The Deoxygenation of Phosphine Oxides under Green Chemical Conditions

Phosphorus Compounds of Natural Origin: Prebiotic, Stereochemistry, Application

Organophosphorus compounds play a vital role as nucleic acids, nucleotide coenzymes, metabolic intermediates and are involved in many biochemical processes. They are part of DNA, RNA, ATP and a number of important biological elements of living organisms. Synthetic compounds of this class have found practical application as agrochemicals, pharmaceuticals, bioregulators, and othrs. In recent years, a large number of phosphorus compounds containing P-O, P-N, P-C bonds have been isolated from natural sources. Many of them have shown interesting biological properties and have become the objects of intensive scientific research. Most of these compounds contain asymmetric centers, the absolute configurations of which have a significant effect on the biological properties of the products of their transformations. This area of research on natural phosphorus compounds is still little-studied, that prompted us to analyze and discuss it in our review. Moreover natural organophosphorus compounds represent interesting models for the development of new biologically active compounds, and a number of promising drugs and agrochemicals have already been obtained on their basis. The review also discusses the history of the development of ideas about the role of organophosphorus compounds and stereochemistry in the origin of life on Earth, starting from the prebiotic period, that allows us in a new way to consider this most important problem of fundamental science.

Microwaves as "Co-Catalysts" or as Substitute for Catalysts in Organophosphorus Chemistry

The purpose of this review is to summarize the importance of microwave (MW) irradiation as a kind of catalyst in organophosphorus chemistry. Slow or reluctant reactions, such as the Diels-Alder cycloaddition or an inverse-Wittig type reaction, may be performed efficiently under MW irradiation. The direct esterification of phosphinic and phosphonic acids, which is practically impossible on conventional heating, may be realized under MW conditions. Ionic liquid additives may promote further esterifications. The opposite reaction, the hydrolysis of P-esters, has also relevance among the MW-assisted transformations. A typical case is when the catalysts are substituted by MWs, which is exemplified by the reduction of phosphine oxides, and by the Kabachnik-Fields condensation affording α-aminophosphonic derivatives. Finally, the Hirao P-C coupling reaction may serve as an example, when the catalyst may be simplified under MW conditions. All of the examples discussed fulfill the expectations of green chemistry.

Synthesis of Aryldiphenylphosphine Oxides by Quaternization of Tertiary Diphenylphosphines with Aryl Bromides Followed by the Wittig Reaction

A two-step method for the synthesis of aryldiphenylphosphine oxides from tertiary diphenylphosphines and aryl bromides is developed. The first step is the quaternization of methyldiphenylphosphine or benzyldiphenylphosphine with aryl bromides. This quaternization can be nickel-catalyzed (metal-free in some cases), and tolerate of a variety of functional groups, furnishing quaternary phosphonium salts in 48-90% yields. The second step is Wittig reactions of these quaternary phosphonium salts with furan-2-carbaldehyde or p-chlorobenzaldehyde to provide aryldiphenylphosphine oxides in 27-90% yields. The use of the Wittig reaction for the synthesis of tertiary phosphine oxides is in contrast to its traditional use for the synthesis of olefins, leaving tertiary phosphine oxide as a byproduct. This quaternization-Wittig method can be applied to synthesize aryldiphenylphosphine oxides that are difficult to access by the alkaline hydrolysis of aryltriphenylphosphonium salts, especially those bearing an electron-deficient aryl group. The ligand-coupling mechanism for the alkaline hydrolysis of (p-acylphenyl)triphenylphosphonim salts is also discussed.

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.

Metal-Free Synthesis of Aryltriphenylphosphonium Bromides by the Reaction of Triphenylphosphine with Aryl Bromides in Refluxing Phenol

A metal-free synthesis of aryltriphenylphosphonium bromides by the reaction of triphenylphosphine with aryl bromides in refluxing phenol is developed. This reaction tolerates hydroxymethyl, hydroxyphenyl, and carboxyl groups in aryl bromides, allowing to synthesize multifunctional aryltriphenylphosphonium bromides, from which facile access to multifunctional aryldiphenylphosphines and their oxides by hydrolysis and subsequent reduction is exemplified. A two-step addition-elimination mechanism, with the elimination step being a fast step, is also proposed.

Application of the Microwave Technique in Continuous Flow Processing of Organophosphorus Chemical Reactions

The microwave (MW) technique is an efficient tool in the realization of organic reactions, as well as in the analytical field and in the food industry. The continuous flow approach is of special interest as a promising way to scale-up MW-assisted syntheses. Besides summarizing the batch precedents, this review focuses on the utilization of the MW technique in the continuous-flow realization of organophosphorus transformations. The advantages of the continuous flow technique against the batch accomplishment are also shown. A few materials chemistry-related applications are also mentioned.

Microwave irradiation and catalysis in organophosphorus reactions

The usual advantage of microwave (MW) assistance is making organic reactions faster and more efficient. In this article we present reaction types from organophosphorus chemistry, when MW-assisted transformations (e.g. the direct esterification and alkylating esterification of phosphinic acids) may be promoted by suitable catalysts, or vice versa, when a catalytic reaction is enhanced by MW irradiation (e.g. the Arbuzov reaction of aryl halides), and when catalysts may be omitted or simplified under MW irradiation as shown by the alkylation of active methylene containing P=O substrates/the Kabachnik–Fields reaction/deoxygenation of phosphine oxides, and the Hirao reaction, respectively.

Catalytic Wittig and aza-Wittig reactions

This review surveys the literature regarding the development of catalytic versions of the Wittig and aza-Wittig reactions. The first section summarizes how arsenic and tellurium-based catalytic Wittig-type reaction systems were developed first due to the relatively easy reduction of the oxides involved. This is followed by a presentation of the current state of the art regarding phosphine-catalyzed Wittig reactions. The second section covers the field of related catalytic aza-Wittig reactions that are catalyzed by both phosphine oxides and phosphines.

Milestones in microwave-assisted organophosphorus chemistry

Our recent results in the field of microwave (MW)-assisted organophosphorus syntheses, especially regarding esterifications, condensations, substitutions and additions are surveyed. Beside making organic chemical reactions more efficient, it was possible to perform transformations that are reluctant on conventional heating. Another option is to substitute catalysts, or to simplify catalyst systems under MW conditions. It is also the purpose of this paper to elucidate the scope and limitations of the MW tool, to interpret the MW effects, and to model the distribution of the local overheatings and their beneficial effect. All these considerations are possible on the basis of the enthalpy of activations determined by us utilizing the Arrhenius equation and the pseudo first order kinetic equation.