Hemilability of phosphine-thioether ligands coordinated to trinuclear Mo3S4 cluster and its effect on hydrogenation catalysis

Ligand-exchange reactions of [Mo3S4(tu)8(H2O)]Cl44H2O (tu = thiourea) with (PhCH2CH2)2PCH2CH2SR ligands, where R = Ph (PS1), pentyl (PS2) or Pr (PS3) afford new complexes isolated as [Mo3S4Cl3(PS1)3]PF6 ([1]PF6), [Mo3S4Cl3(PS2)3]PF6 ([2]PF6) and [Mo3S4Cl3(PS3)3]PF6 ([3]PF6) salts in 30-50% yields as the major reaction products. The crystal structures of [1]PF6 and [2]PF6 were determined by X-ray diffraction (XRD) analysis. Each of the three phosphine-thioether ligands is coordinated in a bidentate chelating mode to a different molybdenum atom of the Mo3S4 trinuclear cluster, herewith all the phosphorus atoms of the phosphino-thioether ligand are located trans to the capping sulfur (3-S). A second product that forms in the reaction of [Mo3S4(tu)8(H2O)]Cl44H2O with PS1 corresponds to the neutral [Mo3S4Cl4(PS1)2(PS1*)] complex. Its XRD analysis reveals both bidentate (PS1) and monodentate (PS1*) coordinating modes of the same ligand. In the latter mode the phosphinethioether is coordinated to a Mo atom only via the P atom. All compounds were characterized by 1H, 31P{1H} NMR, electrospray-ionization (ESI) mass spectrometry and cyclic voltammetry (CV). Reactions of [1]PF6, [2]PF6 and [3]PF6 with an excess of Bu4NCl in CD2Cl2 were followed by 31P{1H} NMR. The spectra indicate equilibrium between cationic [Mo3S4Cl3(PSn)3] + and neutral [Mo3S4Cl4(PSn)2(PSn*)] (n = 1, 2) species. The equilibrium constants were determined as 2.5 ± 0.2103 , 43 ± 2 М -1 and 30 ± 2 М -1 (at 25°C) for [1]PF6, [2]PF6 and [3]PF6, indicating quantitative differences in hemilabile behavior of the phosphino-thioether ligands, depending on the substituent at sulfur. Clusters [1]PF6, [2]PF6 and [3]PF6 were tested as catalysts in reduction of nitrobenzene to aniline with Ph2SiH2 under mild conditions. Significant differencies in the catalytic activity were observed, which can be attributed to different hemilabile behavior of the PS1 and PS2/PS3 ligands

Aerobic addition of secondary phosphine oxides to vinyl sulfides: a shortcut to 1-hydroxy-2-(organosulfanyl)ethyl(diorganyl)phosphine oxides

Secondary phosphine oxides react with vinyl sulfides (both alkyl- and aryl-substituted sulfides) under aerobic and solvent-free conditions (80 °C, air, 7–30 h) to afford 1-hydroxy-2-(organosulfanyl)ethyl(diorganyl)phosphine oxides in 70–93% yields

Aerobic addition of secondary phosphine oxides to vinyl sulfides: a shortcut to 1-hydroxy-2-(organosulfanyl)ethyl- (diorganyl)phosphine oxides

Abstract Secondary phosphine oxides react with vinyl sulfides (both alkyl-and aryl-substituted sulfides) under aerobic and solvent-free conditions (80 °C, air, 7-30 h) to afford 1-hydroxy-2-(organosulfanyl)ethyl(diorganyl)phosphine oxides in 70-93% yields. Findings Tertiary phosphines and phosphine chalcogenides are important organophosphorus compounds that are widely used in industry, organic synthesis, polymer science, medicinal and coordination chemistry Recently, on example of secondary phosphines [27] as well as secondary phosphine sulfides In this letter, we report our serendipitous finding that secondary phosphine oxides 1a-f under aerobic conditions (air, 80 °C, 7-18 h) easily add to vinyl sulfides 2a-c to give unknown 1-hydroxy-2-(organosulfanyl)ethyl(diorganyl)phosphine oxides 3a-h in high yields Importantly, under these conditions, the expected [30] antiMarkovnikov adducts are not observed in detectable amounts ( 31 P NMR). The main byproducts are phosphinic acids, R 2 P(O)OH, formed by air oxidation of secondary phosphine oxides 1a-f. As seen from The presence of an asymmetric carbon atom in the reaction products leads to non-equivalence of both heminal protons in the SCH 2 C* fragment and carbon signals in the arylethyl moiety. In the 1 H NMR spectra of 3a-h, protons of the PCHCH 2 S moiety form an ABMX spin system appearing as three multiplets. Phosphine oxide 3d crystallizes in the centrosymmetric P2 1 /c space group. Within its extended structure, strong intermolecular H-bonding interactions between the O-H hydrogen and P=O oxygen atom of a second molecule {O(1)-H(1)···O(2), 1.80(6) Å; O-H···O angle, 174.9(7)°} leads to the formation of 1D polymeric chains along the b-axis ( In FTIR spectra of 3a-h, absorption bands of the P=O and O-H bonds appear in the regions of 1100-1150 and 3350-3450 cm −1 , respectively. Interestingly, the reaction disclosed is specific for secondary phosphine oxides. Our experiments have shown that their analogues, secondary phosphine sulfides, under similar conditions provide exclusively the anti-Markovnikov adducts (Scheme 2). On the other hand, vinyl ethers and vinyl selenides Beilstein J. Org. Chem. 2015, 11, 1985-1990. 1988 Figure 1: ORTEP drawing (30% thermal ellipsoid) of phosphine oxide 3d. A CIF file with the crystallographic data is available as Supporting Information File 2 and is also available on request from the Cambridge Crystallographic Data Centre as deposition 1046604. (congeners of vinyl sulfides) were found to react with phosphine oxide 1a at 80 °C for about 30 and 20 h, respectively, to deliver difficult-to-separate mixtures of organophosphorus compounds ( 31 P NMR). Scheme 2: Addition of secondary phosphine sulfide to vinyl sulfide under aerobic catalyst-free conditions. To gain a primary insight into the reaction mechanism, several experiments were carried out. On example of phosphine oxide 1a and vinyl sulfide 2c, we have shown that the reaction proceeds in the dark with the same efficiency as in the light. Therefore, the photochemical pathway of the reaction is hardly probable. Also, the reaction was established under an argon atmosphere. Under these conditions (argon, 80 °C for 18 h, exemplified by 1a/2c pair) the formation of products 3a-h does not take place and the starting phosphine oxide remained almost intact ( 31 P NMR). This indicates that the reaction requires the presence of oxygen. In the other experiment, when TEMPO, a widely used radical scavenger, was added (10 mol %) into the reaction system 1a/2c, the product 3d was also formed, however, a longer reaction time was required for complete conversion of secondary phosphine oxide 1a as compared to TEMPO-free conditions (15 vs 11 h). Meanwhile, this observation does not completely exclude a radical mechanism since the cross-coupling reactions between TEMPO and radical intermediates can be reversible Taking these data into account, the following mechanism is suggested (Scheme 3). The first step is assumed to be the generation of phosphinoyl (A) and hydroperoxyl (HOO • ) radicals by the reaction of O 2 with phosphine oxide 1. Earlier, the transfer of a hydrogen atom from the P(O)H species to molecular oxygen has been reported for example for Ph 2 P(O)H Although quantum chemical computations [MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p)] of the model radicals B and C (with R, R' = Me) reveals that the latter is energetically less preferred than the former, their energy difference is too small (4.38 kcal/mol) to completely prohibit the B→C transformation. Beilstein J. Org. Chem. 2015, 11, 1985-1990. 1989 Scheme 3: Putative mechanism. Conclusion In summary, we have disclosed an aerobic addition of secondary phosphine oxides to vinyl sulfides under solvent-and catalyst-free conditions, which provides an efficient approach to hitherto unknown 1-hydroxy-2-(organosulfanyl)ethyl(diorganyl)phosphine oxides in one step. The synthesized phosphine oxides, bearing hydroxy and sulfide functions, represent prospective building blocks for organic synthesis and interesting ligands for metal complexes. The results obtained contribute to the basic chemistry of both phosphine oxides and vinyl sulfides