The catalytic asymmetric Abramov reaction

One-Pot Synthesis of Polymers Containing PC Bonds in the Main Chain

Placement of phosphorus in the polymer main chain leads to organophosphorus polymers with potentially unique chemical and physical properties. Herein, it is demonstrated that the Abramov phosphonylation reaction can be extended to the synthesis of such polymers, by reacting di- or tricarbaldehydes with phosphinic acid (PA) in the presence of N,O-bis(trimethylsilyl)acetamide (BSA). This technique affords polymers with main chain PC bonds, wherein phosphorus (V), aromatic rings, and hydroxymethylene moieties are linked by bis(α-hydroxymethylene)phosphinic acid (BHMPA) units. The resulting polymers are water soluble, display resilience against acid- and base-catalyzed hydrolysis, and exhibit superior thermal stability with high char yield in air (≈83%) and nitrogen (≈76%) atmosphere.

Сatalytic Asymmetric Synthesis of C-Chiral Phosphonates

The current review is devoted to the achievements in the development of methods for the catalytic asymmetric synthesis of phosphonates containing a chiral center in the side chain. C-chiral phosphonates are widely represented among natural compounds with various biological activities as insecticides, herbicides, antibiotics, and bioregulators. Synthetic representatives of this class have found practical application as biologically active compounds. The review summarizes methods of asymmetric metal complex catalysis and organocatalysis as applied to such reactions as phospha-aldol reaction, two-component and three-component phospha-Mannich reaction, phospha-Michael reaction, as well as hydrogenation of unsaturated phosphonates and phosphine oxides, ketophosphonates, and iminophosphonates. Methods for the asymmetric hydride reduction of C=X phosphonates (X=O, S, NR) are also discussed in detail. The review presents updated literature reports, as well as original research by the author.

Visible-Light-Induced α-C(sp3)-H Phosphinylation of Unactivated Ethers under Photocatalyst- and Additive-Free Conditions

A photocatalyst- and additive-free visible-light-induced α-C(sp³)-H phosphinylation of unactivated ethers involving a C-O bond cleavage with molecular oxygen as the sole oxidant at room temperature has been achieved. This method provides a sustainable access to α-hydroxyphosphine oxides in up to 88% yield with good functional group compatibility under mild and neutral conditions (34 examples). Moreover, the subsequent two-step conversion of the resulting dihydroxy diarylphosphine oxides afforded α-phosphinylated cyclic ethers in good overall yields (10 examples).

Asymmetric Hydrophosphonylation of α,β-Unsaturated Ketones Catalyzed by Rare-Earth Metal Complexes Bearing Trost Ligands

Chiral dinuclear rare-earth metal complexes RE₂Lⁿ₂ (n = 1, RE = Y(1), Eu(2), Nd(3), La(4), Gd(5); n = 2, RE = Eu(6), Gd(7)) stabilized by the corresponding Trost ligands H₃L¹ or H₃L² (H₃L¹ = (S,S)-2,6-bis[2-(hydroxydiphenylmethyl)pyrrolidin-1-ylmethyl]-4-methylphenol, H₃L² = (S,S)-2,6-bis[2-(hydroxydiphenylmethyl)pyrrolidin-1-ylmethyl]-4-chlorophenol) were prepared and three unknown complexes 5-7 were characterized by X-ray diffraction analysis. The chiral rare-earth metal complexes 1-7 displayed high reactivity in the asymmetric hydrophosphonylation of α,β-unsaturated ketones, and 5 mol % of complex 7 together with 10 mol % of chiral diamine (1S,2S)-1,2-cyclohexanediamine were proved to be the optimal catalyst combination. Various hydrophosphonylation products with excellent yields and high to excellent enantiomeric excess (ee) values were obtained in toluene (up to 99% yield, >99% ee).

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 Acyl Phosphoramidates Employing a Modified Staudinger Reaction

A one-step synthesis of acyl phosphoramidates from a variety of functionalized acyl azides has been developed employing trimethylsilyl chloride as an activating agent in a modified Staudinger reaction. The methodology was further adapted to include the in situ generation of the acyl azides from a diverse selection of carboxylic acids and hydrazide starting synthons. The reaction scope was extended to include the synthesis of imidodiphosphates and the natural product Microcin C.

Chiral disulfonimides: a versatile template for asymmetric catalysis

Since the emergence of pseudo-C2-symmetric chiral phosphoric acids (CPA), much work has been done to utilize these systems in stereoselective, organocatalytic processes. Despite the success in this field, reasonably basic substrates such as imines are often required to achieve appreciable activation. In order to access a wider variety of potential reaction partners, many related organocatalysts with enhanced Brønsted acidity have since been developed. Chiral disulfonimides (DSIs) have materialized as one such powerful class of organocatalysts and have been shown to expand the list of potential substrates to include aldehydes and ketones via Brønsted, Lewis, or bifunctional acid activation. This versatility renders DSIs amenable to an impressive scope of reaction types, typically with remarkable stereoselectivity induced by asymmetric counteranion-directed catalysis (ACDC). This review serves to provide a complete analysis of the successful applications, mechanistic insights, and unmet challenges exhibited to date in DSI-catalyzed and -assisted processes.

Optical Resolution of Dimethyl α-Hydroxy-Arylmethylphosphonates via Diastereomer Complex Formation Using Calcium Hydrogen O,O′-Dibenzoyl-(2R,3R)-Tartrate; X-Ray Analysis of the Complexes and Products

Two dimethyl α-hydroxy-arylmethylphosphonates (aryl = Ph and 2-MeOPh) were subjected to optical resolution via diastereomer complex formation applying the acidic calcium salt of O,O′-dibenzoyl-(2R,3R)-tartaric acid as the resolving agent. The dominating diastereomer complexes, whose structure was determined by single crystal X-ray measurements, were obtained in 96% and 68% diastereomer excess values, respectively. After decomposing the diastereomer formations by extraction, and after recrystallizations, the major enantiomer (S and R, respectively) of the α-hydroxyphosphonates were prepared in enantiomeric excess values of 96% and 68%, respectively. The stereostructure of the two α-hydroxy-arylmethylphosphonates was again established by X-ray measurements. Detailed study on the X-ray data allowed valuable conclusions on the nature of the coordination in the complexes (intermolecular interactions), and on the H-bonding.

Organophosphorus chemistry based on elemental phosphorus: advances and horizons

The results of studies on the application of elemental phosphorus for the synthesis of important organophosphorus compounds are surveyed and summarized. Currently, this trend represents a synthetically, environmentally and technologically attractive alternative to classical organophosphorus chemistry based on toxic and corrosive phosphorus chlorides. Direct phosphination and phosphinylation of organic compounds with elemental phosphorus (discussed in the first part of the review) basically extend the range of available phosphines, phosphine chalcogenides and phosphinic acids and provides further development of their synthetic potential (discussed in the second part of the review). It is shown that the breakthrough in this area is largely due to the discovery of reactions of elemental phosphorus (white and red) with various electrophiles in superbasic suspensions and emulsions derived from alkali metal hydroxides and to the development of electrochemical, electrocatalytic and catalytic activation of white phosphorus.

The bibliography includes 299 references.

Resolution of Racemic α-Hydroxyphosphonates: Bi(OTf)3-Catalyzed Stereoselective Esterification of α-Hydroxyphosphonates with (+)-Dibenzoyl-l-tartaric Anhydride

A practical and efficient method has been developed for the preparation of optically active α-hydroxyphosphonates through resolution of the racemates. Treatment of racemic diethyl 1-hydroxy-1-phenylmethylphosphonate (1) with (+)-dibenzoyl-L-tartaric anhydride gave two diastereomeric esters 2 and 3 in the presence of bismuth triflate (15 mol %) in an 86:14 ratio. The two diastereomeric esters were separated by simple column chromatography, and the structure for the major diastereomer was determined by X-ray crystallographic analysis. Simple hydrolysis of the isolated major diastereomer in the usual manner afforded (R)-O,O-diethyl-1-[hydroxyl(phenyl)methyl] phosphonate 1. The advantages of the present method are that the operation is simple and easy to handle, along with rapid and good yield preparations of both enantiomers of the racemic α-phosphonates 1. Diastereoselective reactions of various racemic α-hydroxyphosphonates with d-Bz-L-TA in the presence of Bi(OTf)₃ are also described.

Can a Ketone Be More Reactive than an Aldehyde? Catalytic Asymmetric Synthesis of Substituted Tetrahydrofurans

O-heterocycles bearing tetrasubstituted stereogenic centers are prepared via catalytic chemo- and enantioselective nucleophilic additions to ketoaldehydes, in which the ketone reacts preferentially over the aldehyde. Five- and six-membered rings with both aromatic and aliphatic substituents, as well as an alkynyl substituent, are obtained. Moreover, 2,2,5-trisubstituted and 2,2,5,5-tetrasubstituted tetrahydrofurans are synthesized with excellent stereoselectivities. Additionally, the synthetic utility of the described method is demonstrated with a three-step synthesis of the side chain of anhydroharringtonine.

Continuous Flow Alcoholysis of Dialkyl H-Phosphonates with Aliphatic Alcohols

The continuous flow alcoholysis of dialkyl H-phosphonates by aliphatic alcohols in the absence of a catalyst was elaborated using a microwave (MW) reactor equipped with a flow cell. By the precise control of the reaction conditions, the synthesis could be fine-tuned towards dialkyl H-phosphonates with two different and with two identical alkyl groups. In contrast to the "traditional" batch alcoholysis, flow approaches required shorter reaction times, and the products became available at a larger scale.

DDQ-mediated direct C(sp3)–H phosphorylation of xanthene derivatives

A direct route to alkylphosphonates has been achievedviaDDQ-mediated cross-dehydrogenative coupling between diarylphosphine oxides and xanthene derivatives.

Synthesis of chiral γ-aminophosphonates through the organocatalytic hydrophosphonylation of azadienes with phosphites

An organocatalytic enantioselective 1,4-addition of phosphites to azadienes has been successfully developed using quinine as a catalyst, providing an efficient and facile route to optically active γ-aminophosphonates with up to 94% ee.

Efficient synthesis of ether phosphonates using trichloroacetimidate and acetate coupling methods

A series of ether phosphonates have been prepared by trichloroacetimidate and acetate coupling methods. Trichloroacetimidates or acetates were treated with primary and secondary alcohols as O-nucleophiles in the presence of catalytic TMSOTf to afford 21 examples of diethyl alkyloxy(substitutedphenyl)methyl phosphonates via C–O bond formation in 55–90% yields and short reaction time.

An efficient method for the synthesis of various ether phosphonates by trichloroacetimidate and acetate coupling methods is described.

Recent Developments in Organophosphorus Flame Retardants Containing P-C Bond and Their Applications

Organophosphorus compounds containing P-C bonds are increasingly developed as flame retardant additives due to their excellent thermal and hydrolytic stability and ease of synthesis. The latest development (since 2010) in organophosphorus flame retardants containing P-C bonds summarized in this review. In this review, we have broadly classified such phosphorus compounds based on the carbon unit linked to the phosphorus atom i.e., could be a part of either an aliphatic or an aromatic unit. We have only considered those published literature where a P-C bond was created as a part of synthetic strategy to make either an intermediate or a final organophosphorus compound with an aim to use it as a flame retardant. General synthetic strategies to create P-C bonds are briefly discussed. Most popular synthetic strategies used for developing P-C containing phosphorus based flame retardants include Michael addition, Michaelis-Arbuzov, Friedels-Crafts and Grignard reactions. In general, most flame retardant derivatives discussed in this review have been prepared via a one- to two-step synthetic strategy with relatively high yields greater than 80%. Specific examples of P-C containing flame retardants synthesized via suitable synthetic strategy and their applications on various polymer systems are described in detail. Aliphatic phosphorus compounds being liquids or low melting solids are generally applied in polymers via coatings (cellulose) or are incorporated in the bulk of the polymers (epoxy, polyurethanes) during their polymerization as reactive or non-reactive additives. Substituents on the P atoms and the chemistry of the polymer matrix greatly influence the flame retardant behavior of these compounds (condensed phase vs. the gas phase). Recently, aromatic DOPO based phosphinate flame retardants have been developed with relatively higher thermal stabilities (>250 °C). Such compounds have potential as flame retardants for high temperature processable polymers such as polyesters and polyamides. A vast variety of P-C bond containing efficient flame retardants are being developed; however, further work in terms of their economical synthetic methods, detailed impact on mechanical properties and processability, long term durability and their toxicity and environmental impact is much needed for their potential commercial exploitations.

Triazole-Based Anion-Binding Catalysis for the Enantioselective Dearomatization of N-Heteroarenes with Phosphorus Nucleophiles

The first enantioselective synthesis of chiral heterocyclic α-amino phosphonates by nucleophilic dearomatization of quinolines and pyridines using an anion-binding organocatalysis approach is described. Chiral tetrakistriazoles were employed as efficient hydrogen-bond donor catalysts by forming a chiral close ion-pair with the in situ formed N-acyl salts and 2,2,2-trichlorethoxycarbonyl chloride (TrocCl). The ion-pair was subsequently treated with various phosphorus nucleophiles, such as silyl-protected dialkyl- and trialkylphosphites. Thus, the corresponding products were obtained in complete or high regioselectivities and up to 97:3 e.r. for quinolines or up to 89:11 e.r. for the more challenging pyridine substrates. This method allows for rapid access to substituted chiral cyclic α-amino phosphonates, which can be easily transformed into phosphonic acid derivatives.

Phase-transfer catalyzed asymmetric synthesis of α,β-unsaturated γ,γ-disubstituted γ-lactams

The direct, γ-selective vinylogous Michael addition of unsaturated γ-monosubstituted γ-lactams was realized by using chiral phase-transfer catalysis.

An efficient asymmetric hydrophosphonylation of unsaturated amides catalyzed by rare-earth metal amides [(Me3Si)2N]3RE(μ-Cl)Li(THF)3 with phenoxy-functionalized chiral prolinols

An asymmetric hydrophosphonylation reaction of diethyl phosphite with α,β-unsaturated amides catalyzed by [(Me₃Si)₂N]₃RE(μ-Cl)Li(THF)₃ with H₂Lⁿ ((S)-2,4-R₂-6-[[2-(hydroxydiphenylmethyl)pyrrolidin-1-yl]methyl]phenol) was disclosed.

1,1′-Dibenzyl-bis-(triazolyl)diphenylphosphine dioxide: a new efficient organocatalyst for silicon tetrachloride-mediated enantioselective Abramov-type phosphonylation of aldehydes with trialkyl phosphites

Asymmetric phosphonylation of aldehydes with trialkyl phosphites mediated by SiCl₄ and 1,1′-dibenzyl-bis-(triazolyl)diphenylphosphine dioxide organocatalyst.

A Disulfonimide Catalyst for Highly Enantioselective Mukaiyama-Mannich Reaction

A new BINOL-derived chiral disulfonimide has been developed by introducing 4-methyl-3,5-dinitrophenyl substituents at its 3- and 3'-positions. This chiral disulfonimide catalyst displays high catalytic efficacy toward the asymmetric Mukaiyama-Mannich reaction of imines with ketene silyl acetals leading to β-amino acid esters in good yields (up to 99%) with high diastereoselectivities (syn/anti up to 97:3) and enantioselectivities (up to 98% ee). The long-standing problem of the chiral phosphoric acid-catalyzed asymmetric Mukaiyama-Mannich reaction that requires a 2-hydroxyphenyl moiety was solved by this disulfonimide catalyst.

Enantioselective dearomatization of isoquinolines by anion-binding catalysis en route to cyclic α-aminophosphonates

An enantioselective dearomatization of isoquinolines has been developed using chiral anion-binding catalysis. This transformation makes use of silyl phosphite as a nucleophile and generates cyclic α-aminophosphonates.

An enantioselective dearomatization of isoquinolines has been developed using chiral anion-binding catalysis. This transformation, catalyzed by a simple and easy to prepare tert-leucine-based thiourea derivative, makes use of silyl phosphite as a nucleophile and generates cyclic α-aminophosphonates. This is the first time asymmetric anion-binding catalysis has been applied to the synthesis of α-aminophosphonates.

Asymmetric counteranion-directed Lewis acid organocatalysis for the scalable cyanosilylation of aldehydes

Due to the high versatility of chiral cyanohydrins, the catalytic asymmetric cyanation reaction of carbonyl compounds has attracted widespread interest. However, efficient protocols that function at a preparative scale with low catalyst loading are still rare. Here, asymmetric counteranion-directed Lewis acid organocatalysis proves to be remarkably successful in addressing this problem and enabled a molar-scale cyanosilylation in quantitative yield and with excellent enantioselectivity. Also, the catalyst loading could be lowered to a part-per-million level (50 ppm: 0.005 mol%). A readily accessible chiral disulfonimide was used, which in combination with trimethylsilyl cyanide, turned into the active silylium Lewis acid organocatalyst. The nature of a peculiar phenomenon referred to as a "dormant period", which is mainly induced by water, was systematically investigated by means of in situ Fourier transform infrared analysis.

An Ni–PyBisulidine complex for the asymmetric hydrophosphonylation of aldehydes

A novel Ni–PyBisulidine complex has been developed for the asymmetric hydrophosphonylation of aldehydes.