Alkyne Insertion into the M?P and M?H Bonds (M=Pd,Ni, Pt,and Rh): A Theoretical Mechanistic Study of the C?P and C?H Bond‐Formation Steps

In hydrogen‐metal‐phosphorus (H M P) transition metal complexes (proposed as intermediates of H P bond addition to alkynes in the catalytic hydrophosphorylation, hydrophosphinylation, and hydrophospination reactions), alkyne insertion into the metal‐hydrogen bond was found much more facile compared to alkyne insertion into the metal‐phosphorus bond. The conclusion was verified for different metals (Pd, Ni, Pt, and Rh), ligands, and phosphorus groups at various theory levels (B3LYP, B3PW91, BLYP, MP2, and ONIOM). The relative reactivity of the metal complexes in the reaction with alkynes was estimated and decreased in the order of Ni>Pd>Rh>Pt. A trend in relative reactivity was established for various types of phosphorus groups: PR₂>P(O)R₂>P(O)(OR)₂, which showed a decrease in rate upon increasing the number of the oxygen atoms attached to the phosphorus center.     

Ni-catalyzed asymmetric hydrophosphinylation of conjugated enynes and mechanistic studies

The catalytic asymmetric synthesis of P-stereogenic phosphines is an efficient strategy to access structurally diverse chiral phosphines that could serve as organocatalysts and ligands to transition metals and motifs of antiviral drugs. Herein, we describe a Ni catalyzed highly regio and enantioselective hydrophosphinylation reaction of secondary phosphine oxides and enynes. This method afforded a plethora of alkenyl phosphine oxides which could serve as valuable precursors to bidentate ligands. A new type of mechanism was discovered by combined kinetic studies and density functional theory (DFT) calculations, which was opposed to the widely accepted Chalk–Harrod type mechanism. Notably, the alkene moiety which could serve as a directing group by coordinating with the Ni catalyst in the transition state, plays a vital role in determining the reactivity, regio and enantioselectivity.

A Ni-catalyzed hydrophosphinylation reaction of enynes was reported with excellent regio and enantioselectivity. A protonation mechanism was uncovered by combined kinetic studies and DFT calculations, which may lead to the discovery of other hydrofunctionalization reactions.     

Microwave-assisted regioselective addition of P(O)-H bonds to alkenes without added solvent or catalyst

The addition of P(O)-H bonds to alkenes has been accomplished using microwave irradiation in the absence of added solvent and catalyst. In addition to single addition reactions, tandem hydrophosphinylation reactions with alkynes afforded unsymmetrical species such as phosphine oxide-phosphinates. [reaction: see text]     

Markovnikov hydroformylation catalyzed by ROPAC in the presence of phosphine and phosphine oxide ligands

Rhodium-catalyzed hydroformylation of 1-octene in the presence of different phosphine and phosphine oxide ligands has been investigated. The molecular structure of new phosphine ligand, fluorenylidine methyl phenyl diphenylphosphine, was determined by single-crystal X-ray crystallography. Parameters such as different ligands, molar ratio of ligand to rhodium complex, ratio of olefin to rhodium complex, pressure of CO : H₂ mixture, and time of the reaction were studied. The linear aldehyde was the main product when the phosphine ligands were used as auxiliary ligands while the selectivity was changed to the branched products when the related phosphine oxide ligands were used. Under optimized reaction conditions, in the presence of [Rh(acac)(CO)(Ph₃P)]-di(1-naphthyl)phenyl phosphine oxide, conversion of 1-octene reached 97% with 87% selectivity of branched aldehyde.     

Catalytic Asymmetric Hydrophosphinylation of 2-Vinylazaarenes to Access P-Chiral 2-Azaaryl-Ethylphosphine Oxides

A chiral Brønsted acid-catalysed asymmetric hydrophosphinylation of 2-vinylazaarenes by secondary phosphine oxides is described. A variety of P-chiral 2-azaaryl-ethylphosphine oxides are synthesized with high yields and ees, of which both the substituents of phosphines and azaarenes can be flexibly modulated, underscoring an exceptionally broad scope of substrates. These adducts are valuable to asymmetric metal catalysis since the resultant P-chiral tertiary phosphines from the reduction of them are verified as a kind of effective C₁-symmetric chiral 1,5-hybrid P,N-ligands. Importantly, this catalysis platform enables the generic and efficient kinetic resolution of P-chiral secondary phosphine oxides. It thus provides an expedient approach to access the enantiomers of the P-chiral tertiary phosphine oxides derived from asymmetric hydrophosphinylation, further improving the utility of the method.     

A Convenient and Selective Palladium‐Catalyzed Aerobic Oxidation of Alcohols

An efficient procedure for the oxidation of primary and secondary alcohols to aldehydes and ketones, respectively, with molecular oxygen under ambient conditions has been achieved. By applying catalytic amounts of Pd(OAc)₂ in the presence of tertiary phosphine oxides (O?PR₃) as ligands, a variety of substrates are selectively oxidized without formation of ester byproducts. Spectroscopic investigations and DFT calculations suggest stabilization of the active palladium(II) catalyst by phosphine oxide ligands.     

Reactivity of phosphine oxide H<Subscript>3</Subscript>PO in the reactions with ketones

The reactivity of the electrochemically generated phosphine oxide H₃PO towards ketones (acetone, ethyl methyl ketone, methyl n-propyl ketone, and tert-butyl methyl ketone) has been studied. It was found that this reaction led to the formation of mono- and bis(hydroxyalkyl)phosphine oxides of the formulas RR?(OH)P(O)H₂ and [RR?(OH)]₂P(O)H (R = Me; R´ = Me, Et, Pr) and represents the first example of the P—C bond formation involving the intermediate H₃PO.     

Enantioselective Twofold C−H Annulation of Formamides and Alkynes without Built-in Chelating Groups

Twofold C−H annulation of readily available formamides and alkynes without built-in chelating groups was achieved. Ni−Al bimetallic catalysis enabled by a bulky BINOL-derived chiral secondary phosphine oxide (SPO) ligand proved to be critical for high reactivity and high selectivity. This reaction uses readily available formamides as starting materials and provides a concise synthetic pathway to a broad range of chiral ferrocenes in 40–98 % yield and 93–99 % ee.     

Asymmetric Hydrophosphinylation of Alkynes: Facile Access to Axially Chiral Styrene-Phosphines

A Cu/CPA co-catalytic system has been developed for achieving the direct hydrophosphinylation of alkynes with phosphine oxides in delivering novel axially chiral phosphorus-containing alkenes in high yields and excellent enantioselectivities (up to 99 % yield and 99 % ee). DFT calculations were performed to elucidate the reaction pathway and the origin of enantiocontrol. This streamlined and modular methodology establishes a new platform for the design and application of new axially chiral styrene-phosphine ligands.     

Enantioselective Twofold C−H Annulation of Formamides and Alkynes without Built‐in Chelating Groups

Twofold C?H annulation of readily available formamides and alkynes without built‐in chelating groups was achieved. Ni?Al bimetallic catalysis enabled by a bulky BINOL‐derived chiral secondary phosphine oxide (SPO) ligand proved to be critical for high reactivity and high selectivity. This reaction uses readily available formamides as starting materials and provides a concise synthetic pathway to a broad range of chiral ferrocenes in 40–98 % yield and 93–99 % ee.     

The formation of mesitylphosphine and dimesitylphosphine in the reaction of organonickel σ-complex [NiBr(Mes)(bpy)] (Mes = 2,4,6-trimethylphenyl,bpy = 2,2′-bipyridine) with phosphine PH3

Abstract

The reactivity of the previously reported organonickel σ-complex [NiBr(Mes)(bpy)], where Mes = 2,4,6-trimethylphenyl, bpy = 2,2′-bipyridine, toward phosphine PH₃ was investigated. The reaction leads to primary mesitylphosphine MesPH₂ as the main product and dimesitylphosphine Mes₂PH as secondary product with the nickel complex as transmetalating agent. The formed MesPH₂ reacts with an excess of the complex giving Mes₂PH as the major product.     

Reaction of secondary phosphine chalcogenides with 2,2,2-trichloroacetaldehyde

The nucleophilic addition of secondary phosphine chalcogenides to 2,2,2-trichloroacetaldehyde proceeds under mild noncatalytic conditions (12–25dgC, 15–90 min) with the formation of functional tertiary phosphine chalcogenides, containing hydroxy groups in up to 98% yield. Using the method of concurrent reactions the reactivity of secondary phosphine chalcogenides in this reaction was shown to decrease in the order: (PhCH₂CH₂)₂P(O)H ≫ (PhCH₂CH₂)₂P(S)H > (PhCH₂CH₂)₂P(Se)H, and the secondary bis[2-(2-pyridyl) ethyl]-phosphine oxide was more reactive than bis(2-phenethyl)phosphine oxide.     

Scope and Limitations of the s-Block Metal-Mediated Pudovik Reaction

The hydrophosphorylation of phenylacetylene with di(aryl)phosphane oxides Ar₂P(O)H (Pudovik reaction) yields E/Z-isomer mixtures of phenylethenyl-di(aryl)phosphane oxides ( 1 ). Alkali and alkaline-earth metal di(aryl)phosphinites have been studied as catalysts for this reaction with increasing activity for the heavier s-block metals. The Pudovik reaction can only be mediated for di(aryl)phosphane oxides whereas P-bound alkyl and alcoholate substituents impede the P−H addition across alkynes. The demanding mesityl group favors the single-hydrophosphorylated products 1-Ar whereas smaller aryl substituents lead to the double-hydrophosphorylated products 2-Ar . Polar solvents are beneficial for an effective addition. Increasing concentration of the reactants and the catalyst accelerates the Pudovik reaction. Whereas Mes₂P(O)H does not form the bis-phosphorylated product 2-Mes , activation of an ortho-methyl group and cyclization occurs yielding 2-benzyl-1-mesityl-5,7-dimethyl-2,3-dihydrophosphindole 1-oxide ( 3 ).     

Studies on highly regio- and stereoselective fluorohydroxylation reaction of 3-aryl-1,2-allenyl phosphine oxides with Selectfluor

The fluorohydroxylation of allenyl phosphine oxides with Selectfluor in commercial MeCN without prior treatment or a mixed solvent of anhydrous MeCN (refluxed over CaH₂ and distilled before use) and 7.0 equiv of H₂O or MeNO₂/H₂O=10/1 afforded 2-fluoro-3-hydroxy-1(E)-alkenyl diphenyl phosphine oxides in moderate yields with very high regio- and stereoselectivities. The E-stereoselectivity is believed to be controlled by the phosphine oxide functionality. In the reaction of 3-(4-methoxyphenyl)-1,2-propadienyl diphenyl phosphine oxide, further fluorination on the electron-rich phenyl ring was also observed.     

Synthesis of o-(diphenylphosphinoyloxy)anilines by the rhodium-catalyzed reaction of nitroarenes and diphenylphosphine oxide

A rhodium complex Rh₂(OAc)₄ catalyzed the reaction of nitrobenzenes and diphenylphosphine oxide HP(O)Ph₂ giving o-(diphenylphosphinoyloxy)anilines predominantly, which were accompanied by small amounts of the p-isomers. Nitorobenzenes possessing a bulky o-substituent, particularly o-(t-butyl)nitrobenzenes, underwent the reaction in high yields. The reaction is considered to involve the reductive formation of O-phosphinoyl-N-arylhydroxyamines from nitrobenzenes, and o-phosphinoyloxylation by the rearrangement.     

Design and Synthesis of Isocyanide Ligands for Catalysis: Application to Rh‐Catalyzed Hydrosilylation of Ketones

New isocyanide ligands with meta‐terphenyl backbones were synthesized. 2,6‐Bis[3,5‐bis(trimethylsilyl)phenyl]‐4‐methylphenyl isocyanide exhibited the highest rate acceleration in rhodium‐catalyzed hydrosilylation among other isocyanide and phosphine ligands tested in this study. ¹H NMR spectroscopic studies on the coordination behavior of the new ligands to [Rh(cod)₂]BF₄ indicated that 2,6‐bis[3,5‐bis(trimethylsilyl)phenyl]‐4‐methylphenyl isocyanide exclusively forms the biscoordinated rhodium–isocyanide complex, whereas less sterically demanding isocyanide ligands predominantly form tetracoordinated rhodium–isocyanide complexes. FTIR and ¹³C NMR spectroscopic studies on the hydrosilylation reaction mixture with the rhodium–isocyanide catalyst showed that the major catalytic species responsible for the hydrosilylation activity is the Rh complex coordinated with the isocyanide ligand. DFT calculations of model compounds revealed the higher affinity of isocyanides for rhodium relative to phosphines. The combined effect of high ligand affinity for the rhodium atom and the bulkiness of the ligand, which facilitates the formation of a catalytically active, monoisocyanide–rhodium species, is proposed to account for the catalytic efficiency of the rhodium–bulky isocyanide system in hydrosilylation.     

Rhodium/Phosphine catalysed selective hydroformylation of biorenewable olefins

This work reports rhodium catalyzed selective hydroformylation of natural olefins like eugenol, estragole, anethole, prenol and isoprenol using biphenyl based Buchwald phosphine ligands (S‐Phos ( L ₁ ), t‐Bu XPhos ( L ₂ ), Ru‐Phos ( L ₃ ), Johnphos ( L ₄ ) and DavePhos ( L ₅ ). Ru‐Phos ( L ₃ ) ligand exhibited high impact on the hydroformylation of eugenol providing high selectivity (90%) of linear aldehyde as major product. In addition, internal natural olefins like anethole and prenol provided moderate to high selectivity (65% and 85% respectively) of branched aldehydes as a major products. The various reaction parameters such as influence of ligands, P/Rh ratio, syngas pressure, temperature, time and solvents have been studied. A high activity and selectivity gained on the way to the linear aldehydes it may be due to the bulky, steric cyclohexyl and isopropoxy groups present in L ₃ phosphine ligand. Moreover, this catalytic system was smoothly converting natural olefins into corresponding linear and branched aldehydes with higher selectivity under the mild reaction conditions.     

Rhodium‐Catalyzed Enantioposition‐Selective Hydroarylation of Divinylphosphine Oxides with Aryl Boroxines

The rhodium‐catalyzed hydroarylation of divinylphosphine oxides (RP(O)(CH=CH₂)₂) with aryl boroxines ((ArBO)₃) gives the corresponding monoarylation products (RP(O)(CH=CHAr)CH₂CH₃) in high yields. One of the two vinyl groups in the phosphine oxide undergoes oxidative arylation while the other one is reduced to an ethyl moiety. These reactions proceed with high selectivity in terms of the enantiotopic vinyl groups in the presence of (R)‐DTBM‐segphos/Rh to give the P‐stereogenic monoarylation products with high enantioselectivity.     

Nickel/Brønsted acid dual-catalyzed regio- and enantioselective hydrophosphinylation of 1,3-dienes: access to chiral allylic phosphine oxides

While chiral allylic organophosphorus compounds are widely utilized in asymmetric catalysis and for accessing bioactive molecules, their synthetic methods are still very limited. We report the development of asymmetric nickel/Brønsted acid dual-catalyzed hydrophosphinylation of 1,3-dienes with phosphine oxides. This reaction is characterized by an inexpensive chiral catalyst, broad substrate scope, and high regio- and enantioselectivity. This study allows the construction of chiral allylic phosphine oxides in a highly economic and efficient manner. Preliminary mechanistic investigations suggest that the 1,3-diene insertion into the chiral Ni–H species is a highly regioselective process and the formation of the chiral C–P bond is an irreversible step.

Asymmetric hydrophosphinylation of 1,3-dienes with phosphine oxides using an inexpensive chiral catalyst has been demonstrated, providing access to chiral allylic phosphine oxides with broad substrate scope and high regio- and enantioselectivity.     

Ligand effects in the rhodium-catalyzed addition of alkynes to aldehydes and diketones. Modification of the β-diketonate ligand

A catalytic amount of dicarbonylacetonato rhodium(I) and a phosphine ligand bearing bulky electron-donating alkyl groups has been shown to generate an effective catalyst for the addition of alkynes to aldehydes and activated ketones under mild, neutral conditions. While previous studies have shown that modification of the phosphine has significant effects on the activity of the catalyst, the role of the β-diketonate ligand has not been probed. Six different β-diketonate rhodium complexes were synthesized and their ability to catalyze the alkyne addition reaction was evaluated. Changing the structure of the β-diketonate ligand can have a noticeable effect on the reaction rate. Acetylacetonate derivatives with strong electron withdrawing groups have a detrimental effect on the catalytic activity, while bulky and electron rich β-diketonate derivatives provide more efficient catalysts.