Nickel(II)-Magnesium-Catalyzed Cross-Coupling of 1,1-Dibromo-1-alkenes with Diphenylphosphine Oxide: One-Pot Synthesis of (E)-1-Alkenylphosphine Oxides or Bisphosphine Oxides

Synthesis, characterisation and reactivity of group 2 complexes with a thiopyridyl scorpionate ligand

Herein we report the reactivity of the proligand tris(2-pyridylthio)methane (HTptm) with various Alkaline Earth (AE) reagents: (1) dialkylmagnesium reagents and (2) AE bis-amides (AE = Mg-Ba). Heteroleptic complexes of general formulae [Mg(Tptm)(R)] (R = Me, ⁿBu; Tptm = {C(S-C₅H₄N)₃}^-) and [AE(Tptm)(N'')] (AE = Mg-Ba; N'' = {N(SiMe₃)₂}^-) were targeted from the reaction of HTptm with R₂Mg or [AE(N'')₂]₂. Reaction of the proligand with dialkylmagnesium reagents led to formation of [{Mg(κ³C,N,N-C{Bu}{S-C₅H₄N}₂)(μ-S-C₅H₄N)}₂] (1) and [{Mg(κ³C,N,N-C{Me}{S-C₅H₄N}₂)(μ-OSiMe₃)}₂] (2) respectively, as a result of a novel transfer of an alkyl group onto the methanide carbon with concomitant C-S bond cleavage. However, reactivity of bis-amide precursors for Mg and Ca did afford the target species [AE(Tptm)(N'')] (3-AE; AE = Mg-Ca), although these proved susceptible to ligand degradation processes. DFT calculations show that alkyl transfer in the putative [Mg(Tptm)(ⁿBu)] (1m') system and amide transfer in 3-Ca is a facile process that induces C-S bond cleavage in the Tptm ligand. 3-Mg and 3-Ca were also tested as catalysts for the hydrophosphination of selected alkenes and alkynes, including the first example of mono-hydrophosphination of 4-ethynylpyridine which was achieved with high conversions and excellent regio- and stereochemical control.

Room-Temperature-Stable Magnesium Electride via Ni(II) Reduction

Herein, we report the synthesis of highly reduced bipyridyl magnesium complexes and the first example of a stable organic magnesium electride supported by quantum mechanical computations and X-ray diffraction. These complexes serve as unconventional homogeneous reductants due to their high solubility, modular redox potentials, and formation of insoluble, non-coordinating byproducts. The applicability of these reductants is showcased by accessing low-valent (bipy)₂Ni(0) species that are challenging to access otherwise.

Copper-catalyzed stereo- and regioselective hydrophosphorylation of terminal alkynes: scope and mechanistic study

Herein, a protocol for copper-catalyzed highly stereo- and regioselective hydrophosphorylation of terminal alkynes to E-alkenylphosphorus compounds was well developed. It represents a general and practical hydrophosphorylation method, of which diarylphosphine oxide, dialkylphosphine oxide and dialkyl phosphite all had effective P(O)H parts to react with different types of terminal alkynes. Contrary to previous air-sensitive reports, all the reactions proceeded well under air. This methodology is quite attractive owing to the high stereo- and regioselectivity, good functional group tolerance, scalability and facile late-stage derivatization of some natural product derivatives and commercially available herbicides. What's more, investigations on the reaction mechanism with deuterium-labeling experiments and DFT studies firstly disclosed the deprotonation-protonation equilibrium of terminal alkynes and P(O)H part during the catalytic hydrophosphorylation process.

Enantioselective, Copper-Catalyzed Addition of Nucleophilic Silicon to Alkenyl-Substituted Phosphine Oxides

An enantioselective β-silylation of α,β-unsaturated phosphine oxide derivatives using a silylboronic ester as the silicon pronucleophile is reported. The reaction is catalyzed by copper salts in the presence of chiral pyridine–oxazoline (PyOx) ligands. Good to high enantioselectivities (≤95% ee) are obtained for β-aryl-substituted acceptors whereas alkyl residues in the β-position led to a lower ee value for 1° and no reaction for 2° and 3°. The new method represents another way of accessing α-chiral silanes and complements the known β-borylation.

Manganese(I) Catalyzed Alkenylation of Phosphine Oxides Using Alcohols with Liberation of Hydrogen and Water

Herein, a catalytic cross-coupling of methyldiphenylphosphine oxide with arylmethyl alcohols leading to the alkenylphosphine oxides is reported. A manganese pincer catalyst catalyzes the reactions, which provides exclusive formation of trans-alkenylphosphine oxides. Mechanistic studies indicate that reactions proceed via aldehyde intermediacy and the catalyst promotes the C═C bond formation. Reactions are facilitated by dearomatization, and aromatization metal-ligand cooperation operates in catalyst. Use of abundant base metal catalyst and formation of water and H₂ as the only byproducts make this catalytic protocol sustainable and environmentally benign.

Nickel Complexes in C‒P Bond Formation

This review is a comprehensive account of reactions with the participation of nickel complexes that result in the formation of carbon-phosphorus (C‒P) bonds. The catalytic and non-catalytic reactions with the participation of nickel complexes as the catalysts and the reagents are described. The various classes of starting compounds and the products formed are discussed individually. The several putative mechanisms of the nickel catalysed reactions are also included, thereby providing insights into both the synthetic and the mechanistic aspects of this phosphorus chemistry.

Copper-Catalyzed Enaminone C(sp2)-N Bond Phosphonation for Stereoselective Synthesis of Alkenylphosphonates

A direct enaminone C-N bond coupling functionalization that generates a new C-P bond using dialkyl phosphonate for the efficient and stereoselective synthesis of (E)-alkenylphosphonates is reported. The reactions toward target products proceed well with a broad scope, disclosing a valuable new synthetic application of enaminones by the interesting C(sp²)-N bond elaboration.

Palladium-Catalyzed Cascade Difluoroalkylation and Phosphinoylation of 2-Vinyloxy Arylalkynes: Selective Synthesis of Difluoroalkyl-Containing Tetrasubstituted Alkenylphosphine Oxides

A Pd-catalyzed difluoroalkylation/cyclization/phosphinoylation of 2-vinyloxy arylalkynes with ethyl difluoroiodoacetate and diarylphosphine oxides has been successfully developed. This reaction allows the formation of C_sp3-CF₂, C_sp3-C_sp2, and C_sp2-P(O) bonds in one step, providing a straightforward route to difluoroalkyl-containing tetrasubstituted alkenylphosphine oxides with complete stereoselectivities under mild conditions.

Transition metal-free and regioselective vinylation of phosphine oxides and H-phosphinates with VBX reagents

A series of phosphine oxides and H-phosphinates were vinylated in the presence of the iodine(iii) reagents vinylbenziodoxolones (VBX), providing the corresponding alk-1-enyl phosphine oxides and alk-1-enyl phosphinates in good yields with complete chemo- and regioselectivity. The vinylation proceeds in open flask under mild and transition metal-free conditions.

Focusing on the Catal. of the Pd- and Ni-Catalyzed Hirao Reactions

The Hirao reaction involving the phosphinoylation or phosphonation of aryl halides by >P(O)H reagents is a P–C bond forming transformation belonging to the recently very hot topic of cross-couplings. The Pd- or Ni-catalyzed variations take place via the usual cycle including oxidative addition, ligand exchange, and reductive elimination. However, according to the literature, the nature of the transition metal catalysts is not unambiguous. In this feature article, the catalysts described for the Pd(OAc)₂-promoted cases are summarized, and it is concluded that the “(HOY₂P)₂Pd(0)” species (Y = aryl, alkoxy) is the real catalyst. In our model, the excess of the >P(O)H reagent served as the P-ligand. During the less studied Ni(II)-catalyzed instances the “(HOY₂P)(−OY₂P)Ni(II)Cl⁻” form was found to enter the catalytic cycle. The newest conclusions involving the exact structure of the catalysts, and the mechanism for their formation explored by us were supported by our earlier experimental data and theoretical calculations.

Experimental and Theoretical Study on the "2,2'-Bipiridyl-Ni-Catalyzed" Hirao Reaction of >P(O)H Reagents and Halobenzenes: A Ni(0) → Ni(II) or a Ni(II) → Ni(IV) Mechanism?

It was found by us that the P–C coupling reaction of >P(O)H reagents with PhX (X = I and Br) in the presence of NiCl₂/Zn as the precursors for the assumed Ni(0) complexant together with 2,2′-bipyridine as the ligand took place only with PhI at 50/70 °C. M06-2X/6-31G(d,p)//PCM(MeCN) calculations for the reaction of Ph₂P(O)H and PhX revealed a favorable energetics only for the loss of iodide following the oxidative addition of PhI on the Ni(0) atom. However, the assumed transition states with Ni(II) formed after P-ligand uptake and deprotonation could not undergo reductive elimination meaning a “dead-end route”. Hence, it was assumed that the initial complexation of the remaining Ni²⁺ ions with 2,2′-bipyridine may move the P–C coupling forward via a Ni(II) → Ni(IV) transition. This route was also confirmed by calculations, and this mechanism was justified by preparative experiments carried out using NiCl₂/bipyridine in the absence of Zn. Hence, the generally accepted Ni(0) → Ni(II) route was refuted by us, confirming the generality of the Ni(II) → N(IV) protocol, either in the presence of bipyridine, or using the excess of the >P(O)H reagent as the P-ligand. The results of the calculations on the complex forming ability of Ni(0) and Ni(II) with 2,2′-bipyridine or the P-reagents were in accord with our mechanistic proposition.

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).

A surprising mechanism lacking the Ni(0) state during the Ni(II)-catalyzed P–C cross-coupling reaction performed in the absence of a reducing agent – An experimental and a theoretical study

The Hirao reaction, i.e. the P–C coupling between a bromoarene and a >P(O)H reagent performed in most cases in the presence of a Pd(0) complex incorporating a P-ligand may also be carried out applying a Ni(II) catalyst precursor with or without Zn or Mg as the reducing agent. The Ni catalysts may include P- or N-ligands. B3LYP/6-31G(d,p)//PCM(MeCN) quantum chemical calculations suggested that the mechanism of the NiX2 catalyzed (X=Cl or Br) P–C couplings performed in the absence of a reducing agent, and in the excess of the >P(O)H reagent serving as the P-ligand (via its tautomeric >POH form) is completely different from that of the Pd(OAc)2 promoted version, as no reduction of the Ni(II) occurs. In the two variations mentioned, the active catalyst is the dehydrobrominated species derived from primary complex [(HO)Y2P]2Ni(II)Br2, and the [(HO)Y2P]2Pd(0) complex itself, respectively. Both species undergo temporary oxidation (to “Ni(IV)” and “Pd(II)”, respectively) in the catalytic cycle. During the catalysis with “P2Ni(II)X2”, one of the P-ligands serves the >P(O)H function of the ArP(O)H <  product. The consequence of this difference is that in the Ni(II)-catalyzed case, somewhat less >P(O)H-species is needed than in the Pd(0)-promoted instance. Applying 10 % of the Pd(OAc)2 or NiX2 precursor, the optimum quantity of the P-reagent is 1.3 equivalent and, in the first approach, 1.1 equivalent, respectively. Preparative experiments justified the new mechanism explored. The ligation of Ni(II) was also investigated by theoretical calculations. It was proved that the bis-complexation is the most favorable energetically as compared to the mono-, tri- and tetra-ligation.

Radical Cation Salt-initiated Aerobic C-H Phosphorylation of N-Benzylanilines: Synthesis of α-Aminophosphonates

A radical cation salt-initiated phosphorylation of N-benzylanilines was realized through an aerobic oxidation of the sp3 C-H bond, providing a series of α-aminophosphonates in high yields. An investigation of the reaction scope revealed that this mild catalyst system is superior in good functional group tolerance and high reaction efficiency. The mechanistic study implied that the cleavage of the sp3 C-H bond was involved in the rate-determining step.

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)CH₂ CH₃ ) 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.

P-Arylation of secondary phosphine oxides catalyzed by nickel-supported nanoparticles

Nickel-supported nanoparticles were used as catalysts for ligand-free Hirao coupling between secondary phosphine oxides and aryl halides.

Mono and double Mizoroki–Heck reaction of aryl halides with dialkyl vinylphosphonates using a reusable palladium catalyst under aqueous medium

2-Aryl or 2,2-diaryl vinylphosphonates can be selectively obtained by using aryl halides or dialkylvinylphosphonate as the limiting reagent.

Cobalt catalysed, copper assisted C(sp2)–P cross coupling

An efficient C(sp²)–P cross coupling has been achieved using a Co/Cu catalytic system for access to vinyl and aryl phosphonates.

General synthesis of P-stereogenic compounds: the menthyl phosphinate approach

Easily prepared menthyl phosphinates of high diastereoisomeric purity provide versatile intermediates for the synthesis of P-stereogenic compounds.

Decarboxylative functionalization of cinnamic acids

Decarboxylative functionalization of α,β-unsaturated carboxylic acids is an emerging area that has been developed significantly in recent years.

Stereoselective synthesis of vinylphosphonates and phosphine oxides via silver-catalyzed phosphorylation of styrenes

An efficient and stereoselective synthesis of vinylphosphonates and phosphine oxides was developed starting from styrenes using AgNO₃ as the catalyst and K₂S₂O₈ as the oxidant. Various vinyl-phosphonates and phosphine oxides were synthesized in good yields with excellent regioselectivity.