Exploiting the Brønsted Acidity of Phosphinecarboxamides for the Synthesis of New Phosphides and Phosphines

We demonstrate that the synthesis of new N‐functionalized phosphinecarboxamides is possible by reaction of primary and secondary amines with PCO^? in the presence of a proton source. These reactions proceed with varying degrees of success, and although primary amines generally afford the corresponding phosphinecarboxamides in good yields, secondary amines react more sluggishly and often give rise to significant decomposition of the 2‐phosphaethynolate precursor. Of the new N‐derivatized phosphinecarboxamides available, PH₂C(O)NHCy (Cy=cyclohexyl) can be obtained in sufficiently high yields to allow for the exploration of its Brønsted acidity. Thus, deprotonating PH₂C(O)NHCy with one equivalent of potassium bis(trimethylsilyl)amide (KHMDS) gave the new phosphide [PHC(O)NHCy]^?. In contrast, deprotonation with half of an equivalent gives rise to [P{C(O)NHCy}₂]^? and PH₃. These phosphides can be employed to give new phosphines by reactions with electrophiles, thus demonstrating their enormous potential as chemical building blocks.     

Cationic Heterocycles as Ligands: Synthesis and Reactivity with Anionic Nucleophiles of Cationic Triruthenium Clusters Containing C‐Metalated N‐Methylquinoxalinium or N‐Methylpyrazinium Ligands

The cationic cluster complexes [Ru₃(CO)₁₀(μ‐H)(μ‐κ²N,C‐L¹Me)]⁺ ( 3 ⁺; HL¹=quinoxaline) and [Ru₃(CO)₁₀(μ‐H)(μ‐κ²N,C‐L²Me)]⁺ ( 5 ⁺; HL²=pyrazine) have been prepared as triflate salts by treatment of their neutral precursors [Ru₃(CO)₁₀(μ‐H)(μ‐κ²N,C‐Lⁿ)] with methyl triflate. The cationic character of their heterocyclic ligands is responsible for their enhanced tendency to react with anionic nucleophiles relative to that of hydrido triruthenium carbonyl clusters that have neutral N‐heterocyclic ligands. These clusters react instantaneously with methyl lithium and potassium tris‐sec‐butylborohydride (K‐selectride) to give neutral products that contain novel nonaromatic N‐heterocyclic ligands. The following are the products that have been isolated: [Ru₃(CO)₉(μ‐H)(μ₃‐κ²N,C‐L¹Me₂)] ( 6 ; from 3 ⁺ and methyl lithium), [Ru₃(CO)₉(μ‐H)(μ₃‐κ²N,C‐L¹HMe)] ( 7 ; from 3 ⁺ and K‐selectride), [Ru₃(CO)₉(μ‐H)(μ₃‐κ²N,C‐L²Me₂)] ( 8 ; from 5 ⁺ and methyl lithium), and [Ru₃(CO)₉(μ‐H)(μ₃‐κ²N,C‐L²HMe)] ( 11 ; from 5 ⁺ and K‐selectride). Whereas the reactions of 3 ⁺ lead to products that arise from the attack of the corresponding nucleophile at the C atom of the only CH group adjacent to the N‐methyl group, the reactions of 5 ⁺ give mixtures of two products that arise from the attack of the nucleophile at one of the C atoms located on either side of the N‐methyl group. The LUMOs and the atomic charges of 3 ⁺ and 5 ⁺ confirm that the reactions of these clusters with anionic nucleophiles are orbital‐controlled rather than charge‐controlled processes. The N‐heterocyclic ligands of all of these neutral products are attached to the metal atoms in nonconventional face‐capping modes. Those of compounds 6 – 8 have the atoms of a ligand C?N fragment σ‐bonded to two Ru atoms and π‐bonded to the other Ru atom, whereas the ligand of compound 11 has a C? N fragment attached to a Ru atom through the N atom and to the remaining two Ru atoms through the C atom. A variable‐temperature ¹H NMR spectroscopic study showed that the ligand of compound 7 is involved in a fluxional process at temperatures above ?93 °C, the mechanism of which has been satisfactorily modeled with the help of DFT calculations and involves the interconversion of the two enantiomers of this cluster through a conformational change of the ligand CH₂ group, which moves from one side of the plane of the heterocyclic ligand to the other, and a 180° rotation of the entire organic ligand over a face of the metal triangle.     

PIII/PV=O Catalyzed Cascade Synthesis of N‐Functionalized Azaheterocycles

An organocatalytic method for the modular synthesis of diverse N‐aryl and N‐alkyl azaheterocycles (indoles, oxindoles, benzimidazoles, and quinoxalinediones) is reported. The method employs a small‐ring organophosphorus‐based catalyst (1,2,2,3,4,4‐hexamethylphosphetane P‐oxide) and a hydrosilane reductant to drive the conversion of ortho‐functionalized nitroarenes into azaheterocycles through sequential intermolecular reductive C?N cross coupling with boronic acids, followed by intramolecular cyclization. This method enables the rapid construction of azaheterocycles from readily available building blocks, including a regiospecific approach to N‐substituted benzimidazoles and quinoxalinediones.     

Synthesis,Electronic Properties and WOLED Devices of Planar Phosphorus‐Containing Polycyclic Aromatic Hydrocarbons

We describe the synthesis and the physical properties of polyaromatic hydrocarbons (PAHs) containing a phosphorus atom at the edge. In particular, the impact of the successive addition of aromatic rings on the electronic properties was investigated by experimental (UV/Vis absorption, fluorescence, cyclic voltammetry) and theoretical studies (DFT). The physical properties recorded in solution and in the solid state showed that the P‐containing PAHs exhibit properties expected for an emitter in white organic light‐emitting diodes (WOLEDs).     

Paving the Way to Novel Phosphorus‐Based Architectures: A Noncatalyzed Protocol to Access Six‐Membered Heterocycles

Phosphorus‐based heterocycles provide access to materials with properties that are inaccessible from all‐carbon architectures. The unique hybridization of phosphorus gives rise to electron‐accepting capacities, a large variety of coordination reactions, and the possibility of controlling the electronic properties through phosphorus postfunctionalization. Herein, we describe a new noncatalyzed synthetic protocol to prepare fused six‐membered phosphorus heterocycles. In particular, we report the synthesis of novel phosphaphenalenes. These fused systems exhibit the benefits of both five‐ and six‐membered phosphorus heterocycles and enable a series of versatile postfunctionalization reactions. This work thus opens up new horizons in the field of conjugated materials.     

Synthesis and Two‐Photon Absorption Property Characterizations of Small Dendritic Chromophores Containing Functionalized Quinoxaliniod Heterocycles

A series of star‐shaped multi‐polar chromophores (compounds 1 – 3 ) containing functionalized quinoxaline and quinoxalinoid (indenoquinoxaline and pyridopyrazine) units has been synthesized and characterized for their two‐photon absorption (2PA) properties both in the femtosecond and the nanosecond time domain. Under our experimental conditions, these model fluorophores are found to manifest strong and wide‐dispersed two‐photon absorption in the near‐infrared region. It is demonstrated that molecular structures with multi‐branched π frameworks incorporating properly functionalized quinoxalinoid units would possess large molecular nonlinear absorptivities within the studied spectral range. Effective optical‐power attenuation and stabilization behaviors in the nanosecond time domain of a selected representative dye molecule (i.e., compound 2 ) from this model compound set were also investigated and the results indicate that such structural motif could be a useful approach for the molecular design toward strong two‐photon‐absorbing material systems for quick‐responsive and broadband optical‐suppressing‐related applications, particularly to confront long laser pulses.     

Simple Synthesis of Functionalized 2‐Phosphanaphthalenes

A simple synthesis of sodium 2‐phosphanaphthalene‐3‐olate ( 1 ) based on the extrusion of N₂ from phthalazine using Na[OCP] is reported. This heterocycle can be readily functionalized at the negatively charged oxygen center using a variety of electrophilic substrates. The coordination chemistry of both 1 and its neutral derivatives was explored, revealing their facile use as P‐donor ligands for late‐transition‐metal complexes.     

Cobalt(II) Porphyrin‐Catalyzed Intramolecular Cyclopropanation of N‐Alkyl Indoles/Pyrroles with Alkylcarbene: Efficient Synthesis of Polycyclic N‐Heterocycles

A protocol on chemoselective cobalt(II) porphyrin‐catalyzed intramolecular cyclopropanation of N‐alkyl indoles/pyrroles with alkylcarbenes has been developed. The reaction enables the rapid construction of a range of nitrogen‐containing polycyclic compounds in moderate to high yields from readily accessible materials. These N‐containing polycyclic compounds can be converted into a variety of N‐heterocycles with potential synthetic and biological interest. Compared to their N‐tosylhydrazone counterparts, the use of bulky N‐2,4,6‐triisopropylbenzenesulfonyl hydrazones as carbene precursors allows cyclopropanation to occur under milder reaction conditions.     

Solid‐Phase Synthesis of Structurally Diverse Heterocycles by an Amide–Ketone Condensation/N‐Acyliminium Pictet–Spengler Sequence

An efficient approach for the solid‐phase synthesis of structurally diverse heterocyclic compounds is presented. Under acidic reaction conditions, peptidic levulinamides undergo intramolecular ketone–amide condensation reactions to form cyclic N‐acyliminium intermediates. In the presence of a tethered nucleophile, a second cyclization reaction results in the formation of a fused bicyclic ring system. The scope of the methodology was demonstrated by several combinations of substituted ketones and nucleophiles, the latter conveniently originating from amino acids with functionalized side chains, such as tryptophan, substituted phenylalanines, and cysteine. The cyclization sequence provides diastereomerically pure products in high yields. In one extension of the methodology, the resulting relative stereochemistry of the products enables the formation of bridged ring systems by a unique cyclative release mechanism.     

Rapid Synthesis of Fused N‐Heterocycles by Transition‐Metal‐Free Electrophilic Amination of Arene CH Bonds

We disclose an efficient and operationally simple protocol for the preparation of fused N‐heterocycles starting from readily available 2‐nitrobiaryls and PhMgBr under mild conditions. More than two dozen N‐heterocycles, including two bioactive natural products, have been synthesized using this method. A stepwise electrophilic aromatic cyclization mechanism was proposed by DFT calculations.     

Reactivity of Bridged Pentelidene Complexes with Isonitriles: A New Way to Pentel‐Containing Heterocycles

The reaction of [Cp*E{W(CO)₅}₂] (E=P ( 1 a ), As ( 1 b ); Cp*=1,2,3,4,5‐pentamethylcyclopentadienyl) with isonitriles RNC (R=tBu, cyclohexyl (Cy), nBu) depends on the steric demand of the substituent at the isonitrile as well as on the stoichiometry of the starting materials. With tBuNC only the Lewis acid/base adducts [Cp*E{W(CO)₅}₂(CNtBu)] (E=P ( 2 a ), As ( 2 b )) are formed. The use of Cy and n‐butylisonitrile leads first to the formation of the Lewis acid/base adduct, but only at low temperatures. At ambient temperatures, a rearrangement occurs and bicyclo[3.2.0]heptane derivatives of the type [{C(Me)C(CH₂)C(Me)C(Me)C(Me)}C(NR)‐ E{W(CO)₅}₂] (E=P, As; R=Cy, nBu) ( 3 a‐Cy , 3 b‐Cy , 3 a‐nBu and 3 b‐nBu ) are obtained. The use of a further equivalent of isonitrile results in products revealing two new structural motifs, the four‐membered ring derivatives [C(Cp*)N(R)C(NR)E{W(CO)₅}₂] ( 4 : E=P, As; R=Cy, nBu) and the bicyclic complexes [[{C(Me)C‐ (CH₂)C(Me)C(Me)C(Me)}C(NR)₂‐ E{W(CO)₅}₂] ( 5 : E=As; R=Cy). The reaction pathway depends on the substituent at the isonitrile. By treatment of 1 a with two equivalents of CyNC only a 2H‐1,3‐azaphosphet complex 4 a‐Cy (E=P; R=Cy) is formed. Treatment of 1 b with two equivalents of CyNC exclusively leads to the complex 5 b‐Cy (E=As; R=Cy). Treatment of 1 a with two equivalents of nBuNC results in a mixture of complexes, the 2H‐1,3‐azaphosphet 4 a‐nBu (E=P; R=nBu) and the bicyclic complex 5 a‐nBu (E=P; R=nBu). For the arsenidene complex 1 b a mixture of the 2H‐1,3‐azarsete complex 4 b‐nBu (E=As; R=nBu) and the bicyclic complex 5 b‐nBu (E=P, As; R=Cy, nBu) is obtained. Complex 4 b‐nBu is the first example of a 2H‐1,3‐azarsete complex. All products have been characterized by using mass spectrometry, NMR spectroscopy, and X‐ray diffraction analysis.     

Synthesis of Spiro‐Heterocycles with the Thiazolidinone Moiety from Nitrilimines

The 2‐phenylimino‐1,3‐thiazolidin‐4‐one 2 was obtained by thermal cyclization of 4‐amino‐5‐phen‐yl‐3, 5‐thiaaza‐4‐pentenoic acid 1 using DCC as dehydration agent. Treatment of 2‐phenylimino‐1,3‐thiazolidin‐4‐one 2 with various hydrazonoyl halides 3 (nitrilimines 4 precursor) yielded 6‐aryl‐9‐phenyl‐8‐substituted‐1,4,6,7,9‐thiatetrazaspiro‐[4.4]non‐7‐en‐3‐ones 5a‐1. Both analytical and spectroscopic data of all the synthesized compounds are in full agreement with the proposed structures.     

Transition Metal‐Participated Synthesis and Utilization of N‐containing Heterocycles: Exploring for Nitrogen Sources

This account aims to describe our recent efforts on the synthesis and utilization of N‐containing heterocycles, where transition metals participate in the synthesis. A variety of nitrogen sources, including amines, amides, hydrazones, pyrimidines, isocyanides, and copper nitrate, have been disclosed for the synthesis of diverse bioactive and pharmacologically interesting N‐containing heterocycles under the participation of transition metals. The well‐known nitrogen sources, such as amines and amides, were used for the construction of indoles, isatins, and quinolones. Dihydrophthalazines, isoquinolines, indazoles, and pyrazoles were obtained from hydrazones, while various pyrimidine‐containing heterocycles were afforded through regioselective C?H functionalizations using pyrimidine as the directing group. Recent research has focused on the chemistry of isocyanides to achieve several kinds of heterocyclic compounds with high efficiency under the catalysis of transition metals (Pd, Rh, Mn, Cu), through oxidative cyanation reactions, sequential isocyanide insertions into C?H, N?H, or O?H bonds, and tandem radical annulation. More recently, an efficient route to isoxazolines has been reported using copper nitrate as a novel nitrogen source.     

Regioselective Acceptorless Dehydrogenative Coupling of N‐Heterocycles toward Functionalized Quinolines,Phenanthrolines, and Indoles

A new strategy has been developed for the oxidant‐ and base‐free dehydrogenative coupling of N‐heterocycles at mild conditions. Under the action of an iridium catalyst, N‐heterocycles undergo multiple sp³ C? H activation steps, generating a nucleophilic enamine that reacts in situ with various electrophiles to give highly functionalized products. The dehydrogenative coupling can be cascaded with Friedel–Crafts addition, resulting in a double functionalization of the N‐heterocycles.