Syntheses and oxidation of methyloxorhenium(V) complexes with tridentate ligands

Four new methyloxorhenium(V) compounds were synthesized with these tridentate chelating ligands: 2-mercaptoethyl sulfide (abbreviated HSSSH), 2-mercaptoethyl ether (HSOSH), thioldiglycolic acid (HOSOH), and 2-(salicylideneamino)benzoic acid (HONOH). Their reactions with MeReO(3) under suitable conditions led to these products: MeReO(SSS), 1, MeReO(SOS), 2, MeReO(OSO)(PAr(3)), 3, and MeReO(ONO)(PPh(3)), 4. These compounds were characterized spectroscopically and crystallographically. Compounds 1 and 2 have a five-coordinate distorted square pyramidal geometry about rhenium, whereas 3 and 4 are six-coordinate compounds with distorted octahedral structures. The kinetics of oxidation of 2 and 3 in chloroform with pyridine N-oxides follow different patterns. The oxidation of 2 shows first-order dependences on the concentrations of 2 and the ring-substituted pyridine N-oxide. The Hammett analysis of the rate constants gives a remarkably large and negative reaction constant, rho = -4.6. The rate of oxidation of 3 does not depend on the concentration or the identity of the pyridine N-oxide, but it is directly proportional to the concentration of water, both an accidental and then a deliberate cosolvent. The mechanistic differences have been interpreted as reflecting the different steric demands of five- and six-coordinate rhenium compounds.     

Ligand displacement and oxidation reactions of methyl(oxo)rhenium(V) complexes

Compounds that contain the anion [MeReO(edt)(SPh)](-) (3-) were synthesized with the countercations 2-picolinium (PicH+3-) and 2,6-lutidinium (LutH+3-), where edt is 1,2-ethanedithiolate. Both PicH+3- and MeReO(edt)(tetramethylthiourea) (4) were crystallographically characterized. The rhenium atom in each of these compounds exists in a five-coordinate distorted square pyramid. In the solid state, PicH+3- contains an anion with a short (d(SH) = 232 pm) and nearly linear hydrogen-bonded (N-H.S) interaction to the cation. Ligand substitution reactions were studied in chloroform. Displacement of PhSH by PPh(3) follows second-order kinetics, d[MeReO(edt)(PPh(3))]/dt = k[PicH+3-][PPh3], whereas with pyridines an unusual form was found, d[MeReO(edt)(Py)]/dt = k[PyH+3-][Py](2), in which the conversion of PicH+3- to PyH+3- has been incorporated. Further, added Py accelerates the formation of [MeReO(edt)(PPh3)], v = k.[PicH+3-].[PPh3].[Py]. Compound 4, on the other hand, reacts with both PPh(3) and pyridines, L, at a rate given by d[MeReO(edt)(L)]/dt = k.[4].[L]. When PicH+3- reacts with pyridine N-oxides, a three-stage reaction was observed, consistent with ligand replacement of SPh(-) by PyO, N-O bond cleavage of the PyO assisted by another PyO, and eventual decomposition of MeRe(O)(edt)(OPy) to MeReO(3). Each of first two steps showed a large substituent effect; Hammett analysis gave rho(1) = -5.3 and rho(2) = -4.3.     

Kinetics and mechanism of oxygen atom abstraction from a dioxo-rhenium(VII) complex

The kinetics of reaction between triarylphosphanes and two newly prepared dioxorhenium(VII) compounds has been evaluated. The compounds are MeRe(VII)(O)(2)("O,S") in which "O,S" represents an alkoxo, thiolato chelating ligand. With MeReO(3), ligands derived from 1-mercaptoethanol and 1-mercapto-2-propanol form MeRe(O)(2)(met), 2, and MeRe(O)(2)(m2p), 3. These compounds persist in chloroform solution for several hours at room temperature and for 2-3 weeks at -22 degrees C, particularly when water is carefully excluded. They were obtained as red oils with clean (1)H NMR spectra, but attempts to obtain pure, crystalline products were not successful because one decomposition pathway shows a kinetic order >1. The fastest reaction occurs between P(p-MeOC(6)H(4))(3) and 2; k(298) = 215(7) L mol(-1) s(-1) in chloroform at 25(1) degrees C. The other rate constants follow a Hammett correlation against 3sigma, with rho = -0.69(7). This study relates to oxygen atom transfer reactions catalyzed by MeReO(mtp)PPh(3), 1, in which MeRe(O)(2)(mtp), 4, is a postulated intermediate that does not build up to a measurable concentration during the catalytic cycle. Compound 2 does not react with MeSTol, but MeS(O)Tol was formed when tert-butyl hydroperoxide was added. This suggests that equilibrium lies to the left in this reaction, 2 + MeSTol + L = MeReO(met)L + MeS(O)Tol, and is drawn to the right by a reaction between MeReO(met)L and the hydroperoxide. Triphenyl arsane does not react with 2, but thermodynamic versus kinetic barriers were not resolved.     

Kinetics and mechanism of rhenium-catalyzed oxygen atom transfer from pyridine N-oxides to phosphines

The oxygen atom transfer (OAT) reaction cited does not occur on its own in >10 h. Oxorhenium(V) compounds having the formula MeReO(dithiolate)PZ(3) catalyze the reaction; the catalyst most studied was MeReO(mtp)PPh(3), 1, where mtpH(2) = 2-(mercaptomethyl)thiophenol. The mechanism was studied by multiple techniques. Kinetics (initial-rate and full-time-course methods) established this rate law: v = k(c)[1][PyO](2)[PPh(3)](-1). Here and elsewhere PyO symbolizes the general case XC(5)H(4)NO and PicO that with X = 4-Me. For 4-picoline, k(c) = (1.50 +/- 0.05) x 10(4) L mol(-1) s(-1) in benzene at 25.0 degrees C; the inverse phosphine dependence signals the need for the removal of phosphine from the coordination sphere of rhenium prior to the rate-controlling step (RCS). The actual entry of PPh(3) into the cycle occurs in a fast step later in the catalytic cycle, after the RCS; its relative rate constants (k(4)) were evaluated with pairwise combinations of phosphines. Substituent effects were studied in three ways: for (YC(6)H(4))(3)P, a Hammett correlation of k(c) against 3sigma gives the reaction constant rho(c)(P) = +1.03, consistent with phosphine predissociation; for PyO rho(c)(N) = -3.84. It is so highly negative because PyO enters in three steps, each of which is improved by a better Lewis base or nucleophile, and again for (YC(6)H(4))(3)P as regards the k(4) step, rho(4) = -0.70, reflecting its role as a nucleophile in attacking a postulated dioxorhenium(VII) intermediate. The RCS is represented by the breaking of the covalent N-O bond within another intermediate inferred from the kinetics, [MeReO(mtp)(OPy)(2)], to yield the dioxorhenium(VII) species [MeRe(O)(2)(mtp)(OPy)]. A close analogue, [MeRe(O)(2)(mtp)Pic], was identified by (1)H NMR spectroscopy at 240 K in toluene-d(8). The role of the "second" PyO in the rate law and reaction scheme is attributed to its providing nucleophilic assistance to the RCS. Addition of an exogenous nucleophile (tetrabutylammonium bromide, Py, or Pic) caused an accelerating effect. When Pic was used, the rate law took on the new form v = k(NA)[1][PicO][Pic][PPh(3)](-1); k(NA) = 2.6 x 10(2) L mol(-1) s(-1) at 25.0 degrees C in benzene. The ratio k(c)/k(NA) is 58, consistent with the Lewis basicities and nucleophilicities of PicO and Pic.     

Kinetics, mechanism, and computational studies of rhenium-catalyzed desulfurization reactions of thiiranes (thioepoxides)

The oxorhenium(V) dimer {MeReO(edt)}2 (1; where edt = 1,2-ethanedithiolate) catalyzes S atom transfer from thiiranes to triarylphosphines and triarylarsines. Despite the fact that phosphines are more nucleophilic than arsines, phosphines are less effective because they rapidly convert the dimer catalyst to the much less reactive catalyst [MeReO(edt)(PAr3)] (2). With AsAr3, which does not yield the monomer, the rate law is given by v = k[thiirane][1], independent of the arsine concentration. The values of k at 25.0 degrees C in CDCl3 are 5.58 +/- 0.08 L mol(-1) s(-1) for cyclohexene sulfide and ca. 2 L mol(-1) s(-1) for propylene sulfide. The activation parameters for cyclohexene sulfide are deltaH(double dagger) = 10.0 +/- 0.9 kcal mol(-1) and deltaS(double dagger) = -21 +/- 3 cal K(-1) mol(-1). Arsine enters the catalytic cycle after the rate-controlling release of alkene, undergoing a reaction with the Re(VII)(O)(S) intermediate that is so rapid in comparison that it cannot be studied directly. The use of a kinetic competition method provided relative rate constants and a Hammett reaction constant, rho = -1.0. Computations showed that there is little thermodynamic selectivity for arsine attack at O or S of the intermediate. There is, however, a large kinetic selectivity in favor of Ar3AsS formation: the calculated values of deltaH(double dagger) for attack of AsAr3 at Re=O vs Re=S in Re(VII)(O)(S) are 23.2 and 1.1 kcal mol(-1), respectively.     

Kinetics and mechanism of oxygen transfer to methyloxo(dithiolato)rhenium(V) complexes

The kinetics and mechanism of the reactions of the dimeric and monomeric methyloxo(dithiolato)rhenium(V) complexes [(o-SC6H4CH2S)Re(O)CH3]2 and [(o-SC6H4CH2S)PyRe(O)CH3] (Py = pyridine) with XO, sulfoxides, and pyridine N-oxides are studied. In these reactions, an oxygen atom from XO is transferred to rhenium, from which it later removed. A reaction scheme is proposed to interpret the kinetic data. This scheme features the formation of a monomeric (sulfoxide)- or (pyridine N-oxide)(dithiolato)methyloxorhenium(V) complex followed by its bimolecular oxidation in a rate-controlling step. Several sulfoxides (methyl, methyl phenyl, and substituted diphenyl) all react at similar rates. Activation parameters are determined for dimethyl sulfoxide and di-4-tolyl sulfoxide from temperature-dependent studies. The reactions with pyridine N-oxides show autocatalysis in which the catalyst is confirmed to be pyridine formed in the reactions.     

A non-radical chain mechanism for oxygen atom transfer with a thiorhenium(V) catalyst

The compound MeRe(S)(mtp)(PPh3), 2, where mtpH2 is 2-(mercaptomethyl)thiophenol, was used to catalyze the reaction between pyridine N-oxides, PyO, and triphenylphosphine. The rate law is -d[PyO]/dt=kc'[2].[PyO](1/2), with kc' at 25.0 degrees C in benzene=0.68 (4-picoline N-oxide) and 3.5x10(-3) dm(3/2) mol(-1/2) s(-1) (4-NO2-pyridine N-oxide). A chain mechanism with three steady-state thiorhenium species as chain carriers is implicated.     

Multielectron atom transfer reactions of perchlorate and other substrates catalyzed by rhenium oxazoline and thiazoline complexes: reaction kinetics, mechanisms, and density functional theory calculations

The title complexes, the Re(O)L(2)(Solv)(+) complexes (L = hoz, 2-(2'-hydroxyphenyl)-2-oxazoline(-) or thoz, 2-(2'-hydroxyphenyl)-2-thiazoline(-); Solv = H(2)O or CH(3)CN), are effective catalysts for the following fundamental oxo transfer reaction between closed shell molecules: XO + Y --> X + YO. Among suitable oxygen acceptors (Y's) are organic thioethers and phosphines, and among suitable oxo donors (XO's) are pyridine N-oxide (PyO), t-BuOOH, and inorganic oxyanions. One of the remarkable features of these catalysts is their high kinetic competency in effecting perchlorate reduction by pure atom transfer. Oxo transfer to rhenium(V) proceeds cleanly to afford the cationic dioxorhenium(VII) complex Re(O)(2)L(2)(+) in a two-step mechanism, rapid substrate (XO) coordination to give the precursor adduct cis-Re(V)(O)(OX)L(2)(+) followed by oxygen atom transfer (OAT) as the rate determining step. Electronic variations with PyO derivatives demonstrated that electron-withdrawing substituents accelerate the rate of Re(VII)(O)(2)L(2)(+) formation from the precursor adduct cis-Re(V)(O)(OX)L(2)(+). The activation parameters for OAT with picoline N-oxide and chlorate have been measured; the entropic barrier to oxo transfer is essentially zero. The potential energy surface for the reaction of Re(O)(hoz)(2)(OH(2))(+) with PyO was defined, and all pertinent intermediates and transition states along the reaction pathway were located by density functional theory (DFT) calculations (B3LYP/6-31G). In the second half of the catalytic cycle, Re(O)(2)L(2)(+) reacts with oxygen acceptors (Y's) in second-order reactions with associative transition states. The rate of OAT to substrates spans a remarkable range of 0.1-10(6) L mol(-)(1) s(-)(1), and the substrate reactivity order is Ph(3)P > dialkyl sulfides > alkyl aryl sulfides > Ph(2)S approximately DMSO, which demonstrates electrophilic oxo transfer. Competing deactivation and inhibitory pathways as well as their relevant kinetics are also reported.     

Synthesis and characterization of dimetallic oxorhenium(V) and dioxorhenium(VII) compounds, and a study of stoichiometric and catalytic reactions

Chelating dithiolate ligands--e.g., mtp from 2-(mercaptomethyl)thiophenol, edt from 1,2-ethanedithiol, and pdt from 1,3-propanedithiol--stabilize high-valent oxorhenium(V) against hydrolytic and oxidative decomposition. In addition to the dithiolate chelating to a single rhenium, one sulfur forms a coordinate bond to the other rhenium. In one arrangement this gives a dimer with a nearly planar diamond core with different internal Re-S distances. The new compounds are [MeReO(edt)](2) (2) and [MeReO(pdt)](2) (3), which can be compared to the previously known [MeReO(mtp)](2) (1). Another mode of synthesis leads to [ReO](2)(mtp)(3) (5) and [ReO](2)(edt)(3) (6). They, too, have similar Re(2)S(2) cores that involve donor atoms from two of the dithiolate ligands; the third dithiolate chelates one of the rhenium atoms. Gentle hydrolysis of 1 affords [Bu(n)4][[MeReO(mtp)](2)(mu-OH)] (7) in low yield. It appears to be the first example of this structural type for rhenium. The use of dithioerythritol as a starting material allowed the synthesis of a dioxorhenium(VII) compound, [MeReO(2)](2)(dte) (8). Its importance lies in understanding the role such compounds are believed to play as intermediates in oxygen atom catalysis. Ligation of the dimers 1-3 converts them into monomeric compounds, MeReO(dithiolate)L. These reactions go essentially to completion for L = PPh(3), but reach an equilibrium for L = NC(5)H(4)R. With R = 4-Ph, the values of K/10(3) L mol(-1) for the reactions (1-3) + 2L = 2MeReO(dithiolate)L are identical within 3 sigma: 1.15(3) (1), 1.24(4) (2), and 1.03(16) (3). The rates of monomer formation follow the rate law -d ln [dimer]/dt = k(a)[L] + k(b)[L](2). These trends were found: (1) phosphines are slow to react compared to pyridines, (2) the edt dimer 2 reacts much more rapidly than 1 and 3. Dimer 1 and MeReO(mtp)PPh(3) both catalyze oxygen atom transfer: PicO + PPh(3) --> Pic + Ph(3)PO. Compound 1 is ca. 90 times more reactive, which can be attributed to its lability toward small ligands as opposed to the low rate of displacement of PPh(3) from the mononuclear catalyst. The kinetics of this reaction follows the rate law -d[PicO]/dt = k[PicO][1]/[1 + kappa[PPh(3)]], with k = 5.8 x 10(6) L mol(-1) s(-1) and kappa = 3.5 x 10(2) L mol(-1) at 23 degrees C in benzene. A mechanism has been proposed to account for these findings.     

Kinetics and mechanisms of catalytic oxygen atom transfer with oxorhenium(V) oxazoline complexes

The rhenium(V) monooxo complexes (hoz)2Re(O)Cl (1) and [(hoz)2Re(O)(OH2)][OTf] (2) have been synthesized and fully characterized (hoz = 2-(2'-hydroxyphenyl)-2-oxazoline). A single-crystal X-ray structure of 2 has been solved: space group = P1, a = 13.61(2) A, b = 14.76(2) A, c = 11.871(14) A, alpha = 93.69(4) degrees, beta = 99.43(4) degrees, gamma = 108.44(4) degrees, Z = 4; the structure was refined to final residuals R = 0.0455 and Rw = 0.1055. 1 and 2 catalyze oxygen atom transfer from aryl sulfoxides to alkyl sulfides and oxygen-scrambling between sulfoxides to yield sulfone and sulfide. Superior catalytic activity has been observed for 2 due to the availability of a coordination site on the rhenium. The active form of the catalyst is a dioxo rhenium(VII) intermediate, [Re(O)2(hoz)2]+ (3). In the presence of sulfide, 3 is rapidly reduced to [Re(O)(hoz)2]+ with sulfoxide as the sole organic product. The transition state is very sensitive to electronic influences. A Hammett correlation plot with para-substituted thioanisole derivatives gave a reaction constant rho of -4.6 +/- 0.4, in agreement with an electrophilic oxygen transfer from rhenium. The catalytic reaction features inhibition by sulfides at high concentrations. The equilibrium constants for sulfide binding to complex 2 (cause of inhibition), K2 (L x mol(-1)), were determined for a few sulfides: Me2S (22 +/- 3), Et2S (14 +/- 2), and tBu2S (8 +/- 2). Thermodynamic data, obtained from equilibrium measurements in solution, show that the S=O bond in alkyl sulfoxides is stronger than in aryl sulfoxides. The Re=O bond strength in 3 was estimated to be about 20 kcal x mol(-1). The high activity and oxygen electrophilicity of complex 3 are discussed and related to analogous molybdenum systems.     

Issues of microscopic reversibility and an isomeric intermediate in ligand substitution reactions of five-coordinate oxorhenium(V) dithiolate complexes.

Ligand substitution reactions between five-coordinate oxorhenium(V) dithiolates, [CH(3)ReO(SCH(2)C(6)H(4)S)X], or MeReO(mtp)X, and entering ligands Y have been studied; Y is a phosphine and X is a phosphine (usually) or a pyridine. Many of them occur in two distinct stages, and other two-stage reactions merge to a single kinetic term when the successive rate constants are quite different in value. An intermediate can be detected directly by electronic and NMR spectroscopy. Just for phosphines, the range of rate constants is remarkably large; in the first stage, k spans the range 10(-)(4)-10(1) L mol(-)(1) s(-)(1) at 25 degrees C in benzene; in the second, which also shows a first-order dependence on the concentration of the entering ligand, the range is 10(-)(4)-10(3) L mol(-)(1) s(-)(1). Spectroscopic evidence shows that the intermediate has the same composition as the product; the metastable form is designated as MeReO(mtp)Y. The structures of all the isolated products MeReO(mtp)Y have a single stereochemistry: Me and -SCH(2) lie in trans positions, as do Y and -SAr. This structure is believed to be reversed in the transient, Y and -SCH(2) occupying trans positions. Further support for this assignment comes from the (31)P splitting of the (1)H NMR spectrum, where additional coupling indicates unusual four-bond coupling from a W-pattern of the hydrogen and phosphorus atoms. The intermediate does not undergo an intramolecular rearrangement to the final product; instead, it reacts with a ligand of the same type in an intermolecular reaction leading to rearrangement. The activation parameters were determined for selected reactions, and the results support a mechanism with considerable associative character; DeltaS() values are ca. -125 J K(-)(1) mol(-)(1). Because ligand Y must enter the coordination sphere from the vacant coordination position trans to the Re=O group, a means must be devised for the leaving group X to gain that position. To account for the intervention of the isomer while honoring the principle of microscopic reversibility, two mechanisms are proposed. One involves a C(3) ("turnstile") rotation of a specific group of three ligands in the six-coordinate transition state. Turnstile rotation of the groups X, Me, and Y can accomplish the needed transposition; the transition state passes through an approximate trigonal prismatic configuration, giving rise to a different and less stable isomer. The alternative mechanism, which may more easily accommodate data for Y = Me(2)bpy, involves rearrangement of the common octahedral intermediate to a pentagonal pyramid. The arrangement of ligands in the intermediate, governed by their sizes, determines that isomerization accompanies product formation. Following either rearrangement, a second reaction, between MeReO(SCH(2)C(6)H(4)S)Y and Y, then ensues by the same mechanism. The second rearrangement process then generates the more stable isomer of the product. Results are also presented from a study of monomerization of the dimeric rhenium species, [MeReO(mtp)](2), with phosphines(X) of various size and basicity. The results support a mechanism with two intermediates on the pathway to MeReO(mtp)X.     

Kinetic and mechanistic studies of sulfur transfer from imidomethylrhenium sulfides

The bis(2,6-diisopropylphenylimido)methylrhenium(VII) sulfide dimer, [CH(3)Re(NAr)(2)](2)(mu-S)(2) (1), reacts with a 1:1 amount of a phosphine or an alkyl isocyanide to yield a dimeric rhenium(VI) species, [CH(3)Re(NAr)(2)](2)(mu-S) (2), which has been structurally characterized. The two rhenium atoms in 2 are within bonding distance, 280 pm, more than 90 pm shorter than in 1. With excess L, 1 reacts to give a monomeric rhenium(V) complex, CH(3)Re(NAr)(2)L(2) (3A, L = PZ(3), Z = alkyl, aryl; 3B, L = isocyanide). The rate of formation of 3A is first-order with respect to [1] and second-order with respect to monodentate phosphine concentrations. With bidentate phosphines, however, the order with respect to the phosphine drops to unity. The addition of another (nonoxidizable) coordinating ligand, such as pyridine or one of its derivatives, accelerates the formation of 3A. In the presence of a pyridine ligand the reaction is first-order with respect to phosphine concentration, both monodentate and bidentate. The reactions between phosphines and 2 are slower than those with 1, which excludes [CH(3)Re(NAr)(2)](2)(mu-S) from being the intermediate in the reactions of 1. To account for that, we have proposed an intervening species that partitions between transformation to 3 with excess L and to 2 otherwise.     

Thermodynamics, kinetics, and mechanism of the reversible formation of oxorhenium(VII) catecholate complexes

Mononuclear Re(V) compounds MeReO(mtp)NC(5)H(4)X, 3, where mtpH(2) is 2-(mercaptomethyl)thiophenol have been prepared from the monomerization of [MeReO(mtp)](2) by pyridines with electron-donating substituents in the para or meta position; X = 4-Me, 4-Bu(t), 3-Me, 4-Ph, and H. Analogous compounds, MeReO(edt)N(5)H(4)X, 4, edtH(2) = 1,2-ethanedithiol, were prepared similarly. The equilibrium constants for the reaction, dimer + 2Py = 2M-Py, are in the range (2.5-31.6) x 10(2) L mol(-1). Both groups of monomeric compounds react with quinones (phenanthrenequinone, PQ, and 3,5-tert-butyl-1,2-benzoquinone, DBQ), displacing the pyridine ligand and forming Re(VII) catecholate complexes MeReO(dithiolate)PCat and MeReO(dithiolate)DBCat. With PQ, the reaction MeReO(dithiolate)Py + PQ = MeReO(dithiolate)PCat + Py is an equilibrium; values of K(Q) for different Py ligands lie in the ranges 9.2-42.7 (mtp) and 3.2-11.2 (edt) at 298 K. These second-order rate constants (L mol(-1) s(-1)) at 25 degrees C in benzene were obtained for the PQ reactions: k(f) = (5.3-15.5) x 10(-2) (mtp), (6.6-16.4) x 10(-2) (edt); k(r) = (3.63-5.71) x 10(-3) (mtp), (14.7-22.0) x 10(-3) (edt). The ranges in each case refer to the series of pyridine ligands, the forward rate constant being the largest for C(5)H(5)N, with the lowest Lewis basicity. The reactions of MeReO(dithiolate)Py with DBQ proceed to completion. Values of k(f)/L mol(-1) s(-1) fall in a narrow range, 4.02 (X = Bu(t)) to 8.4 (X = H) with the dithiolate being mtp.     

New oxorhenium(V) compound for catalyzed oxygen atom transfer from picoline N-oxide to triarylphosphines

The synthesis and characterization of a new oxorhenium(V) compound is reported; it is [MeReO(edt)(bpym)], 8, where edt = 1,2-ethanedithiolate and bpym = 2,2'-bipyrimidine. Compound 8 was characterized by NMR spectroscopy and single-crystal X-ray analysis. It exists as a six-coordinate Re(V) compound comparable to the previously known [MeReO(edt)(bpy)] and [MeReO(mtp)(bpy)]. Compound 8 catalyzes the oxygen-atom-transfer reaction PicO + PZ3 --> Pic + Z3PO, whereas the other two do not. The kinetics of this reaction with catalyst 8 follows the rate law -d[PicO]/dt = k[8][PicO]/(1 + c[PZ3]). With different phosphines, the rate law has the same k value, 4.17 L mol(-1) s(-1), but different c values. For tritolylphosphine, c = 67.5 L mol(-1) in benzene at 25 degrees C. A mechanism has been proposed to account for these findings. The data establish that an open coordination site on rhenium is necessary for oxygen-atom-transfer reactions.     

Catalytic deoxygenation of pyridine N-oxides with N-fused porphyrin rhenium complexes

Deoxygenation reactions of pyridine N-oxide derivatives catalyzed by N-fused porphyrin rhenium(VII) trioxo complexes are developed, affording the corresponding pyridine derivatives in quantitative yields with excellent turnover numbers up to 340,000.     

A new mode of reactivity for pyridine N-oxide: C-H activation with uranium(IV) and thorium(IV) bis(alkyl) complexes

Uranium(IV) and thorium(IV) bis(alkyl) complexes of the type (C5Me5)2AnR2 (An = U, Th; R = CH3, CH2Ph) activate the sp2 and sp3 hybridized C-H bonds in pyridine N-oxide and lutidine N-oxide to produce the corresponding cyclometalated complexes, (C5Me5)2An(R)[eta2-(O,C)-ONC5H4] and (C5Me5)2An(R)[eta2-(O,C)-ON-2-CH2-5-CH3-C5H3]. These provide rare examples of C-H activation chemistry mediated by actinide metal centers. This chemistry is in contrast to the known oxygen atom transfer reactivity patterns of pyridine N-oxides with oxophilic metal complexes and constitutes a new mode of reactivity for pyridine N-oxides.     

Arylation of pyridine N-oxides via a ligand-free Suzuki reaction in water

We report a practical and highly efficient protocol for the arylation of pyridine N-oxides with arylboronic acid through palladium-catalyzed Suzuki reaction in water.This ligand-free Suzuki reaction is performed in the presence of diisopropylamine and gives 2-or 3-arylated pyridyl N-oxide derivatives in good to excellent yields within 1 h.     

Oxo transfer reactions mediated by bis(dithiolene)tungsten analogues of the active sites of molybdoenzymes in the DMSO reductase family: comparative reactivity of tungsten and molybdenum.

The discovery of tungsten enzymes and molybdenum/tungsten isoenzymes, in which the mononuclear catalytic sites contain a metal chelated by one or two pterin-dithiolene cofactor ligands, has lent new significance to tungsten-dithiolene chemistry. Reaction of [W(CO)(2)(S(2)C(2)Me(2))(2)] with RO(-) affords a series of square pyramidal desoxo complexes [W(IV)(OR')(S(2)C(2)Me(2))(2)](1)(-), including R' = Ph (1) and Pr(i)() (3). Reaction of 1 and 3 with Me(3)NO gives the cis-octahedral complexes [W(VI)O(OR')(S(2)C(2)Me(2))(2)](1)(-), including R' = Ph (6) and Pr(i)() (8). These W(IV,VI) complexes are considered unconstrained versions of protein-bound sites of DMSOR and TMAOR (DMSOR = dimethylsulfoxide reductase, TMAOR = trimethylamine N-oxide reductase) members of the title enzyme family. The structure of 6 and the catalytic center of one DMSO reductase isoenzyme have similar overall stereochemistry and comparable bond lengths. The minimal oxo transfer reaction paradigm thought to apply to enzymes, W(IV) + XO --> W(VI)O + X, has been investigated. Direct oxo transfer was demonstrated by isotope transfer from Ph(2)Se(18)O. Complex 1 reacts cleanly and completely with various substrates XO to afford 6 and product X in second-order reactions with associative transition states. The substrate reactivity order with 1 is Me(3)NO > Ph(3)AsO > pyO (pyridine N-oxide) > R(2)SO > Ph(3)PO. For reaction of 3 with Me(3)NO, k(2) = 0.93 M(-)(1) s(-)(1), and for 1 with Me(2)SO, k(2) = 3.9 x 10(-)(5) M(-)(1) s(-)(1); other rate constants and activation parameters are reported. These results demonstrate that bis(dithiolene)W(IV) complexes are competent to reduce both N-oxides and S-oxides; DMSORs reduce both substrate types, but TMAORs are reported to reduce only N-oxides. Comparison of k(cat)/K(M) data for isoenzymes and k(2) values for isostructural analogue complexes reveals that catalytic and stoichiometric oxo transfer, respectively, from substrate to metal is faster with tungsten and from metal to substrate is faster with molybdenum. These results constitute a kinetic metal effect in direct oxo transfer reactions for analogue complexes and for isoenzymes provided the catalytic sites are isostructural. The nature of the transition state in oxo transfer reactions of analogues is tentatively considered. This research presents the first kinetics study of substrate reduction via oxo transfer mediated by bis(dithiolene)tungsten complexes.     

Palladium-catalyzed C-H functionalization of pyridine N-oxides: highly selective alkenylation and direct arylation with unactivated arenes

Two catalytic protocols of the oxidative C-C bond formation have been developed on the basis of the C-H bond activation of pyridine N-oxides. Pd-catalyzed alkenylation of the N-oxides proceeds with excellent regio-, stereo-, and chemoselectivity, and the corresponding ortho-alkenylated N-oxide derivatives are obtained in good to excellent yields. Direct cross-coupling reaction of pyridine N-oxides with unactivated arene was also developed in the presence of Pd catalyst and Ag oxidant, which affords ortho-arylated pyridine N-oxide products with high site-selectivity.     

Alkene cyclopropanation catalyzed by Halterman iron porphyrin: participation of organic bases as axial ligands

With the iron(III) complex of the Halterman iron porphyrin [P*Fe(Cl)] and ethyl diazoacetate (EDA) as catalyst and carbene source, respectively, styrene-type substrates were converted to cyclopropyl esters with high trans/cis ratio (not less than 12) and high enantioselectivity for the trans-isomers (74-86% ee). The isomeric distribution of the cyclopropyl esters so obtained is akin to that obtained from the previously reported Ru(II) counterpart [P*Ru(CO)]. A linear Hammett correlation log(k(X)/k(H)) = sigma(+)rho was observed with rho = -0.57 suggesting the involvement of an electrophilic cyclopropanating species derived from the iron(II) center as the reactive intermediate in the catalytic cycle. This is further supported by a dramatic decrease in the enantioselectivity and trans/cis ratio observed in an experiment of styrene cyclopropanation when the reaction mixture was deliberately exposed to air. Axial ligand effects on the selectivities was also investigated. Substantial improvement in trans/cis ratios could be achieved by addition of organic bases such as pyridine (py) and 1-methylimidazole (MeIm) to the catalytic reaction. The existence of axially ligated iron carbene moieties, [P*Fe(CHCO(2)Et)(py)] and [P*Fe(CHCO(2)Et)(MeIm)], was established by electrospray mass spectrometry. Study of secondary kinetic isotope effect indicated that a more product-like transition state was generated by addition of MeIm.