Reduction Of Phosphine Oxides Research Articles

Chiral Ru/Ir bimetallic dendronized polymer catalysts constructed through sequential metal coordination and applied in asymmetric hydrogenation of quinaldine

A new kind of chiral conjugated copolymer P-2 with (S)-BINAP [BINAP=2,2′-bis(diarylphosphino)-1,1′-binaphthyl] and fluorene as the building blocks was synthesized involving Pd[P(p-tolyl)3]3-catalyzed Suzuki-coupling reaction together with reduction of phosphine oxide with HSiCl3/NEt3. The chiral polymer was used to construct chiral Ru/Ir bimetallic dendronized polymer catalysts P-5-m with different generations of Fréchet-type dendritic DPEN [DPEN=1,2-diphenylethylenediamine] ligands through a programmed metal–ligand coordination strategy. These dendronized polymer catalysts were applied to the Ir-catalyzed asymmetric hydrogenation of quinaldine and exhibited excellent activity with moderate enantioselectivity. The reaction rate was increased with increasing generation of the dendronized polymer catalysts, which are better than that obtained from the monomeric catalyst. This interesting positive dendrimer effect might be due to effective shielding of the catalytically active iridium center by the bulky dendritic pendants, which reduced the formation of the inactive iridium dimer.

Electrochemical and Chemical Reduction of Phosphine Oxides to the Corresponding Phosphines

Phosphines (R3P) have been used as reductants in Wittig, Staudinger, Appel, and Mitsunobu reactions as well as ligands in various transition metal catalysts. Though phosphines can be synthesized by reduction of the corresponding phosphine oxides (R3P=O) with hydrosilanes, P=O bond is so strong that the reduction proceeds only under harsh conditions, and C-P bonds would be cleaved. We reported that Ph3P was obtained from Ph3P=O via Ph3PCl2.1 Since Ph3PCl2 is moisture-sensitive, Ph3PCl2 is hard to be stored and should be prepared just before the reduction. We investigated direct reduction of R3P=O to R3P under mild conditions. In this paper, we describe that the reduction proceeds electrochemically and chemically under 45 ºC in the presence of an efficient oxophile, Me3SiCl (Fig. 1).Fig. 1. Reduction of Phosphine Oxides 1 A typical procedure for electrochemical reduction (ECR) is as follows. In an undivided cell fitted with Zn anode (1.0 x 1.5 cm2) and Cu cathode (1.0 x 1.5 cm2) was placed an acetonitrile (5 mL) solution of 1 (2 mmol), Me3SiCl (6 mmol),2 and Bu4NBr (2 mmol). The electroreduction was carried out under constant current conditions (100 mA, 4 F/mol-1) at 45 ºC. Usual work-up and purification by silica gel column chromatography gave 2(Table 1, ECR). Table 1. Me3SiCl-Assisted Electrochemical (ECR) and Chemical Reduction (CR) of Phosphine Oxides 1 The reduction of triarylphosphine oxides 1a and 1c-1e proceeded smoothly to give the corresponding triarylphosphines 2a and 2c-2e in 80-96% yields (Table 1, Runs 1, 3-5, ECR), whereas tris(o-tolyl)phosphine oxide 1b was reduced less effectively to give 2b in 22% yield, presumably due to steric hindrance of the o-substituents (Run 2). Electroreduction of alkyl-diarylphosphine oxides 1f and 1g was performed in a similar manner to give the corresponding phosphine 2f and 2g in 82% and 46% yields, respectively (Runs 6, 7). It is worthy to note that no appreciable peaks except for 1 and 2 were observed in 31P NMR of the crude products. Electroreduction of triphenyl phosphate 1hwas not successful (Run 8). .Similar reduction of 1 was performed chemically (CR) by using Mg as an electron source. A typical procedure is as follows: A mixture of 1 (2 mmol), magnesium powder (8 mmol),3 and Me3SiCl (6.0 mmol)2 in 1,3-dimethylimidazolidinone (8 mL) was stirred for 2 hours at room temperature under Ar atmosphere. Usual work-up followed by silica gel column chromatography gave 2 (Table 1, CR).4 Proper choice of the solvent is essential: the reduction of 1 proceeded smoothly only in DMI, and 2was obtained in less than 10% yields in MeCN and DMF, and no reduction occurred in THF. Chemical shift of Ph3P=O (1a, 31P NMR: d 27.3 ppm) did not shift by the addition of Me3SiCl. CV curve of 1a showed a single, irreversible wave at -3.0 V versus Ag/Ag+, whereas Me3SiCl did not show any reduction wave at range of 0 - -3.5 V. Addition of Me3SiCl caused significant increase of the reduction current without change of the reduction potential of 1a. From these facts, we propose a plausible mechanism of the reduction of phosphine oxide 1 (Fig. 2). In the initial stage of the reduction of 1, one-electron reduction would occur to form an anion radical [R3P·-O]- 3, which would be trapped with Me3SiCl to give radical [R3P·-OSiMe3] 4. Further one-electron reduction of 4, leading to anion 5, and subsequent P-O bond cleavage followed by silylation with Me3SiCl would give 2 together with (Me3Si)2O. Fig. 2. A Plausible MechanismIn conclusion, R3P=O is reduced to the corresponding R3P electrochemically and chemically in the presence of Me3SiCl under mild conditions. 1Yano, T.; Hoshino, M.; Kuroboshi, M.; Tanaka, H. Synlett 2010, 801-803. 2Though the reduction proceeded with t BuMe2SiCl and i Pr3SiCl similarly, no 2 was obtained with MgCl2 and SiCl4, probably because 2 is easily adsorbed on MgCl2 and SiO2 derived from SiCl4, to prevent electron transfer from Mg. 3Mg turnings gave 2in lower yield, whereas zinc powder and aluminum chips did not proceed the reduction at all. 4Though CuCl activated Mg to give 2 quantitatively, 2 formed complex with Cu species.

Enantioselective Synthesis of Biaryl Compounds via Suzuki–Miyaura Cross-Coupling Using a Palladium Complex of 7′-Butoxy-7-(diphenylphosphino)-8,8′-biquinolyl: Investigation of a New Chiral Ligand Architecture

7,7′-Dihydroxy-8,8′-biquinolyl was converted to the title phosphine in four-steps via Mitsunobu monoetherification, trifylation, phosphination, and phosphine oxide reduction (63% overall yield). Enantiomerically pure phosphine was combined with Pd2dba3 and investigated for the synthesis of axially chiral biaryl compounds from Ar1Br and Ar2B(OH)2 in the presence of K3PO4 in toluene solvent (6 examples, 4–97% yield, 4–74% ee). The analogous carbocyclic ligand 2-(diphenylphosphino)-2′-methoxy-1,1′-binaphthyl (MOP) was studied for comparative purposes and found to be effective for the synthesis of hindered 2,2′-disubstituted 1,1′-binaphthyls (78–83% yield, 20–38% ee).

Organocatalytic Chemoselective Reduction of Phosphine Oxides to Phosphines

Organocatalytic Chemoselective Reduction of Phosphine Oxides to Phosphines

Highly Chemoselective Metal-Free Reduction of Phosphine Oxides to Phosphines

Unprecedented chemoselective reductions of phosphine oxides to phosphines proceed smoothly in the presence of catalytic amounts of specific Brønsted acids. By utilizing inexpensive silanes, e.g., PMHS or (EtO)(2)MeSiH, other reducible functional groups such as ketones, aldehydes, olefins, nitriles, and esters are well-tolerated under optimized conditions.

ChemInform Abstract: Reduction of Phosphine Oxides to Phosphines with the InBr3/TMDS System.

AbstractFor the most phosphines, it is necessary to protect them as borane adducts.

Reduction of phosphine oxides to phosphines with the InBr3/TMDS system

An efficient method for the reduction of phosphine oxide derivatives into their corresponding phosphines is described. The system InBr3/TMDS allows the reduction of different secondary and tertiary phosphine oxides as well as aliphatic and aromatic phosphine oxides.

ChemInform Abstract: Titanium(IV) Catalysis in the Reduction of Phosphine Oxides.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Phosphine-Catalyzed Reductions of Alkyl Silyl Peroxides by Titanium Hydride Reducing Agents: Development of the Method and Mechanistic Investigations

A method that allows for the reduction of protected hydroperoxides by employing catalytic amounts of phosphine is presented. The combination of a titanium(IV) alkoxide and a siloxane allowed for the chemoselective reduction of phosphine oxides in the presence of alkyl silyl peroxides. Subsequent reduction of the peroxide moiety by phosphine provided the corresponding silylated alcohols in useful yields. Mechanistic experiments, including crossover experiments, support a mechanism in which the peroxide group was reduced and the silyl group was transferred in a concerted step. Labeling studies with (17)O-labeled peroxides demonstrate that the oxygen atom adjacent to the silicon atom is removed from the silyl peroxide.

An Overview of the Synthesis of Alkenylphosphines

Alkenylphosphines are compounds of great interest in view of their structures. Hence, these phosphorus derivatives bear a carbon- carbon double bond, directly linked to the phosphorus atom. This insaturation may be used as a coordinating part in presence of a metal or as a reactive part in contact with an electrophile. Thus, alkenylphosphines have found numerous applications in various fields of chemistry. They are useful intermediates in organic synthesis, as precursors of polyphosphines or dendrimers, or as dienophiles in cycloaddition reactions. Due to the presence of both the phosphorus atom and the carbon-carbon double bond, they are of great interest in coordination chemistry. Recently, they were also used as ligand in homogeneous catalysis. Alkenylphosphines are mainly prepared by reaction between halophosphines and alkenyl organometallic species, by hydrophosphination of alkynes or by C-P cross-coupling reactions. The aim of this review will be to describe these methods, emphasizing particularly on the catalytic ones. The new methodologies involving the functionalization of alkynylphosphines and the bis-functionalization of alkynes, which recently appeared in the literature will also be documented. Less commonly used reactions such as alkylation of primary or secondary vinylphosphines, reduction of phosphine-oxides or phosphinates will be briefly reported.

LiAlH4/silica chloride as a new chemoselective system for reduction of carbonyl compounds and phosphine oxides

LiAlH4/silica chloride system was used as an efficient and chemoselective reductant for organic substrates. The reduction of phosphine oxides was achieved with this system. Good chemoselectivity in reduction of aldehydes and ketones in the presence of other carbonyl compounds was observed.

A New Class of 3′‐Sulfonyl BINAPHOS Ligands: Modulation of Activity and Selectivity in Asymmetric Palladium‐Catalysed Hydrophosphorylation of Styrene

AbstractPalladium‐catalysed monophosphorylation of (R)‐2,2′‐bisperfluoroalkanesulfonates of BINOL (RF=CF3 or C4F9) by a diaryl phosphinate [Ar2P(O)H] followed by phosphine oxide reduction (Cl3SiH) then lithium diisopropylamide‐mediated anionic thia‐Fries rearrangement furnishes enantiomerically‐pure (R)‐2′‐diarylphosphino‐2′‐hydroxy‐3′‐perfluoralkanesulfonyl‐1,1′‐binaphthalenes [(R)‐8ab and (R)‐8g–j], which can be further diversified by Grignard reagent (RMgX)‐mediated CF3‐displacement [→(R)‐8c–f]. Coupling of (R)‐8a–j with (S)‐1,1′‐binaphthalene‐2,2′‐dioxychlorophosphine (S)‐9 generates 3′‐sulfonyl BINAPHOS ligands (R,S)‐10a–j in good yields (43–82%). These new ligands are of utlility in the asymmetric hydrophosphonylation of styrene (1) by 4,4,5,5‐tetramethyl‐1,3,2‐dioxaphospholane 2‐oxide (2), for which a combination of the chiral ligands with either [Pd(Cp)(allyl)] or [Pd(allyl)(MeCN)2]+/NaCH(CO2Me)2 proves to be a convenient and active pre‐catalyst system. A combination of an electron‐rich phosphine moiety and an electron‐deficient 3′‐sulfone moiety provides the best enantioselectivity to date for this process, affording the branched 2‐phenethenephosphonate, (−)‐iso‐3, in up to 74% ee with ligand (R,S)‐10i, where Ar=p‐anisyl and the 3′‐SO2R group is triflone.

Formation of Cyclic Phosphonium Salts in Trichlorosilane Reduction of Phosphine Oxides Bearing a Pendant Hydroxyl Group and Their Hydrolysis to Cyclic Phosphine Oxides

An unusual ring closure to cyclic phosphonium salts was observed during trichlorosilane reduction of phosphine oxides with a pendant hydroxylmethyl group. Alkaline hydrolysis of the cyclic phosphonium salts afforded a cyclic phosphine oxide and a ring-fission product.

Syntheses and reactions of the bis-boryloxide O(Bpin)2(pin = O2C2Me4)

The reaction of the phosphine oxides, OPEt31 and OPn-Bu32 with pinacolborane (HBpin) results in phosphine oxide reduction and the formation of O(Bpin)23. In contrast, the phosphine oxide OPn-Bu3 reacts with HB(C6F5)2 or B(C6F5)3 to give only the donor-acceptor adducts. Compound 3 reacts with HNPt-Bu3 to give the phosphinimonium borate salt, [t-Bu3PNH2][(Bpin(OBpin)2]6, while reaction with Cp2ZrMe2 affords the species Cp2Zr(OBpin)27.

Novel chiral diphosphine ligands with a pinene core obtained via an allylphosphinite–allylphosphine oxide rearrangement

A number of new chiral diphosphine ligands with a pinene framework were synthesized and tested in some transition metal-catalyzed asymmetric reactions (enantioselectivities up to 84% ee were achieved). The [2,3]-sigmatropic allylphosphinite–allyl phosphine oxide rearrangement was applied as a key step of the synthesis. An improved procedure for the phosphine oxide reduction and a new protocol for the deprotection of phosphine–borane complexes were developed.

ChemInform Abstract: Stereospecific Reduction of Phosphine Oxides to Phosphines by the Use of a Methylation Reagent and Lithium Aluminum Hydride.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Stereospecific reduction of phosphine oxides to phosphines by the use of a methylation reagent and lithium aluminum hydride.

[figure: see text] Various phosphine oxides are efficiently reduced by the use of a methylation reagent and lithium aluminum hydride. Optically active P-chirogenic phosphine oxides are also reduced with inversion of configuration at phosphorus atom by treatment with methyl triflate, followed by reaction with LiAlH4.

ChemInform Abstract: Reduction of Phosphine Oxides, a Mild and Practical Synthesis of Secondary P‐Ethynylphosphines.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Reduction of Phosphine Oxides, A Mild and Practical Synthesis of Secondary P-Ethynylphosphines

Reduction of Phosphine Oxides, A Mild and Practical Synthesis of Secondary P-Ethynylphosphines

Titanium (IV) catalysis in the reduction of phosphine oxides

Phosphine oxides can be reduced by triethoxysilane, or more conveniently polymethylhydrosiloxane, and catalytic titanium (IV) isopropoxide and provides a system for the efficient one-pot generation of phosphonium salts; the reduction occurs with retention of configuration at phosphorus.