Visible Light Induced Rhodium(I)‐Catalyzed C−H Borylation

An efficient visible light induced rhodium(I)‐catalyzed regioselective borylation of aromatic C?H bonds is reported. The photocatalytic system is based on a single NHC?Rh^I complex capable of both harvesting visible light and enabling the bond breaking/forming at room temperature. The chelating nature of the NHC‐carboxylate ligand was critical to ensure the stability of the Rh^I complex and to provide excellent photocatalytic activities. Experimental mechanistic studies evidenced a photooxidative ortho C?H bond addition upon irradiation with blue LEDs, leading to a cyclometalated Rh^III‐hydride intermediate.     

meta‐Selective C−H Activation of Arenes at Room Temperature Using Visible Light: Dual‐Function Ruthenium Catalysis

Ruthenium‐catalyzed meta‐C?H activation of arenes at room temperature is reported to proceed under blue‐light irradiation. A variety of heteroarenes are compatible with this photochemical process, which leads to the corresponding meta C?C coupling products in good to very good yields. Initial mechanistic studies suggest a single‐electron transfer process occurs between a photoexcited Ru^II‐cyclometalated complex and alkyl halides, enabling meta‐C?H functionalization reaction via carbon‐centered radicals.     

Radical‐Promoted C−C Bond Cleavage: A Deconstructive Approach for Selective Functionalization

Just as “Deconstructivism” appeared as a novel movement in architecture in the 1980s, deconstructive approaches have recently emerged as excellent strategies for scaffold hopping modifications in chemistry. The deconstruction and functionalization of cyclic molecules mainly involves the cleavage of the carbon–carbon (C?C) bond followed by the construction of new bonds. The cleavage of inert C?C single bonds, especially in unstrained cycles, and their subsequent functionalization is still one of the most sought‐after challenges in chemistry. In this vein, radical‐mediated strategies provide an excellent approach for achieving this aim. This minireview is an outline of the history of homolytic cleavage and highlights the recent advances in exploring new chemical space by deconstructive functionalization.     

Metal Insertion into C−C Bonds in Solution

What kind of ligated metal center is necessary for insertion into the “hidden” C−C bond? How can one tune the metal center for C−C bond activation by variation of the steric and electronic properties of ligands? What are the possible mechanisms of C−C bond activation in various reaction systems? A systematic look at the available data on C−C bond activation in solution provides some answers to these questions.     

Direct Aryl C−H Amination with Primary Amines Using Organic Photoredox Catalysis

The direct catalytic C−H amination of arenes is a powerful synthetic strategy with useful applications in pharmaceuticals, agrochemicals, and materials chemistry. Despite the advances in catalytic C−H functionalization, the use of aliphatic amine coupling partners is limited. Described herein is the construction of C−N bonds, using primary amines, by direct C−H functionalization with an acridinium photoredox catalyst under an aerobic atmosphere. A wide variety of primary amines, including amino acids and more complex amines are competent coupling partners. Various electron‐rich aromatics and heteroaromatics are useful scaffolds in this reaction, as are complex, biologically active arenes. We also describe the ability to functionalize arenes that are not oxidized by an acridinium catalyst, such as benzene and toluene, thus supporting a reactive amine cation radical intermediate.     

Exchanging Alkyl Groups through Unstrained C−C Bond Cleavage in the Presence of a Copper Catalyst

Although numerous reports exist on strained C−C bond cleavage reactions in aryl substitutions, the cleavage methodology for unstrained C−C bonds in alkylation reactions has not yet been established. We found that unstrained allylic C−C bonds can be cleaved using α‐radicals to form C(sp³)−C(sp³) bonds in the presence of a copper catalyst. In this reaction, the property of leaving and loading radicals is very important for radical fragmentations. In this paper, we investigated the effects of these properties in cleavage reactions for unstrained C−C bonds.     

Transition‐Metal‐Mediated Cleavage of C−C Bonds in Aromatic Rings

Metal‐mediated cleavage of aromatic C?C bonds has a range of potential synthetic applications: from direct coal liquefaction to synthesis of natural products. However, in contrast to the activation of aromatic C?H bonds, which has already been widely studied and exploited in diverse set of functionalization reactions, cleavage of aromatic C?C bonds remains Terra incognita. This Minireview summarizes the recent progress in this field and outlines key challenges to be overcome to develop synthetic methods based on this fundamental organometallic transformation.     

Modular Functionalization of Arenes in a Triply Selective Sequence: Rapid C(sp2) and C(sp3) Coupling of C−Br,C−OTf,and C−Cl Bonds Enabled by a Single Palladium(I) Dimer

Full control over multiple competing coupling sites would enable straightforward access to densely functionalized compound libraries. Historically, the site selection in Pd⁰‐catalyzed functionalizations of poly(pseudo)halogenated arenes has been unpredictable, being dependent on the employed catalyst, the reaction conditions, and the substrate itself. Building on our previous report of C−Br‐selective functionalization in the presence of C−OTf and C−Cl bonds, we herein complete the sequence and demonstrate the first general arylations and alkylations of C−OTf bonds (in <10 min), followed by functionalization of the C−Cl site (in <25 min), at room temperature using the same air‐ and moisture‐stable Pd^I dimer. This allowed the realization of the first general and triply selective sequential C−C coupling (in 2D and 3D space) of C−Br followed by C−OTf and then C−Cl bonds.     

Catalytic Ester and Amide to Amine Interconversion: Nickel‐Catalyzed Decarbonylative Amination of Esters and Amides by C−O and C−C Bond Activation

An efficient nickel‐catalyzed decarbonylative amination reaction of aryl and heteroaryl esters has been achieved for the first time. The new amination protocol allows the direct interconversion of esters and amides into the corresponding amines and represents a good alternative to classical rearrangements as well as cross coupling reactions.     

C−H and C−N Activation at Redox‐Active Pyridine Complexes of Iron

Pyridine activation by inexpensive iron catalysts has great utility, but the steps through which iron species can break the strong (105–111 kcal mol⁻¹) C−H bonds of pyridine substrates are unknown. In this work, we report the rapid room‐temperature cleavage of C−H bonds in pyridine, 4‐tert‐butylpyridine, and 2‐phenylpyridine by an iron(I) species, to give well‐characterized iron(II) products. In addition, 4‐dimethylaminopyridine (DMAP) undergoes room‐temperature C−N bond cleavage, which forms a dimethylamidoiron(II) complex and a pyridyl‐bridged tetrairon(II) square. These facile bond‐cleaving reactions are proposed to occur through intermediates having a two‐electron reduced pyridine that bridges two iron centers. Thus, the redox non‐innocence of the pyridine can play a key role in enabling high regioselectivity for difficult reactions.     

Selective C−O Bond Cleavage of Sugars with Hydrosilanes Catalyzed by Piers’ Borane Generated In Situ

Described herein is the selective reduction of sugars with hydrosilanes catalyzed by using Piers’ borane [(C₆F₅)₂BH] generated in situ. The hydrosilylative C−O bond cleavage of silyl‐protected mono‐ and disaccharides in the presence of a (C₆F₅)₂BH catalyst, generated in situ from (C₆F₅)₂BOH, takes place with excellent chemo‐ and regioselectivities to provide a range of polyols. A study of the substituent effects of sugars on the catalytic activity and selectivity revealed that the steric environment around the anomeric carbon (C1) is crucial.     

Palladium‐Catalyzed Chemo‐ and Enantioselective C−O Bond Cleavage of α‐Acyloxy Ketones by Hydrogenolysis

A chemoselective C−O bond cleavage of the ester alkyl side‐chain of α‐acyloxy ketones was realized for the first time by a highly efficient palladium‐catalyzed hydrogenolysis (S/C=6000, the highest catalytic efficiency by far). Furthermore, a kinetic resolution of α‐acyloxy ketones was first developed by enantioselective hydrogenolysis with good yields and up to 99 % ee.     

Palladium‐Catalyzed Hydrolytic Cleavage of Aromatic C−O Bonds

Metallic palladium surfaces are highly selective in promoting the reductive hydrolysis of aromatic ethers in aqueous phase at relatively mild temperatures and pressures of H₂. At quantitative conversions, the selectivity to hydrolysis products of PhOR ethers was observed to range from 50 % (R=Ph) to greater than 90 % (R=n ‐C₄H₉, cyclohexyl, and PhCH₂CH₂). By analysis of the evolution of products with and without incorporation of H₂¹⁸O, the pathway was concluded to be initiated by palladium metal catalyzed partial hydrogenation of the phenyl group to an enol ether. Water then rapidly adds to the enol ether to form a hemiacetal, which then undergoes elimination to cyclohexanone and phenol/alkanol products. A remarkable feature of the reaction is that the stronger Ph−O bond is cleaved rather than the weaker aliphatic O−R bond.     

Phosphaborenes: Accessible Reagents for the Synthesis of C−C/P−B Isosteres

Formal exchange of C=C units with isoelectronic B=N or B=P units can provide access to molecules with unique electronic or chemical properties. Herein, we report the simple solution‐phase generation of highly reactive phosphaborenes, RP=BR, and demonstrate their use for the introduction of P=B units into organic systems. Ring opening of a P−B‐containing cyclobutene isostere provided access to unique 1,4‐boraphosphabutadiene systems with conjugated main‐group multiple bonds.     

Development of High Performance Heterogeneous Catalysts for Selective Cleavage of C−O and C−C Bonds of Biomass‐Derived Oxygenates

The environmental impact of CO₂ emissions via the use of fossil resources as chemical feedstock and fuels has stimulated research to utilize renewable biomass feedstock. The biogenic compounds such as polyols are highly oxygenated and their valorization requires the new methods to control the oxygen to carbon ratio of the chemicals. The catalytic cleavage of C?O bonds and C?C bonds is promising methods, but the conventional catalyst systems encounter the difficulty to obtain the high yields of the desired products. This review describes our recent development of the high performance heterogeneous catalysts for the valorization of the biogenic chemicals such as glycerol, furfural, and levulinic acid via selective cleavage of C?O bonds and C?C bonds in the liquid‐phase. Selective C?O bond cleavage by hydrogenolysis enables production of various diols useful as engineering plastics, antifreeze, and cosmetics in high yields. The success of the selective C?C bond scission of levulinic acid can be applied to a wide range of the biogenic oxygenates such as carboxylic acids, esters, lactones, and primary alcohols, in which the selective C?C bond scission at adjacent to the oxygen functional groups are achieved. Furthermore, valorization of glycerol by selective acetylation and acetalization, and of levulinic acid by hydrogenation is described. Our catalysts show excellent performance compared to the reported catalysts in the aforementioned valorization.     

Photochemical Intramolecular C−H Addition of Dimesityl(hetero)arylboranes through a [1,6]‐Sigmatropic Rearrangement

A new reaction mode for triarylboranes under photochemical conditions was discovered. Photoirradiation of dimesitylboryl‐substituted (hetero)arenes produced spirocyclic boraindanes, where one of the C−H bonds in the ortho ‐methyl groups of the mesityl substituents was formally added in a syn fashion to a C−C double bond of the (hetero)aryl group. Quantum chemical calculations and laser flash photolysis measurements indicated that the reaction proceeds through a [1,6]‐sigmatropic rearrangement. This behavior is reminiscent of the photochemical reaction mode of arylalkenylketones, thus demonstrating the isosteric relation between tricoordinate organoboron compounds and the corresponding pseudo‐carbocationic species in terms of pericyclic reactions. Despite the disrupted π‐conjugation, the resulting spirocyclic boraindanes exhibited a characteristic absorption band at relatively long wavelengths (λ =370—400 nm).     

Photoinduced Remote Functionalisations by Iminyl Radical Promoted C−C and C−H Bond Cleavage Cascades

A photoinduced cascade strategy leading to a variety of differentially functionalised nitriles and ketones has been developed. These reactions rely on the oxidative generation of iminyl radicals from simple oximes. Radical transposition by C(sp³)−(sp³) and C(sp³)−H bond cleavage gives access to distal carbon radicals that undergo S_H2 functionalisations. These mild, visible‐light‐mediated procedures can be used for remote fluorination, chlorination, and azidation, and were applied to the modification of bioactive and structurally complex molecules.     

Triaminotriborane(3): A Homocatenated Boron Chain Connected by B−B Multiple Bonds

A triaminotriborane(3) was isolated as purple crystals through the reduction of (TMP)BCl₂ (TMP=2,2,6,6‐tetramethylpiperidino) by sodium naphthalenide. Single‐crystal X‐ray diffraction and computational studies of the obtained triaminotriborane(3) revealed a bent structure of the [B(NR₂)]₃ chain. The bond lengths between the central and terminal boron atoms were similar to those observed in neutral diborene species. The multiple‐bonding character may be best described by a three‐center two‐electron π‐bond along the B₃ chain. The distance between the two terminal boron atoms (2.177 Å) in the solid‐state structure implies a weak interaction between them. When an excess amount of Li was used as the reducing agent, the reaction yielded an unusual dianionic species. The isolation and characterization of these two reduction products are reported herein.