Yangyang Cheng

Palladium‐Catalyzed Desulfitative CH Arylation of Heteroarenes with Sodium Sulfinates

The structural motif with aryl–heteroaryl bonds is a predominant substructure of many biologically active compounds, natural products, pharmaceuticals, and functional materials. Transition-metal-catalyzed direct C H arylation of heteroaromatics has recently received a great deal of attention as an efficient approach for the straightforward synthesis of aryl–heteroaryl units. Over the past several years, the scope of the arylating reagents has been extended from aryl halides to various surrogates such as arenesulfonates, arylsilanes, aryl boronic acids, aryltrifluoroborates, diaryl iodonium salts, and aromatic carboxylic acids. Despite remarkable advances in these types of transformations, the search for new reliable alternative arylating reagents is still critically important from both scientific and practical standpoints. Arylsulfonyl chlorides (ArSO2Cl) have been widely used as sulfonylating agents for formation of S O, S N and S C bonds, and have recently been demonstrated as electrophilic partners in transition-metal-catalyzed desulfitative C C cross-coupling reactions. Despite readily availability, inexpensiveness, and high versatility, the active arylsulfonyl chlorides generally suffer from moisture sensitivity with the release of the acidic HCl gas. Thus, the air stable and easy to handle arylsulfinic acids (or salts) would serve as the ideal arylating reagent for the C C bond-forming reactions through the liberation of sulphur dioxide gas. Nevertheless, to date, sulfinic acids (or salts) are rarely used as the aryl source in transition-metal-catalyzed desulfinative reactions, whereas current research is mainly focused on sulfonylation reagents. The presently known desulfitative C C bond formation reactions include Heck-type coupling, and coupling between aldehydes or nitriles and arenesulfinic acid salts to aryl ketones; however, the extension of the reaction to the direct C H arylation of heteroaromatics remains unresolved. In line with our continuous efforts to forge aryl–heteroaryl bonds, we herein wish to develop an efficient desulfitative C H arylation of a wide range of heteroarenes with arenesulfinic acid sodium salts to enrich the current synthetic methodologies. However, we may face a series of hindrances: 1) In comparison with alkenes, it is well known that N-heteroarenes themselves are susceptible to oxidative homocoupling and decomposition with transition-metal-catalyzed oxidative conditions; and 2) aromatic sulfinic acid sodium salts were observed to easily encounter desulfitative self-coupling in the presence of a Pd species and an oxidant (Scheme 1). Thus, to achieve the heterocou-

Rh(III)-Catalyzed Decarboxylative ortho-Heteroarylation of Aromatic Carboxylic Acids by Using the Carboxylic Acid as a Traceless Directing Group

Highly selective decarboxylative ortho-heteroarylation of aromatic carboxylic acids with various heteroarenes has been developed through Rh(III)-catalyzed two-fold C-H activation, which exhibits a wide substrate scope of both aromatic carboxylic acids and heteroarenes. The use of naturally occurring carboxylic acid as the directing group avoids troublesome extra steps for installation and removal of an external directing group.

Regiospecific N‐Heteroarylation of Amidines for Full‐Color‐Tunable Boron Difluoride Dyes with Mechanochromic Luminescence

Colors to dye for: Palladium-catalyzed regiospecific N-heteroarylations of amidines with 2-halo-N-heteroarenes leads to a structurally diverse library of BF2 /amidine-based complexes. These dyes not only present full-visible-color solid-state emissions with large Stokes shifts and high fluorescence quantum yields, but also exhibit a full-color-tunable mechanofluorochromic nature.

Palladium(II)‐Catalyzed Oxidative CH/CH Cross‐Coupling between Two Structurally Similar Azoles

Power of two: A widely functional-group tolerant, selective and rapid oxidative cross-coupling between two structurally similar azoles has been carried out by using a palladium/copper co-catalytic twofold C-H activation method (see scheme).

Unparalleled Ease of Access to a Library of Biheteroaryl Fluorophores via Oxidative Cross-Coupling Reactions: Discovery of Photostable NIR Probe for Mitochondria

The development of straightforward accesses to organic functional materials through C-H activation is a revolutionary trend in organic synthesis. In this article, we propose a concise strategy to construct a large library of donor-acceptor-type biheteroaryl fluorophores via the palladium-catalyzed oxidative C-H/C-H cross-coupling of electron-deficient 2H-indazoles with electron-rich heteroarenes. The directly coupled biheteroaryl fluorophores, named Indazo-Fluors, exhibit continuously tunable full-color emissions with quantum yields up to 93% and large Stokes shifts up to 8705 cm(-1) in CH2Cl2. By further fine-tuning of the substituent on the core skeleton, Indazo-Fluor 3l (FW = 274; λem = 725 nm) is obtained as the lowest molecular weight near-infrared (NIR) fluorophore with emission wavelength over 720 nm in the solid state. The NIR dye 5h specifically lights up mitochondria in living cells with bright red luminescence. Typically, commercially available mitochondria trackers suffer from poor photostability. Indazo-Fluor 5h exhibits superior photostability and very low cytotoxicity, which would be a prominent reagent for in vivo mitochondria imaging.

Cobalt-Catalyzed Oxidative C−H/C−H Cross-Coupling between Two Heteroarenes

The first example of cobalt-catalyzed oxidative C-H/C-H cross-coupling between two heteroarenes is reported, which exhibits a broad substrate scope and a high tolerance level for sensitive functional groups. When the amount of Co(OAc)2 ⋅4 H2 O is reduced from 6.0 to 0.5 mol %, an excellent yield is still obtained at an elevated temperature with a prolonged reaction time. The method can be extended to the reaction between an arene and a heteroarene. It is worth noting that the Ag2 CO3 oxidant is renewable. Preliminary mechanistic studies by radical trapping experiments, hydrogen/deuterium exchange experiments, kinetic isotope effect, electron paramagnetic resonance (EPR), and high resolution mass spectrometry (HRMS) suggest that a single electron transfer (SET) pathway is operative, which is distinctly different from the dual C-H bond activation pathway that the well-described oxidative C-H/C-H cross-coupling reactions between two heteroarenes typically undergo.

Nickel Catalysis Enables Oxidative C(sp2)–H/C(sp2)–H Cross‐Coupling Reactions between Two Heteroarenes

Nickel can be used to promote oxidative C(sp(2) )-H/C(sp(2) )-H cross-coupling between two heteroarenes. The reaction scope can be extended to aromatic carboxamides as the coupling partner. The reaction exhibits high functional-group compatibility and broad substrate scope. The silver oxidant can be recycled to reduce costs and waste, which is very useful for practical applications.

Novel Ruthenium Sensitizers with a Phenothiazine Conjugated Bipyridyl Ligand for High-Efficiency Dye-Sensitized Solar Cells.

Two efficient ruthenium sensitizers with a phenothiazine-modified bipyridine as an ancillary ligand, coded SCZ-1 and SCZ-2, have been developed as dyes in dye-sensitized solar cells (DSSCs). Both sensitizers exhibit low-energy metal-to-ligand charge transfer (MLCT) bands centered at 539 nm with high molar extinction coefficients of 1.77 × 10(4) M(-1) cm(-1) for SCZ-1 and 1.66 × 10(4) M(-1) cm(-1) for SCZ-2, which are significantly higher than the corresponding value for the reference N719 (1.27 × 10(4) M(-1) cm(-1)), indicating that the light-harvesting capacity of ruthenium sensitizers can be reinforced by introducing phenothiazine moieties into the bipyridine ligand. Under AM 1.5G irradiation (100 mW cm(-2)), SCZ-1 and SCZ-2 sensitized DSSC devices show impressive power conversion efficiencies (PCE) up to 10.4% by using of iodide-based electrolytes, which exceeds that of N719 (9.9%) under the same conditions. Both of the open circuit voltage (VOC) and fill factor (FF) of SCZ-sensitized solar cells approximate to those of N719-sensitized cell. The relatively higher efficiencies of the SCZ-sensitized cells than that of N719-sensitized cell come from their higher short-circuit photocurrent density (JSC), which may be mainly attributed to the high absorption coefficient. The absorption spectrum and device efficiency of SCZ-1 are both quite close to those of SCZ-2, suggesting that the difference in alkyl chains on the N atom of phenothiazine is not a decisive factor in affecting the photovoltaic performance of dyes.

Oxygen as an oxidant in palladium/copper-cocatalyzed oxidative C-H/C-H cross-coupling between two heteroarenes

The palladium/copper-cocatalyzed oxidative C-H/C-H cross-coupling between two heteroarenes by using molecular oxygen as an oxidant instead of metal oxidants has been developed for the first time to construct biheteroaryl motifs. A relatively broad range of thiophenes, furans and indoles can smoothly couple with various N-heteroarenes in satisfactory yields. This catalytic system with O2 as the terminal oxidant offers clear advantages of economically feasible and eco-friendly processes.