Monodentate Phosphine Ligands Research Articles

Palladium‐Catalyzed Direct Stereoselective Synthesis of Deoxyglycosides from Glycals

AbstractPalladium(II) in combination with a monodentate phosphine ligand enables the unprecedented direct and α‐stereoselective catalytic synthesis of deoxyglycosides from glycals. Initial mechanistic studies suggest that in the presence of N‐phenyl‐2‐(di‐tert‐butylphosphino)pyrrole as the ligand, the reaction proceeds via an alkoxy palladium intermediate that increases the proton acidity and oxygen nucleophilicity of the alcohol. The method is demonstrated with a wide range of glycal donors and acceptors, including substrates bearing alkene functionalities.

Hidden Enantioselective Hydrogenation of N-Silyl Enamines and Silyl Enol Ethers in Net C═N and C═O Hydrosilylations Catalyzed by Ru–S Complexes with One Monodentate Chiral Phosphine Ligand

Ruthenium thiolate complexes with one chiral monodentate phosphine ligand are applied to enantioselective hydrosilylation of enolizable imines and ketones. The structural features of the catalyst exclude the presence of more than one phosphine ligand at the ruthenium center in the enantioselectivity-determining step. The enantiomeric excesses obtained in these reduction reactions are moderate (up to 66% ee), but the stereochemical outcome enables an experimental analysis of the reaction pathways operative in this catalysis. A two-step sequence consisting of successive N–Si/O–Si dehydrogenative coupling and enamine/enol ether hydrogenation is the prevailing mechanism of action. Both steps involve cooperative bond activation at the Ru–S bond of the coordinatively unsaturated ruthenium complex: Si–H bond activation in the dehydrogenative coupling and heterolytic H–H splitting in the hydrogenation. Previously documented side reactions such as deprotonation/protonation equilibria as well as competing direct C═...

Tantallacyclopentadiene as a unique metal-containing diene ligand coordinated to nickel for preparing tantalum-nickel heterobimetallic complexes.

A mononuclear tantallacyclopentadiene complex, TaCl3(C4H2tBu2) (3), serves as a unique ligand to nickel: the addition of Ni(COD)2 to 3 selectively afforded heterobimetallic Ta-Ni complex 4. The cyclooctadiene ligand bound to the nickel center in complex 4 was readily substituted by monodentate and bidentate phosphine ligands, such as dimethylphenylphosphine, 1,2-bis(diphenylphosphino)ethane, and 1,2-bis(diethylphosphino)ethane, to give the corresponding phosphine complexes 5, 6a, and 6b. We also examined a ligand substitution reaction with 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) to produce the corresponding Ta-Ni complex 7. These newly prepared Ta-Ni heterobimetallic complexes were characterized spectroscopically together with the crystal structures of 4, 6a, and 7.

Mechanistic Investigation of Iridium-Catalyzed C–H Borylation of Methyl Benzoate: Ligand Effects in Regioselectivity and Activity

The Ir-catalyzed C–H borylation of methyl benzoate has been studied with DFT methodology in order to understand the experimentally observed ligand-induced regioselectivity and activity when different [(ligand)Ir(Bpin)3] catalysts are employed. While bidentate ligands such as 4,4′-di-tBu-2,2′-bipyridine (dtbpy) completely inhibit ortho-borylation, the use of selected triphenylphosphine derivatives enables the reaction on that position, avoiding the meta- and para-regioisomers. The analysis of the catalytic cycles for the borylation reactions with dtbpy, PPh3, P(p-CF3C6H4)3, and P(m,m-(CF3)2C6H3)3 allows the interpretation of the observed ligand effects. The different reactivity observed for the different monodentate phosphine ligands can also be rationalized in terms of catalyst stability.

ChemInform Abstract: Minimizing Aryloxy Elimination in RhI‐Catalyzed Asymmetric Hydrogenation of β‐Aryloxyacrylic Acids Using a Mixed‐Ligand Strategy.

AbstractAn efficient asymmetric hydrogenation of β‐aryloxyacrylic acids is realized using a Rh(I)‐complex based on the heterocombination of a readily available chiral monodentate secondary phosphine oxide (SPO) and an achiral monodentate phosphine ligand as the catalyst.

Highly active rhodium/phosphorus catalytic system for the hydroformylation of α-methylstyrene

Rhodium catalyzed hydroformylation of α-methylstyrene was investigated in the presence of monodentate phosphine ligands L1–L6. We found that the phosphine with good π-acceptability could efficiently improve the activity of the α-methylstyrene hydroformylation. The big steric hindrance of α-C in α-methylstyrene enhanced the regioselectivity towards the linear aldehyde, which resulted in 3-phenylbutanal as the predominant product (>99.0%). When tris(N-pyrrolyl)phosphine (L1) modified Rh(acac)(CO)2 was employed as the catalyst, the TOF could reach up to 5786 h−1 in the α-methylstyrene hydroformylation at relatively mild conditions (110 °C, 6 MPa).

Cycloplatinated(II) complex bearing 2-vinylpyridine and monodentate phosphine ligands: Optical properties and kinetic study

The 2-vinylpyridinate complex [PtMe(Vpy)(DMSO)], C, is successfully prepared from dimeric platinum(II) complex cis,cis-[Me2Pt(μ-SMe2)2PtMe2], B, in the presence of DMSO and 2-vinylpyridine ligand. The DMSO molecule in complex C can be readily substituted by methyldiphenylphosphine (PPh2Me), allowing synthesis of [PtMe(Vpy)(PPh2Me)], 1, which has been fully characterized spectroscopically. Complex 1 is emissive in solid state and solution at room temperature and at 77 K. This complex displays an intense phosphorescence emission (with a significantly long lifetime) which is originated from mixed 3ILCT/3MLCT excited state. Additionally, complex 1 undergoes oxidative addition with alkyl halide (MeI) bearing [PtMe2I(Vpy)(PPh2Me)], 2, after 2 h. Kinetic studies suggest that the latter reaction (oxidative addition) proceed by a classical SN2 mechanism. The rate of oxidative addition reaction at different temperatures is measured and high negative ΔS# value is obtained for the reaction which supported the proposed mechanism. On the basis of NMR spectroscopy, when complex 1 reacts with MeI under extended time (5 days), complex 2 gradually converts to a mixture of platinum complexes trans-[PtMeI(PPh2Me)2], 3, [{PtMe2(Vpy)2(μ-I)2], 4, and a free ligand Z/E-[C8H9N], 5, based on the carbon–carbon bond coupling.

Minimizing Aryloxy Elimination in Rh(I) -Catalyzed Asymmetric Hydrogenation of β-Aryloxyacrylic Acids using a Mixed-Ligand Strategy.

The first example of efficient asymmetric hydrogenation of challenging β-aryloxyacrylic acids was realized using a Rh(I) -complex based on the heterocombination of a readily available chiral monodentate secondary phosphine oxide (SPO) and an achiral monodentate phosphine ligand as the catalyst. Excellent enantioselectivities (92->99 % ee) were achieved for a wide variety of chiral β-aryloxypropionic acids with minor aryloxy elimination in most cases. The resultant products were readily transformed into biologically active compounds through simple synthetic manipulations.

Steve Buchwald @60

Steve Buchwald @60

Quantum Chemical Investigation on Phosphine‐Functionalized Charge‐Neutral Osmium(II) Complexes Bearing Bidentate/Tetradentate Pyridylpyrazolate‐Based Ligands

AbstractBy altering the monodentate phosphine ligand (PR3), a series of charge‐neutral OsII complexes 1a–5a [Os(fppz)2(PR3)2] and 1b–5b [Os(L‐NPh)(PR3)2] [fppz = 3‐(trifluoromethyl)‐5‐(2‐pyridyl)pyrazole; PR3 = PMe3 (1a/1b), PPhMe2 (2a/2b), PPh2Me (3a/3b), PPh3 (4a/4b), P(OPh)3 (5a/5b); L‐NPh is two bis(pyridylpyrazolate] fragments connected by a phenylamino bridge at the 6‐position of the pyridyl rings] have been investigated by density functional theory (DFT) methods. The P(OPh)3 substitution in 5a (5b) rendered a considerable stabilization to the HOMO energy levels and an enlargement of the HOMO–LUMO energy gap compared with those of 1a–4a (1b–4b), which led to a clear blueshift in the emission spectra. With the enhancement of the electron‐withdrawing strength of the phosphine ligands, the emissions are successively blueshifted from 1a/1b to 4a/4b. The results also revealed a general relationship between the variations of the emissions and changes to the Os–P bond lengths. The phenylamido bridge in 1b–5b redshifted the absorption and emission spectra compared with those of 1a–5a and decreased the Os(d) participation in the HOMOs, which led to dominant L‐ligand‐based 3π–π* character for their emissive states compared with dominant metal‐to‐ligand charge‐transfer (3MLCT) characters for 1a–5a.

A kinetic approach to carbon–iodide bond activation by rollover cycloplatinated(II) complexes containing monodentate phosphine ligands

New rollover cyclometalated platinum(II) complex [PtMe(bpy-H) (PPh2Me)] (bpy-H = κ2N,C-2,2′-bipyridine) was synthesized by the reaction of [PtMe(bpy-H)(DMSO)] with 1 equiv. of methyldiphenylphosphine. The reaction of the rollover cyclometalated Pt(II) complex with MeI gave the rollover cyclometalated Pt(IV) complex [PtMe2(bpy-H) (PPh2Me)I]. The rollover cycloplatinated(II) complexes [PtMe(bpy-H) (L)] (L = PPh3 and PPh2Me) have a metal-to-ligand charge-transfer (MLCT) band, which were used to easily follow the kinetics of their reactions with MeI. On the basis of the kinetic data, the classical SN2 mechanism was suggested. The rates of the reactions at different temperatures were measured and consistent with the proposed mechanism, large negative ΔS‡ values were found. The PPh3 complex [PtMe(bpy-H) (PPh3)] reacted almost 4 or 5 times slower with MeI as compared to that of the PPh2Me complex [PtMe(bpy-H) (PPh2Me)] in CHCl3. This was attributed to the stronger donor ability and the less steric hindrance of the PPh2Me group as compared with those of the PPh3 group. The rate of MeI oxidative addition reactions of rollover cyclometalated platinum(II) complexes, [PtMe(bpy-H) (L)], were compared to classical cyclometalated complexes [PtMe(ppy) (L)] (ppy = 2-phenylpyridinate) and found to be lower than those reported for ppy analog. This was attributed to the existence of extra electronegative nitrogen atom in bpy-H ligand compared to ppy, which makes the bpy-H ligand slightly weaker donors, thus decreasing the reaction rates for [PtMe(bpy-H) (L)] (compared to [PtMe(ppy) (L)]).

Cationic gold clusters ligated with differently substituted phosphines: effect of substitution on ligand reactivity and binding.

We present a systematic study of the effect of the number of methyl (Me) and cyclohexyl (Cy) functional groups in monodentate phosphine ligands on the solution-phase synthesis of ligated sub-nanometer gold clusters and their gas-phase fragmentation pathways. Small mixed ligand cationic gold clusters were synthesized using reactions between pre-formed triphenylphosphine ligated (PPh3) gold clusters and monodentate Me- and Cy-substituted phosphine ligands in solution and characterized using electrospray ionization mass spectrometry (ESI-MS) and collision-induced dissociation (CID) experiments. Under the same experimental conditions, larger gold-PPh3 clusters undergo efficient exchange of unsubstituted PPh3 ligands for singly Me- and Cy-substituted PPh2Me and PPh2Cy ligands. The efficiency of reaction decreases with an increasing number of Me or Cy groups in the substituted phosphine ligands. CID experiments performed for a series of mixed-ligand gold clusters indicate that loss of a neutral Me-substituted ligand is preferred over loss of a neutral PPh3 ligand while the opposite trend is observed for Cy-substituted ligands. The branching ratio of the competing ligand loss channels is strongly correlated with the electron donating ability of the phosphorous lone pair as determined by the relative proton affinity of the ligand. The results indicate that the relative ligand binding energies increase in the order PMe3 < PPhMe2 < PPh2Me < PPh3 < PPh2Cy < PPhCy2 < PCy3. Furthermore, the difference in relative ligand binding energies increases with the number of substituted PPh(3-m)Me(m) or PPh(3-m)Cy(m) ligands (L) on each cluster. This study provides the first experimental determination of the relative binding energies of ligated gold clusters containing differently substituted monophosphine ligands, which are important to controlling their synthesis and reactivity in solution. The results also indicate that ligand substitution is an important parameter that must be considered in theoretical modeling of these complex systems.

Hydrogen‐Borrowing and Interrupted‐Hydrogen‐Borrowing Reactions of Ketones and Methanol Catalyzed by Iridium

Reported herein is the use of catalytic [{Ir(cod)Cl}2] to facilitate hydrogen-borrowing reactions of ketone enolates with methanol at 65 °C. An oxygen atmosphere accelerates the process, and when combined with the use of a bulky monodentate phosphine ligand, interrupts the catalytic cycle by preventing enone reduction. Subsequent addition of pro-nucleophiles to the reaction mixture allowed a one-pot methylenation/conjugate addition protocol to be developed, which greatly expands the range of products that can be made by this methodology.

Complexes of Copper(I) Thiocyanate with Monodentate Phosphine and Pyridine Ligands and the P(,N)‐Donor Diphenyl(2‐pyridyl)phosphine

AbstractCopper(I) thiocyanate derivatives were prepared by the reaction of CuNCS with pyridine (py) and tertiary monophosphine ligands [PR3 in general; in detail: PPh3, triphenylphosphine, P(4‐FPh)3, tris(4‐fluorophenyl)phosphine)], as well as the potentially bidentate ligand diphenyl(2‐pyridyl)phosphine (PPh2py). Mechanochemical methods were used in some cases to investigate stoichieometries that were not easily accessible by conventional solution syntheses. Three forms of the resulting adducts of CuNCS/PR3/py‐base (1:3–n:n) stoichiometry―all containing four‐coordinate copper(I) atoms and monodentate N‐thiocyanate groups―were confirmed crystallographically. Mononuclear arrays are defined for [(PPh2py)3–n(py)nCuNCS], n = 0, 1, 2, the monodentate thiocyanate being N‐coordinated in all; two polymorphs are observed for the n = 2 complex, both crystallizing in monoclinic P21 (Z = 2) cells with similar cell dimensions, but with aromatic components eclipsed about the Cu–P bond in the PPh3 complex, and staggered in the PPh2py complex. Bridging thiocyanate groups are found in the 1:1:1 CuNCS/PPh2py/2‐methylpyridine (mpy) and P(4‐FPh)3/mpy complexes, wherein centrosymmetric dimers with eight‐membered central rings are obtained: [(R3P)(mpy)Cu(NCS)2Cu(PR3)(mpy)], as is also the case in the parent 1:2 CuNCS/PPh2py adduct [(pyPh2P)2Cu(NCS)2Cu(PPh2py)2]. For the 1:1:1 CuNCS/P(4‐FPh)3/py and PPh3/Brmpy (Brmpy = 3‐bromo‐4‐methylpyridine) adducts, and, likely, CuNCS/PPh2py/py (1:1:1), single‐stranded polymers of the form [···Cu(NCS)(PR3)(py‐base)(Cu)···](∞|∞) with linearly bridging NCS ligands were obtained. Some derivatives, representative of all forms, display medium to strong green to blue luminescence when excited with radiation at 365 nm. The 31P CPMAS NMR spectroscopic data clearly differentiate the inequivalent phosphorus positions within each system, showing a wide range of 1J(31P,63/65Cu) values ranging from 965 Hz for [Cu(NCS)(PPh2py)3] to 1540 Hz for dimeric [(4‐FPh)3P(mpy)Cu(NCS)2Cu(P(4‐FPh)3)(mpy)], reflecting the large variations in the Cu–P bond length.

ChemInform Abstract: Chiral Monodentate Phosphine Ligands for the Enantioselective α‐ and γ‐Arylation of Aldehydes.

AbstractChiral trialkyl monodentate phosphine ligands (I) elaborated on the binepine scaffold are synthesized and applied to the Pd‐catalyzed intramolecular asymmetric α‐arylation of aldehydes as well as the intermolecular asymmetric γ‐arylation of α,β‐unsaturated aldehydes.

Synthesis, Characterization, and X‐ray Structures of Ru3(CO)9(dotpm)(L) Complexes [L = PPh3, P(C6H4Cl‐p)3, and PPh2(C6H4Br‐p)

AbstractThe reaction of Ru3(CO)10(dotpm) (1) [dotpm = (bis(di‐ortho‐tolylphosphanyl)methane)] and one equivalent of L [L = PPh3, P(C6H4Cl‐p)3 and PPh2(C6H4Br‐p)] in refluxing n‐hexane afforded a series of derivatives [Ru3(CO)9(dotpm)L] (2–4), respectively, in ca. 67–70 % yield. Complexes 2–4 were characterized by elemental analysis (CHN), IR, 1H NMR, 13C{1H} NMR and 31P{1H} NMR spectroscopy. The molecular structures of 2, 3, and 4 were established by single‐crystal X‐ray diffraction. The bidentate dotpm and monodentate phosphine ligands occupy equatorial positions with respect to the Ru triangle. The effect of substitution resulted in significant differences in the Ru–Ru and Ru–P bond lengths.

Chiral monodentate phosphine ligands for the enantioselective α- and γ-arylation of aldehydes

The synthesis of chiral variants of monodentate trialkyl and dialkylbiaryl phosphine ligands elaborated on the binepine scaffold is described. Their application in the Pd-catalyzed intramolecular asymmetric α-arylation of aldehydes and the intermolecular asymmetric γ-arylation of α,β-unsaturated aldehydes provides a mean of validating the design of these ligands. For the first reaction, excellent reactivities have been obtained while only modest enantioselectivities were measured. Aside from enantioselectivity, the second reaction offers additional challenges associated with intramolecularity and regioselectivity. With the formal chiral trialkyl monodentate phosphine ligands, good yield, high olefin stereocontrol, and perfect γ-selectivity were obtained while the enantioselectivity remained in the low but promising range.

Mechanisms and Origins of Switchable Chemoselectivity of Ni-Catalyzed C(aryl)–O and C(acyl)–O Activation of Aryl Esters with Phosphine Ligands

Many experiments have shown that nickel with monodentate phosphine ligands favors the C(aryl)-O activation over the C(acyl)-O activation for aryl esters. However, Itami and co-workers recently discovered that nickel with bidentate phosphine ligands can selectively activate the C(acyl)-O bond of aryl esters of aromatic carboxylic acids. The chemoselectivity with bidentate phosphine ligands can be switched back to C(aryl)-O activation when aryl pivalates are employed. To understand the mechanisms and origins of this switchable chemoselectivity, density functional theory (DFT) calculations have been conducted. For aryl esters, nickel with bidentate phosphine ligands cleaves C(acyl)-O and C(aryl)-O bonds via three-centered transition states. The C(acyl)-O activation is more favorable due to the lower bond dissociation energy (BDE) of C(acyl)-O bond, which translates into a lower transition-state distortion energy. However, when monodentate phosphine ligands are used, a vacant coordination site on nickel creates an extra Ni-O bond in the five-centered C(aryl)-O cleavage transition state. The additional interaction energy between the catalyst and substrate makes C(aryl)-O activation favorable. In the case of aryl pivalates, nickel with bidentate phosphine ligands still favors the C(acyl)-O activation over the C(aryl)-O activation at the cleavage step. However, the subsequent decarbonylation generates a very unstable tBu-Ni(II) intermediate, and this unfavorable step greatly increases the overall barrier for generating the C(acyl)-O activation products. Instead, the subsequent C-H activation of azoles and C-C coupling in the C(aryl)-O activation pathway are much easier, leading to the observed C(aryl)-O activation products.

Rhenium and technetium bi- and tricarbonyl complexes in a new strategy for biomolecule incorporation using click chemistry.

A versatile strategy to prepare fac-[M(I)(CO)3](+) and cis-[M(I)(CO)2](+) (M = Re, (99m)Tc) complexes was developed using Huisgen click chemistry and monodentate phosphine ligands to readily incorporate biomolecules and tailor the chemical properties.

Silver‐Catalyzed Hydrogenation of Aldehydes in Water

Silver bullet: The first silver-catalyzed hydrogenation in water was developed. A silver complex containing a bulky monodentate phosphine ligand was used to generate alcohols from a broad range of aldehydes, including aliphatic, aromatic, and heterocyclic aldehydes. This method provides a direct and efficient route to alcohols from aldehydes and opens a new avenue in silver catalysis.