Photoinduced Dehydrogenative Borylation via Dihydrogen Bond Bridged Electron Donor and Acceptor Complexes.

Air-stable amine- and phosphine-boranes are discovered as donors to integrate with pyridinium acceptor for generating photoactive electron-donor-acceptor (EDA) complexes. Experimental results and DFT calculations suggest a dihydrogen bond bridging the donor and acceptor. Irradiating the EDA complex enables an intra-complex single electron transfer to give a boron-centered radical for dehydrogenative borylation with no need of external photosensitizer and radical initiator. The deprotonation of Wheland-like radical intermediate rather than its generation is believed to determine the good ortho-selectivity based on DFT calculations. A variety of α-borylated pyridine derivatives have been readily synthesized with good functional group tolerance.

Charge Separation in Metal-Organic Framework Enables Heterogeneous Thiol Catalysis.

Photoinduced metal-organic framework (MOF) enabled heterogeneous thiol catalysis has been achieved for the first time. MOF Zr-TPDCS-1, consisting of Zr6 -clusters and TPDCS linkers (TPDCS=3,3'',5,5''-tetramercapto[1,1':4',1''-terphenyl]-4,4''-dicarboxylate), effectively catalyzed the borylation, silylation, phosphorylation, and thiolation of organic molecules. Upon irradiation, the fast electron transfer from TPDCS to Zr6 -cluster is believed to facilitate the formation of the thiyl radical, a hydrogen atom transfer catalyst, which competently abstracts the hydrogen from borane, silane, phosphine, or thiol for generating the corresponding element radical to engender the chemical transformations. The elaborate control experiments evidenced the generation of thiyl radicals in MOF and illustrated a radical reaction pathway. The gram-scale reaction worked well, and the product was conveniently separated via centrifugation and vacuum with a turnover number (TON) of ≈3880, highlighting the practical application potential of heterogeneous thiyl-radical catalysis.

Ir-Catalyzed Selective B(3)-H Amination of o-Carboranes with NH3.

Ammonia gas, NH3, is a cheap and widely used industrial feedstock, which has received tremendous research interests in its functionalization. This work reports a breakthrough in catalytic selective cage B(3)-H amination of o-carboranes with NH3 via Ir-catalyzed B-H/N-H dehydrocoupling, offering convenient and efficient access to a series of 3-NH2-o-carborane derivatives in moderate to high isolated yields with a broad substrate scope. The employment of readily available NH3 gas as the aminating reagent with H2 as the sole byproduct endows the protocol economy, practicability, and environmental friendliness. A plausible reaction mechanism is proposed on the basis of control experiments.

Bifunctional Metal-Organic Layer with Organic Dyes and Iron Centers for Synergistic Photoredox Catalysis.

Here we report the design of a bifunctional metal-organic layer (MOL), Hf-EY-Fe, by bridging eosin Y (EY)-capped Hf6 secondary building units (SBUs) with Fe-TPY (TPY = 4'-(4-carboxyphenyl)[2,2':6',2''-terpyridine]-5,5''-dicarboxylate) ligands. With the organic dye EY as an efficient photosensitizer and TPY-Fe(OTf)2 as the catalytic center, Hf-EY-Fe efficiently catalyzes aminotrifluoromethylation, hydroxytrifluoromethylation, and chlorotrifluoromethylation of alkenes. Hf-EY-Fe also catalyzes the synthesis of CF3-substituted derivatives of large bioactive molecules such as rotenone, estrone, and adapalene with sizes of up to 2.2 nm. The proximity between EY and iron centers and their site isolation in Hf-EY-Fe enhance catalytic activity while inhibiting their mutual deactivation, leading to high turnover numbers of up to 1840 and good recyclability of the MOL catalyst.

Bifunctional Metal-Organic Layers for Tandem Catalytic Transformations Using Molecular Oxygen and Carbon Dioxide.

Tandem catalytic reactions improve atom- and step-economy over traditional synthesis but are limited by the incompatibility of the required catalysts. Herein, we report the design of bifunctional metal-organic layers (MOLs), HfOTf-Fe and HfOTf-Mn, consisting of triflate (OTf)-capped Hf6 secondary building units (SBUs) as strong Lewis acidic centers and metalated TPY ligands as metal active sites for tandem catalytic transformations using O2 and CO2 as coreactants. HfOTf-Fe effectively transforms hydrocarbons into cyanohydrins via tandem oxidation with O2 and silylcyanation whereas HfOTf-Mn converts styrenes into styrene carbonates via tandem epoxidation and CO2 insertion. Density functional theory calculations revealed the involvement of a high-spin FeIV (S = 2) center in the challenging oxidation of the sp3 C-H bond. This work highlights the potential of MOLs as a tunable platform to incorporate multiple catalysts for tandem transformations.

Metal-Organic Layers Hierarchically Integrate Three Synergistic Active Sites for Tandem Catalysis.

We report the design of a bifunctional metal-organic layer (MOL), Hf12 -Ru-Co, composed of [Ru(DBB)(bpy)2 ]2+ [DBB-Ru, DBB=4,4'-di(4-benzoato)-2,2'-bipyridine; bpy=2,2'-bipyridine] connecting ligand as a photosensitizer and Co(dmgH)2 (PPA)Cl (PPA-Co, dmgH=dimethylglyoxime; PPA=4-pyridinepropionic acid) on the Hf12 secondary building unit (SBU) as a hydrogen-transfer catalyst. Hf12 -Ru-Co efficiently catalyzed acceptorless dehydrogenation of indolines and tetrahydroquinolines to afford indoles and quinolones. We extended this strategy to prepare Hf12 -Ru-Co-OTf MOL with a [Ru(DBB)(bpy)2 ]2+ photosensitizer and Hf12 SBU capped with triflate as strong Lewis acids and PPA-Co as a hydrogen transfer catalyst. With three synergistic active sites, Hf12 -Ru-Co-OTf competently catalyzed dehydrogenative tandem transformations of indolines with alkenes or aldehydes to afford 3-alkylindoles and bisindolylmethanes with turnover numbers of up to 500 and 460, respectively, illustrating the potential use of MOLs in constructing novel multifunctional heterogeneous catalysts.

Copper-Catalyzed Electrochemical Selective B-H Oxygenation of o-Carboranes at Room Temperature.

Copper-catalyzed electrochemical selective cage B-H oxygenation of o-carboranes has been achieved for the first time. Under a constant electric current (4.0 mA) at room temperature, copper-catalyzed cross-coupling of carboranyl amides with lithium phenolates results in the formation of B(4,5)-diphenolated o-carboranes via direct B-H activation, whereas the use of lithium tert-butoxide affords B(4)-monooxygenated products. This reaction does not require any additional chemical oxidants and generates H2 and a lithium salt as byproducts. Control experiments indicated that a high-valent Cu(III) species is likely involved in the reaction process.

Metal-Organic Layers for Synergistic Lewis Acid and Photoredox Catalysis.

We report the design of a new multifunctional metal-organic layer (MOL), Hf12-Ir-OTf, comprising triflate (OTf)-capped Hf12 secondary building units (SBUs) and photosensitizing Ir(DBB)[dF(CF3)ppy]2+ [DBB-Ir-F, DBB = 4,4'-di(4-benzoato)-2,2'-bipyridine; dF(CF3)ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine] bridging ligands. Hf12-Ir-OTf effectively catalyzed dehydrogenative cross-couplings of heteroarenes with ethers, amines, and unactivated alkanes with turnover numbers of 930, 790, and 950, respectively. Hf12-Ir-OTf also competently catalyzed late-stage functionalization of bioactive and drug molecules such as caffeine, Fasudil, and Metyrapone. The superior catalytic performance of Hf12-Ir-OTf over a mixture of photoredox catalyst and stoichiometric amounts of Brønsted acids or substoichiometric amounts (20 mol %) of Lewis acids is attributed to the close proximity (1.2 nm) between photoredox and Lewis acid catalysts in Hf12-Ir-OTf, which not only facilitates the reaction between the carbon radical and the activated heteroarene but also accelerates the electron transfer from the nitrogen radical intermediate to the Ir(IV) species in the catalytic cycle.

Metal-Organic Framework with Dual Active Sites in Engineered Mesopores for Bioinspired Synergistic Catalysis.

Here we report the design of an enzyme-inspired metal-organic framework (MOF), 1-OTf-Ir, by installing strong Lewis acid and photoredox sites in engineered mesopores. Al-MOF (1), with mixed 2,2'-bipyridyl-5,5-dicarboxylate (dcbpy) and 1,4-benzenediacrylate (pdac) ligands, was oxidized with ozone and then triflated to generate strongly Lewis acidic Al-OTf sites in the mesopores, followed by the installation of [Ir(ppy)2(dcbpy)]+ (ppy = 2-phenylpyridine) sites to afford 1-OTf-Ir with both Lewis acid and photoredox sites. 1-OTf-Ir effectively catalyzed reductive cross-coupling of N-hydroxyphthalimide esters or aryl bromomethyl ketones with vinyl- or alkynyl-azaarenes to afford new azaarene derivatives. 1-OTf-Ir enabled catalytic synthesis of anticholinergic drugs Pheniramine and Chlorpheniramine.

Controlled functionalization of o-carborane via transition metal catalyzed B-H activation.

Carboranes, a class of carbon-boron molecular clusters, are often viewed as three-dimensional analogues of benzene. They are finding increasing applications as useful functional building blocks in materials science, medicine, organometallic/coordination chemistry and more. Thus, functionalization of carboranes has received considerable attention. In comparison with the weakly acidic cage C-H bonds that can be readily functionalized, selective cage B-H functionalization among ten chemically similar BH vertices in o-carboranes is very challenging. Only in the recent few years, considerable progress has been made in transition metal catalyzed vertex-specific BH functionalization. This review summarizes recent advances in this research area.

Catalytic Cascade Dehydrogenative Cross-Coupling of BH/CH and BH/NH: One-Pot Process to Carborano-Isoquinolinone.

A proof-of-principle study of cascade dehydrogenative cross-coupling of carboranyl carboxylic acid with readily available benzamide has been achieved, resulting in the facile synthesis of previously inaccessible carborano-isoquinolinone derivatives in a simple one-pot process, in which two cage B-H, one aryl C-H, and one N-H bond were sequentially activated to construct efficiently new B-C and B-N bonds, respectively. Under suitable reaction conditions, such cascade cyclization can be stopped at the first B-H/C-H cross-coupling step to give a series of α-carboranyl benzamides, suggesting the preferential occurrence of B-C cross-coupling over that of B-N. The carboxylic acid directing group plays a key role in the B-C cross-coupling step, which is then removed through in situ decarboxylation. The CV results combined with control experiments indicate that high-valent Ir(V)-species may be involved in the reaction pathways, which is crucial for such cascade dehydrogenative cross-coupling reactions. The isolation and structural identification of a key intermediate, its controlled transformations, and deuterium labeling experiments support a new Ir-nitrene-mediated amination for B-H/N-H dehydrocoupling.

Catalytic Regioselective Cage B(8)-H Arylation of o-Carboranes via "Cage-Walking" Strategy.

A proof-of-concept example of catalytic regioselective cage B(8)-H functionalization of o-carboranes has been disclosed for the first time. Under the help of an acylamino directing group at cage B(3), a series of B(8)-arylated, B(4,7,8)-triarylated and B(4,7,8)-trifluorinated o-carborane derivatives were conveniently prepared. On the basis of isolation of a key intermediate, deuterium labeling experiments and DFT calculations, a reaction mechanism involving a high-valent palladium induced "cage-walking" from B(4) to B(8) vertex is proposed to account for the regioselective B(8)-H activation.

Metal-Organic Layers as Multifunctional Two-Dimensional Nanomaterials for Enhanced Photoredox Catalysis.

Metal-organic layers (MOLs) have recently emerged as a novel class of molecular two-dimensional (2D) materials with significant potential for catalytic applications. Herein we report the design of a new multifunctional MOL, Hf12-Ir-Ni, by laterally linking Hf12 secondary building units (SBUs) with photosensitizing Ir(DBB)[dF(CF3)ppy]2+ [DBB-Ir-F, DBB = 4,4'-di(4-benzoato)-2,2'-bipyridine; dF(CF3)ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine] bridging ligands and vertically terminating the SBUs with catalytic Ni(MBA)Cl2 [MBA = 2-(4'-methyl-[2,2'-bipyridin]-4-yl)acetate] capping agents. Hf12-Ir-Ni was synthesized in a bottom-up approach and characterized by TEM, AFM, PXRD, TGA, NMR, ICP-MS, UV-vis, and luminescence spectroscopy. The proximity between photosensitizing Ir centers and catalytic Ni centers (∼0.85 nm) in Hf12-Ir-Ni facilitates single electron transfer, leading to a 15-fold increase in photoredox reactivity. Hf12-Ir-Ni was highly effective in catalytic C-S, C-O, and C-C cross-coupling reactions with broad substrate scopes and turnover numbers of ∼4500, ∼1900, and ∼450, respectively.

Transition-Metal-Catalyzed Selective Cage B-H Functionalization of o-Carboranes.

Carboranes are a class of carbon-boron molecular clusters with unusual thermal and chemical stabilities. They have been proved as very useful building blocks in supramolecular design, optoelectronics, nanomaterials, boron neutron capture therapy agents and organometallic/coordination chemistry. Thus, the functionalization of o-carboranes has received growing interests. Over the past decades, most of the works in this area have been focused on cage carbon functionalization as the weakly acidic cage C-H proton can be readily deprotonated by strong bases. In sharp contrast, selective cage B-H activation/functionalization among chemically very similar ten B-H vertices is very challenging. Considering the differences in electron density of ten cage B-H bonds in o-carborane and the nature of transition metal complexes, we have tackled this selectivity issue by means of organometallic chemistry. Our strategy is as follows: using electron-rich transition metal catalysts for the functionalization of the most electron-deficient B(3,6)-H vertices (bonded to both cage CH vertices); using electron-deficient transition-metal catalysts for the functionalization of relatively electron-rich B(8,9,10,12)-H vertices (with no bonding to both cage CH vertices); and using the combination of directing groups and electrophilic transition metal catalysts for the functionalization of B(4,5,7,11)-H vertices (bonded to only one cage CH vertex). Successful applications of such a strategy result in the preparation of a large variety of cage B-functionalized carboranes in a regioselective and catalytic manner, which are inaccessible by other means. It is believed that as this field progresses, other cage B-functionalized carboranes are expected to be synthesized, and the results detailed in this concept article will further these efforts.

Transition Metal Catalyzed, Regioselective B(4)-Halogenation and B(4,5)-Diiodination of Cage B-H Bonds in o-Carboranes.

Transition metal catalyzed, regioselective carborane-cage B(4)-H iodination, bromination, and chlorination as well as B(4,5)-H diiodination were achieved by using NXS (X=I, Br), FeCl3 , or IOAc as the halogenating agent, respectively. A series of previously inaccessible B(4)-halogenated o-carboranes were synthesized in a simple one-pot process, and proved to be valuable synthons for the functionalization of carboranes. Mono- and di-selectivity can be controlled by in situ removal of the carboxy directing group. The resultant 4-I-o-C2 B10 H11 can serve as a versatile feedstock for the construction of cage B-C(sp2 ), B-C(sp), B-O, and B-N bonds.

Transition Metal Catalyzed Direct Amination of the Cage B(4)-H Bond in o-Carboranes: Synthesis of Tertiary, Secondary, and Primary o-Carboranyl Amines.

Transition metal catalyzed regioselective amination of the cage B(4)-H bond in o-carboranes has been achieved for the first time using O-benzoyl hydroxylamines or organic azides as the amination reagents, leading to the preparation of a series of tertiary and secondary carboranyl amines. Both amination reactions proceeded under mild conditions without the addition of any external oxidants. Hydrogenolysis of the resultant product 4-N(CH2Ph)2-o-carborane afforded the primary carboranyl amine, 4-amino-o-carborane, in quantitative yield.

Palladium-Catalyzed Regioselective Diarylation of o-Carboranes By Direct Cage B-H Activation.

Palladium-catalyzed intermolecular coupling of o-carborane with aromatics by direct cage B-H bond activation has been achieved, leading to the synthesis of a series of cage B(4,5)-diarylated-o-carboranes in high yields with excellent regioselectivity. Traceless directing group -COOH plays a crucial role for site- and di-selectivity of such intermolecular coupling reaction. A Pd(II)-Pd(IV)-Pd(II) catalytic cycle is proposed to be responsible for the stepwise arylation.

Rhodium-Catalyzed Regioselective Hydroxylation of Cage B-H Bonds of o-Carboranes with O2 or Air.

A rhodium-catalyzed hydroxylation of a cage B4-H bond in o-carboranes with either O2 or air as the oxygen source is described, and serves as a new methodology for the regioselective generation of a series of 4-OH-o-carboranes in a one-pot process. The use of either O2 or air as both the oxidant and the oxygen source makes this protocol very environmentally friendly and practical.

Palladium-catalyzed regioselective intramolecular coupling of o-carborane with aromatics via direct cage B-H activation.

Palladium-catalyzed intramolecular coupling of o-carborane with aromatics via direct cage B-H bond activation has been achieved, leading to the synthesis of a series of o-carborane-functionalized aromatics in high yields with excellent regioselectivity. In addition, the site selectivity can also be tuned by the substituents on cage carbon atom.

Palladium-Catalyzed Direct Dialkenylation of Cage B-H Bonds in o-Carboranes through Cross-Coupling Reactions.

Palladium-catalyzed direct dialkenylation of cage B(4,5)-H bonds in o-carboranes has been achieved with the help of a carboxylic acid directing group, leading to the preparation of a series of 4,5-[trans-(ArCH=CH)]2-ocarboranes in high yields with excellent regioselectivity. The traceless directing group, eliminated during the course of the reaction, is responsible for controlling regioselectivity and dialkenylation. A possible catalytic cycle is proposed, involving a tandem sequence of Pd(II) -initiated cage B-H activation, alkene insertion, ß-H elimination, reductive elimination, and decarboxylation.