A mechanistic study of oxygen atom transfer from N-sulfonyloxaziridine to enolates

Enolate additions to chiral N-sulfonyloxaziridines providing enantiomerically enriched α-hydroxy carbonyl compounds is a reaction of importance, yet a clear understanding of the factors governing stereoinduction in these transformations remains ambiguous. This is despite, previous computational studies, one by Bach et al. employing truncated model systems exploring oxygen atom transfer to an unsubstituted lithium enolate and another by our own group. In clarifying this reactivity we report here a computational study examining oxygen atom transfer from 1-S-(+)-(10-camphorsulfonyl)oxaziridine, viz., archetypal Davis chiral oxaziridine to substituted Li, Na, K enolates offering improved mechanistic understanding. From this investigation, a revised model is offered revealing the metal cation, chelation effects and sterics as decisive stereocontrolling factors in enolate additions to chiral N-sulfonyloxaziridines affording enantiomerically enriched α-hydroxy carbonyl compounds.     

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.     

Oxygen atom transfer from a trans-dioxoruthenium(VI) complex to nitric oxide

In aqueous acidic solutions trans-[Ru(VI)(L)(O)(2)](2+) (L=1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) is rapidly reduced by excess NO to give trans-[Ru(L)(NO)(OH)](2+). When ≤1 mol equiv NO is used, the intermediate Ru(IV) species, trans-[Ru(IV)(L)(O)(OH(2))](2+), can be detected. The reaction of [Ru(VI)(L)(O)(2)](2+) with NO is first order with respect to [Ru(VI)] and [NO], k(2)=(4.13±0.21)×10(1) M(-1) s(-1) at 298.0 K. ΔH(≠) and ΔS(≠) are (12.0±0.3) kcal mol(-1) and -(11±1) cal mol(-1) K(-1), respectively. In CH(3)CN, ΔH(≠) and ΔS(≠) have the same values as in H(2)O; this suggests that the mechanism is the same in both solvents. In CH(3)CN, the reaction of [Ru(VI)(L)(O)(2)](2+) with NO produces a blue-green species with λ(max) at approximately 650 nm, which is characteristic of N(2)O(3). N(2)O(3) is formed by coupling of NO(2) with excess NO; it is relatively stable in CH(3)CN, but undergoes rapid hydrolysis in H(2)O. A mechanism that involves oxygen atom transfer from [Ru(VI)(L)(O)(2)](2+) to NO to produce NO(2) is proposed. The kinetics of the reaction of [Ru(IV)(L)(O)(OH(2))](2+) with NO has also been investigated. In this case, the data are consistent with initial one-electron O(-) transfer from Ru(IV) to NO to produce the nitrito species [Ru(III)(L)(ONO)(OH(2))](2+) (k(2)>10(6) M(-1) s(-1)), followed by a reaction with another molecule of NO to give [Ru(L)(NO)(OH)](2+) and NO(2)(-) (k(2)=54.7 M(-1) s(-1)).     

Oxygen atom transfer reactivity from a dioxo-Mo(VI) complex to tertiary phosphines: synthesis, characterization, and structure of phosphoryl intermediate complexes

The oxygen atom transfer (OAT) reactivity of TpMoO2Cl with PMe3, PEt3, and PPhMe2 (where Tp = hydrotris(3,5-dimethylpyrazol-1-yl)borate) has been investigated. The OAT reactions proceed through a diamagnetic Mo(IV) phosphoryl intermediate complex of general formula TpMoOCl(OPR3) (OPR3 = OPMe3, OPEt3, OPPhMe2), which have been isolated and characterized by 1H and 31P NMR, UV-visible, and infrared spectroscopies and electrospray ionization mass spectrometry. Solid-state crystal structures of TpMoOCl(OPMe3) and TpMoOCl(OPPhMe2) are also reported, the oxygen-to-phosphorus distances agree with a double-bond formulation and a single bond between the metal and the phosphoryl oxygen atom. The stability of the phosphoryl intermediate complexes depends on the steric properties of the coordinated phosphine-oxides. These intermediate complexes have been converted to solvent-coordinated species, TpMoOCl(solv) (solv = acetonitrile or dmf), and the coordinated solvents exchange with the bulk solvent.     

Ab initio study of oxygen atom transfer from hydrogen peroxide to trimethylamine

Unlike what has been theoretically proposed for ammonia oxidation with hydrogen peroxide, trimethylamine oxidation occurs with a concerted mechanism, which is favored even when an explicit water molecule is added or continuum solvent (water) is simulated.     

Thermodynamic and kinetic characteristics of one-electron transfer reactions in N-alkylaniline-tetracyanoethylene charge transfer complex systems

One-electron transfer reactions in systems containing an N,N-dialkylaniline (D) and tetracyanoethylene (A) in methylene chloride proceed through the formation of AD and AD₂ charge transfer complexes. The tetracyanoethylene -system and nitrogen unshared electron pair of the N,N-dialkylaniline participate in the formation of the charge transfer complex. The equilibrium constants for complex formation K and rates of formation of the TCE^– radical-anions k were determined for this reaction. The sign of the regression coefficient in the correlation between the thermodynamic (K) and kinetic indices (k) for the one-electron transfer reactions in charge transfer complex systems indicates the formation of shortlived intermediates between the starting compounds and final Reaction products (in the case of increasing K(AD) values with increasing k(TCE^–)) or of several longlived intermediate complexes (in the case of increasing K(AD) values with decreasing k(TCE^–).Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 23, No. 6, pp. 692–699, November–December, 1987.     

Proton-promoted oxygen atom transfer vs proton-coupled electron transfer of a non-heme iron(IV)-oxo complex

Sulfoxidation of thioanisoles by a non-heme iron(IV)-oxo complex, [(N4Py)Fe(IV)(O)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), was remarkably enhanced by perchloric acid (70% HClO(4)). The observed second-order rate constant (k(obs)) of sulfoxidation of thioaniosoles by [(N4Py)Fe(IV)(O)](2+) increases linearly with increasing concentration of HClO(4) (70%) in acetonitrile (MeCN)at 298 K. In contrast to sulfoxidation of thioanisoles by [(N4Py)Fe(IV)(O)](2+), the observed second-order rate constant (k(et)) of electron transfer from one-electron reductants such as [Fe(II)(Me(2)bpy)(3)](2+) (Me(2)bpy = 4,4-dimehtyl-2,2'-bipyridine) to [(N4Py)Fe(IV)(O)](2+) increases with increasing concentration of HClO(4), exhibiting second-order dependence on HClO(4) concentration. This indicates that the proton-coupled electron transfer (PCET) involves two protons associated with electron transfer from [Fe(II)(Me(2)bpy)(3)](2+) to [(N4Py)Fe(IV)(O)](2+) to yield [Fe(III)(Me(2)bpy)(3)](3+) and [(N4Py)Fe(III)(OH(2))](3+). The one-electron reduction potential (E(red)) of [(N4Py)Fe(IV)(O)](2+) in the presence of 10 mM HClO(4) (70%) in MeCN is determined to be 1.43 V vs SCE. A plot of E(red) vs log[HClO(4)] also indicates involvement of two protons in the PCET reduction of [(N4Py)Fe(IV)(O)](2+). The PCET driving force dependence of log k(et) is fitted in light of the Marcus theory of outer-sphere electron transfer to afford the reorganization of PCET (λ = 2.74 eV). The comparison of the k(obs) values of acid-promoted sulfoxidation of thioanisoles by [(N4Py)Fe(IV)(O)](2+) with the k(et) values of PCET from one-electron reductants to [(N4Py)Fe(IV)(O)](2+) at the same PCET driving force reveals that the acid-promoted sulfoxidation proceeds by one-step oxygen atom transfer from [(N4Py)Fe(IV)(O)](2+) to thioanisoles rather than outer-sphere PCET.     

Thermodynamic and kinetic aspects of metal binding to the histidine-rich protein, Hpn

The histidine-rich protein, Hpn, binds to essential metals Ni2+, Cu2+, Zn2+ and a therapeutic metal Bi3+ with the in vitro affinities in the order of Cu2+ > Ni2+ > Bi3+ > Zn2+. In contrast, the in vivo (in E. coli) protection by the protein is in the order of Ni2+ > Bi3+ > Cu2+ approximately Zn2+. The release of Ni2+ from the protein follows a two-step process consisting of a rapidly established equilibrium and subsequently a rate-determining step (dissociation of Hpn-Ni...EDTA to Ni-EDTA). Our work suggests the nickel storage and homeostasis in H. pylori as the primary role of Hpn.     

Protic conversion of nitrile into azavinylidene complexes of rhenium, a mechanistic theoretical study

The mechanism of the protonation of the rhenium nitrile chloro-complexes [ReCl(NCCH3)(PH3)4] (2), taken as models of the real systems [ReCl(NCR)(dppe)(2)] (dppe = Ph2PCH2CH2PPh2), leading to the azavinylidene products [ReCl(NC(H)CH3)(PH3)4]+ (3) was investigated by theoretical methods at the B3LYP level of theory. Electrostatic and molecular orbital arguments and thermodynamic, kinetic, and steric factors are analyzed and indicate that the chlorine atom is the most probable site of the initial proton attack, although the direct protonation of the nitrile carbon atom is also possible as a concurrent process. For the cis-isomer of 2, the initially formed chloro-protonated species cis-[Re(ClH)(NCCH3)(PH3)4]+ further converts to the azavinylidene cis-3 via either an acid-independent 1,4-proton shift or an acid-base catalyzed pathway involving a second protonation of the nitrile carbon atom to give cis-[Re(ClH)(NC(H)CH3)(PH3)4]2+ followed by elimination of the proton from the chlorine atom.     

Thermodynamics of hydrogen atom transfer to a high-valent iron imido complex

Thermodynamic investigations relevant to hydrogen atom transfer by the high-valent iron imido complex [LMesFe[triple bond]NAd]OTf have been undertaken. The complex is found to be weakly oxidizing by cyclic voltammetry (E1/2 = -0.98 V vs Cp2Fe+/Cp2Fe in MeCN). A combination of experimental and computational studies has been used to determine the acidity of LMesFe-N(H)Ad+ (pKa = 37 in MeCN), allowing the N-H BDFE (88(5) kcal/mol) to be calculated from a thermodynamic cycle. Consistent with this value, [LMesFe[triple bond]NAd]OTf reacts with 9,10-dihydroanthracene (C-H BDE = 78(1) kcal/mol) to form anthracene.     

Pyridylazole chelation of oxorhenium(V) and imidorhenium(V). Rates and trends of oxygen atom transfer from Re(V)O to tertiary phosphines

The concerned azoles are 2-(2-pyridyl)benzoxazole (pbo) and 2-(2-pyridyl)benzthiazole (pbt). These react with ReOCl(3)(PPh(3))(2) in benzene, affording Re(V)OCl(3)(pbo) and Re(V)OCl(3)(pbt), which undergo facile oxygen atom transfer to PPh(2)R (R = Ph, Me) in dichloromethane solution, furnishing Re(III)(OPPh(2)R)Cl(3)(pbo) and Re(III)(OPPh(2)R)Cl(3)(pbt). The oxo species react with aniline in toluene solution, yielding the imido complexes Re(V)(NPh)Cl(3)(pbo) and Re(V)(NPh)Cl(3)(pbt). The X-ray structures of pbt, ReOCl(3)(pbt), Re(OPPh(3))Cl(3)(pbt), and Re(NPh)Cl(3)(pbo) are reported. The lattice of pbt consists of stacked dimers. In all the complexes the azole ligand is N,N-chelated and the ReCl(3) moiety is meridionally disposed. In ReOCl(3)(pbt) the metal-oxo bond length is 1.607(9) A. The second-order rates and the associated activation parameters of the oxygen atom transfer reactions of the Re(V)O chelates with PPh(2)R are reported. The large and negative entropy of activation (approximately -24 eu) is consistent with an associative pathway involving nucleophilic phosphine attack. The rate increases with phosphine basicity (PPh(2)Me > PPh(3)) and azole heteroatom electronegativity (O(pbo) > S(pbt)). Logarithmic rate constants for ReOCl(3)(pbo), ReOCl(3)(pbt), and ReOCl(3)(pal) are found to correlate linearly with Re(VI)O/Re(V)O reduction potentials (pal is pyridine-2-(N-p-tolyl)aldimine). The relatively low rate constant of ReOCl(3)(pbt) compared to that of ReOCl(3)(pal) is consistent with the observed shortness of the metal-oxo bond in the former. Crystal data are as follows: (pbt) empirical formula C(12)H(8)N(2)S, crystal system orthorhombic, space group Pca2(1), a = 13.762(9) A, b = 12.952(8) A, c = 11.077(4) A, V = 1974(2) A(3), Z = 8; (ReOCl(3)(pbt)) empirical formula C(12)H(8)Cl(3)N(2)OSRe, crystal system monoclinic, space group P2(1)/c, a = 11.174(7) A, b = 16.403(10) A, c = 7.751(2) A, beta = 99.35(4) degrees, V = 1401.8(13) A(3), Z = 4; (Re(NPh)Cl(3)(pbo)) empirical formula C(18)H(13)Cl(3)N(3)ORe, crystal system monoclinic, space group P2(1)/c, a = 9.566(6) A, b = 16.082(8) A, c = 11.841(5) A, beta = 94.03(4) degrees, V = 1817(2) A(3), Z = 4.     

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.     

PtCl(PHCy2)[(PCy2O)2H]: a hydrogen bonded phosphinito complex obtained by direct reaction of molecular oxygen with a terminal phosphido complex

Exposure of solid trans-[Pt(PHCy2)2(PCy2)Cl] (1) to dry oxygen unexpectedly leads to [PtCl(PHCy2)[(PCy2O)2H]] (2) as the major product, the formation of which has been followed by NOESY 1H NMR techniques.     

Tryptophan-based peptides to synthesize gold and silver nanoparticles: a mechanistic and kinetic study

Synthetic oligopeptides with a tryptophan residue at the C-terminus have been used for the synthesis of gold and silver nanoparticles at pH 11. The tryptophan residue in the peptides is responsible for the reduction of metal ions to the respective metals, possibly through electron transfer. A mechanistic pathway has been proposed to explain the reductive properties of the tryptophan moiety of the peptide based on some spectroscopic techniques, such as UV-visible and fluorescence spectroscopy. This study reveals that some of the peptide molecules are converted to its corresponding ditryptophan, kynurenine form and some cross-linked products, all of which are highly fluorescent species. The resultant peptide-functionalized metal nanoparticles have also been characterized by UV-visible spectroscopy, transmission electron microscopy, and Fourier transform IR spectroscopy and thermogravimatric analysis.     

A mechanistic and kinetic study of the formation of metal nanoparticles by using synthetic tyrosine-based oligopeptides

Synthetic oligopeptides containing redox-active tyrosine residues have been employed to prepare gold and silver nanoparticles. In this reduction process an electron from the tyrosinate ion of the peptide is transferred to the metal ion at basic pH through the formation of a tyrosyl radical, which is eventually converted to its dityrosine form during the reaction. This reaction mechanism was confirmed from UV-visible, fluorescence, and EPR spectroscopy and was found to be pH-dependent. Transmission electron microscopy measurement shows that the average size and the monodispersity of gold nanoparticles increase as the number of tyrosine residues in the peptide increases. The kinetic study, based on spectrophotometric measurements of the surface plasmon resonance optical property, shows that the rate of formation of gold nanoparticles was much faster at higher pH than at lower pH and was also dependent on the number of tyrosine residues present in the peptide. The dityrosine form of the peptide was found to retain reducing properties like those of tyrosine in basic medium.     

Thermodynamic, kinetic, and computational study of heavier chalcogen (S, Se, and Te) terminal multiple bonds to molybdenum, carbon, and phosphorus

Enthalpies of chalcogen atom transfer to Mo(N[t-Bu]Ar)3, where Ar = 3,5-C6H3Me2, and to IPr (defined as bis-(2,6-isopropylphenyl)imidazol-2-ylidene) have been measured by solution calorimetry leading to bond energy estimates (kcal/mol) for EMo(N[t-Bu]Ar)3 (E = S, 115; Se, 87; Te, 64) and EIPr (E = S, 102; Se, 77; Te, 53). The enthalpy of S-atom transfer to PMo(N[ t-Bu]Ar) 3 generating SPMo(N[t-Bu]Ar)3 has been measured, yielding a value of only 78 kcal/mol. The kinetics of combination of Mo(N[t-Bu]Ar)3 with SMo(N[t-Bu]Ar)3 yielding (mu-S)[Mo(N[t-Bu]Ar)3]2 have been studied, and yield activation parameters Delta H (double dagger) = 4.7 +/- 1 kcal/mol and Delta S (double dagger) = -33 +/- 5 eu. Equilibrium studies for the same reaction yielded thermochemical parameters Delta H degrees = -18.6 +/- 3.2 kcal/mol and Delta S degrees = -56.2 +/- 10.5 eu. The large negative entropy of formation of (mu-S)[Mo(N[t-Bu]Ar)3]2 is interpreted in terms of the crowded molecular structure of this complex as revealed by X-ray crystallography. The crystal structure of Te-atom transfer agent TePCy3 is also reported. Quantum chemical calculations were used to make bond energy predictions as well as to probe terminal chalcogen bonding in terms of an energy partitioning analysis.     

Oxidation of nitrite by a trans-dioxoruthenium(VI) complex: direct evidence for reversible oxygen atom transfer

Reaction of trans-[Ru(VI)(L)(O)(2)](2+) (1, L = 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane, a tetradentate macrocyclic ligand with N(2)O(2) donor atoms) with nitrite in aqueous solution or in H(2)O/CH(3)CN produces the corresponding (nitrato)oxoruthenium(IV) species, trans-[Ru(IV)(L)(O)(ONO(2))](+) (2), which then undergoes relatively slow aquation to give trans-[Ru(IV)(L)(O)(OH(2))](2+). These processes have been monitored by both ESI/MS and UV/vis spectrophotometry. The structure of trans-[Ru(IV)(L)(O)(ONO(2))](+) (2) has been determined by X-ray crystallography. The ruthenium center adopts a distorted octahedral geometry with the oxo and the nitrato ligands trans to each other. The Ru=O distance is 1.735(3) A, the Ru-ONO(2) distance is 2.163(4) A, and the Ru-O-NO(2) angle is 138.46(35) degrees . Reaction of trans-[Ru(VI)(L)((18)O)(2)](2+) (1-(18)O(2)) with N(16)O(2)(-) in H(2)O/CH(3)CN produces the (18)O-enriched (nitrato)oxoruthenium(IV) species 2-(18)O(2). Analysis of the ESI/MS spectrum of 2-(18)O(2) suggests that scrambling of the (18)O atoms has occurred. A mechanism that involves linkage isomerization of the nitrato ligand and reversible oxygen atom transfer is proposed.     

A bifurcated pathway of oxygen atom transfer reactions from a monooxo molybdenum(VI) complex under electrospray ionisation mass spectrometric conditions

A stable molybdenum(V) complex, LMoOCl2(where L is hydrotris(3,5-dimethylpyrazolyl)borate), has been oxidized under mass spectrometric conditions. The oxidized species reacts with tertiary phosphines and the products have been detected by mass spectrometry. The product distribution has been followed by isotope labeling experiments, and energy dependent electrospray mass spectrometry. These experiments reveal not only oxygen atom transfer but also loss of a chlorine atom from the resulting species.