Transition metal-catalyzed C–H bond functionalizations have been the focus of intensive research over the last decades for the formation of C–C bonds from unfunctionalized arenes, heteroarenes, alkenes These direct transformations provide new approaches in synthesis with high atom- and step-economy compared to the traditional catalytic cross-coupling reactions However, such methods still suffer from several limitations including functional group tolerance and the lack of regioselectivity In addition, they often require harsh reaction conditions and some of them need the use of strong oxidant, in a stoichiometric amount, avoiding these processes to be truly eco-friendly The use of photoredox catalysis has contributed to a significant expansion of the scope of C(sp2)–H bond functionalizations which include the direct arylations, (perfluoro)alkylations, acylations, and even cyanations Most of these transformations involve the photochemical induced generation of a radical followed by its regioselective a

Photoredox Catalysis for Building C-C Bonds from C(sp2)-H Bonds.

Reactions that involve the addition of carbon-centered radicals to basic heteroarenes, followed by formal hydrogen atom loss, have become widely known as Minisci-type reactions. First developed into a useful synthetic tool in the late 1960s by Minisci, this reaction type has been in constant use over the last half century by chemists seeking to functionalize heterocycles in a rapid and direct manner, avoiding the need for de novo heterocycle synthesis. Whilst the originally developed protocols for radical generation remain in active use today, they have been joined in recent years by a new array of radical generation strategies that allow use of a wider variety of radical precursors that often operate under milder and more benign conditions. The recent surge of interest in new transformations based on free radical reactivity has meant that numerous choices are now available to a synthetic chemist looking to utilize a Minisci-type reaction. Radical-generation methods based on photoredox catalysis and electrochemistry have joined approaches which utilize thermal cleavage or the in situ generation of reactive radical precursors. This review will cover the remarkably large body of literature that has appeared on this topic over the last decade in an attempt to provide guidance to the synthetic chemist, as well as a perspective on both the challenges that have been overcome and those that still remain. As well as the logical classification of advances based on the nature of the radical precursor, with which most advances have been concerned, recent advances in control of various selectivity aspects associated with Minisci-type reactions will also be discussed.

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Recent Advances in Minisci-Type Reactions.

N-Heterocyclic carbenes (NHCs) have become one of the most widely studied class of ligands in molecular chemistry and have found applications in fields as varied as catalysis, the stabilization of reactive molecular fragments, and biochemistry. More recently, NHCs have found applications in materials chemistry and have allowed for the functionalization of surfaces, polymers, nanoparticles, and discrete, well-defined clusters. In this review, we provide an in-depth look at recent advances in the use of NHCs for the development of functional materials.

N-Heterocyclic Carbenes in Materials Chemistry.

Decarboxylative reactions with and without light – a comparison

Herein, we report a conceptually novel borylation reaction proceeding via a mild photoinduced decarboxylation of redox-activated aromatic carboxylic acids This work constitutes the first application of cheap and easily prepared N-hydroxyphthalimide esters as aryl radical precursors and does not require the use of expensive transition metals or ligands The reaction is operationally simple, scalable, and displays broad scope and functional group tolerance

Transition-Metal-Free, Visible-Light-Enabled Decarboxylative Borylation of Aryl N-Hydroxyphthalimide Esters

Herein we present the first example of aryl radical formation via the visible light-mediated decarboxylation of aryl carboxylic acids using photoredox catalysis. This method constitutes a mild protocol for the decarboxylation of cheap and abundant aryl carboxylic acids and tolerates both electron-rich substrates and those lacking ortho-substitution. The in situ formation of an acyl hypobromite is proposed to prevent unproductive hydrogen atom abstraction and trapping of the intermediate aroyloxy radical, enabling mild decarboxylation.

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Mild, visible light-mediated decarboxylation of aryl carboxylic acids to access aryl radicals

Tuning the binding mode of N-heterocyclic carbenes on metal surfaces is crucial for the development of new functional materials. To understand the impact of alkyl side groups on the formation of NHC species at the Au(111) surface, we combined scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. We reveal two significantly different binding modes depending on the alkyl chain length. In the case of a short alkyl substituent, an up-standing configuration with one Au adatom is preferred, whereas the longer alkyl groups result exclusively in NHC-Au-NHC complexes lying flat on the surface. Our study highlights how well-defined structural modifications of NHCs allow for controlling the local binding motif on surfaces, which is important to design designated catalytic sites at interfaces.

Elucidating the Binding Modes of N-Heterocyclic Carbenes on a Gold Surface

Patterned monolayers of N-heterocyclic carbenes (NHCs) on gold surfaces were obtained by microcontact printing of NHC-CO2 adducts and NHC(H)[HCO3 ] salts. The NHC-modified areas showed an increased conductivity compared to unmodified gold surface areas. Furthermore, the remaining surface areas could be modified with a second, azide-functionalized carbene, facilitating further applications and post-printing modifications. Thorough elucidation by a variety of analytical methods offers comprehensive evidence for the viability of the methodology reported here. The protocol enables facile access to versatile, microstructured NHC-modified gold surfaces with highly stable patterns, enhanced conductivity, and the option for further modification.

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Versatile Micropatterns of N‐Heterocyclic Carbenes on Gold Surfaces: Increased Thermal and Pattern Stability with Enhanced Conductivity

Direct Dearomatization of Pyridines via an Energy-Transfer-Catalyzed Intramolecular [4+2] Cycloaddition

N-Heterocyclic carbenes (NHCs), which react with the surface of Au electrodes, have been successfully applied in pentacene transistors. With the application of NHCs, the charge-carrier mobility of pentacene transistors increased by five times, while the contact resistance at the pentacene-Au interface was reduced by 85 %. Even after annealing the NHC-Au electrodes at 200 °C for 2 h before pentacene deposition, the charge-carrier mobility of the pentacene transistors did not decrease. The distinguished performance makes NHCs as excellent alternatives to thiols as metal modifiers for the application in organic field-effect transistors (OFETs).

Matthias Freitag

Papers

Mild, visible light-mediated decarboxylation of aryl carboxylic acids to access aryl radicals

Elucidating the Binding Modes of N-Heterocyclic Carbenes on a Gold Surface

Versatile Micropatterns of N‐Heterocyclic Carbenes on Gold Surfaces: Increased Thermal and Pattern Stability with Enhanced Conductivity

Direct Dearomatization of Pyridines via an Energy-Transfer-Catalyzed Intramolecular [4+2] Cycloaddition

N-Heterocyclic-Carbene-Treated Gold Surfaces in Pentacene Organic Field-Effect Transistors: Improved Stability and Contact at the Interface.