In Situ Phosphine Oxide Reduction: A Catalytic Appel Reaction

Electrophilic C(sp2)-H Cyanation with Inorganic Cyanate (OCN-) by PIII/PV=O-Catalyzed Phase Transfer Activation

An organophosphorus -catalyzed method for the direct electrophilic cyanation of C(sp2)-H nucleophiles with sodium cyanate (NaOCN) is reported. The catalytic deoxyfunctionalization of the OCN- anion is enabled by the use of a small-ring phosphacyclic (phosphetane) catalyst in combination with a terminal hydrosilane O-atom acceptor and a malonate-derived bromenium donor. In situ spectroscopy under single-turnover conditions demonstrate that insoluble inorganic cyanate anion is activated by bromide displacement on a bromophosphonium catalytic intermediate to give a reactive N-bound isocyanatophosphonium ion, which delivers electrophilic "CN+" equivalents to nucleophilic (hetero)arenes and alkenes with loss of a phosphine oxide. These results demonstrate the feasibility of deoxyfunctionalization of insoluble inorganic salts by PIII/PV=O catalyzed phase transfer activation.

Chiral Bisphosphine-Catalyzed Asymmetric Staudinger/aza-Wittig Reaction: An Enantioselective Desymmetrizing Approach to Crinine-Type Amaryllidaceae Alkaloids

An unprecedented chiral bisphosphine-catalyzed asymmetric Staudinger/aza-Wittig reaction of 2,2-disubstituted cyclohexane-1,3-diones is reported, enabling the facile access of a broad range of cis-3a-arylhydroindoles in high yields with excellent enantioselectivities. The key to the success of this work relies on the first application of chiral bisphosphine DuanPhos to the asymmetric Staudinger/aza-Wittig reaction. An effective reductive system has been established to address the challenging PV═O/PIII redox cycle associated with the chiral bisphosphine catalyst. In addition, comprehensive experimental and computational investigations were carried out to elucidate the mechanism of the asymmetric reaction. Leveraging the newly developed chemistry, the enantioselective total syntheses of several crinine-type Amaryllidaceae alkaloids, including (+)-powelline, (+)-buphanamine, (+)-vittatine, and (+)-crinane, have been accomplished with remarkable conciseness and efficiency.

(3+2) Annulation of 4-Acetoxy Allenoate with Aldimine Enabled by AgF-Assisted P(III)/P(V) Catalysis

A phosphine-catalyzed (3+2) annulation of 4-acetoxy allenoate and aldimine with the assistance of AgF is described. The success of this reaction hinges on the metathesis between the enolate-phosphonium zwitterion and AgF, leading to a key intermediate comprising of silver enolate and a fluorophosphorane P(V)-moiety. The former is able to undergo a Mannich reaction with aldimine, whereas the latter initiates a cascade sequence of AcO-elimination/aza-addition, thus furnishing the P(III)/P(V) catalysis. By taking advantage of the silver enolate, a preliminary attempt at an asymmetric variant was conducted with the combination of an achiral phosphine catalyst and a chiral bis(oxazolinyl)pyridine ligand (PyBox), giving moderate enantioselectivity.

Organocatalytic Stereospecific Appel Reaction

Herein we report a new method for the catalytic Appel reaction by P(III)/P(V) redox cycling at very low catalyst loadings of 1-2 mol % using low amounts of hexachloroacetone as the halogen source and phenylsilane as the terminal reductant. Twenty-six alcohols and nine epoxides containing a wide variety of functional groups were converted to the respective chlorides and dichlorides in yields of up to 97%, enantiospecificities of up to >99%, and enantiomeric ratios of up to >99:1.

Mechanistic Insights into the Appel Reaction Mediated by a Poly-Phosphamide Material as a Heterogeneous Catalyst

Due to notable thermochemical stability, polyphosphamides are often regarded as flame retardants, while molecular phosphamides can serve as versatile Lewis base to catalyze diverse organic transformations. Being chemically analogous to phosphine oxide, phosphamide can also be considered as a mediator for the phosphine-mediated reaction. Herein, an amorphous polymeric material consisting of phosphamide (-NH-P(O)) in the repeating unit (POP) has been prepared via condensation of tris(2-aminoethyl)amine (TREN) and phenyl phosphinic dichloride (PPDC). The POP is isolated as a metal-free and pure organic material which is made of a strong covalent bond and the phosphamide unit is deployed in the organic framework. The presence of phosphamide in the repeating unit of the isolated amorphous POP material can be confirmed by 31P CPMAS NMR, FTIR, and Raman studies. The core-level N 2p and P 2p X-ray photoelectron spectra are in accordance with the presence of tertiary amine nitrogen attached to carbon and secondary amine nitrogen attached to phosphorus. Elemental analyses have depicted approximately 19.7% of phosphorus content in the material, which is being utilized to study the catalytic Appel reaction with 76% conversion of alcohol to a corresponding halide and TON of 462. Quasi in situ Raman study has identified that amino phosphine formed via in situ reduction of the phosphamide unit of the POP catalyzes the halogenation of primary and secondary alcohols with wide substrate scope and functional group tolerance. Kinetic studies have established a first-order dependence with respect to alcohol, while deuterium labeling experiments emphasize that the deprotonation of alcohol is the rate-limiting step. High thermal stability of the material, scope of easy catalyst recyclability, and a cumulative TON of 1386 have led the POP as an emerging pure organic material to be explored further for other phosphine-mediated organocatalysis.

Enantioselective Palladium-Catalyzed Domino Carbonylative Heck Esterification of o-Iodoalkenylbenzenes with Arylboronic Acids

The current investigation presents an innovative palladium-catalyzed asymmetric carbonylative Heck esterification method. This approach facilitates the efficient synthesis of various chiral γ-ketoacid esters by utilizing o-alkenyliodobenzenes and arylboronic acids as primary substrates. This reaction achieves the creation of three carbon-carbon bonds, two carbon-oxygen bonds, and the establishment of a quaternary carbon center within a single step. The α-chiral γ-ketoacid esters were obtained in yields ranging from good to high yields, displaying enantiomeric excesses (ee's) levels up to 92% under mild reaction conditions.

Deoxygenation of Phosphine Oxides by PIII/PV═O Redox Catalysis via Successive Isodesmic Reactions

Deoxygenation of phosphine oxides is of great significance to synthesis of phosphorus ligands and relevant catalysts, as well as to the sustainability of phosphorus chemistry. However, the thermodynamic inertness of P═O bonds poses a severe challenge to their reduction. Previous approaches in this regard rely primarily on a type of P═O bond activation with either Lewis/Brønsted acids or stoichiometric halogenating reagents under harsh conditions. Here, we wish to report a novel catalytic strategy for facile and efficient deoxygenation of phosphine oxides via successive isodesmic reactions, whose thermodynamic driving force for breaking the strong P═O bond was compensated by a synchronous formation of another P═O bond. The reaction was enabled by P^III/P═O redox sequences with the cyclic organophosphorus catalyst and terminal reductant PhSiH₃. This catalytic reaction avoids the use of the stoichiometric activator as in other cases and features a broad substrate scope, excellent reactivities, and mild reaction conditions. Preliminary thermodynamic and mechanistic investigations disclosed a dual synergistic role of the catalyst.

Immobilization of Phosphamide on the TiO2 Surface for Heterogeneous Phase Catalytic Appel Reaction

Global consumption of triphenylphosphine (Ph₃P) for phosphorus-mediated organic synthesis and production of the dead-end triphenylphosphine oxide (Ph₃PO) waste is exceptionally high. Recycling Ph₃PO and/or use of it as a reaction mediator gained significant attention. On the other end, phosphamides, traditionally used as a flame redundant, are stable analogues to Ph₃PO. Herein, via a low temperature condensation reaction of methyl 4-(aminomethyl)benzoate (AMB) and diphenyl phosphinic chloride (DPPC), methyl 4-((N,N-diphenylphosphinamido)methyl)benzoate (1) has been synthesized and hydrolysis of the ester functional group of 1 leads to a phosphamide with a carboxylate terminal, 4-((N,N-diphenylphosphinamido)methyl)benzoic acid (2). The presence of phosphamide functionality (NH─P═O) in 2 can be confirmed by its characteristic Raman vibration at 999 cm^-1 with expected P-N and P═O bonds distances from the single-crystal X-ray structure. In-situ hydrolysis of [Ti(OiPr)₄] in the presence of 2 followed by hydrothermal heating results in immobilization of 2 on a ca. 5 nm TiO₂ surface (2@TiO₂). The covalent attachment of 2 via coordination through the carboxylate terminal to the TiO₂ nanocrystal's surface has been established via multiple spectroscopic and microscopic studies. 2@TiO₂ is further used as the heterogeneous mediator for the catalytic Appel reaction, halogenation of alcohol (typically mediated by phosphine), with a fair catalytic conversion and a recorded TON up to 31. The major advantage of the heterogeneous approach studied herein is the recovery of used 2@TiO₂ from the reaction mixture via centrifugation only leaving the organic product in the supernatant, which is limiting in Ph₃P-mediated homogeneous catalysis. Time-resolved Raman spectroscopy confirms amino phosphine as the active species formed in-situ during the catalytic Appel reaction. Post-catalytic characterization of the material recovered after catalysis from the reaction mixture confirms the chemical integrity and that can further be utilized for another two catalytic runs. The developed reaction scheme showcases the use of a phosphamide as a reactive analogue to Ph₃PO for an organic reaction in a heterogeneous approach, and the same strategy can be explored further as a general scheme for other phosphorus-mediated reactions.

Catalytic Vilsmeier-Haack Reactions for C1-Deuterated Formylation of Indoles

The Vilsmeier-Haack reaction is a powerful tool to introduce formyl groups into electron-rich arenes, but its wide application is significantly restricted by stoichiometric employment of caustic POCl₃. Herein, we reported a catalytic version of the Vilsmeier-Haack reaction enabled by a P(III)/P(V)═O cycle. This catalytic reaction provides a facile and efficient route for the direct construction of C1-deuterated indol-3-carboxaldehyde under mild conditions with stoichiometric DMF-d7 as the deuterium source. The products feature a remarkably higher deuteration level (>99%) than previously reported ones and are not contaminated by the likely unselective deuteration at other sites. The present transformation can also be used to transfer other carbonyl groups. Further downstream derivatizations of these deuterated products manifested their potential applications in the synthesis of deuterated bioactive molecules. Mechanistic insight was disclosed from studies of kinetics and intermediates.

A Facile Synthetic Method for Anhydride from Carboxylic Acid with the Promotion of Triphenylphosphine Oxide and Oxaloyl Chloride

A highly efficient synthesis reaction of carboxylic anhydrides catalyzed by triphenylphosphine oxide is described for the quick synthesis of a range of symmetric carboxylic anhydrides and cyclic anhydrides under mild and neutral conditions with a high yield. The system adopts the strong reactive intermediate Ph₃PCl₂ as the catalyst of carboxylic acid salt; driven by catalytic reaction, the synthesis takes a relatively short time to complete.

Ligand-Enabled Disproportionation of 1,2-Diphenylhydrazine at a PV -Center

We present herein the synthesis of a nearly square-pyramidal chlorophosphorane supported by the tetradentate bis(amidophenolate) ligand, N,N'-bis(3,5-di-tert-butyl-2-phenoxy)-1,2-phenylenediamide. After chloride abstraction the resulting phosphonium cation efficiently promotes the disproportionation of 1,2-diphenylhydrazine to aniline and azobenzene. Mechanistic studies, spectroscopic analyses and theoretical calculations suggest that this unprecedented reactivity mode for PV -centres is induced by the high electrophilicity at the cationic PV -center, which originates from the geometry constraints imposed by the rigid pincer ligand, combined with the ability of the o-amidophenolate moieties to act as electron reservoir. This study illustrates the promising role of cooperativity between redox-active ligands and phosphorus for the design of organocatalysts able to promote redox processes.

Visible Light-Driven, Gold(I)-Catalyzed Preparation of Symmetrical (Hetero)biaryls by Homocoupling of Arylazo Sulfones

The preparation of symmetrical (hetero)biaryls via arylazo sulfones has been successfully carried out upon visible light irradiation in the presence of PPh3AuCl as the catalyst. The present protocol led to the efficient synthesis of a wide range of target compounds in an organic-aqueous solvent under photocatalyst-free conditions.

A rational synthesis of ultrasmall palladium-based alloys with superhydrophilicity as biocompatible agents and recyclable catalysts

As essential controlling parameters, the local surface area (size distribution) and polarity property of the surface molecules can determine the catalytic activity and biocompatibility directly. Here, ultrasmall palladium-based alloys (FePd, FePdCo, and FePdCu NCs) were developed to serve as artificial degradation catalysts with superhydrophilicity (SPh), biocompatibility, and reusability, which were referred to as "biocatalysts". As synthesized in aqueous solvent with negative surface potential while dispersing in water medium, because of the surface biological molecules effect. The obtained alloys illustrated a size distribution of about 3.5 nm. Additionally, owing to SPh property, these alloys could be stored in water up to 30 days without any precipitation and retained their monodisperse morphology in colloidal solutions. The cytotoxicity assessment of the alloys by exposing to L-929 cells over 3 days indicated that it maintained cell viability of >80% even up to 390 μg mL-1 (concentration of alloys). Furthermore, they exhibited an obvious enhancement in the catalytic performance for degrading Rhodamine B (RhB) and 4-nitrophenol (4-NP). The recyclable utilization of biocatalysts demonstrates durable stability even after 8 reduction cycles.

An environmentally benign and high-rate Appel type reaction

A green, high rate, high yielding, and selective Appel reaction that make use of 1,3-dihalo-5,5-dimethylhydantoins and N-halo-succinimides as halogenation reagents. The new Appel reaction method was developed for batch and continuous flow reactors.

Synthesis of Dipeptide, Amide, and Ester without Racemization by Oxalyl Chloride and Catalytic Triphenylphosphine Oxide

An efficient triphenylphosphine oxide-catalyzed amidation and esterification for the rapid synthesis of a series of dipeptides, amides, and esters is described. This reaction is applicable to challenging couplings of hindered carboxylic acids with weakly nucleophilic amines or alcohols, giving the products in good yields (67-90%) without racemization. This system employs the highly reactive intermediate Ph₃PCl₂ as the activator of the carboxylate in a catalytic manner and drives the reaction to completion in a short reaction time (less than 10 min).

Uniting Amide Synthesis and Activation by PIII/PV-Catalyzed Serial Condensation: Three-Component Assembly of 2-Amidopyridines

An organophosphorus (P^III/P^V redox) catalyzed method for the three-component condensation of amines, carboxylic acids, and pyridine N-oxides to generate 2-amidopyridines via serial dehydration is reported. Whereas amide synthesis and functionalization usually occur under divergent reaction conditions, here a phosphetane catalyst (together with a mild bromenium oxidant and terminal hydrosilane reductant) is shown to drive both steps chemoselectively in an auto-tandem catalytic cascade. The ability to both prepare and functionalize amides under the action of a single organocatalytic reactive intermediate enables new possibilities for the efficient and modular preparation of medicinal targets.

Catalytic Hydrodefluorination via Oxidative Addition, Ligand Metathesis, and Reductive Elimination at Bi(I)/Bi(III) Centers

Herein, we report a hydrodefluorination reaction of polyfluoroarenes catalyzed by bismuthinidenes, Phebox-Bi(I) and OMe-Phebox-Bi(I). Mechanistic studies on the elementary steps support a Bi(I)/Bi(III) redox cycle that comprises C(sp²)-F oxidative addition, F/H ligand metathesis, and C(sp²)-H reductive elimination. Isolation and characterization of a cationic Phebox-Bi(III)(4-tetrafluoropyridyl) triflate manifests the feasible oxidative addition of Phebox-Bi(I) into the C(sp²)-F bond. Spectroscopic evidence was provided for the formation of a transient Phebox-Bi(III)(4-tetrafluoropyridyl) hydride during catalysis, which decomposes at low temperature to afford the corresponding C(sp²)-H bond while regenerating the propagating Phebox-Bi(I). This protocol represents a distinct catalytic example where a main-group center performs three elementary organometallic steps in a low-valent redox manifold.

Phosphorus-Based Catalysis

Phosphorus-based organocatalysis encompasses several subfields that have undergone rapid growth in recent years. This Outlook gives an overview of its various aspects. In particular, we highlight key advances in three topics: nucleophilic phosphine catalysis, organophosphorus catalysis to bypass phosphine oxide waste, and organophosphorus compound-mediated single electron transfer processes. We briefly summarize five additional topics: chiral phosphoric acid catalysis, phosphine oxide Lewis base catalysis, iminophosphorane super base catalysis, phosphonium salt phase transfer catalysis, and frustrated Lewis pair catalysis. Although it is not catalytic in nature, we also discuss novel discoveries that are emerging in phosphorus(V) ligand coupling. We conclude with some ideas about the future of organophosphorus catalysis.

Main Group Redox Catalysis of Organopnictogens: Vertical Periodic Trends and Emerging Opportunities in Group 15

A growing number of organopnictogen redox catalytic methods have emerged-especially within the past 10 years-that leverage the plentiful reversible two-electron redox chemistry within Group 15. The goal of this Perspective is to provide readers the context to understand the dramatic developments in organopnictogen catalysis over the past decade with an eye toward future development. An exposition of the fundamental differences in the atomic structure and bonding of the pnictogens, and thus the molecular electronic structure of organopnictogen compounds, is presented to establish the backdrop against which organopnictogen redox reactivity-and ultimately catalysis-is framed. A deep appreciation of these underlying periodic principles informs an understanding of the differing modes of organopnictogen redox catalysis and evokes the key challenges to the field moving forward. We close by addressing forward-looking directions likely to animate this area in the years to come. What new catalytic manifolds can be developed through creative catalyst and reaction design that take advantage of the intrinsic redox reactivity of the pnictogens to drive new discoveries in catalysis?

1,3-Diphenyldisiloxane Enables Additive-Free Redox Recycling Reactions and Catalysis with Triphenylphosphine

The recently reported chemoselective reduction of phosphine oxides with 1,3-diphenyldisiloxane (DPDS) has opened up the possibility of additive-free phosphine oxide reductions in catalytic systems. Herein we disclose the use of this new reducing agent as an enabler of phosphorus redox recycling in Wittig, Staudinger, and alcohol substitution reactions. DPDS was successfully utilized in ambient-temperature additive-free redox recycling variants of the Wittig olefination, Appel halogenation, and Staudinger reduction. Triphenylphosphine-promoted catalytic recycling reactions were also facilitated by DPDS. Additive-free triphenylphosphine-promoted catalytic Staudinger reductions could even be performed at ambient temperature due to the rapid nature of phosphinimine reduction, for which we characterized kinetic and thermodynamic parameters. These results demonstrate the utility of DPDS as an excellent reducing agent for the development of phosphorus redox recycling reactions.

Organophosphorus-catalyzed relay oxidation of H-Bpin: electrophilic C-H borylation of heteroarenes

A nontrigonal phosphorus triamide (1, P{N[o-NMe-C₆H₄]₂}) is shown to catalyze C–H borylation of electron-rich heteroarenes with pinacolborane (HBpin) in the presence of a mild chloroalkane reagent. C–H borylation proceeds for a range of electron-rich heterocycles including pyrroles, indoles, and thiophenes of varied substitution. Mechanistic studies implicate an initial P–N cooperative activation of HBpin by 1 to give P-hydrido diazaphospholene 2, which is diverted by Atherton–Todd oxidation with chloroalkane to generate P-chloro diazaphospholene 3. DFT calculations suggest subsequent oxidation of pinacolborane by 3 generates chloropinacolborane (ClBpin) as a transient electrophilic borylating species, consistent with observed substituent effects and regiochemical outcomes. These results illustrate the targeted diversion of established reaction pathways in organophosphorus catalysis to enable a new mode of main group-catalyzed C–H borylation.

A nontrigonal phosphorus triamide (1, P{N[o-NMe-C₆H₄]₂}) is shown to catalyze C–H borylation of electron-rich heteroarenes with pinacolborane (HBpin) in the presence of a mild chloroalkane reagent.

Catalytic Activation of N2O at a Low-Valent Bismuth Redox Platform

Herein we present the catalytic activation of N₂O at a Bi^I⇄Bi^III redox platform. The activation of such a kinetically inert molecule was achieved by the use of bismuthinidene catalysts, aided by HBpin as reducing agent. The protocol features remarkably mild conditions (25 °C, 1 bar N₂O), together with high turnover numbers (TON, up to 6700) and turnover frequencies (TOF). Analysis of the elementary steps enabled structural characterization of catalytically relevant intermediates after O-insertion, namely a rare arylbismuth oxo dimer and a unique monomeric arylbismuth hydroxide. This protocol represents a distinctive example of a main-group redox cycling for the catalytic activation of N₂O.

P(III)/P(V)-Catalyzed Methylamination of Arylboronic Acids and Esters: Reductive C-N Coupling with Nitromethane as a Methylamine Surrogate

The direct reductive N-arylation of nitromethane by organophosphorus-catalyzed reductive C-N coupling with arylboronic acid derivatives is reported. This method operates by the action of a small ring organophosphorus-based catalyst (1,2,2,3,4,4-hexamethylphosphetane P-oxide) together with a mild terminal reductant hydrosilane to drive the selective installation of the methylamino group to (hetero)aromatic boronic acids and esters. This method also provides for a unified synthetic approach to isotopically labeled N-methylanilines from various stable isotopologues of nitromethane (i.e., CD₃NO₂, CH₃¹⁵NO₂, and ¹³CH₃NO₂), revealing this easy-to-handle compound as a versatile precursor for the direct installation of the methylamino group.

Construction of P-Chiral Alkenylphosphine Oxides through Highly Chemo-, Regio-, and Enantioselective Hydrophosphinylation of Alkynes

Alkenylphosphine oxides have a wide spectrum of practical applications. However, chemo-, regio-, and enantiocontrolled construction of this structural motif still constitutes a significant synthetic challenge. Here we show that these compounds can be efficiently accessed by using a palladium/Xiao-Phos catalytic system, which leads to the highly regioselective formation of the anti-Markovnikov adducts through addition of a secondary phosphine oxide to an alkyne. Diverse (hetero)aryl and alkyl alkynes, as well as both terminal and internal alkynes can be employed as substrates. The kinetic resolution process makes it possible to produce alkenylphosphine oxide and recovered secondary phosphine oxides with high ee values. Further transformations of these two P-chiral scaffolds confirm the high practicability and application prospect of our synthetic strategies. Initial mechanistic studies strongly suggested that hydropalladation is likely responsible for the conversion process.

Let's Make White Phosphorus Obsolete

Industrial and laboratory methods for incorporating phosphorus atoms into molecules within the framework of Green Chemistry are in their infancy. Current practice requires large inputs of energy, involves toxic intermediates, and generates substantial waste. Furthermore, a negligible fraction of phosphorus-containing waste is recycled which in turn contributes to negative environmental impacts, such as eutrophication. Methods that begin to address some of these drawbacks are reviewed, and some key opportunities to be realized by pursuing organophosphorus chemistry under the principles of Green Chemistry are highlighted. Methods used by nature, or in the chemistry of other elements such as silicon, are discussed as model processes for the future of phosphorus in chemical synthesis.

An Improved PIII/PV═O-Catalyzed Reductive C-N Coupling of Nitroaromatics and Boronic Acids by Mechanistic Differentiation of Rate- and Product-Determining Steps

Experimental, spectroscopic, and computational studies are reported that provide an evidence-based mechanistic description of an intermolecular reductive C–N coupling of nitroarenes and arylboronic acids catalyzed by a redox-active main-group catalyst (1,2,2,3,4,4-hexamethylphosphetane P-oxide, i.e., 1·[O]). The central observations include the following: (1) catalytic reduction of 1·[O] to P^III phosphetane 1 is kinetically fast under conditions of catalysis; (2) phosphetane 1 represents the catalytic resting state as observed by ³¹P NMR spectroscopy; (3) there are no long-lived nitroarene partial-reduction intermediates observable by ¹⁵N NMR spectroscopy; (4) the reaction is sensitive to solvent dielectric, performing best in moderately polar solvents (viz. cyclopentylmethyl ether); and (5) the reaction is largely insensitive with respect to common hydrosilane reductants. On the basis of the foregoing studies, new modified catalytic conditions are described that expand the reaction scope and provide for mild temperatures (T ≥ 60 °C), low catalyst loadings (≥2 mol%), and innocuous terminal reductants (polymethylhydrosiloxane). DFT calculations define a two-stage deoxygenation sequence for the reductive C–N coupling. The initial deoxygenation involves a rate-determining step that consists of a (3+1) cheletropic addition between the nitroarene substrate and phosphetane 1; energy decomposition techniques highlight the biphilic character of the phosphetane in this step. Although kinetically invisible, the second deoxygenation stage is implicated as the critical C–N product-forming event, in which a postulated oxazaphosphirane intermediate is diverted from arylnitrene dissociation toward heterolytic ring opening with the arylboronic acid; the resulting dipolar intermediate evolves by antiperiplanar 1,2-migration of the organoboron residue to nitrogen, resulting in displacement of 1·[O] and formation of the target C–N coupling product upon in situ hydrolysis. The method thus described constitutes a mechanistically well-defined and operationally robust main-group complement to the current workhorse transition-metal-based methods for catalytic intermolecular C–N coupling.

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.

Why do silanes reduce electron-rich phosphine oxides faster than electron-poor phosphine oxides?

Organophosphine-mediated reactions that generate P[double bond, length as m-dash]O-bonded byproducts can be transformed into catalytic processes by reducing the R3P[double bond, length as m-dash]O byproduct back to PR3in situ with a silane. DFT calculations explain why the most readily reduced phosphine oxides are those incorporating electron-rich (e.g. alkyl) substituents rather than electron-deficient (e.g. aryl) substituents.

Convergent Synthesis of Polysubstituted Furans via Catalytic Phosphine Mediated Multicomponent Reactions

Tri- or tetrasubstituted furans have been prepared from terminal activated olefins and acyl chlorides or anhydrides by a multicomponental convergent synthesis mode. Instead of stoichiometric nBu₃P, only catalytic nBu₃P or nBu₃P=O is needed to furnish the furans in modest to excellent yields with a good functional group tolerance under the aid of reducing agent silane. This synthetic method features a silane-driven catalytic intramolecular Wittig reaction as a key annulation step and represents the first successful application of catalytic Wittig reaction in multicomponent cascade reaction.

Sulphide as a leaving group: highly stereoselective bromination of alkyl phenyl sulphides

A conceptionally novel nucleophilic substitution approach to synthetically important alkyl bromides is presented. Using molecular bromine (Br2), readily available secondary benzyl and tertiary alkyl phenyl sulphides are converted into the corresponding bromides under exceptionally mild, acid- and base-free reaction conditions. This simple transformation allows the isolation of elimination sensitive benzylic β-bromo carbonyl and nitrile compounds in mostly high yields and purities. Remarkably, protic functionalities such as acids and alcohols are tolerated. Enantioenriched benzylic β-sulphido esters, readily prepared by asymmetric sulpha-Michael addition, produce the corresponding inverted bromides with high stereoselectivities, approaching complete enantiospecificity at -40 °C. Significantly, the reported benzylic β-bromo esters can be stored without racemisation for prolonged periods at -20 °C. Their synthetic potential was demonstrated by the one-pot preparation of γ-azido alcohol (S)-5 in 90% ee. NMR studies revealed an initial formation of a sulphide bromine adduct, which in turn is in equilibrium with a postulated dibromosulphurane intermediate that undergoes C-Br bond formation.

Reduction Rate of 1-Phenyl Phospholane 1-Oxide Enhanced by Silanol Byproducts: Comprehensive DFT Study and Kinetic Modeling Linked to Reagent Design

Important stoichiometric transformations like Wittig and Appel reactions have been implemented in a catalytic fashion in the past decade. The phosphine oxide generated in situ can be reintroduced as phosphine into the catalytic cycle using mild and selective silane reagents (redox-driven catalysis). While the field of experimental investigation has been fully expanding in the past decade, theoretical studies are still sparse. In this present work, density functional theory (DFT) has been used to characterize the free energy surfaces of the reduction of 1-phenyl phospholane 1-oxide with four different silanes. Found stationary points have been studied in-depth to highlight mechanistic peculiarities, like the effect of substituents at the silicon center and the parallel and competitive reactivity between the precursor silanes and their semioxidized byproducts. Calculated thermodynamic parameters in combination with "real" values for concentrations have been used in the formulation of rate equations for simple bimolecular and monomolecular steps of the mechanism. The deterministic integration concentrations versus time of such rate equations led to a realistic description of the systems under study and paved the way to strategic and rational design of new silanes with increased reactivity.

Driving Recursive Dehydration by PIII/PV Catalysis: Annulation of Amines and Carboxylic Acids by Sequential C-N and C-C Bond Formation

A method for the annulation of amines and carboxylic acids to form pharmaceutically relevant azaheterocycles via organophosphorus P^III/P^V redox catalysis is reported. The method employs a phosphetane catalyst together with a mild bromenium oxidant and terminal hydrosilane reductant to drive successive C–N and C–C bond-forming dehydration events via the serial action of a catalytic bromophosphonium intermediate. These results demonstrate the capacity of P^III/P^V redox catalysis to enable iterative redox-neutral transformations in complement to the common reductive driving force of the P^III/P^V couple.

Catalytic Staudinger Reduction at Room Temperature

We report an efficient catalytic Staudinger reduction at room temperature that enables the preparation of a structurally diverse set of amines from azides in excellent yields. The reaction is based on the use of catalytic amounts of triphenylphosphine as a phosphine source and diphenyldisiloxane as a reducing agent. Our catalytic Staudinger reduction exhibits a high chemoselectivity, as exemplified by reduction of azides over other common functionalities, including nitriles, alkenes, alkynes, esters, and ketones.