A study on the deoxygenation of trialkyl-, dialkyl-phenyl- and alkyl-diphenyl phosphine oxides by hydrosilanes

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.

A Study on the Direct Esterification of Monoalkylphosphates and Dialkylphosphates; The Conversion of the Latter Species to Trialkylphosphates by Alkylating Esterification

The microwave (MW)-assisted direct esterification of certain P-acids is a green method. Quantum chemical calculations revealed that the activation enthalpy (ΔH#) for the exothermic monoalkylphosphate → dialkylphosphate transformation was on the average 156.6 kJ mol−1, while ΔH# for the dialkylphosphate → trialkylphosphate conversion was somewhat higher, 171.2 kJ mol−1, and the energetics of the elemental steps of this esterification was less favorable. The direct monoesterification may be performed on MW irradiation in the presence of a suitable ionic liquid additive. However, the second step, with the less favorable energetics as a whole, could not be promoted by MWs. Hence, dialkylphosphates had to be converted to triesters by another method that was alkylation. In this way, it was also possible to synthesize triesters with different alkyl groups. Eventually a green, P-chloride free MW-promoted two-step method was elaborated for the synthesis of phosphate triesters.

Synthesis of Mixed Arylalkyl Tertiary Phosphines via the Grignard Approach

Trialkyl and triaryl phosphines are important classes of ligands in the field of catalysis and materials research. The wide usability of these low-valent phosphines has led to the design and development of new synthesis routes for a variety of phosphines. In the present work, we report the synthesis and characterization of some mixed arylalkyl tertiary phosphines via the Grignard approach. A new asymmetric phosphine is characterized extensively by multi-spectroscopic techniques. IR and UV-Vis spectra of some selected compounds are also compared and discussed. Density functional theory (DFT)-calculated results support the formation of the new compounds.

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.

Preparation of Palladium(II) Complexes of 1-substituted-3-phospholene Ligands and their Evaluation as Catalysts in Hydroalkoxycarbonylation

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A series of palladium(II) complexes incorporating 1-substituted-3-methyl-3- phospholenes as the P-ligands were prepared from phospholene oxides by deoxygenation followed by complexation with PdCl2(PhCN)2. The two 1-substituted-3-methyl-3- phospholene ligands were trans position to each other in the Pd(II)-complexes. As the ligands contain a P-stereogenic center, the Pd-complexes were obtained as a 1:1 mixture of two stereoisomers, the homochiral (R,R and S,S) and the meso (R,S) forms, when racemic starting materials were used. An optically active Pd-complex containing (R)-1-propyl- 3-phospholene ligand was also prepared. Catalytic activity of an aryl- and an alkyl-3- phospholene-palladium(II)-complex was evaluated in hydroalkoxycarbonylation of styrene. The alkyl-derivative showed higher activity and selectivity towards the formation of the esters of 3-phenylpropionic acid. However, the overall activity of these PdCl2(phospholene)2-type complexes was low.

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.

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.