Synthesis of Phosphonates via Michaelis-Arbuzov Reaction

Combined Computational and Experimental Study Reveals Complex Mechanistic Landscape of Brønsted Acid-Catalyzed Silane-Dependent P═O Reduction

The reaction mechanism of Brønsted acid-catalyzed silane-dependent P═O reduction has been elucidated through combined computational and experimental methods. Due to its remarkable chemo- and stereoselective nature, the Brønsted acid/silane reduction system has been widely employed in organophosphine-catalyzed transformations involving P(V)/P(III) redox cycle. However, the full mechanistic profile of this type of P═O reduction has yet to be clearly established to date. Supported by both DFT and experimental studies, our research reveals that the reaction likely proceeds through mechanisms other than the widely accepted "dual activation mode by silyl ester" or "acid-mediated direct P═O activation" mechanism. We propose that although the reduction mechanisms may vary with the substitution patterns of silane species, Brønsted acid generally activates the silane rather than the P═O group in transition structures. The proposed activation mode differs significantly from that associated with traditional Brønsted acid-catalyzed C═O reduction. The uniqueness of P═O reduction originates from the dominant Si/O═P orbital interactions in transition structures rather than the P/H-Si interactions. The comprehensive mechanistic landscape provided by us will serve as a guidance for the rational design and development of more efficient P═O reduction systems as well as novel organophosphine-catalyzed reactions involving P(V)/P(III) redox cycle.

Water-Promoted Mild and General Michaelis-Arbuzov Reaction of Triaryl Phosphites and Aryl Iodides by Palladium Catalysis

A Pd-catalyzed relatively general Michaelis-Arbuzov reaction of triaryl phosphites and aryl iodides for preparing useful aryl phosphonates was developed. Interestingly, water can greatly facilitate the reaction through a water-participating phosphonium intermediate rearrangement process, which also makes the reaction conditions rather mild. In comparison with the known methods, this reaction is milder and more general, as it exhibits excellent functional group tolerance, can be applied to various triaryl phosphites and aryl iodides, and can be extended to aryl phosphonites and phosphinites. A gram-scale reaction with a low catalyst loading also revealed its practicality and potential in large-scale preparation.

Development of a General Organophosphorus Radical Trap: Deoxyphosphonylation of Alcohols

Here we report the design of a general, redox-switchable organophosphorus alkyl radical trap that enables the synthesis of a broad range of C(sp3)-P(V) modalities. This "plug-and-play" approach relies upon in situ activation of alcohols and O═P(R2)H motifs, two broadly available and inexpensive sources of molecular complexity. The mild, photocatalytic deoxygenative strategy described herein allows for the direct conversion of sugars, nucleosides, and complex pharmaceutical architectures to their organophosphorus analogs. This includes the facile incorporation of medicinally relevant phosphonate ester prodrugs.

Sequential KOtBu/FeCl3-catalyzed reductive phosphonylation of tertiary amides for the synthesis of α-amino phosphonates and phosphines

A simple, mild and efficient sequential KO^tBu/FeCl₃-catalyzed reductive phosphonylation of tertiary amides is herein described. This process first involved the KO^tBu-catalyzed selective semi-reduction of tertiary amides to hemiaminal intermediates by TMDS (1,1,3,3-tetramethyldisiloxane) and then the FeCl₃-catalyzed nucleophilic addition of the hemiaminal intermediates to phosphonates, which allowed the straightforward synthesis of α-amino phosphonates in moderate to good yields. This method applied well to amides and lactams that bear no strong acidic α-hydrogens, and various functional groups, including methoxy, methylthio, cyano, halogen, and heterocycles, could be tolerated.

Improvements, Variations and Biomedical Applications of the Michaelis-Arbuzov Reaction

Compounds bearing the phosphorus–carbon (P–C) bond have important pharmacological, biochemical, and toxicological properties. Historically, the most notable reaction for the formation of the P–C bond is the Michaelis–Arbuzov reaction, first described in 1898. The classical Michaelis–Arbuzov reaction entails a reaction between an alkyl halide and a trialkyl phosphite to yield a dialkylalkylphosphonate. Nonetheless, deviations from the classical mechanisms and new modifications have appeared that allowed the expansion of the library of reactants and consequently the chemical space of the yielded products. These involve the use of Lewis acid catalysts, green methods, ultrasound, microwave, photochemically-assisted reactions, aryne-based reactions, etc. Here, a detailed presentation of the Michaelis–Arbuzov reaction and its developments and applications in the synthesis of biomedically important agents is provided. Certain examples of such applications include the development of alkylphosphonofluoridates as serine hydrolase inhibitors and activity-based probes, and the P–C containing antiviral and anticancer agents.

5-Phenyl-3-(2-phosphonoethyl)-1,2,3-triazol-1-ium chloride

This new triazole-functionalized phospho­nic acid, PTEPHCl, was synthesized by the ‘click’ reaction of the alkyl azide diethyl-(2-azido­eth­yl)phospho­nate with phenyl­acetyl­ene to give the dieth­yl[2-(4-phenyl-1H-1,2,3-triazol-1-yl)eth­yl]phospho­nate ester, which was then hydrolyzed under acidic conditions (HCl) to give the ‘free’ phospho­nic acid. The use of HCl for the hydrolysis caused protonation of the triazole ring, rendering the compound cationic, charged-balanced by a Cl⁻ anion.

The new triazole-functionalized phospho­nic acid 5-phenyl-3-(2-phosphono­eth­yl)-1,2,3-triazol-1-ium chloride, C₁₀H₁₃N₃O₃P⁺·Cl⁻ (PTEPHCl), was synthesized by the ‘click’ reaction of the alkyl azide diethyl-(2-azido­eth­yl)phospho­nate with phenyl­acetyl­ene to give the dieth­yl[2-(4-phenyl-1H-1,2,3-triazol-1-yl)eth­yl]phospho­nate ester, which was then hydrolyzed under acidic conditions (HCl) to give the ‘free’ phospho­nic acid. The use of HCl for the hydrolysis caused protonation of the triazole ring, rendering the compound cationic, charged-balanced by a Cl⁻ anion. There are extensive hydrogen-bonding inter­actions in the structure of PTEPHCl, involving the phospho­nic acid (–PO₃H₂) group, the triazolium ring and the Cl⁻ anion.