Reduction Of Tertiary Phosphine Oxides Research Articles

Reduction of tertiary phosphine oxides to phosphine-boranes using Ti(Oi-Pr)4/BH3-THF

A new method for reduction of tertiary phosphine oxides leading to the formation of tertiary phosphine-boranes has been developed. The BH 3 -THF/Ti(O i -Pr) 4 reducing system enables conversion of triaryl, diarylalkyl and trialkylphosphine oxides directly to their borane analogues in good to high yields. In contrast to the previously reported protocols, the presence of activating groups in the structure of starting material is not necessary for the reaction to occur. The reaction is highly stereoselective and proceeds with predominant retention of configuration at the phosphorus atom. A plausible mechanism of reduction of the P O bond by BH 3 -THF/Ti(O i -Pr) 4 has been proposed.

Reduction of Tertiary Phosphine Oxides by BH3 Assisted by Neighboring­ Activating Groups

Tertiary sulfanylphosphine and aminoalkylphosphine oxides can be easily converted into the corresponding tertiary sulfanylphosphine- and aminoalkylphosphine-boranes, respectively, through the facile P=O bond reduction by borane complexes. The easy reduction of the strong P=O bond by BH3, a mild reducing agent, has been achieved through an intramolecular P=O -- B complexation directed by proximal SH or NH activating groups located at the α- or β-position to the P=O bond. A generalized reduction mechanism has been proposed.

Cover Picture: Brønsted Acid Promoted Reduction of Tertiary Phosphine Oxides (Z. Anorg. Allg. Chem. 14/2017)

Cover Picture: Brønsted Acid Promoted Reduction of Tertiary Phosphine Oxides (Z. Anorg. Allg. Chem. 14/2017)

Brønsted Acid Promoted Reduction of Tertiary Phosphine Oxides

Recently, Bronsted acids, such as phosphoric acids, carboxylic acids, and triflic acid, were found to catalyze the reduction of phosphine oxides to the corresponding phosphines. In this study, we fully characterize the HCl, HOTf, and Me2SiHOTf adducts of triphenylphosphine oxide and find that the thermally stable adduct Ph3POH+OTf– is efficiently converted into triphenylphosphine at 100 °C in the presence of readily available hydrosiloxanes. Under the same reaction conditions, also Ph3POSiMe2H+OTf– selectively affords triphenylphosphine indicating that silylated phosphine oxides are likely intermediates in this process.

The Reduction of Tertiary Phosphine Oxides by Silanes

The deoxygenation of tertiary phosphine oxides is an important reaction, as it provides useful intermediates and reagents, makes the available P-ligands useful in transition metal complexes, and regenerates wastes produced by the Wittig-, Mitsunobu and Appel reactions. Methods for the reduction of the P=O moiety comprising different metal and other hydrides, silanes and other species are surveyed, especially from the point of view of environmentally-friendly (green) chemistry, and practical applicability to be able to select the most suitable reagent. Oxalyl chloride converting the phosphine oxide to the corresponding chlorophosphonium salt may also be used, but this step should be followed by a reduction. Obviously, silanes are the reagents of choice. The most widespread, but corrosive trichlorosilane may be substituted by more userfriendly alternatives, such as phenylsilane, tetramethyldisiloxane (TMDS) and polymethylhydrosiloxane (PMHS). The latter two species are cheap, but of lower reactivity, however, they may be useful in microwave-assisted and solvent-free deoxygenations. Stereochemical and mechanistic aspects of the most often applied silane reductions are also discussed. In most cases, the P-configuration is preserved. Keywords: Phosphine oxides, reduction, phosphines, silanes, hydrides, user-friendly methods.

Organocatalyzed Reduction of Tertiary Phosphine Oxides

AbstractA novel selective catalytic reduction method of tertiary phosphine oxides to the corresponding phosphines has been developed. Notably, the reaction proceeds smoothly with low catalyst loadings of 1–5 mol% even at low temperature (70 °C). Under the optimized conditions various phosphine oxides could be selectively reduced and the desired phosphines were usually obtained in excellent yields above 90%. Furthermore, we have developed a one‐pot reaction sequence for the preparation of valuable phosphine⋅borane adducts. Simple addition of BH3⋅THF subsequent to the reduction step gave the desired adducts in yields up to 99%.magnified image

New Insights into the Mechanism of Reduction of Tertiary Phosphine Oxides by Means of Phenylsilane

ABSTRACTThe mechanism of the reduction of phosphine oxides by PhSiH3 was established on the basis of kinetic measurements and Density Functional Theory (DFT) calculations. In particular, it has been proved that the model reaction between tri‐n‐butylphosphine oxide and phenylsilane occurs via a nonpolar mechanism. The data presented herein allow prediction and verification of the applicability of the new reduction reagents and conditions for industrially attractive processes.

Highly efficient reduction of tertiary phosphine oxides and sulfides with amine-assisted aluminum hydrides under mild conditions

Reduction of tertiary phosphine oxides and sulfides into the corresponding phosphines with amine-assisted aluminum hydrides has been studied. The method is characterized by mild conditions, short reaction time, high efficiency, and expanded substrate scope. The new method is an alternative to the currently used methods of reducing phosphine oxides or recycling phosphines engaged in organic reactions.

ChemInform Abstract: Metal‐Free Reduction of Tertiary Phosphine Oxides with Hantzsch Ester.

Abstract The (COCl) 2 /Hantzsch ester is found to be an effective system for the metal-free reduction of tertiary phosphine oxides. The reaction proceeds under mild conditions, and is applicable to triarylphosphine oxides and alkyldiarylphosphine oxides to produce the corresponding tertiary phosphines in good to excellent yields. This new finding provides a practical, convenient and metal-free method for the reduction of tertiary phosphine oxides to tertiary phosphines, and shows potential application in organic synthesis.

Metal-free reduction of tertiary phosphine oxides with Hantzsch ester

Abstract The (COCl) 2 /Hantzsch ester is found to be an effective system for the metal-free reduction of tertiary phosphine oxides. The reaction proceeds under mild conditions, and is applicable to triarylphosphine oxides and alkyldiarylphosphine oxides to produce the corresponding tertiary phosphines in good to excellent yields. This new finding provides a practical, convenient and metal-free method for the reduction of tertiary phosphine oxides to tertiary phosphines, and shows potential application in organic synthesis.

Unraveling the Catalytic Cycle of Tertiary Phosphine Oxides Reduction with Hydrosiloxane and Ti(OiPr)4 through EPR and 29Si NMR Spectroscopy

The reduction of tertiary phosphine oxides using tetramethyldisiloxane (TMDS) as a mild reducing agent and catalytic amount of TiIV isopropoxide has been studied in detail. An extensive EPR study has revealed the presence of at least five TiIII species, and structures have been proposed. Thus, a single electron transfer (SET) mechanism consisting of a back and forth oxido-reduction of Ti from the IV to the III oxidation state could be proposed. Reduction of TiIV produces a Si• that undergoes a O2– abstraction from PV compounds leading to PIII compounds and Si–O• species. Reoxidation of TiIII by the latter gives silanol species. This mechanism was further probed by 29Si NMR analysis of the reaction mixture as a function of time and by the reduction of optically active P-stereogenic tertiary phosphine oxides. A practical reduction protocol of Ph3PO with these environmentally benign reagents on a 100 g scale has been developed.

Mechanistic Insight into the Reduction of Tertiary Phosphine Oxides by Ti(OiPr)4/TMDS

The reduction mechanism of tertiary phosphine oxides by Ti(OiPr)4/hydrosiloxane was studied. Strong improvement was achieved using a drying agent. ESR spectra of the reaction mixture give evidence for a single electronic transfer (SET) mechanism.

Reduction of Tertiary Phosphine Oxides with DIBAL-H

The reduction of tertiary phosphine oxides (TPOs) and sulfides with diisobutylaluminum hydride (DIBAL-H) has been studied in detail. An extensive solvent screen has revealed that hindered aliphatic ethers, such as MTBE, are optimum for this reaction at ambient temperature. Many TPOs undergo considerable reduction at ambient temperature and then stall due to inhibition. 31P and 13C NMR studies using isotopically labeled substrates as well as competition studies have revealed that the source of this inhibition is tetraisobutyldialuminoxane (TIBAO), which builds up as the reaction proceeds. TIBAO selectively coordinates the TPO starting material, preventing further reduction. Several strategies have been found to circumvent this inhibition and obtain full conversion with this extremely inexpensive reducing agent for the first time. Practical reduction protocols for these critical targets have been developed.