Ortho -Trialkylstannyl Arylphosphanes by C-P and C-Sn Bond Formation in Arynes

Synthesis of Phosphachromones by Cyclized Coupling of Ethyl Hydrogen (Phenylethynyl)phosphonate with Arynes

A straightforward and efficient strategy for the synthesis of phosphachromones has been reported via the insertion of arynes into P-O bonds. This operationally simple reaction is compatible with different functional groups, affording various phosphachromones by the simultaneous formation of C-P and C-O bonds in one step with moderate to good yields, and the Fries rearrangement involving phosphorus atoms is a key step in the reaction.

Synthesis of triarylphosphines from arylammonium salts via one-pot transition-metal-free C-P coupling

A nucleophilic aromatic substitution (S_NAr) reaction that allowed transition-metal-free C-P bond construction via C-N bond cleavage was developed. The coupling between aryltrimethylammonium salts and secondary phosphines from the in situ reduction of diarylphosphine oxides led to the formation of diverse triarylphosphines with various functional groups. This one-pot process was not only a pertinent S_NAr precedent but also a favorable transition-metal-free alternative for C-P coupling.

The bis(Biphenyl)phosphorus Fragment in Trivalent and Tetravalent P-Environments

Diaryl substituted phosphorus (III) compounds are commonly used motifs in synthesis. Although the basic synthetic routes to these molecules starting from PCl3 are well reported, sterically hindered aryl substituents can be difficult to introduce, especially if the P atom is in ortho position to another group. This work explores the chemistry of the bis(biphenyl)phosphorus(III) fragment. As third substituents, H, M, Cl, NR2, two group 14 element substituents and also Li were introduced in high-yielding processes offering a wide chemical variety of the bis(biphenyl) phosphine motif. In addition, also a tetravalent phosphine borane adduct was isolated. All structures were thoroughly investigated by heteronuclear NMR spectroscopic analysis. Furthermore, the reaction conditions are discussed in connection with the structures and four crystal structures of the aminophosphine, phosphine, phosphine borane and phosphide are provided. The latter crystallized as a dimer with a unique planar P2Li2 ring, which is stabilized by the non-covalent C⋯Li interaction arising from the biphenyl motif and represents a rare example of a donor-free planar P2Li2 ring.

The Aryne Phosphate Reaction*

Condensed phosphates are a critically important class of molecules in biochemistry. Non‐natural analogues are important for various applications, such as single‐molecule real‐time DNA sequencing. Often, such analogues contain more than three phosphate units in their oligophosphate chain. Consequently, investigations into phosphate reactivity enabling new ways of phosphate functionalization and oligophosphorylation are essential. Here, we scrutinize the potential of phosphates to act as arynophiles, paving the way for follow‐up oligophosphorylation reactions. The aryne phosphate reaction is a powerful tool to—depending on the perspective—(oligo)phosphorylate arenes or arylate (oligo‐cyclo)phosphates. Based on Kobayashi‐type o‐silylaryltriflates, the aryne phosphate reaction enables rapid entry into a broad spectrum of arylated products, like monophosphates, diphosphates, phosphodiesters and polyphosphates. The synthetic potential of these new transformations is demonstrated by efficient syntheses of nucleotide analogues and an unprecedented one‐flask octaphosphorylation.

The Aryne-Abramov Reaction as a 1,2-Benzdiyne Platform for the Generation and Solvent-Dependent Trapping of 3-Phosphonyl Benzynes

Synthetic methodology utilizing two aryne intermediates (i.e., a formal benzdiyne) enables the rapid generation of structurally complex molecules with diverse functionality. This report describes the sequential generation of two ortho-benzyne intermediates for the synthesis of 2,3-disubstituted aryl phosphonates. Aryl phosphonates have proven useful in medicinal chemistry and materials science, and the reported methodology provides a two-step route to functionally dense variants by way of 3-phosphonyl benzyne intermediates. The process begins with regioselective trapping of a 3-trifloxybenzyne intermediate by an O-silyl phosphite in an Abramov-like reaction to bond the strained Csp carbons with phosphorus and silicon. Standard aryne-generating conditions follow to convert the resulting 2-silylphenyl triflate into a 3-phosphonyl benzyne, which readily reacts with numerous aryne trapping reactants to form a variety of 2,3-difunctionalized aryl phosphonate products. DFT computational studies shed light on important mechanistic details and revealed that 3-phosphonyl benzynes are highly polarizable. Specifically, the distortion in the internal bond angles at each of the Csp atoms was strongly influenced by both the electronegativity of the phosphonate ester groups as well as the dielectric of the computational solvation model. These effects were verified experimentally as the regioselectivity of benzyl azide trapping increased with more electronegative esters and/or increasingly polar solvents. Conversely, replacing the conventional solvent, acetonitrile, with nonpolar alternatives provided attenuated or even inverted selectivities. Overall, these studies showcase new reactivity of benzyne intermediates and extend the aryne relay methodology to include organophosphonates. Furthermore, this work demonstrates that the regioselectivity of aryne trapping reactions could be tuned by simply changing the solvent.

Synthesis of Stannylated Aryl Imines and Amines via Aryne Insertion Reactions into Sn-N Bonds

The reaction of in situ generated arynes with stannylated imines to provide ortho-stannyl-aniline derivatives is reported. The readily prepared trimethylstannyl benzophenone imine is introduced as an efficient reagent to realize the aryne σ-insertion reaction. The imine functionality is an established N-protecting group and insertions proceed with good yields and good to excellent regioselectivities. The product anilines are valuable starting materials for follow-up chemistry thanks to the rich chemistry offered by the trimethylstannyl moiety.

Gold-Catalyzed Formal Hexadehydro-Diels-Alder/Carboalkoxylation Reaction Cascades

A dual gold-catalyzed hexadehydro-Diels-Alder/carboalkoxylation cascade reaction is reported. In this transformation, the gold catalyst participated in the hexadehydro-Diels-Alder step, switching the mechanism from a radical type to a cationic one, and then the catalyst activated the resulting aryne to form an ortho-Au phenyl cation species, which underwent a carboalkoxylation rearrangement rather than the expected aryne-ene reaction.

Arynes as Radical Acceptors: TEMPO-Mediated Cascades Comprising Addition, Cyclization, and Trapping

The application of arynes as radical acceptors is described. The stable radical TEMPO (2,2,6,6-tetramethyl piperidine 1-oxyl) is shown to add to various ortho-substituted benzynes generating the corresponding aryl radicals which engage in 5-exo or 6-endo cyclizations. The cyclized radicals are eventually trapped by TEMPO. The introduced method provides ready access to various dihydrobenzofurans, oxindoles, and sultones by a conceptually novel approach.

Phenanthrenes/dihydrophenanthrenes: the selectivity controlled by different benzynes and allenes

A method for the intermolecular annulation of benzynes with allenes is disclosed. This protocol utilized allenes as an unconventional diene component for the selective synthesis of phenanthrenes and dihydrophenanthrenes under the control of different benzyne precursors, featuring high atom-economy and good functional group compatibility. Density functional theory (DFT) calculations reveal that different migratory routes of the aromatic C-H bond are crucial for the observed selectivity.

Aryne Precursors for Selective Generation of 3-Haloarynes: Preparation and Application to Synthetic Reactions

The synthesis and reaction of new 3-haloaryne precursors 2a-2h were studied. The ortho-(trimethylsilyl)aryl triflate precursors 2a-2h were prepared by a simple procedure involving O-trimethylsilylation and migration of a trimethylsilyl group followed by triflation. The remarkable feature of new precursors is the selective generation of 3-haloarynes by suppressing the competitive thia-Fries rearrangement, which is the problem in the reaction using the well-known 3-haloaryne precursors. The advantage of new precursor 2a over a typical precursor 1 was confirmed by the direct comparisons in several reactions. The application of precursors 2a-2h to the syntheses of heterocycles was also reported.

Reaction of Vinyl Aziridines with Arynes: Synthesis of Benzazepines and Branched Allyl Fluorides

We report the cycloaddition between vinyl aziridines and arynes. Depending on the reaction conditions and the choice of the aryne precursor, the aziridinium intermediate can be trapped through two distinct mechanistic pathways. The first one proceeds through a formal [5+2] cycloaddition to furnish valuable multi-substituted benzazepines. In the second pathway, the aziridinium is intercepted by a fluoride ion to afford allylic fluorides in good yields. Both reactions proceed stereospecifically and furnish enantiopure benzazepines and allylic fluorides.

Orthophosphate and Sulfate Utilization for C-E (E = P, S) Bond Formation via Trichlorosilyl Phosphide and Sulfide Anions

Reduction of phosphoric acid (H₃PO₄) or tetra- n-butylammonium bisulfate ([TBA][HSO₄]) with trichlorosilane leads to the formation of the bis(trichlorosilyl)phosphide ([P(SiCl₃)₂]^-, 1) and trichlorosilylsulfide ([Cl₃SiS]^-, 2) anions, respectively. Balanced equations for the formation of the TBA salts of anions 1 and 2 were formulated based on the identification of hexachlorodisiloxane and hydrogen gas as byproducts arising from these reductive processes: i) [H₂PO₄]^- + 10HSiCl₃ → 1 + 4O(SiCl₃)₂ + 6H₂ for P and ii) [HSO₄]^- + 9HSiCl₃ → 2 + 4O(SiCl₃)₂ + 5H₂ for S. Hydrogen gas was identified by its subsequent use to hydrogenate an alkene ((-)-terpinen-4-ol) using Crabtree's catalyst ([(COD)Ir(py)(PCy₃)][PF₆], COD = 1,5-cyclooctadiene, py = pyridine, Cy = cyclohexyl). Phosphide 1 was generated in situ by the reaction of phosphoric acid and trichlorosilane and used to convert an alkyl chloride (1-chlorooctane) to the corresponding primary phosphine, which was isolated in 41% yield. Anion 1 was also prepared from [TBA][H₂PO₄] and isolated in 62% yield on a gram scale. Treatment of [TBA]1 with an excess of benzyl chloride leads to the formation of tetrabenzylphosphonium chloride, which was isolated in 61% yield. Sulfide 2 was used as a thionation reagent, converting benzophenone to thiobenzophenone in 62% yield. It also converted benzyl bromide to benzyl mercaptan in 55% yield. The TBA salt of trimetaphosphate ([TBA]₃[P₃O₉]·2H₂O), also a precursor to anion 1, was found to react with either trichlorosilane or silicon(IV) chloride to provide bis(trimetaphosphate)silicate, [TBA]₂[Si(P₃O₉)₂], characterized by NMR spectroscopy, X-ray crystallography, and elemental analysis. Trichlorosilane reduction of [TBA]₂[Si(P₃O₉)₂] also provided anion 1. The electronic structures of 1 and 2 were investigated using a suite of theoretical methods; the computational studies suggest that the trichlorosilyl ligand is a good π-acceptor and forms σ-bonds with a high degree of s character.

Trialkylborane-Mediated Multicomponent Reaction for the Diastereoselective Synthesis of Anti-δ,δ-Disubstituted Homoallylic Alcohols

The trialkylborane/O₂-mediated reaction of propargyl acetates having a tributylstannyl group at an alkyne terminus with aldehydes in a THF-H₂O solvent system gave anti-δ,δ-disubstituted homoallylic alcohols with good to high diastereoselectivity. Intriguingly, two alkyl groups derived from trialkylborane were embedded into the reaction product. The trialkylborane plays a key role not only as a radical initiator but also as a source of alkyl radicals.

Diphosphination of Arynes with Diphosphines

A diphosphination of arynes with diphosphines has been developed. The reaction of stable aryne precursors, 2-(trimethylsilyl)aryl triflates, with tetraaryldiphosphines proceeds in the presence of fluorine- or carbonate-based activators to deliver the corresponding diphosphinated products, sterically and electronically tuned 1,2-bis(diphenylphosphino)benzene (dppbz) derivatives, which can find wide application in transition metal catalysis and material science. Additionally, preliminary computational studies on the reaction mechanism are also reported.

Palladium-catalyzed carbon-sulfur or carbon-phosphorus bond metathesis by reversible arylation

Compounds bearing aryl-sulfur and aryl-phosphorus bonds have found numerous applications in drug development, organic materials, polymer science, and homogeneous catalysis. We describe palladium-catalyzed metathesis reactions of both compound classes, each of which proceeds through a reversible arylation manifold. The synthetic power and immediate utility of this approach are demonstrated in several applications that would be challenging to achieve by means of traditional cross-coupling methods. The C(sp²)-S bond metathesis protocol was used in the depolymerization of a commercial thermoplastic polymer and in the late-stage derivatization of a drug. The C(sp²)-P variant led to the convenient preparation of a variety of phosphorus heterocycles, including a potential chiral ligand and fluorescent organic materials, via a ring-closing transformation.

Decarbethoxylative Arylation Employing Arynes: A Metal-Free Pathway to Arylfluoroamides

An efficient, metal-free decarbethoxylative arylation protocol for the synthesis of α-aryl-α-fluoroamides from fluoromalonamates, under ambient reaction conditions using aryne as an electrophilic arylating agent, is reported. This decarbethoxylative arylation proceeds under mild conditions and provides a practical and effective entry to a wide range of α-aryl-α-fluoroacetamides. Interestingly, the use of the tert-butyl ester of fluoromalonamate prevented the otherwise rapid decarboxylation step, affording the arylated fluoromalonamate in moderate yield.

Synthesis of aryl triflones by insertion of arynes into C–SO2CF3 bonds

A new approach toward the synthesis of aryl triflones was achieved by the formal insertion of arynes into C–SO₂CF₃ bonds.

Insertion of Arynes into P-O Bonds: One-Step Simultaneous Construction of C-P and C-O Bonds

The insertion of arynes into P-O bonds for the preparation of o-hydroxy-substituted arylphosphine oxides, -phosphinates, and -phosphonates is described. This novel reaction leads to the simultaneous formation of C-P and C-O bonds in one step with good yields and regioselectivities under mild and transition-metal-free conditions. The easy follow-up transformations of the resulting o-hydroxyl group extend these reactions to the facile construction of other ortho-substituted arylphosphorus compounds.

Aryne 1,2,3-Trifunctionalization with Aryl Allyl Sulfoxides

An aryne 1,2,3-trisubstitution with aryl allyl sulfoxides is accomplished, featuring an incorporation of C-S, C-O, and C-C bonds on the consecutive positions of a benzene ring. The reaction condition is mild with broad substrate scope. Preliminary mechanistic study suggests a cascade formal [2 + 2] reaction of aryne with S═O bond, an allyl S → O migration, and a Claisen rearrangement.

Transition metal-free N-arylation of secondary amides through iodonium salts as aryne precursors

A transition metal-free approach for N-arylation of secondary amides was developed via diaryliodonium salts as aryne precursors.