgPF 6 under irradiation from blue light at room temperature.…”

Cobalt/Organophotoredox Dual-Catalysis-Enabled Cyclization of 1,5,10-Enediynes Involving Metallole-Mediated Remote C(sp3)–H Bond Activation Leading to Axially Chiral Aryl Alkenes

“…Metallacycles, such as metallacyclopentadienes (metalloles) and metallacyclopentenes, are key intermediates in the transition-metal-catalyzed cycloaddition of alkynes and alkenes, which enables quick access to various carbo- and heterocyclic frameworks from simple unsaturated substrates. Although these metallacycles have been exploited for reactions with π-components in most cases, they can also be involved in C–H bond activation. − The cycloaddition through metallacycle-mediated site-selective C–H bond activation (CMCA) can concisely assemble complex polycyclic molecules, which are difficult to access by the conventional cycloaddition reactions based on only π-bond activation. For example, in 1994, Vollhardt et al reported that the CpCo I -mediated CMCA of bis­(trimethylsilyl)­acetylene and an alkyne bearing a furan moiety afforded a fused furan product (Scheme A) .…”