Fu-She Han

Metallosupramolecular Polyelectrolytes Self-Assembled from Various Pyridine Ring-Substituted Bisterpyridines and Metal Ions: Photophysical, Electrochemical, and Electrochromic Properties

This work presents several metallosupramolecular coordination polyelectrolytes (MEPEs) self-assembled from rigid, pi-conjugated, pyridine ring functionalized bisterpyridines and metal ions. The MEPEs are water-soluble and display different colors spanning the entire visible regions. Optical, electrochemical, and electrochromic properties of the obtained MEPEs are presented. The results show that the properties are profoundly affected by the nature of the substituents at the peripheral pyridine rings. Namely, MEPEs assembled from the electron-rich OMe group modified ligands exhibit high switching reversibility and stability and show a lower switching potential than the unsubstituted and electron-deficient Br-substituted analogues. The response times can be tuned either by the design of the ligands or by the choice of the metal ions to cover a broad time scale from under 1 s to several minutes. The optical memory is enhanced from 30 s to longer than 15 min as a comparison of unsubstituted and substituted MEPEs shows. Thus, the significantly enhanced stability and the ease of tuning the properties render this type of supramolecular assembly attractive as electrochromic materials for applications in a large variety of areas. Most importantly, we presented the structure-property relationships of MEPEs, which lays the groundwork for further design of new bisterpyridine-based metallosupramolecular functional materials.

[NiCl2(dppp)]‐Catalyzed Cross‐Coupling of Aryl Halides with Dialkyl Phosphite, Diphenylphosphine Oxide, and Diphenylphosphine

We present a general approach to C-P bond formation through the cross-coupling of aryl halides with a dialkyl phosphite, diphenylphosphine oxide, and diphenylphosphane by using [NiCl(2) (dppp)] as catalyst (dppp=1,3-bis(diphenylphosphino)propane). This catalyst system displays a broad applicability that is capable of catalyzing the cross-coupling of aryl bromides, particularly a range of unreactive aryl chlorides, with various types of phosphorus substrates, such as a dialkyl phosphite, diphenylphosphine oxide, and diphenylphosphane. Consequently, the synthesis of valuable phosphonates, phosphine oxides, and phosphanes can be achieved with one catalyst system. Moreover, the reaction proceeds not only at a much lower temperature (100-120 °C) relative to the classic Arbuzov reaction (ca. 160-220 °C), but also without the need of external reductants and supporting ligands. In addition, owing to the relatively mild reaction conditions, a range of labile groups, such as ether, ester, ketone, and cyano groups, are tolerated. Finally, a brief mechanistic study revealed that by using [NiCl(2) (dppp)] as a catalyst, the Ni(II) center could be readily reduced in situ to Ni(0) by the phosphorus substrates due to the influence of the dppp ligand, thereby facilitating the oxidative addition of aryl halides to a Ni(0) center. This step is the key to bringing the reaction into the catalytic cycle.

Pd(II)-Catalyzed Enantioselective Synthesis of P-Stereogenic Phosphinamides via Desymmetric C–H Arylation

We present the enantioselective synthesis of P-stereogenic phosphinamides through Pd-catalyzed desymmetric ortho C-H arylation of diarylphosphinamides with boronic esters. The method represents the first example of the synthesis of P-stereogenic phosphorus compounds via the desymmetric C-H functionalization strategy. The reaction proceeded efficiently with a wide array of reaction partners to afford the P-stereogenic phosphinamides in up to 74% yield and 98% ee. The efficiency was further demonstrated by gram scale syntheses. Moreover, the flexible conversion of the P-stereogenic phosphinamides into various types of P-stereogenic phosphorus derivatives was also elaborated. Thus, the protocol provides a novel tool for the efficient and versatile synthesis of P-stereogenic compounds.

Reliable and Diverse Synthesis of Aryl Azides through Copper‐Catalyzed Coupling of Boronic Acids or Esters with TMSN3

Organic azides are versatile compounds in a wide spectrum of areas. They are commonly used as photoaffinity labeling reagents for biomolecules, and they also serve as a latent group for accessing a rich variety of functional groups including amines, amides, and carbamates, as well as aziridines, the ring-opening and expansion reactions of which are extremely attractive for constructing numerous N-containing structural motifs. Particularly, with the recent advent of the click chemistry of azides and alkynes for accessing triACHTUNGTRENNUNGazoles,[3] and the [3 +2] cycloaddition of azides and cyanides for the construction of structurally more interesting tetrazoles, azides have found widespread applications in drug discovery, biochemistry, materials science, and supramolecular chemistry. Thus, practical routes to access various azides are required. Although a myriad of approaches for the synthesis of aliphatic azides are available, methods for the preparation of aryl azides are rather limited, with the conversion of amines to the azides through their diazonium salts or the reactions of organometallic aryls with p-tosyl azide being the major choice. However, general use of these processes is restricted owing to the poor functional-group compatibility and harsh reaction conditions. Recently, copper-catalyzed couplings of aryl amines, halides, and boronic acids with an azide source such as TfN3 [12a] and NaN3, [13,14] have been reported. Although powerful, the generality of these methods remains to be determined. More significantly, hazards associated with the azide sources and tBuONO are important deterrents to their practical use. Herein, we present a less hazardous, simple, and highly efficient protocol for the diverse synthesis of aryl azides through coppercatalyzed coupling of boronic acids or esters with TMSN3. Although TMSN3 is a more reliable azide source, [9b] its use in metal-catalyzed organic azide formation is virtually unknown. In the initial studies, coupling of 4-methoxylphenylboronic acid (1) with TMSN3 was carried out to optimize the reaction conditions. As shown in Table 1, either in the absence or in the presence of tetrabutylammonium fluoride (TBAF), the CuI-catalyzed coupling was inefficient in MeOH at room temperature (Table 1, entries 1 and 2). The reaction was also sluggish by heating under reflux without using TBAF (Table 1, entry 3). Pleasingly, when TBAF was added under reflux conditions, the desired coupling reaction occurred quite smoothly to give the 4-methoxylphenyl azide (2) in 89 % yield (Table 1, entry 4). The control experiment showed that no reaction occurred in the absence of CuI (Table 1, entry 5). It is thus evident that in addition to the catalyst, the use of TBAF as an additive coupled with an elevated reaction temperature plays a pivotal role in effecting the reaction. Here, TBAF is assumed to promote the cleavage of the N Si bond in TMSN3 to form a more nucleophilic N3 anion. With these preliminary results in hand, we investigated further the effect of catalysts, additives, and solvents on this transformation. A survey of the catalysts revealed that although FeCl2 was inefficient (Table 1, entry 6), rapid conversion as well as excellent yields were observed for various copper catalysts that have different redox states (e.g., Cu and Cu) and counter ions (Table 1, entries 7–11), and CuCl appeared to be the most appealing catalyst in terms of the yield and reaction time (Table 1, entry 7). Thus, by using CuCl as the catalyst, the effect of additives was then evaluated. The results showed that TBAF was more effective than KF, CsF, and NH4F as shown by a comparison of the corresponding results (Table 1, entry 7 vs. entries 12–14). More interestingly, we observed a strong dependence of the reaction efficiency on the nature of solvents. Namely, the reaction was completely suppressed when aprotic solvents [a] Y. Li, Prof. Dr. L.-X. Gao, Prof. Dr. F.-S. Han Changchun Institute of Applied Chemistry Chinese Academy of Sciences, 5625 Renmin Street Changchun 130022, Jilin (China) Tel: (+86) 431-85262936 E-mail : fshan@ciac.jl.cn [b] Y. Li Graduate School of Chinese Academy of Sciences Beijing, 100864 (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000971.

Aryl Phosphoramides: Useful Electrophiles for Suzuki–Miyaura Coupling Catalyzed by a NiCl2/dppp System (dppp=1,3‐Bis(diphenylphosphino)propane)

The transition-metal-catalyzed cross-coupling of phenol derivatives has emerged as an extremely attractive method in synthetic chemistry, because of the many advantages pertaining to the use of phenol derivatives. So far, aryl triflates or nonaflates have frequently been investigated, but their high cost and instability are deterrents to their extensive use. As more affordable and manipulatable substitutes aryl sulfonates, ethers, and phosphates are viable electrophiles. However, the successful coupling of these inert electrophiles often exhibits marked substrate specificity and necessitates rigorous catalysis, wherein air-sensitive, transition-metal complexes and bulky, electron-rich phosphine or N-heterocyclic-carbene ligands are usually required. More significantly, although the coupling reactions of phenol derivatives have been extensively studied, couplings under milder, more appealing Suzuki–Miyaura conditions are relatively rare. 7,8c] Particularly, the Suzuki– Miyaura coupling of aryl phosphates has not been realized and has been shown to be extremely difficult, as exemplified by the Pdand Ni-catalyzed coupling reactions of an O,Odiphenyl-O-vinylphosphate, which occur exclusively with vinyl functions under Suzuki–Miyaura conditions. Recently, the Suzuki–Miyaura reactions of aryl esters, carbamates, carbonates, and sulfamates have been successfully achieved by using [NiACHTUNGTRENNUNG(PCy3)2Cl2] (Cy=cyclohexyl) as the catalyst or a catalyst system composed of [NiACHTUNGTRENNUNG(PCy3)2Cl2] and PCy3 (or PCy3HBF4) ligands. The ready availability of aryl substrates and the facile handling of the catalysts render these processes highly attractive. Despite these significant advances, the coupling efficiency of most of these processes (except for aryl sulfamates) is moderate for non-fused, aromatic aryl substrates, even in the presence of a large excess of the boronic acid (4.0–5.0 equivalents) and the base (7.2–9.0 equiv). Therefore, the development of a system that allows efficient Suzuki–Miyaura coupling remains a challenge. Herein, we present the first Suzuki– Miyaura reactions of aryl phosphoramides as a highly efficient and broadly applicable methodology for the diverse synthesis of biaryls and heterobiaryls, which uses a cheaper, as well as much more stable, catalyst system consisting of NiCl2 and 1,3-bis(diphenylphosphino)propane (dppp). During our search for a new methodology for phenol activation, we shifted our attention from the conventionally investigated aryl carbonyland sulfonyl-based derivatives to aryl phosphoramides (Scheme 1) and chose bis(2-oxo-3-

Nickel-Catalyzed Cross-Coupling of Phenols and Arylboronic Acids Through an In Situ Phenol Activation Mediated by PyBroP

A new method for the Suzuki-Miyaura cross-coupling of phenols and arylboronic acids through in situ phenol activation mediated by PyBroP is presented. The reaction proceeds efficiently by using cost-effective, markedly stable [NiCl(2)(dppp)] (dppp=1,3-bis(diphenylphosphino)propane) as the catalyst in only 5 mol % loading, as well as in the absence of extra ligands. The method exhibits broad applicability and high efficiency towards a wide range of both phenols and boronic acids, including activated, nonactivated, deactivated, and heteroaromatic coupling partners. In addition, various functional groups, such as ether, amino, cyano, ester, and ketone groups, are compatible with this transformation. Notably, arylboronic acids containing an unprotected NH(2) group and 2-heterocyclic boronic acids, which are generally problematic for coupling under conventional conditions, are also viable substrates, although moderate yields were obtained for sterically hindered substrates. Consequently, the in situ cross-coupling methodology coupled with the use of an inexpensive and stable nickel catalyst provides a rapid and efficient pathway for the assembly of biaryls and heterobiaryls with structural diversity from readily available phenol compounds.

Luminescence properties of metallo-supramolecular coordination polymers assembled from pyridine ring functionalized ditopic bis-terpyridines and Ru(II) ion

We present the study of photophysical properties of several Ru(II)-based metallo-supramolecular coordination polymers (MEPEs) constructed from various ditopic bis-terpyridines (bis-tpy) bearing electron-rich (OMe) or electron-deficient (Br) groups at the 6-position of the pyridyl periphery and having different spacers for connecting the two tpy moieties. The MEPEs are distinct from the frequently reported analogues derived from unsubstituted tpy. The photophysical properties such as emission spectroscopic properties and lifetime at both room temperature and 77 K are presented. The MEPEs are luminescent at room temperature with quantum yields (Φlum) < 10−6 and lifetimes on a timescale of nanoseconds (τ = ca. 6.6–22.4 ns), and display strong luminescence at 77 K with Φlum = ca. 0.21–2.5 × 10−2 and lifetimes on a timescale of microseconds (τ = ca. 2.4–7.9 μs). More importantly, we find that introduction of either an electron-rich or an electron-deficient functional group at the 6-position of the ligands results in a clearly decreased absorption intensity of the derived MEPEs. This result is in contrast to the reported data in the literature where an increased intensity was observed when substitution occurred at the 4′-position of tpys, regardless of the electron-rich or electron-deficient nature of the substituents. In accordance with the absorption behaviour, substitution at the 6-position also leads to a notably different effect on the luminescent properties of the complexes. Thus, our results provide an alternative strategy for the de novo design of new materials.

PdCl2-catalyzed efficient allylation and benzylation of heteroarenes under ligand, base/acid, and additive-free conditions

We present a PdCl(2)-catalyzed protocol for highly efficient allylation and benzylation of a rich variety of N-, O-, and S-containing heteroarenes under base/acid, additive, and ligand-free conditions. The method represents the very few examples for simple, universally applicable, clean, and atom-efficient functionalization of heteroarenes.

The development of copper-catalyzed aerobic oxidative coupling of H-tetrazoles with boronic acids and an insight into the reaction mechanism.

The development of a highly efficient and practical protocol for the direct C-N coupling of H-tetrazole and boronic acid was presented. A careful and patient optimization of a variety of reaction parameters revealed that this conventionally challenge reaction could indeed proceed efficiently in a very simple system, that is, just by stirring the tetrazoles and boronic acids under oxygen in the presence of different Cu(I) or Cu(II) salts with only 5 mol % loading in DMSO at 100 °C. Most significantly, the reaction could proceed very smoothly in a regiospecific manner to afford the 2,5-disubstituted tetrazoles in high to excellent yields. A mechanistic study revealed that both tetrazole and DMSO are crucial for the generation of catalytically active copper species in the reaction process in addition to their role as reactant and solvent, respectively. It is demonstrated that in the reaction cycle, the Cu(I) catalyst could be oxidized to Cu(II) by oxygen to form a [CuT2D] complex (T = tetrazole anion; D = DMSO) through an oxidative copper amination reaction. The Cu(II) complex thus formed was confirmed to be the real catalytically active copper species. Namely, the Cu(II) complex disproportionates to aryl Cu(III) and Cu(I) in the presence of boronic acid. Facile elimination of the Cu(III) species delivers the C-N-coupled product. The results presented herein not only provide a reliable and efficient protocol for the synthesis of 2,5-disubstituted tetrazoles, but most importantly, the mechanistic results would have broad implications for the de novo design and development of new methods for Cu-catalyzed coupling reactions.

PdII‐Catalyzed Mild CH ortho Arylation and Intramolecular Amination Oriented by a Phosphinamide Group

A novel protocol for the Pd-catalyzed ortho-arylation of aryl phosphinamide with boronic acid is reported. By using phosphinamide as a new directing group, the reaction proceeds efficiently under mild conditions at 408C. Mechanistic studies reveal that the reaction proceeds via a PdII to Pd0 cycle. The phosphinamide group is also shown to be an effective orienting group for direct C-H amination.