Beijing National Laboratory of Molecular Sciences (BNLMS) and Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry and Green Chemistry Center, Peking University, 202 Chengfu Road, 098#, Beijing 100871, China State Key Laboratory of Organometallic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 200060, China

Transition-Metal-Free Coupling Reactions

As two coexisting and fast-growing research fields in modern synthetic chemistry, the merging of organocatalysis and C–H bond functionalization is well foreseeable, and the joint force along this line has been demonstrated to be a powerful approach in making inert C–H bond functionalization more viable, predictable, and selective. In this review, we provide a comprehensive summary of organocatalysis in inert C–H bond functionalization over the past two decades. The review is arranged by types of inert C–H bonds including alkane C–H, arene C–H, and vinyl C–H as well as those activated benzylic C–H, allylic C–H, and C–H bonds alpha to the heteroatom such as nitrogen and oxygen. In each section, the discussion is classified by the explicit organocatalytic mode involved.

Organocatalysis in Inert C–H Bond Functionalization

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC–main group element adducts, and might be...

NHCs in Main Group Chemistry.

Transformations of aromatic compounds into the corresponding amines, amides, and imides through carbon–hydrogen (C–H) bond functionalization represent one of the most step- and atom-economical methods for the synthesis of arylamine compounds. Because arylamines are privileged structures in materials- and biology-oriented functional molecules, the development of novel and efficient synthetic methods for aromatic C–H amination has received significant attention from a wide range of research fields including materials and pharmaceuticals. This review covers recent advances in catalytic aromatic C–H amination reactions. An array of recently developed new reactions are categorized by the nature of aromatic substrates: (1) 5-membered heteroarenes, (2) arenes having a nitrogen moiety in the molecule (intramolecular C–H amination), (3) arenes having a directing group, (4) simple arenes with excess amounts, and (5) simple arenes as the limiting reagents.

Catalytic Methods for Aromatic C–H Amination: An Ideal Strategy for Nitrogen-Based Functional Molecules

Oxidative addition and reductive elimination are key steps in a wide variety of catalytic reactions mediated by transition-metal complexes. Historically, this reactivity has been considered to be the exclusive domain of d-block elements. However, this paradigm has changed in recent years with the demonstration of transition-metal-like reactivity by main-group compounds. This Review highlights the substantial progress achieved in the past decade for the activation of robust single bonds by main-group compounds and the more recently realized activation of multiple bonds by these elements. We also discuss the significant discovery of reversible activation of single bonds and distinct examples of reductive elimination at main-group element centers. The review consists of three major parts, starting with oxidative addition of single bonds, proceeding to cleavage of multiple bonds, and culminated by the discussion of reversible bond activation and reductive elimination. Within each subsection, the discussion is...

Oxidative Addition and Reductive Elimination at Main-Group Element Centers

A metal-free amination of benzoxazoles--the first example of an iodide-catalyzed oxidative amination of heteroarenes.

Intramolecular germylene, stannylene, and plumbylene Lewis pairs were reacted with hexanal and yielded the cyclic addition products only with the germanium and tin reagents. In further reactivity studies, the hydroboration of aldehydes and ketones catalyzed by intramolecular germylene, stannylene, and plumbylene Lewis pairs was studied. In the case of the cyclic germylene Lewis pair, the product of the oxidative addition of pinacolborane at the germylene moiety was observed. According to stoichiometric as well as catalytic experiments, the intramolecular germylene Lewis pair acts as a catalyst in the hydroboration of aldehydes and ketones. The homologous stannylene Lewis pair forms a reactive tin hydride during the catalysis, which can also act as a catalyst in this transformation.

Diverse Activation Modes in the Hydroboration of Aldehydes and Ketones with Germanium, Tin, and Lead Lewis Pairs.

N-Heterocyclic carbenes are shown to cleanly abstract dihydrogen from organotin di- and trihydrides to intermediately form the reactive stannylene species [R2Sn] and [R′SnH], respectively, as well as the corresponding dihydro imidazole [R = Ph, trip: 2,4,6-triisopropylphenyl; R′ = trip, Ar*: 2,6-bis(2′,4′,6′-triisopropylphenyl)phenyl]. Depending on the molar ratio of the carbene, these intermediates undergo further reactions to form tetraorgano distannanes and carbene adducts in excellent yields. Since the side products can be removed as volatiles, the method offers a facile, clean and high yielding route to stannylene derivatives with strongly decreased steric protection compared to previously known compounds. On stoichiometric reaction hydrogen was completely removed from an organotin trihydride and a novel tin cluster with overall composition R6Sn6 (R = trip) was structurally characterized.

Hydrogen abstraction from organotin di- and trihydrides by N-heterocyclic carbenes: a new method for the preparation of NHC adducts to tin(II) species and observation of an isomer of a hexastannabenzene derivative [R6Sn6]

The weakly coordinating anion [Me3 NB12 Cl11 ](-) has been prepared by a simple two-step procedure. The anion [Me3 NB12 Cl11 ](-) is easily obtained in batches of up to 20 g by chlorination of the known [H3 NB12 H11 ](-) anion with SbCl5 at about 190 °C and subsequent N-methylation with methyl iodide. Starting from Na[Me3 NB12 Cl11 ], several synthetically useful salts with reactive cations ([NO](+) , [Ph3 C](+) , and [(Et3 Si)2 H](+) ) were prepared. Full spectroscopic (NMR, IR, Raman, TGA, MS) characterization and single-crystal X-ray diffraction studies confirmed the identity and purity of the products. The thermal, chemical, and electrochemical stability as well as the basicity of the [Me3 NB12 Cl11 ](-) anion is similar to that of the structurally related weakly coordinating 1-carba-closo-dodecaborate and closo-dodecaborate anions. The facile preparation of the [Me3 NB12 Cl11 ](-) anion and its ideal chemical and physical properties make it a cheap alternative to other classes of weakly coordinating anions.

Synthesis and Properties of the Weakly Coordinating Anion [Me3NB12Cl11]−

Although hydrides of the group 14 elements are well-known as versatile starting materials in many chemical transformations, a hydride of lead in oxidation state II is so far unknown. In this work, we finally complete the jigsaw puzzle by reporting the isolation of the first low valent organolead hydride. The thermolabile dimeric organolead hydride was synthesized at low temperature and features a hydride 1H NMR signal (in solution 35.61 ppm; in the solid state 31.1 ppm) at the lowest field observed so far for a diamagnetic compound in agreement with quantum chemical predictions.

Christian P. Sindlinger

Papers

A metal-free amination of benzoxazoles--the first example of an iodide-catalyzed oxidative amination of heteroarenes.

Diverse Activation Modes in the Hydroboration of Aldehydes and Ketones with Germanium, Tin, and Lead Lewis Pairs.

Hydrogen abstraction from organotin di- and trihydrides by N-heterocyclic carbenes: a new method for the preparation of NHC adducts to tin(II) species and observation of an isomer of a hexastannabenzene derivative [R6Sn6]

Synthesis and Properties of the Weakly Coordinating Anion [Me3NB12Cl11]−

Low-Valent Lead Hydride and Its Extreme Low-Field 1H NMR Chemical Shift