Hydride Abstraction Research Articles

Formation of a cationic alkylidene complex via formal hydride abstraction: synthesis and structural characterization of [W(PMe3)4(η2-CHPMe2)H]X (X = Br, I)

W(PMe(3))(4)(η(2)-CH(2)PMe(2))H reacts with aryl halides to give the alkylidene complex, [W(PMe(3))(4)(η(2)-CHPMe(2))H](+), which reacts with LiAlD(4) to give selectively W(PMe(3))(4)(η(2)-CHDPMe(2))H, in which the deuterium resides in the methylene group; subsequent migration of deuterium from the methylene group provides a means to measure the rate constant for the formation of the 16-electron species [W(PMe(3))(5)] from W(PMe(3))(4)(η(2)-CH(2)PMe(2))H.

Mechanism of Hydride Abstraction by Cyclopentadienone‐Ligated Carbonylmetal Complexes (M = Ru, Fe)

AbstractCyclopentadienone‐ligated ruthenium complexes, such as Shvo's catalyst, are known to oxidize reversibly alcohols to the corresponding carbonyl compounds. The mechanism of this reaction has been the subject of some controversy, but it is generally believed to proceed through concerted transfer of proton and hydride, respectively, to the cyclopentadienone ligand and the ruthenium center. In this paper we further study the hydride transfer process as an example of a coordinatively directed hydride abstraction by adding quantitative understanding to some features of this mechanism that are not well understood. We find that an oxidant as weak as acetone can be used to re‐oxidize the intermediate ruthenium hydride without catalyst re‐oxidation becoming rate‐limiting. Furthermore, C–H cleavage is a significantly electrophilic event, as demonstrated by a Hammett reaction parameter of ρ = –0.89. We then describe how the application of our mechanistic insights obtained from the study have enabled us to extend the ligand‐directed hydride abstraction strategy to include a rare example of an iron(0) oxidation catalyst.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Selective Oxidation of Alcohols by Orthorhombic Mo–V–O Phase with Molecular Oxygen

Abstract We have investigated heterogeneous oxidation of alcohols using crystalline molybdenum vanadium oxide (Mo–V–O) as catalyst with molecular oxygen as oxidant. The major product from primary alcohol was aldehyde, the secondary alcohol was mainly dehydrated to olefin, and the oxidation of cyclic alcohols chiefly afforded ketones. A Hammett plot suggested that hydride abstraction might be involved in the reaction step. We have discussed possible reaction mechanism based on substrate adsorption and activation on catalytically active sites.

Hydride Abstraction from the Octahydrotriborate Ion and Formation of Bridged Triborane(7)-Phosphane Complexes

Reaction of two equivalents of (Bu4N)[B 3 H8] with a variety of metal halides (TiBr4, TiCl4, WCl6, or GeCl4) results in nearly instantaneous hydride abstraction and the formation of THF.B3H7. Subsequent reactions with bidentate phosphanes lead to the first triborane(7)-phosphane complexes with a bridging phosphane ligand: (B3H7)-P(Ph2)-(CH2) m -P(Ph2)-(B3H7) ( 1 , m = 1; 2 , m = 2; 3 , m = 4; 4 , m = 6). (Compounds 1–4 also can be made in “one-pot” syntheses.) Compounds 1–4 and the previously reported Ph3P·B3H7 are characterized by 11B and 31P NMR spectroscopy.

Hydride abstraction of methylamine with Cu +( 1S) in the gas phase: A density functional theory study

The gas-phase hydride abstraction of methylamine with Cu +( 1S) is theoretically investigated by using density functional theory. Geometries for all the stationary points involved are fully optimized at both the B3LYP/6-311++G(d,p) and B3LYP/6-311++G(3df,2p) levels and the reaction is analyzed in terms of the topology of potential energy surface. Approach of Cu + towards methylamine could form either “classical” N or “nonclassical” η 1-methyl-H attached complex with the former being the global minimum. Both complexes are found to be key intermediates for the hydride abstraction, which could transfer into each other via two parallel routes, i.e., concerted metal movement and stepwise C–H activation-rearrangement. A charge-transfer process is detected for the “nonclassical” complex converting to a precursor species (CuH–NH 2CH 2 +), which accounts for the final products by a nonreactive dissociation.

A Tantalum(V) Carbene Complex: Formation of a Carbene−Bis(phenoxide) Ligand by Sequential Proton and Hydride Abstraction

Proton and subsequent hydride abstraction from the bis(phenoxide) ligand of the trimethyltantalum(V) complex affords a cationic tantalum(V) carbene complex, in which two phenoxide groups are linked to the carbene center. The electrophilic nature of the carbene functionality is demonstrated by the reaction with PPh3.

C–H activation in reactions of protonated hydrogen peroxide with propane: Remarkable regioselectivity for hydride abstraction and mechanism for acid catalyzed hydrogen exchange

Reactions between deuterium labelled alkanes and H 2OOH + have been investigated by conducting gas phase reactions within the cell of an FT-ICR mass spectrometer. Propane is exceptional in giving off a hydride (deuteride) thereby producing the propyl cation and two water molecules, as well as demonstrating high activity for proton induced H/D exchange. The detailed mechanistic scenario has been modelled using accurate quantum chemical methods (MP2 and G3). The calculations show that hydrogen exchange takes place via a synchronous flip–flop mechanism. Results of reactions with partially deuteriated propanes demonstrate quite exceptional regioselectivity for hydride abstraction in favour of the secondary positions (2-H) compared to the primary (1-H). This observation is understood on the basis of transition state theory.

Ion/neutral complexes generated during unimolecular fragmentation: Intra-complex hydride abstraction by tert-butyl cations from electron-rich and electron-poor 1,3-diphenylpropanes

The hydride transfer and proton transfer occurring between the constituents of ion/neutral complexes of tert-butyl cations and electron-rich and electron-poor 1,3-diphenylpropanes, [(CH 3) 3C + C 6H 5CH 2CH 2CH 2C 6H 4X] were investigated by use of CI/MIKE spectrometry of nine 1-( tert-butylphenyl)-3-arylpropanes and 16 site-specific deuterium-labeled isotopomers. The competition between H − abstraction by the (CH 3) 3C + ion from the neutral arene and H + transfer to it was found to be strongly affected by the electron-donating substituents, in particular by X = OCH 3, on the one hand, and X = F and CF 3, on the other, suggesting that the 1,3-diphenylpropane molecule within the I/N complex acts and reacts as a bidentate solvating partner to the carbocation. The effect of the substituents X on the regioselectivity of the intra-complex hydride abstraction from the two benzylic CH 2 groups, k γ-H/ k α-H, and their influence on the kinetic isotope effect ( k H/ k D) γ, operating during the abstraction from the substituted benzylic moiety, were determined in a semi-quantitative approach by assuming ( k H/ k D) α = 1.60, the generally observed value for the unsubstituted benzylic moiety. The regioselectivity range was found to span almost three orders of magnitude, from k γ-H/ k α-H ≥ 11.2 for the complex [(CH 3) 3C + C 6H 5C αH 2CH 2C γH 2C 6H 4( p-OCH 3)] to k γ-H/ k α-H ≤ 0.04 for the complex [(CH 3) 3C + C 6H 5C αH 2CH 2C γH 2C 6H 4( p-CF 3)].

Synthesis and Dynamic Stereochemistry of Azulene-Substituted 9-Fluorenyl, 9,10-Dihydro-10,10-dimethyl-9-anthryl, 10,11-Dihydro-5H-dibenzo[a,d]cyclohepten-5-yl, and 5H-Dibenzo[a,d]cyclohepten-5-yl Cations. Correlations of Stabilities of the Carbocations and Rotational Barrier of Azulene Ring

Abstract The carbocations in the title: 9-(1-Azulenyl)-9-fluorenyl, 9-(1-azulenyl)-9,10-dihydro-10,10-dimethyl-9-anthryl, 5-(1-azulenyl)-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-yl, and 5-(1-azulenyl)-5H-dibenzo[a,d]cyclohepten-5-yl cations (4c, 5a–c, 6a–c, and 7a–c) were prepared by hydride abstraction of the corresponding hydrocarbon derivatives and their properties were fully characterized. The pKR+ values of 4c, 5a–c, 6a–c, and 7a–c were determined as −2.7, 0.5–2.7, 8.4–9.1, and 7.4–8.5, respectively, which were higher by 8.1, 6.0–8.2, 15.1–16.1, and 13.1–14.3 pK units, respectively, than those of the corresponding phenyl derivatives. Conformations and energy barriers for 4c, 5a–c, 6a–c, and 7a–c were studied by means of dynamic nuclear magnetic resonance spectroscopy (DNMR) techniques. The thermodynamic stability of the cations and the barrier to the rotation of the 1-azulenyl group were significantly affected by not only the electron density effect but also the dynamic stereochemistry of the connected-ring systems.

Determination of a Large Reorganization Energy Barrier for Hydride Abstraction by Glucose Oxidase

Glucose oxidase mediates the aerobic oxidation of simple sugars to lactones using a noncovalently bound flavin cofactor. The chemical mechanism of this reaction has been uncertain for many years. Here it is shown, using enzymes reconstituted with chemically modified cofactors, that sugar oxidation most likely occurs by concerted hydride (H-) abstraction. Studies of the kinetics and thermodynamics together with the application of Marcus theory reveal a large reorganization energy barrier. The magnitude of this intrinsic contribution appears characteristic of H- transfer in proteins and in solution. The observation that neither the thermodynamics nor reorganization energy is significantly altered in the glucose oxidase active site raises questions concerning how the redox reaction may be catalyzed.

Activation of alkanes on zeolites: A DFT study of hydride abstraction and dehydrogenation on extra‐framework aluminum species

AbstractAn AlOH2+ · T6 cluster was used to represent, at zeroth order, an extra‐framework aluminum species coordinated with the zeolite Y structure. This cluster was used to study the hydride abstraction from alkanes, at B3LYP/6‐311++G(d,p)//B3LYP/6‐31G(d,p) level, to form an alkoxide and an Al‐hydride · T6 species. It was found that all reactions were endothermic with respect to the isolated reactants and presented high activation energy. The endothermicity and activation energy depend on the type of hydride being abstracted. Methane showed the highest values (ΔH = +41.5 kcal/mol and ΔH‡ = 91.7 kcal/mol); isobutane has the lowest (ΔH = +36.8 kcal/mol and ΔH‡ = 46.7 kcal/mol). Dehydrogenation to form an alkene and an adsorbed hydrogen molecule on the AlOH2+ · T6 cluster was also investigated and computed slightly more endothermic than the respective hydride abstraction. However, dehydrogenation showed a significantly lower activation enthalpy, especially for linear alkanes. For isobutane the results indicated a similar energy barrier for hydride abstraction and dehydrogenation. Larger, more realistic clusters and periodic reactivity studies are nevertheless needed to describe, in a more refined way, the energetics of these relevant catalytic reactions in zeolitic systems. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Cationic Polymerization of 2‐Vinylthiophene by Chloroarylmethanes as Surface Initiators on Silica and Consecutive Hydride Abstraction by Acceptors

AbstractSummary: 2‐Vinylthiophene (2‐VT) has been cationically polymerized using chloroarylmethane derivatives as the surface polymerization initiator on silica. By applying this procedure a soluble fraction of poly(vinylthiophene) (PVT) and PVT/silica composites can be simultaneously synthesized. The mass balance of the products (soluble fraction and hybrid particle fraction) depends significantly on temperature and 2‐VT/silica ratio. The hydride abstraction reaction of PVT both in solution and immobilized on silica particle surface has been studied using 2,3‐dichloro‐5,6‐dicyano‐1,4‐quinone (DDQ), tetrachloro‐1,4‐quinone (chloranile = ClA) and triphenylmethylium as reagents. The transformation process of PVT towards conjugated polymers has been studied with UV‐vis spectroscopy and ESR spectroscopy. Cyclic voltammetry shows that chloranil is complexated with the formed polymer. Radical formation increases with increasing degree of conversion. The soluble fraction of the conjugated PVT sections formed is capable of reacting with each other as evidenced by GPC data. Structure of PVT/silica and resulting hybrid materials have been investigated by solid state 13C {1H} CP MAS NMR‐spectroscopy showing a reaction of methine and methylene hydrogen atoms after treatment with DDQ or chloranil as hydride acceptors. For all poly‐(2‐vinylthiophene)/hydride acceptor systems studied, chloranil has been found to be the best reagent for the transformation of PVT towards conjugated polymers.Transformation of PVT in poly(2‐ethinylthiophene) (PET) and PVT‐PET copolymers.magnified imageTransformation of PVT in poly(2‐ethinylthiophene) (PET) and PVT‐PET copolymers.

Resonance energies of the allyl cation and allyl anion: contribution by resonance and inductive effects toward the acidity and hydride abstraction enthalpy of propene.

Density functional theory was employed to calculate the acidities and hydride abstraction enthalpies of propene (3) and propane (4), along with their vinylogues (5 and 6, respectively). The same reaction enthalpies were calculated for the propene vinylogues in which the terminal vinyl group was rotated perpendicular to the rest of the conjugated system (7). The contribution by resonance and inductive effects toward the acidity and hydride abstraction enthalpy of each vinylogue of 5 (n = 1-3) was computed and extrapolated to n = 0 (the parent propene system). The resonance energies of the allyl cation and anion were determined to be about 20-22 and 17-18 kcal/mol, respectively. Comparisons are made to resonance energies calculated using other methodologies.

Hydride abstraction from 1,3,5‐cycloheptatriene by gaseous carbenium ions, as studied by Fourier transform ion cyclotron resonance kinetics and deuterium labeling

AbstractThe gas‐phase interaction of protonated acetone and tert‐butyl cations with 1,3,5‐cycloheptatriene was studied by Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometry. In addition to proton transfer giving C7H9+ ions, hydride abstraction from cycloheptatriene giving C7H7+ ions was observed. Deuterium labeling experiments combined with the determination of the reaction kinetics excluded the formation of C7H7+ ions by consecutive proton transfer and H2 expulsion under these conditions. The kinetic isotope effect measured for the hydride transfer channel was found to be in the range 1.65±0.1, very close to that known to operate during hydride transfer from simple alkylbenzenes to t‐C4H9+ ions. Copyright © 2003 John Wiley & Sons, Ltd.

Formation of keto-carbeniums from 1,2,2-triarylacenaphthen-1-ols by breaking a long CC bond: unusual alcohol protonolysis accompanied by formal hydride abstraction

Oxidation of 1,2,2-tris(4-dimethylaminophenyl)- and 2,2-bis(4-dimethylaminophenyl)-1-phenylacenaphthen-1-ols 1a, b with I 2 induced the C 1C 2 bond fission to give 8-aroylnaphthalen-1-yl-bis(4-dimethylaminophenyl)carbenium derivatives 2a, b + , the intramolecular Lewis acid–base pairs. Treatment of 1 with HBF 4 did not induce the expected COH bond heterolysis but caused fission of COH and C 1C 2 bonds to give exactly the same carbenium 2 + .

Transition from hydrogen atom to hydride abstraction by Mn4O4(O2PPh2)6 versus [Mn4O4(O2PPh2)6]+: O-H bond dissociation energies and the formation of Mn4O3(OH)(O2PPh2)6.

Synthesis, characterization, and reactions of the novel manganese-oxo cubane complex [Mn(4)O(4)(O(2)PPh(2))(6)](ClO(4)), 1+ (ClO(4)(-)), are described. Cation 1+ is composed of the [Mn(4)O(4)](7+) core surrounded by six bidentate phosphinate ligands. The proton-coupled electron transfer (pcet) reactions of phenothiazine (pzH), the cation radical (pzH(.+)(ClO(4)(-)), and the neutral pz* radical with 1+ are reported and compared to Mn(4)O(4)(O(2)PPh(2))(6) (1). Compound 1+ (ClO(4)(-)) reacts with excess pzH via four sequential reduction steps that transfer a total of five electrons and four protons to 1+. This reaction forms the doubly dehydrated manganese cluster Mn(4)O(2)(O(2)PPh(2))(6) (2) and two water molecules derived from the corner oxygen atoms. The first pcet step forms the novel complex Mn(4)O(3)(OH)(O(2)PPh(2))(6) (1H) and 1 equiv of the pz+ cation by net hydride transfer from pzH. Spectroscopic characterization of isolated 1H is reported. Reduction of 1 by pzH or a series of para-substituted phenols also produces 1H via net H atom transfer. A lower limit to the homolytic bond dissociation energy (BDE) (1H --> 1 + H) was estimated to be >94 kcal/mol using solution phase BDEs for pzH and para-substituted phenols. The heterolytic BDE was estimated for the hydride transfer reaction 1H --> 1+ + H(-) (BDE approximately 127 kcal/mol). These comparisons reveal the O-H bond in 1H to be among the strongest of any Mn-hydroxo complex measured thus far. In three successive H atom transfer steps, 1H abstracts three hydrogen atoms from three pzH molecules to form complex 2. Complex 2 is shown to be identical to the "pinned butterfly" cluster produced by the reaction of 1 with pzH (Ruettinger, W. F.; Dismukes, G. C. Inorg. Chem. 2000, 39, 1021-1027). The Mn oxidation states in 2 are formally Mn(4)(2II,2III), and no further reduction occurs in excess pzH. By contrast, outer-sphere electron-only reductants such as cobaltacene reduce both 1+ and 1 to the all Mn(II) oxidation level and cause cluster fragmentation. The reaction of pzH(.+) with 1+ produces 1H and the pz+ cation by net hydrogen atom transfer, and terminates at 1 equiv of pzH(.+) with no further reaction at excess. By contrast, pz* does not react with 1+ at all, indicating that reduction of 1+ by electron transfer to form pz+ does not occur without a proton (pcet to 1+ is thermodynamically required). Experimental free energy changes are shown to account for these pcet reactions and the absence of electron transfer for any of the phenothiazine series. Hydrogen atom abstraction from substrates by 1 versus hydride abstraction by 1(+ )()illustrates the transition to two-electron one-proton pcet chemistry in the [Mn(4)O(4)](7+) core that is understood on the basis of free energy consideration. This transition provides a concrete example of the predicted lowest-energy pathway for the oxidation of two water molecules to H(2)O(2) as an intermediate within the photosynthetic water-oxidizing enzyme (vs sequential one-electron/proton steps). The implications for the mechanism of photosynthetic water splitting are discussed.

Investigation of the synthesis of {Mo(η5-C5H5)(CO)3}+fragments partnered with the monoanionic carboranes [closo-CB11H11Br]−, [closo-CB11H6Br6]−and [closo-HCB11Me11]−by silver salt metathesis and hydride abstraction

Abstraction of iodide from [Cp(CO)3MoI] (Cp = η5-C5H5) using the silver salt of the weakly coordinating anion [closo-CB11H11Br]−, initially affords an intermediate dimeric complex, [MoCp(CO)3I·Ag(CB11H11Br)]2, which has a central {AgI}2 core appended by two carborane anions in the solid state. Prolonged reaction does result in elimination of AgI to form [MoCp(CO)3(CB11H11Br)], but not cleanly. This complex can be isolated in good yield by hydride abstraction from [Cp(CO)3MoH] using the new trityl salt [CPh3][closo-CB11H11Br]. Similarly, reaction of this hydride source with [CPh3][closo-CB11H6Br6] in CD2Cl2 results in hydride abstraction, but a mixture of products is formed, suggested to consist of the hydride bridged dimer, [{Cp(CO)3Mo}2(μ-H)][closo-CB11H6Br6], the anion coordinated complex [Cp(CO)3Mo(closo-CB11H6Br6)] and the solvent adduct [Cp(CO)3Mo(ClCD2Cl)][closo-CB11H6Br6]. Reaction of Ag[closo-HCB11Me11] with [Cp(CO)3MoX] (X = Cl, I) results in a intermediate complex which has a [Cp(CO)3MoX]6Ag2 central core (crystallographically characterised for X = I) with no interactions from the permethylated carborane anions. Prolonged reaction does result in partial AgX elimination to form the halide-bridged dimers [{CpMo(CO)3}2(μ-X)][closo-HCB11Me11]. Hydride abstraction from [Cp(CO)3MoH] using the new trityl salt [CPh3][closo-HCB11Me11] resulted in the clean formation of the hydride-bridged dimer [{Cp(CO)3Mo}2(μ-H)][closo-HCB11Me11]. The two anions [closo-CB11H6Br6]− and [closo-HCB11Me11]− have been compared with [B(ArF′)4]− [ArF′ = 3,5-bis(trifluoromethyl)phenyl] in Bullock's ionic hydrogenation of 3-pentanone using [Cp(CO)2(PPh3)Mo]+ catalysts. The permethylated anion slightly outperforms its hexabrominated congener in catalysis, but is still slower than [B(ArF′)4]−.

Trimethyltriazacyclohexane as bridging ligand for triangular Cu3 units and C-H hydride abstraction into a Cu6 cluster.

Reaction of [Cu(MeCN)4](BF4) with N,N',N"-trimethyl-1,3,5-triazacyclohexane in CH2Cl2 leads to two Cu6 clusters containing the triazacyclohexane as a ligand to triangular Cu3 units with the abstraction and incorporation of chloride and hydride.

Mass-Spectrometric Study of Ion-Molecule Reactions of CH5+, C2H5+, and C3H5+ with C8–C18 Alcohols in an Ion Trap

Abstract Chemical ionization of n-alkyl alcohols (n-CxH2x+1–OH = M; x=8–18) by the CH5+, C2H5+, and C3H5+ ions has been studied under a reactant-ion selective mode of an ion-trap type of GC/MS. In all reactions, [M – H]+, alkyl CyH2y+1+ (y = 3−x), and alkenyl CyH2y−1+ (y = 4−x) ions were observed. On the basis of observed initial distributions and calculated thermochemical data, it was concluded that the major reactions for the formation of CyH2y+1+ were proton transfer to a hydroxy group in the CH5+ reactions, proton transfer and/or hydroxide-ion abstraction in the C2H5+ reactions, and CzH2zOH−-ion (z < x) abstraction in the C3H5+ reactions, while the major reactions for the formation of CyH2y−1+ ions were proton transfer to alkyl chain in the CH5+ reactions, hydride-ion abstraction in the C2H5+ reactions, and alkanide-ion abstraction in the C3H5+ reactions.

Reactions of carbocations with unsaturated hydrocarbons: electrophilic alkylation or hydride abstraction?

Benzhydryl cations were used as reference electrophiles to determine the hydride donor reactivities of unsaturated hydrocarbons. The kinetics of the reactions were followed by UV-vis spectroscopy and conductivity measurements, and it was found that the second-order rate constants for the hydride transfer processes were almost independent of the solvents or counterions employed. The rate constants correlate linearly with the previously published empirical electrophilicity parameters E of the benzhydrylium ions. Therefore, the linear free energy relationship log k(20 degrees C) = s(E + N) could be employed to characterize the hydride reactivities of the hydrocarbons by the nucleophilicity parameters N and s. The similarity of the slopes s for hydride donors and pi-nucleophiles allows a direct comparison of the reactivities of these different functional groups based on their nucleophilicity parameters N. Since nucleophilicity parameters of -5 < N < 0 have been found for a large variety of allylic and bisallylic hydride donors, a rule of thumb is derived that hydride transfer processes may compete with carbon-carbon bond-forming reactions when carbocations are combined with olefins of pi-nucleophilicity N < 0.