Phosphite Ligands Research Articles

Highly regioselective homogeneous isomerization-hydroformylation of 2-butene with water- and air-stable phosphoramidite bidentate ligand

• The linear selectivity of valeraldehyde was over 99% with the yield of more than 93%. • Diphosphoramidite ligand is water and air stable, proved by hydrolysis, oxidation experiments and cyclic voltammetry measurements. • Diphosphoramidite ligand coordination to rhodium as ee configuration was confirmed by NMR and IR. • The Rh/diphosphoramidite catalyst could work well for beyond 40 h. Highly selective isomerization-hydroformylation of 2-butene was achieved with the presence of Rh(acac)(CO) 2 and a phosphoramidite bidentate ligand which bearing 2,2′-dihydroxy-1,1′-binaphthyl backbone and N-indolyl substitute. The molar ratio of n- to isovaleraldehyde (217) is distinctly higher than the reported systems. NMR and IR revealed that the five-coordinate HRh(ligand)(CO) 2 was an equatorial-equatorial configuration which contributed to the n-selectivity of valeraldehyde. The strong π-acceptor ability of ligand was suggested to play a key role in fast isomerization of 2-butene. Hydrolysis and oxidation experiments demonstrated that the ligand was water- and air-stable. Cyclic voltammetry measurement confirmed that this phosphoramidite ligand is more difficult to be oxidized, compared with the phosphine, phosphinite and phosphite ligands. Inspiringly, recycling experiments showed the catalytic system could work for at least 7 runs with unchanged selectivity.

Diastereoselective Rhodium‐Catalyzed [(3+2+2)] Carbocyclization Reactions with Tethered Alkynylidenecyclopropanes: Synthesis of the Tremulane Sesquiterpene Natural Products

AbstractThe development of a diastereoselective intramolecular rhodium‐catalyzed [(3+2+2)] carbocyclization reactions of allyl ester‐tethered alkynylidenecyclopropanes (ACPs) for the construction of the 5,7,5‐tricyclic scaffold of the tremulane sesquiterpene natural products is described. This work illustrates that the stereoelectronic nature of the phosphite ligand is critical for attaining high levels of stereocontrol, which is supported by density functional theory (DFT) studies that indicate the migratory insertion step favors the formation of the cis‐diastereoisomer. Consequently, several strategies were devised to access the thermodynamically more stable trans‐diastereoisomer for the synthesis of the tremulanes; however, the attempts to either switch the sense of stereocontrol in the carbocyclization reaction or epimerize the C7 stereocenter of the cycloadduct provided only modest improvements. Hence, we elected to highlight the synthetic utility of this strategy with the development of concise and highly efficient syntheses of (±)‐epi‐tremulenolide A, (±)‐epi‐tremulenediol A and (±)‐epi‐ceriponol D.

Solvent effects in hydroformylation of long-chain olefins

With the increasing concern about climate change, a transformation of chemical processes from petrochemicals towards renewable, sustainable feedstock is indispensable. This transition, however, requires a careful assessment of the transferability of kinetic and thermodynamic data and reaction mechanisms. Here, the rhodium-catalyzed hydroformylation reaction of a long chain olefin (1-decene) from renewable resources with synthesis gas (CO/H2) is investigated. The bidentate phosphite ligand (BiPhePhos) shows a high degree of selectivity towards the desired n-aldehyde. This selectivity can be rationalized by kinetic discrimination at the branching point, i.e. the transition state of the hydride insertion step. Formation of the iso-aldehyde is kinetically and thermodynamically discriminated. The complex multi-step reactions of the desired hydroformylation reaction pathway with respect to the side-reactions such as olefin isomerization and hydrogenation are elucidated in detail. The solvent effects of a mixed polar/non-polar thermomorphic solvent system (DMF/dodecane) on chemical equilibria and activation energies were investigated using the COSMO-RS solvent model. Solvent screening for the overall rate-determining step gives a perspective as to solvent control of the process.

Preparation of a dicationic, p-cymene ruthenium(II) dimer, [(p-cymene)Ru(µ-Cl)(P{OCH2}3CEt)]2 2+: structure, characterization and reactivity

Reaction of (p-cymene)RuCl2(P{OCH2}3CEt) (P{OCH2}3CEt = 4-ethyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane) with silver tetrakis[(3,5-trifluoromethyl)phenyl]borate (Ag[BArF′]) in the presence of acetonitrile produces the monomeric, cationic, acetonitrile ruthenium(II) compound, [(p-cymene)RuCl(P{OCH2}3CEt)(NCMe)][BArF′] (1), in near quantitative yields (97%). Chloroform solutions of 1 cause precipitation of the binuclear, dicationic ruthenium(II) dimer complex, [(p-cymene)Ru(µ-Cl(P{OCH2}3CEt)]2[BArF′]2 (2), with bridging chloride ligands (73%). Complex 2 can be readily converted back to 1 by treatment with acetonitrile. Complexes 1 and 2 were characterized by 1H, 13C{1H}, 31P{1H}, 19F{1H} nuclear magnetic resonance spectroscopy and infrared spectroscopy. Complex 2 was also characterized by single crystal X-ray crystallography, which revealed a rhomboid Ru2Cl2 core. The two ruthenium ions in 2 are displaced at 3.668 Å, which precludes Ru–Ru bonding. To our knowledge, this is the first cymene ruthenium(II) dimeric complex with bridging halide ligands supported by a monodentate phosphite ligand.

Advancement of Chiral Monodentate Phosphite Ligands in Asymmetric Hydrogenation

Advancement of Chiral Monodentate Phosphite Ligands in Asymmetric Hydrogenation

Rhodium‐Catalyzed Regioselective 3‐Fluoroallylic Cyanation

AbstractA rhodium catalyzed allylic cyanation of 3‐fluoroallylic trifluoroacetates with TMSCN and a bulky phosphite ligand gives 3‐fluoroallylic nitriles with a good substrate scope and high Z/E‐selectivity. For dialkylated substrates the fluorine substituent controls the regioselectivity in favor of the distal product. For the stereoselectivity of the allylic cyanation selected scalemic substrates showed inversion of the stereocenter and some or no stereoerosion.magnified image

Ruthenium(II)–arene and triruthenium-carbonyl cluster complexes with new water-soluble phopsphites based on glucose: Synthesis, characterization and antiproliferative activity

New water-soluble 3,5,6-bicyclophosphite ligands based on glucose modified with uracil, 5-fluorouracil or thymine are reported. The phosphite ligands were subsequently reacted with bis[dichlorido(η6-p-cymene)ruthenium(II)] and trirutheniumdodecacarbonyl to afford monoruthenium analogues of RAPTA-C and triruthenium clusters with 1–3 phosphite ligands, respectively. The influence of ligands on the stability of the compounds and the antiproliferative activity of the compounds was investigated.

Interaction of norbornadiene with allyl acetate in the presence of Ni0 complexes: a DFT modeling

Catalytic allylation of norbornadiene in the presence of Ni0 phosphite complexes resulting in 3-methylenetricyclo[4.2.1.02,5]non-7-ene was studied within the framework of the density functional theory using the PBE functional and the L11, L2, and L3 basis sets. According to calculations, the formation of the first C-C bond more likely proceeds involving the η3-allyl rather than η1-allyl ligand. The presence of one phosphite ligand favors the formation of the second C-C bond. Hydride transfer from the allyl moiety to the acetate ligand proceeds via an intermediate with agostic interaction. The highest energy on the reaction pathway (22.7 kcal mol−1) corresponds to the transition state of the formation of the first C-C bond.

Chemistry of vinylidene complexes. XXV. Synthesis and reactions of binuclear µ-vinylidene RePt complexes containing phosphite ligands. Spectroscopic, structural and electrochemical study

Reactions of Cp(CO)2ReCCHPh with Pt[P(OR)3]4 (R = Pri, Et, Ph) gave binuclear μ-vinylidene complexes Cp(CO)2RePt(μ-CCHPh)[P(OR)3]2. Treatment of the previously synthesized Cp(CO)2Re(μ-CCHPh)Pt(PPh3)2 with triisopropylphosphite or triethylphosphite resulted in a stepwise substitution of PPh3 ligands, leading to the disubstituted Cp(CO)2RePt(μ-CCHPh)[P(OR)3]2 and monosubstituted Cp(CO)2RePt(μ-CCHPh)[P(OR)3](PPh3) (R = Pri or Et) species, while no triphenylphosphine ligand substitution in the reaction with P(OPh)3 occurs at all. The monosubstituted Cp(CO)2RePt(μ-CCHPh)[P(OR)3](PPh3) (R = Pri, Et, Ph) species were also obtained by reacting Cp(CO)2ReCCHPh with mixed-ligand complexes Pt(PPh3)3L (L = P(OPri)3, P(OEt)3, P(OPh)3). Reactions of Cp(CO)2RePt(μ-CCHPh)LL′ (L = L′ = P(OPri)3, P(OEt)3, P(OPh)3; L = P(OPri)3, P(OEt)3, P(OPh)3, L′ = PPh3) with Co2(CO)9 yield tricarbonyl vinylidene species Cp(CO)2RePt(μ-CCHPh)[P(OR)3](CO) (R = Pri, Et, Ph). The obtained compounds were characterized by IR and 1H, 13C, 31P NMR spectroscopy. The molecular structures of Cp(CO)2RePt(μ-CCHPh)[P(OPri)3]2, Cp(CO)2RePt(μ-CCHPh)[P(OPri)3](PPh3) and Cp(CO)2RePt(μ-CCHPh)[P(OPri)3](CO) were determined by X-ray diffraction study. The redox properties of the new complexes and their reactions of chemical oxidation were studied.

Effects of Substitution Pattern in Phosphite Ligands Used in Rhodium-Catalyzed Hydroformylation on Reactivity and Hydrolysis Stability

The stability of homogeneous catalytic systems is an industrially crucial topic, which, however, receives comparatively little attention from academic research. Phosphites are among the most frequently used ligands in industrial, rhodium-catalyzed n-regioselective hydroformylation. However, they are particularly vulnerable to hydrolysis. Since the decomposition of ligands should be dependent on the substitution patterns, phenyl, tert-butyl and condensed ring systems of benzopinacolphosphites were evaluated concerning their activity, regioselectivity and hydrolysis stability. A series of twelve strongly related phosphites were synthesized, tested in the hydroformylation of isomeric n-octenes, and studied in hydrolysis experiments using in situ NMR spectroscopy. Our results show that substituents in the ortho-position, especially tert-butyl substituents, enhance hydrolysis stability while maintaining compelling activity and regioselectivity. In contrast, substituents in the para-position may destabilize the phosphite.

N-Heterocyclic Carbene Ligand-Controlled Regioselectivity for Nickel-Catalyzed Hydroarylation of Vinylarenes with Benzothiazoles.

A facile regioselective switch for nickel-catalyzed hydroarylation of vinylarenes with benzothiazoles has been developed, which relies on the simple structural variation of novel Ni(II) complexes of the type Ni(NHC)[P(OR)3]Br2. Using magnesium turnings as the reductant, Ni(IMes)[P(OEt)3]Br2 afforded branched products, while Ni(IPr*OMe)[P(OEt)3]Br2 created steric demand to afford linear products. This work also provides a rare example of the rational design of heteroleptic Ni(II) complexes that display the required air stability, reactivity, and regioselectivity via synergism between NHC and phosphite ligands.

Boosting the hydrolytic stability of phosphite ligand in hydroformylation by the construction of superhydrophobic porous framework

The development of a catalyst that delivers high activities and selectivities with excellent durability is of great importance. Numerous efficient catalysts suffer from the inherent hydrolysis liabilities, plaguing their practical applications. Herein, we show that this challenge can be tackled by constructing them into superhydrophobic porous frameworks, as exemplified by a water-sensitive phosphite ligand, tris(2-tert-butylphenyl) phosphite. The efficiency and long-term stability of the developed system are remarkably high in the hydroformylation of internal olefins after metalation with Rh species, superior to the corresponding homogeneous analogues. The significantly boosted hydrolytic stability allows for catalytic transformations using water as a green solvent, which not only facilitates the isolation of the products, but also furnishes the aldehydes with higher regioselectivities for the desired linear form in comparison with that operated under benchmark conditions using toluene as a reaction medium. Given these promising results, we anticipate the strategy advanced herein will form the basis for constructive perspectives in the enhancement of the water resistance of catalysts and the development of high performance hydroformylation catalysts.

Novel Full Hydrogenation Reaction of Methyl Esters of Palm Kernel and Sunflower Oils Into Methyl Stearate Catalyzed by Rhodium, Ruthenium and Nickel Complexes of Bidentate Hexasulfonated o-Phenylendiphosphite Ligands

High catalytic activities (TOF = 8680 h−1) have been achieved by novel rhodium catalysts modified with the chelating sulfonated phosphite ligand hexasulfonated o-phenylendiphosphite in the hydrogenation reaction of renewable methyl esters of sunflower oil under mild reaction conditions and a low rhodium concentration of 50 ppm in methanol.

Mercapto-benzothiazolyl based ruthenium(ii) borate complexes: synthesis and reactivity towards various phosphines.

The synthesis and reactivity of ruthenium complexes containing an anionic boron based ligand, supported by mercapto-benzothiazolyl heterocycles are presented. Specifically, the reaction of [(η6-p-cymene)Ru{P(OMe)2OR}Cl2], (1a: R = Me; 1b: R = H) with [H2B(mbz)2]- (mbz = 2-mercaptobenzothiazolyl) at room temperature afforded a series of borate complexes, namely [(L)Ru{κ3-H,S,S'-H2B(L)2}P(O)(OMe)(HL)], 2, [Ru{κ3-H,S,S'-H2B(L)2}2], 3 and [(κ2-N,S-L)P(OMe)3Ru{κ3-H,S,S'-H2B(L)2}], 4a; (L = C7H4NS2). The pivotal feature of 2 is the coordination of the Ru centre with a phosphorus atom of secondary phosphine oxide and mercapto-benzothiazolyl ligands. On the other hand, 3 features dual RuH-B interactions between Ru and B-H bonds of [H2B(mbz)2]-. Interestingly, along with 3, compound [(κ2-N,S-L)P(OPh)3Ru{κ3-H,S,S'-H2B(L)2}], 4b (L = C7H4NS2), was isolated upon treatment of the same borate with [(η6-p-cymene)RuCl2P(OPh)3], 1c, which is stabilized by δ-B-H interactions and one phosphite ligand. Furthermore, compound 3 promptly reacts with P(OR)3 to generate [(OR)3PRu-{κ2-S,S'-H2B(L)2}{κ3-H,S,S'-H2B(L)2}], (5a: R = Me, 5b: R = Ph; L = C7H4NS2) by breaking one of the RuH-B interactions. Upon heating, compound 5a converts into [(OMe)2OPRu{κ2-S,S'-HB(L)2}{κ3-H,S,S'-H2B(L)2}], 6a (L = C7H4NS2) by release of methane gas. Compound 6a is a unique example wherein the boron atom of the borate ligand is bound to an oxygen atom of the phosphite unit. In contrast, the thermolysis of 3 with PR2R' yielded [Ru{κ3-H,S,S'-H2B(L2)}(PR2R')2(L)], (7a: R = Me, R' = Ph; 7b: R = Ph; R' = Me; L = C7H4NS2), respectively, revealing the incorporation of two triphosphine ligands in the coordination sphere of ruthenium. Density functional theory (DFT) calculations were undertaken to provide an insight into the electronic structures of the complexes.

X-ray crystallography and electrochemistry reveal electronic and steric effects of phosphine and phosphite ligands in complexes RuII(κ4-bda)(PR3)2 and RuII(κ3-bda)(PR3)3 (bda = 2,2′-bipyridine-6,6′-dicarboxylato)

We have examined coordination of PR3 = triphenylphosphine, triethylphosphine, triisopropyl phosphite, trimethyl phosphite, and 1,3,5-triaza-7-phosphaadamantane (PTA) to the fragment RuII(bda) to better understand how different phosphine and phosphite ligands influence the electronic and structural properties of the RuII complexes. PTA and P(OMe)3 afforded complexes with three phosphorus ligands bound to Ru, with the bda being tridentate (κ3-N,N,O) in complexes 4 and 5; for the other three phosphorus ligands, even in the presence of >2 equiv, only RuII(κ4-bda)(PR3)2 species 1–3 were seen. Both experimental and computational methods were used to study the complexes. Steric effects are the main factor determining whether bis- or tris(PR3) complexes are formed. Cyclic voltammetry studies of the complexes revealed an increase in RuIII/II potential upon having another phosphorus ligand in the equatorial position. Computational studies predict that the additional phosphine ligand in the equatorial plane of 4 engages in significant orbital mixing with the ruthenium center that results in lower energy bonding as compared to the axial phosphine ligands. This work provides the first evaluation of phosphorus ligand steric and electronic effects on the RuII(bda) fragment.

Acyl Glycosides through Stereospecific Glycosyl Cross-Coupling: Rapid Access to C(sp3)-Linked Glycomimetics.

Replacement of a glycosidic bond with hydrolytically stable C–C surrogates is an efficient strategy to access glycomimetics with improved physicochemical and pharmacological properties. We describe here a stereoretentive cross-coupling reaction of glycosyl stannanes with C(sp2)- and C(sp3)-thio(seleno)esters suitable for the preparation C-acyl glycosides as synthetic building blocks to obtain C(sp3)-linked and fluorinated glycomimetics. First, we identified a set of standardized conditions employing a Pd(0) precatalyst, CuCl additive, and phosphite ligand that provided access to C-acyl glycosides without deterioration of anomeric integrity and decarbonylation of the acyl donors (>40 examples). Second, we demonstrated that C(sp3)-glycomimetics could be introduced into the anomeric position via a direct conversion of C1 ketones. Specifically, the conversion of the carbonyl group into a CF2 mimetic is an appealing method to access valuable fluorinated analogues. We also illustrate that the introduction of other carbonyl-based groups into the C1 position of mono- and oligosaccharides can be accomplished using the corresponding acyl donors. This protocol is amenable to late-stage glycodiversification and programmed mutation of the C–O bond into hydrolytically stable C–C bonds. Taken together, stereoretentive anomeric acylation represents a convenient method to prepare a diverse set of glycan mimetics with minimal synthetic manipulations and with absolute control of anomeric configuration.

Efficient modular phosphorus-containing ligands for stereoselective catalysis

Abstract Over several years, our research team has contributed to ligand design for metal-catalyzed stereoselective transformations. This has been achieved with the synthesis and application to organic transformations of interest of an array of structurally diverse P-containing ligands. These range from highly modular enantiopure phosphine–phosphite ligands to supramolecularly regulated enantioselective phosphorus-based catalysts. Our research in supramolecular interactions has also led to the discovery of an unprecedented halogen-bonded rhodium-catalyst.

Comparative Study of Novel Phosphordiamidite and Phosphite Ligands Used in Alkene Hydroformylation; Synthesis, Characterization, Metalation, and Catalytic Evaluation

Three novel ligands have been prepared by the reactions of 1,1′‐(chlorophosphinediyl)bis(1H‐pyrrole) with either 2‐pyridinemethanol or 2,6‐pyridinedimethanol or the reaction of 1,1′‐biphenyl‐2,2′‐diyl phosphorochloridite with 2‐pyridinemethanol. Measurement of |1JP–Se| values demonstrate that the phosphite donor is less basic than the phosphoramidite donors. Coordination preferences of the ligands in octahedral cis‐tetracarbonylmolybdenum(0) and square planar cis‐dichloropalladium(II) complexes have been evaluated using multinuclear NMR and X‐ray crystallography. Rhodium(I) complexes of the bidentate ligands have been evaluated as catalysts for styrene hydroformylation, and their activities are nearly double those of catalysts containing traditional phosphites. The regioselectivities of the RhI complexes of the bidentate ligands are not significantly different at 80 °C and 20 atm and are not affected by the addition of a lithium salt. In contrast, changing reaction conditions causes the % n‐aldehyde to vary from 15 to 50 %. The regioselectivity of the catalyst containing the bidentate phosphoramidite/pyridine ligand was more sensitive to pressure than temperature with both effects being significant.

Syntheses, structures and photoelectrochemical properties of phosphite-stabilized titanium-oxo clusters containing 2,2′-biphenolato ligands

The solvothermal reaction of Ti(OiPr)4, phosphorous acid and 2,2′-biphenol under different conditions offer three titanium-oxo clusters, namely, [Ti8(μ3-O)2(μ2-O)2(μ2-OiPr)4(OiPr)8(O3PH)4bph2], (bph = 2,2′-biphenolato) [Ti8(μ4-O)2(μ2-O)2(μ2-OEt)8(OEt)8(O3PH)2bph2], and [Ti4(μ3-O)2(μ2-OiPr)2(OiPr)4{O2P(OiPr)2}2bph2], respectively. The phosphite ligands induce the formation of different core structures compared to the well-studied phosphonate-stabilized titanium-oxo clusters. The 2,2′-biphenolato ligands introduce the ligand-to-core charge-transfer (LCCT) transition and result in narrow band gap of these clusters. Photocurrent response measurements show that the clusters with {Ti8P4} and {Ti8P2} core structures have higher photoelectrochemical activity than that of {Ti4P2}.

CO Release from N,C,S-Pincer Iron(III) Carbonyl Complexes Induced by Visible-to-NIR Light Irradiation: Mechanistic Insight into Effects of Axial Phosphorus Ligands.

Light-induced CO release from newly synthesized N,C,S-pincer iron(III) carbonyl complexes with two phosphorus ligands- trans-[Fe(L-κ3 N,C,S)(CO)(PR2R')2]PF6 ([1]PF6, R = Me, R' = Ph; [2]PF6, R = R' = Me; [3]PF6, R = R' = OEt)-were investigated. All the iron(III) carbonyl complexes were stable in solution and showed light-inducible CO release under ambient conditions. Studies on the wavelength dependence of photoreaction revealed that the phosphite complex [3]PF6 exhibited the most extended photosensitivity including all visible and a part of near-IR light (390-800 nm wavelengths). The phosphine complexes [1]PF6 and [2]PF6 showed sensitivity to only the higher-energy region of visible light (390-450 nm). Quantum-chemical calculations and spectroscopic data suggested that all complexes [1]PF6-[3]PF6 have dπ-dπ excitation modes to depopulate Fe-C(carbonyl) bonding and potentially induce the CO release by irradiation of light in the near-IR region, although moderately weakened Fe-C(carbonyl) bonding due to stronger π-backbonding by the phosphite ligand rendered the excitation effective on the CO release exclusively in [3]PF6.