Copper-Mediated Annulation of Phosphorus-Containing Arenes with Alkynes: An Approach to Phosphindolium Salts

Dithieno[3,2-b; 2',3'-f]phosphepinium-Based Near-Infrared Fluorophores: px-π* Conjugation Inherent to Seven-Membered Phosphacycles

Positively charged phosphorus-containing heterocycles are characteristic core skeletons for functional molecules. While various phosphonium-containing five- or six-membered-ring compounds have been reported, the seven-membered-ring phosphepinium have not been fully studied yet. In this study, dithieno[3,2-b; 2',3'-f]phosphepinium ions containing electron-donating aminophenyl groups were synthesized. An X-ray crystallographic analysis of the resulting donor-acceptor-donor dyes revealed a bent conformation of the central seven-membered ring. These compounds exhibit fluorescence in the near-infrared region with a bathochromic shift of ca. 70 nm compared to a phosphepine oxide congener and a large Stokes shift. High fluorescence quantum yields were obtained even in polar solvents due to the suppression of the nonradiative decay process. A theoretical study revealed that the phosphepinium skeleton is highly electron-accepting owing to the orbital interaction between a px orbital of the phosphonium moiety and a π* orbital of the 1,3,5-hexatriene moiety. Due to the lower-lying px orbital in the phosphonium moiety compared to that of the phosphine oxide and the bent conformation of the seven-membered ring, the phosphepinium ring permits effective px-π* conjugation. A large structural relaxation with a contribution of a quinoidal resonance structure is suggested in the excited state, which should be responsible for the bright emission with a large Stokes shift.

Couple-close construction of non-classical boron cluster-phosphonium conjugates

Heteropolycyclic molecular systems, which are essential components in the fields of materials and pharmacology, frequently consist of 2D extended organic aromatic rings. Here, we introduce a type of inorganic-organic hybrid 3D conjugates by merging an aromatic boron cluster with a phosphine and a π-conjugated unit. To achieve this, a couple-close synthetic strategy via B-H activation of nido-carboranes with alkynes has been developed, which leads to diverse boron cluster-extended phosphoniums in a twisted structure with high yields under mild conditions. Experimental and theoretical results reveal that the fusion between the boron cluster and the formed borophosphonium heterocycle facilitates electron delocalization throughout the structure. The unusual framework demonstrates distinct properties from bare boron clusters and pure aromatic ring-extended counterparts, such as improved thermal/chemical stability and photophysical properties. Thus, the boron cluster-based 3D conjugates expand the library of aromatic-based heterocyclics, showcasing great potential in functional materials.

Synthesis, Photophysical and Electrochemical Properties of Spiro-Phosphonium Compounds

A series of spiro-phosphonium compounds have been synthesized by copper-mediated coupling reaction of phosphacyclic compounds with alkynes. Their photophysical properties are tuned by varying substituents and exhibit different luminescent colors from blue to green, and finally, yellow. The fluorescence quantum efficiency of diethyl spiro-xanthenebenzophosphole 3aa in solid and liquid states reached 31% and 76%, respectively. Diphenyl spiro-xanthenebenzophosphole 3ad displayed relatively low cytotoxicity toward lung cancer cells A549 and was able to effectively penetrate the cell membrane and maintain strong staining. Moreover, density functional theory (DFT) and time-dependent DFT calculations have been performed to explore the origin of their photophysical properties.

Advancement in Synthetic Strategies of Phosphorus Heterocycles: Recent Progress from Synthesis to Emerging Class of Optoelectronic Materials

Organophosphorus heterocycles have long been acknowledged for their significant potential across diverse fields, including catalysis, material science, and drug development. Incorporating phosphorus functionalities into organic compounds offers a means to effectively tailor their medicinal properties, augment biological responses, and enhance selectivity and bioavailability. The distinctive physical and photoelectric characteristics of phosphorus-containing conjugated compounds have garnered considerable interest as promising materials for organic optoelectronics. These compounds find extensive utility in various applications such as light-emitting diodes, photovoltaic cells, phosphole-based fluorophores, and semiconductors.

Rhodium(III)-Catalyzed Triple Aryl/Alkenyl C-H Bond Activation of Aryl Enaminones to Access Naphtho[1,8-bc]pyrans

Rhodium(III)-catalyzed triple C-H bond activation of aryl enaminones was achieved to access naphtho[1,8-bc]pyrans by oxidative annulation to internal alkynes. 1-Naphthols might be formed as the only products, depending on the steric and/or electronic environment around the aroyl moiety of the aryl enaminones or the electronic impact from the alkynes. With propargyl alcohols as the masked terminal alkynes, aryl enaminones underwent rhodium(III)- or rhodium(I)-catalyzed internal alkenyl C-H bond activation to afford functionalized but-2-ene-1,4-diones. The resultant naphtho[1,8-bc]pyrans are highly fluorescent and can be further transformed by chlorination, bromination, and difluoromethylation, demonstrating potential practicability of the synthetic protocol.

Two-Fold Intramolecular Phosphacyclization: From Fluorescent Diphosphapyrene Salts to Pentavalent Derivatives

The synthesis of π-extended pyrene-based luminescent compounds containing two six-membered phosphacycles has been realized through a two-step synthesis. It involves a Cu(II)-mediated double cyclization of tertiary diphosphane derivatives to afford dicationic molecules with quaternized phosphorus centers. Subsequent transformation of diphosphonium species into the corresponding P-oxide derivatives has been successfully achieved through Pd(0)-assisted cleavage of the P-Ph bonds, which opens a promising way for the functionalization of polyaromatic P-systems.

A 2,2'-diphosphinotolane as a versatile precursor for the synthesis of P-ylidic mesoionic carbenes via reversible C-P bond formation

A metal-templated synthetic route to cyclic (aryl)(ylidic) mesoionic carbenes (CArY-MICs) featuring an endocyclic P-ylide is presented. This approach, which requires metal templates with two cis-positioned open coordination sites, is based on the controlled cyclisation of a P,P'-diisopropyl-substituted 2,2'-diphosphinotolane (1) and leads to chelate complexes coordinated by a phosphine donor and the CArY-MIC carbon atom. The C-P bond formation involved in the former partial cyclisation of 1 proceeds under mild conditions and was shown to be applicable all over the d-block. In the presence of a third fac-positioned open coordination site, the P-C bond formation was found to be reversible, as shown for a series of molybdenum complexes. DFT modelling studies are in line with an interpretation of the target compounds as CArY-MICs.

Cationic Organophosphorus Chromophores: A Diamond in the Rough among Ionic Dyes

Tunable electron-accepting properties of the cationic phosphorus center, its geometry and unique preparative chemistry that allows combining this unit with diversity of π-conjugated motifs, define the appealing photophysical and electrochemical characteristics of organophosphorus ionic chromophores. This Minireview summarizes the achievements in the synthesis of the π-extended molecules functionalized with P-cationic fragments, modulation of their properties by means of structural modification, and emphasizes the important effect of cation-anion interactions, which can drastically change physical behavior of these two-component systems.

Toward N,P-Doped π-Extended PAHs: A One-Pot Synthesis to Diannulated 1,4,2-Diazaphospholium Triflate Salts

A novel method for the one-pot synthesis of diannulated 1,4,2-diazaphospholium triflate salts by a Me₃SiOTf-mediated self-condensation of dichlorophosphaneyl aza-(poly)cyclic aromatic hydrocarbons (aza-(P)AHs; namely, pyridine, quinoline, phenanthridine, and benzo[d]thiazole) is reported. The diannulated 1,4,2-diazaphospholium triflate salts are characterized by multinuclear NMR spectroscopy, X-ray analysis, as well as their calculated NICS values, underlining their aromatic character. Quantum mechanical calculations shed light on the intermolecular reaction mechanism. Follow up chemistry, such as the halogenation reaction with XeF₂ or SO₂Cl₂ with the dipyridinium derivative selectively yields the respective dihalo-σ⁴,λ⁵- and tetrahalo-σ⁵,λ⁶-diazaphospholium triflate salts. The dihalo-σ⁴,λ⁵-diazaphospholium triflate salt serves well as a surrogate for the introduction of the cationic 2-(1,2'-bipyridin)-1-iumyl ligand (1,2'-bipyl), the monocationic structural isomer of the prototypical 2,2'-bipyridine ligand (2,2'-bipy).

Intramolecular Phosphacyclization: Polyaromatic Phosphonium P-Heterocycles with Wide-Tuning Optical Properties

Rationally designed cationic phospha-polyaromatic fluorophores were prepared through intramolecular cyclization of the tertiary ortho-(acene)phenylene-phosphines mediated by CuII triflate. As a result of phosphorus quaternization, heterocyclic phosphonium salts 1 c-3 c, derived from naphthalene, phenanthrene, and anthracene cores, exhibited very intense blue to green fluorescence (Φem =0.38-0.99) and high photostability in aqueous medium. The structure-emission relationship was further investigated by tailoring the electron-donating functions to the anthracene moiety to give dyes 4 c-6 c with charge-transfer character. The latter significantly decreases the emission energy to reach near-IR region. Thus, the intramolecular phosphacyclization renders an ultra-wide tuning of fluorescence from 420 nm (1 c) to 780 nm (6 c) in solution, extended to 825 nm for 6 c in the solid state with quantum efficiency of approximately 0.07. The physical behavior of these new dyes was studied spectroscopically, crystallographically, and electrochemically, whereas computational analysis was used to correlate the experimental data with molecular electronic structures. The excellent stability, water solubility, and attractive photophysical characteristics make these phosphonium heterocycles powerful tools in cell imaging.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure. The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them. Accordingly, this review should be of particular interest also for scientists from industrial R&D sector. Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date.

P–C reductive elimination in Ru(ii) complexes to convert triarylphosphine ligands into five- or six-membered phosphacycles

η⁴-Phosphacycle complexes were obtained from PPh₂Ar (Ar = Ph, 1-Naph) via cyclometallation, alkyne insertion and P–C reductive elimination at a Ru centre.

Late stage modifications of P-containing ligands using transition-metal-catalysed C–H bond functionalisation

Recent advances in C–H bond functionalisation of phosphorus derivatives are presented, with a special attention to preparation of phosphine ligands.