Kinetic Isotope Effect as a Tool To Investigate the Oxygen Reduction Reaction on Pt-based Electrocatalysts – Part I: High-loading Pt/C and Pt Extended Surface

Kinetic isotope effect (KIE) was used to study the rate-determining step for oxygen reduction reaction (ORR) on dispersed Pt/C electrocatalyst and polycrystalline Pt (Pt-poly). KIE is defined as the ratio of the kinetic current measured in protonated electrolyte versus deuterated electrolyte, with KIE values larger than one indicating proton participation in the rate-determining step. The effect of poisoning anions on the platinum rate determining step is investigated by assessing the KIE in perchloric (non-poisoning) and sulfuric acid-based electrolytes. The kinetics currents were calculated using the Koutechy-Levich and Tafel analysis. A KIE of 1 was observed for Pt/C (with a 40 wt.% Pt loading) and Pt-poly, thus indicating that, on 40 wt. % Pt/C and Pt-poly, the rate determining step is proton independent.     

Theoretical studies of the transition-state structures and free energy barriers for base-catalyzed hydrolysis of amides

The transition-state structures and free energy barriers for the rate-determining step (i.e. the formation of a tetrahedral intermediate) of base-catalyzed hydrolysis of a series of amides in aqueous solution have been studied by performing first-principle electronic structure calculations using a hybrid supermolecule-polarizable continuum approach. The calculated results and a revisit of recently reported experimental proton inventory data reveal that the favorable transition-state structure optimized for the tetrahedral intermediate formation of hydroxide ion-catalyzed hydrolysis of formamide may have three solvating water molecules remaining on the attacking hydroxide oxygen and two additional water molecules attached to the carbonyl oxygen of formamide. The calculated results have also demonstrated interesting substituent effects on the optimized transition-state geometries, on the transition-state stabilization, and on the calculated free energy barriers for the base-catalyzed hydrolysis of amides. When some or all of the hydrogen atoms of formamide are replaced by methyl groups, the total number of water molecules hydrogen-bonding with the attacking hydroxide in the transition state decreases from three for formamide to two for N-methylacetamide, N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA). The larger substituents of the amide hinder the solvent water molecules approaching the attacking hydroxide oxygen in the transition state and, therefore, destabilize the transition-state structure and increase the free energy barrier. By using the optimized most favorable transition-state structures, the calculated free energy barriers, i.e., 21.6 (or 21.7), 22.7, 23.1, and 26.0 kcal/mol for formamide, N-methylacetamide, DMF, and DMA, respectively, are in good agreement with the available experimental free energy barriers, i.e., 21.2, 21.5, 22.6, and 24.1 kcal/mol for formamide, N-methylacetamide, DMF, and DMA, respectively.     

Influence of donor-acceptor distance variation on photoinduced electron and proton transfer in rhenium(i)-phenol dyads

A homologous series of four molecules in which a phenol unit is linked covalently to a rhenium(I) tricarbonyl diimine photooxidant via a variable number of p-xylene spacers (n = 0-3) was synthesized and investigated. The species with a single p-xylene spacer was structurally characterized to get some benchmark distances. Photoexcitation of the metal complex in the shortest dyad (n = 0) triggers release of the phenolic proton to the acetonitrile/water solvent mixture; a H/D kinetic isotope effect (KIE) of 2.0 ± 0.4 is associated with this process. Thus, the shortest dyad basically acts like a photoacid. The next two longer dyads (n = 1, 2) exhibit intramolecular photoinduced phenol-to-rhenium electron transfer in the rate-determining excited-state deactivation step, and there is no significant KIE in this case. For the dyad with n = 1, transient absorption spectroscopy provided evidence for release of the phenolic proton to the solvent upon oxidation of the phenol by intramolecular photoinduced electron transfer. Subsequent thermal charge recombination is associated with a H/D KIE of 3.6 ± 0.4 and therefore is likely to involve proton motion in the rate-determining reaction step. Thus, some of the longer dyads (n = 1, 2) exhibit photoinduced proton-coupled electron transfer (PCET), albeit in a stepwise (electron transfer followed by proton transfer) rather than concerted manner. Our study demonstrates that electronically strongly coupled donor-acceptor systems may exhibit significantly different photoinduced PCET chemistry than electronically weakly coupled donor-bridge-acceptor molecules.     

Direct examination of H2O2 activation by a heme peroxidase

Described here is the application of oxygen isotope fractionation together with computational methods, to elucidate a mechanism of enzymatic H2O2 activation. Horseradish peroxidase (HRP) has been the subject of intensive experimental and computational studies, yet questions remain as to the reversibility of the O-O cleavage step. New insight is afforded by the competitive oxygen kinetic isotope effect (18O KIE) upon H2O2 consumption determined under turnover conditions. The 18O KIE is compared to isotope effects calculated for the O-O heterolysis transition state and potential intermediates using density functional theory. In addition, experiments in enriched water provide evidence for HRP-catalyzed scrambling of the 18O label into the unreacted H2O2. The results provide an unprecedented view of H2O2 activation by a heme peroxidase and challenge the assumption of rate-limiting O-O heterolysis.     

Mechanisms of electron transfer in catalysis by copper zinc superoxide dismutase

Activated oxygen intermediates during copper zinc superoxide dismutase (SOD) catalysis were investigated using an isotope fractionation technique and natural abundance reagents. Competitive oxygen kinetic isotope effects (KIEs) are reported for the enzyme-catalyzed disproportionation of superoxide as well as the stoichiometric reaction of reduced SOD with molecular oxygen. Analysis within the context of quantum mechanical electron transfer theory provides evidence against an outer-sphere mechanism for O2*- oxidation. A CuII-O2-I intermediate is, therefore, proposed. The SOD-catalyzed oxidation of O2*- is characterized by an inverse (<1) KIE which is similar to those determined for the analogous reactions of synthetic copper compounds. An inverse kinetic isotope effect upon the enzymatic reduction of O2*- is also observed and proposed to arise from rate-determining proton transfer which leads to the formation of HO2* in the SOD active site.     

Turnover is rate-limited by deglycosylation for Micromonospora viridifaciens sialidase-catalyzed hydrolyses: conformational implications for the Michaelis complex

A panel of seven isotopically substituted sialoside natural substrate analogues based on the core structure 7-(5-acetamido-3,5-dideoxy-d-glycero-α-d-galacto-non-2-ulopyranosylonic acid)-(2→6)-β-D-galactopyranosyloxy)-8-fluoro-4-methylcoumarin (1, Neu5Acα2,6GalβFMU) have been synthesized and used to probe the rate-limiting step for turnover by the M. viridifaciens sialidase. The derived kinetic isotope effects (KIEs) on k(cat) for the ring oxygen ((18)V), leaving group oxygen ((18)V), anomeric carbon ((13)V), C3-carbon ((13)V), C3-R deuterium ((D)V(R)), C3-S deuterium ((D)V(S)), and C3-dideuterium ((D)(2)V) are 0.986 ± 0.003, 1.003 ± 0.005, 1.021 ± 0.006, 1.001 ± 0.008, 1.029 ± 0.007, 0.891 ± 0.008, and 0.890 ± 0.006, respectively. The solvent deuterium KIE ((D(2)O)V) for the sialidase-catalyzed hydrolysis of 1 is 1.585 ± 0.004. In addition, a linear proton inventory was measured for the rate of hydrolysis, under saturating condition, as a function of n, the fraction of deuterium in the solvent. These KIEs are compatible with rate-determining cleavage of the enzymatic tyrosinyl β-sialoside intermediate. Moreover, the secondary deuterium KIEs are consistent with the accumulating Michaelis complex in which the sialosyl ring of the carbohydrate substrate is in a (6)S(2) skew boat conformation. These KIE measurements are also consistent with the rate-determining deglycosylation reaction occurring via an exploded transition state in which synchronous charge delocalization is occurring onto the ring oxygen atom. Finally, the proton inventory and the magnitude of the solvent KIE are consistent with deglycosylation involving general acid-catalyzed protonation of the departing tyrosine residue rather than general base-assisted attack of the nucleophilic water.     

An altered mechanism of hydrolysis for a metal-complexed phosphate diester

Isotope effects in the nucleophile and in the leaving group were measured to gain information about the mechanism and transition state of the hydrolysis of methyl p-nitrophenyl phosphate complexed to a dinuclear cobalt complex. The complexed diester undergoes hydrolysis about 1011 times faster than the corresponding uncomplexed diester. The kinetic isotope effects indicate that this rate acceleration is accompanied by a change in mechanism. A large inverse 18O isotope effect in the bridging hydroxide nucleophile (0.937 +/- 0.002) suggests that nucleophilic attack occurs before the rate-determining step. Large isotope effects in the nitrophenyl leaving group (18Olg = 1.029 +/- 0.002, 15N = 1.0026 +/- 0.0002) indicate significant fission of the P-O ester bond in the transition state of the rate-determining step. The data indicate that in contrast to uncomplexed diesters, which undergo hydrolysis by a concerted mechanism, the reaction of the complexed diester likely proceeds via an addition-elimination mechanism. The rate-limiting step is expulsion of the p-nitrophenyl leaving group from the intermediate, which proceeds by a late transition state with extensive bond fission to the leaving group. This represents a substantial change in mechanism from the hydrolysis of uncomplexed aryl phosphate diesters.     

Efficient synthesis of 2-arylquinazolines via copper-catalyzed dual oxidative benzylic C–H aminations of methylarenes

A novel copper-catalyzed dual oxidative benzylic C–H aminations of methylarenes with 2-aminobenzoketones in the presence of ammonium acetate was developed. This reaction represents a new avenue for 2-arylquinazolines with good yields. A key intermediate was detected and the kinetics isotope effect(KIE) indicated that C–H bond cleavage was the rate-determining step.     

Flash photolytic generation of ortho-quinone methide in aqueous solution and study of its chemistry in that medium

Flash photolysis of o-hydroxybenzyl alcohol, o-hydroxybenzyl p-cyanophenyl ether, and (o-hydroxybenzyl)trimethylammonium iodide in aqueous perchloric acid and sodium hydroxide solutions, and in acetic acid and biphosphate ion buffers, produced o-quinone methide as a short-lived transient species that underwent hydration back to benzyl alcohol in hydrogen-ion catalyzed (k(H+) = 8.4 x 10(5) M(-1) s(-1)) and hydroxide-ion catalyzed (k(HO)- = 3.0 x 10(4) M(-1) s(-1)) reactions as well as an uncatalyzed (k(UC) = 2.6 x 10(2) s(-1)) process. The hydrogen-ion catalyzed reaction gave the solvent isotope effect k(H+)/k(D)+ = 0.42, whose inverse nature indicates that this process occurs by rapid and reversible equilibrium protonation of the carbonyl oxygen atom of the quinone methide, followed by rate-determining capture of the carbocation so produced by water. The magnitude of the rate constant of the uncatalyzed reaction, on the other hand, indicates that this process occurs by simple nucleophilic addition of water to the methylene group of the quinone methide. Decay of the quinone methide is also accelerated by acetic acid buffers through both acid- and base-catalyzed pathways, and quantitative analysis of the reaction products formed in these solutions shows that this acceleration is caused by nucleophilic reactions of acetate ion rather than by acetate ion assisted hydration. Bromide and thiocyanate ions also accelerate decay of the quinone methide through both hydrogen-ion catalyzed and uncatalyzed pathways, and the inverse nature of solvent isotope effects on the hydrogen-ion catalyzed reactions shows that these reactions also occur by rapid equilibrium protonation of the quinone methide carbonyl oxygen followed by rate-determining nucleophilic capture of the ensuing carbocation. Assignment of an encounter-controlled value to the rate constant for the rate-determining step of the thiocyanate reaction leads to pK(a) = -1.7 for the acidity constant of the carbonyl-protonated quinone methide.     

Theoretical study on the hydrolysis mechanism of N,N-dimethyl-N'-(2-oxo-1, 2-dihydro-pyrimidinyl)formamidine: water-assisted mechanism and cluster-continuum model

The hydrolysis reaction of N,N-dimethyl-N'-(2-oxo-1, 2-dihydro-pyrimidinyl)formamidine (DMPFA), a model compound of the antivirus drug amidine-3TC (3TC = 2', 3'-dideoxy-3'-thiacytidine), is investigated by the hybrid density functional theory B3LYP/6-31+G (d,p) method. The hydrolysis reaction of the title compound is predicted to undergo via two pathways, each of which is a stepwise process. Path A is the addition of H2O to the C=N double bond in the amidine group to form a tetrahedral structure in its first step, and then the transfer of the H atom of hydroxyl leads to the corresponding products via four possible channels. Path B simultaneously involves the nucleophilic attack of H2O to the C atom of the C=N bond and the proton transfer to the N atom of amino group leading to the cleavage of the C-N single bond in the amidine group. The results indicate that path A is more favorable than path B in the gas phase. Moreover, to simulate the title reaction in aqueous solution, water-assisted mechanism and the cluster-continuum model, based on the SCRF/CPCM model, are taken into account in our work. The results indicate that it is rational for two water molecules served as a bridge to assist in the first step of path A and that cytosine rather than the cytosine-substituted formamide should be released from the tetrahedral intermediate via s six-membered cycle transition state (channel 2). Our calculations exhibit that the process toward the tetrahedral intermediate is the rate-determining step both in the gas phase and in aqueous solution.     

Multiple isotope effect study of the acid-catalyzed hydrolysis of methyl formate

Multiple isotope effects have been measured for the acid-catalyzed hydrolysis of methyl formate in 0.5 M HCl at 20 degrees C. The isotope effects in the present investigation include the carbonyl carbon (13k = 1.028 +/- 0.001), the carbonyl oxygen (18k = 0.9945 +/- 0.0009), the nucleophile oxygen (18k = 0.995 +/- 0.001), and the formyl hydrogen ((D)k = 0.81 +/- 0.02). Determination of the carbonyl carbon, carbonyl oxygen, and formyl hydrogen isotope effects was performed via isotopic analysis of residual substrate. However, determination of the oxygen nucleophile isotope effect required analysis of the oxygen atoms of the product (formic acid), which exchange with the solvent (water) under acid conditions. This necessitated measurement of the rate of exchange of these oxygen atoms under the conditions for hydrolysis (k(ex) = 0.0723 min(-1)) and correction of the raw isotope ratios measured during the nucleophile-O isotope effect experiment. These results, along with the previously reported isotope effect for the leaving oxygen (18k = 1.0009) and the ratio of the rate of hydrolysis to that of exchange of the carbonyl oxygen with water (k(h)/k(ex) = 11.3), give a detailed picture of the transition-state structure for the reaction.     

Accumulation of tetrahedral intermediates in cholinesterase catalysis: a secondary isotope effect study

In a previous communication, kinetic β-deuterium secondary isotope effects were reported that support a mechanism for substrate-activated turnover of acetylthiocholine by human butyrylcholinesterase (BuChE) wherein the accumulating reactant state is a tetrahedral intermediate ( Tormos , J. R. ; et al. J. Am. Chem. Soc. 2005 , 127 , 14538 - 14539 ). In this contribution additional isotope effect experiments are described with acetyl-labeled acetylthiocholines (CL(3)COSCH(2)CH(2)N(+)Me(3); L = H or D) that also support accumulation of the tetrahedral intermediate in Drosophila melanogaster acetylcholinesterase (DmAChE) catalysis. In contrast to the aforementioned BuChE-catalyzed reaction, for this reaction the dependence of initial rates on substrate concentration is marked by pronounced substrate inhibition at high substrate concentrations. Moreover, kinetic β-deuterium secondary isotope effects for turnover of acetylthiocholine depended on substrate concentration, and gave the following: (D3)k(cat)/K(m) = 0.95 ± 0.03, (D3)k(cat) = 1.12 ± 0.02 and (D3)βk(cat) = 0.97 ± 0.04. The inverse isotope effect on k(cat)/K(m) is consistent with conversion of the sp(2)-hybridized substrate carbonyl in the E + A reactant state into a quasi-tetrahedral transition state in the acylation stage of catalysis, whereas the markedly normal isotope effect on k(cat) is consistent with hybridization change from sp(3) toward sp(2) as the reactant state for deacylation is converted into the subsequent transition state. Transition states for Drosophila melanogaster AChE-catalyzed hydrolysis of acetylthiocholine were further characterized by measuring solvent isotope effects and determining proton inventories. These experiments indicated that the transition state for rate-determining decomposition of the tetrahedral intermediate is stabilized by multiple protonic interactions. Finally, a simple model is proposed for the contribution that tetrahedral intermediate stabilization provides to the catalytic power of acetylcholinesterase.     

Iron substitution for sodium in a carboxylate-bridged, heterodinuclear sodium-iron complex

The synthesis and structural characterization of the carboxylate-bridged, heterodinuclear iron-sodium complex [NaFe(PIC2DET)(mu-O2CTrp)3] (2), where PIC2DET (1) is a 2,3-diethynyltriptycene-linked dipicolinic methyl ester ligand and Trp is 9-triptycenyl, are described. The metal ions in 2 are bridged by three triptycene carboxylates with an Fe...Na distance of 3.181(2) A, and each is coordinated to a pyridine nitrogen and carbonyl oxygen atom of 1, forming two five-membered chelate rings. A linkage isomer in which Fe1 is bound by the other ester oxygen atom of 1 was identified by X-ray crystallographic analysis. Treatment of 2 with Fe(OTf)2.2MeCN resulted in substitution of sodium by iron(II) to give the cationic diiron(II) complex [Fe2(PIC2DET)(mu-O2CTrp)3][OTf] (3). This reaction was investigated by UV-vis, IR, MS, and stopped-flow spectroscopy. The substitution is first order with respect to 2 and zero order with respect to Fe(OTf)2.2MeCN (kobs = 21 +/- 2 s-1), consistent with a dissociative mechanism. A positive enthalpy of activation (DeltaH = 59 +/- 6 kJ mol-1) and negative entropy of activation (DeltaS = -20 +/- 6 J mol-1 K-1) were calculated from the temperature dependence of the rate-determining dissociation step.     

Cationic Iridium/Chiral Bisphosphine‐Catalyzed Enantioselective Hydroacylation of Ketones

A facile and convenient synthesis of the chiral phthalide framework catalyzed by cationic iridium was developed. The method utilized cationic iridium/bisphosphine‐catalyzed asymmetric intramolecular carbonyl hydroacylation of 2‐keto benzaldehydes to furnish the corresponding optically active phthalide products in good to excellent enantioselectivities (up to 98% ee). The mechanistic studies using a deuterium‐labelled substrate suggested that the reaction involved an intramolecular carbonyl insertion mechanism to iridium hydride intermediate. In addition, we investigated the kinetic isotope effect (KIE) of intramolecular hydroacylation with deuterated substrate and determined that the C?H activation step is not included in the turnover‐limiting step.     

Co(+)-assisted decomposition of h6-acetone and d6-acetone: acquisition of reaction rate constants and dynamics of the dissociative mechanism

Reaction rate constants have been acquired for the gaseous unimolecular decomposition reaction of the Co(+)(OC(CH(3))(2)) cluster ion and its deuterium labeled analog. Each rate constant is measured at a well resolved cluster internal energy within the range 12,300-16,100 cm(-1). The weighted, averaged kinetic isotope effect (KIE), k(H)/k(D) = 1.54 ± 0.05, is about three times smaller than the KIE measured for the rate-determining rate constants in the similar Ni(+)(OC(CH(3))(2)) decomposition reaction. These reactions likely follow the same oxidative addition-reductive elimination mechanism. Thus, this unexpected change in the KIE magnitudes is not due to differences in the dissociative reaction coordinates. Rather, we propose that the unique dissociation dynamics of these two similar systems is due to differences in the low-lying electronic structure of each transition metal ion.     

Combined experimental and theoretical study on aromatic hydroxylation by mononuclear nonheme iron(IV)-oxo complexes

The hydroxylation of aromatic compounds by mononuclear nonheme iron(IV)-oxo complexes, [FeIV(Bn-tpen)(O)]2+ (Bn-tpen=N-benzyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine) and [FeIV(N4Py)(O)]2+ (N4Py=N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), has been investigated by a combined experimental and theoretical approach. In the experimental work, we have performed kinetic studies of the oxidation of anthracene with nonheme iron(IV)-oxo complexes generated in situ, thereby determining kinetic and thermodynamic parameters, a Hammett rho value, and a kinetic isotope effect (KIE) value. A large negative Hammett rho value of -3.9 and an inverse KIE value of 0.9 indicate that the iron-oxo group attacks the aromatic ring via an electrophilic pathway. By carrying out isotope labeling experiments, the oxygen in oxygenated products was found to derive from the nonheme iron(IV)-oxo species. In the theoretical work, we have conducted density functional theory (DFT) calculations on the hydroxylation of benzene by [FeIV(N4Py)(O)]2+. The calculations show that the reaction proceeds via two-state reactivity patterns on competing triplet and quintet spin states via an initial rate determining electrophilic substitution step. In analogy to heme iron(IV)-oxo catalysts, the ligand is noninnocent and actively participates in the reaction mechanism by reshuttling a proton from the ipso position to the oxo group. Calculated kinetic isotope effects of C6H6 versus C6D6 confirm an inverse isotope effect for the electrophilic substitution pathway. Based on the experimental and theoretical results, we have concluded that the aromatic ring oxidation by mononuclear nonheme iron(IV)-oxo complexes does not occur via a hydrogen atom abstraction mechanism but involves an initial electrophilic attack on the pi-system of the aromatic ring to produce a tetrahedral radical or cationic sigma-complex.     

Mechanistic insights from reactions between copper(II)-phenoxyl complexes and substrates with activated C-H bonds

The reactivities of two copper(II)-phenoxyl analogues of the oxidized, active form of the metalloenzyme galactose oxidase, [1tBu2]+ and [2tBu2]+, have been studied using the substrates benzyl alcohol and 9,10-dihydroanthracene, for a total of four reactions. The reaction stoichiometries in all cases show a 2:1 ratio of oxidant to benzaldehyde or anthracene product, indicating that [1tBu2]+ and [2tBu2]+ behave ultimately as only one-electron oxidants, but the reaction kinetics each indicate that only a single copper(II)-phenoxyl complex is involved in the rate-determining step. For each substrate, rate laws indicate that [1tBu2]+ and [2tBu2]+ react by different mechanisms: one proceeds by a simple bimolecular reaction, while the other first enters into a substrate-binding equilibrium before subsequently reacting by an intramolecular reaction. The reactions proceeding by the latter mechanism have faster overall rates, which correlates to a lower entropic barrier for the substrate-binding mechanism. Correlation of the reaction rates with the C-H bond dissociation energies of substrates as well as significant deuterium kinetic isotope effects indicates that the rate-determining steps involve hydrogen atom abstraction from the activated C-H bonds. A variable-temperature study (268-308 K) of the nonclassical KIE of the [1tBu2]+/benzyl alcohol reaction (kH/kD = 15 at 298 K) failed to show evidence for quantum tunneling. The rapid sequence by which a second 1 equiv of copper(II)-phenoxyl oxidant completes the reaction after the rate- and product-determining hydrogen atom abstraction step cannot be probed kinetically. Comparisons are made to the reactivities of other copper(II)-phenoxyl complexes reported in the literature and to galactose oxidase itself.     

Hydride reduction of NAD+ analogues by isopropyl alcohol: kinetics, deuterium isotope effects and mechanism

Observed pseudo-first-order rate constants (k(obs)) of the hydride-transfer reactions from isopropyl alcohol (i-PrOH) to two NAD(+) analogues, 9-phenylxanthylium ion (PhXn(+)) and 10-methylacridinium ion (MA(+)), were determined at temperatures ranging from 49 to 82 degrees C in i-PrOH containing various amounts of AN or water. Formations of the alcohol-cation ether adducts (ROPr-i) were observed as side equilibria. The equilibrium constants for the conversion of PhXn(+) to PhXnOPr-i in i-PrOH/AN (v/v = 1) were determined, and the equilibrium isotope effect (EIE = K(i-PrOH)/K(i-PrOD)) at 62 degrees C was calculated to be 2.67. The k(H) of the hydride-transfer step for both reactions were calculated on the basis of the k(obs) and K. The corresponding deuterium kinetic isotope effects (e.g., KIE(OD)(H) = k(H)(i-PrOH)/k(H)(i-PrOD) and KIE(beta-D6)(H) = k(obs)(i-PrOH)/k(obs)((CD3)2CHOH)), as well as the activation parameters, were derived. For the reaction of PhXn(+) (62 degrees C) and MA(+) (67 degrees C), primary KIE(alpha-D)(H) (4.4 and 2.1, respectively) as well as secondary KIE(OD)(H) (1.07 and 1.18) and KIE(beta-D6)(H) (1.1 and 1.5) were observed. The observed EIE and KIE(OD)(H) were explained in terms of the fractionation factors for deuterium between OH and OH(+)(OH(delta+)) sites. The observed inverse kinetic solvent isotope effect for the reaction of PhXn(+) (k(obs)(i-PrOH)/k(obs)(i-PrOD) = 0.39) is consistent with the intermolecular hydride-transfer mechanism. The dramatic reduction of the reaction rate for MA(+), when the water or i-PrOH cosolvent was replaced by AN, suggests that the hydride-transfer T.S. is stabilized by H-bonding between O of the solvent OH and the substrate alcohol OH(delta+). This result suggests an H-bonding stabilization effect on the T.S. of the alcohol dehydrogenase reactions.     

Pd(II)-catalyzed intramolecular amidoarylation of alkenes with molecular oxygen as sole oxidant

Stereoselective palladium-catalyzed synthesis of structurally versatile indoline derivatives, using molecular oxygen as the sole oxidant, is described. New C-N and C-C bonds form across an alkene in an intramolecular manner. The C-N bond-forming step proceeds via a syn-amidopalladation pathway. The moderate kinetic isotope effects (intramolecular KIE = 3.56) suggest that electrophilic aromatic substitution occurs in the arylation step.     

Dinuclear Zn(II) complex catalyzed phosphodiester cleavage proceeds via a concerted mechanism: a density functional theory study

Density functional theory (DFT) calculations were used to study the mechanism for the cleavage reaction of the RNA analogue HpPNP (HpPNP = 2-hydroxypropyl-4-nitrophenyl phosphate) catalyzed by the dinuclear Zn(II) complex of 1,3-bis(1,4,7-triazacyclonon-1-yl)-2-hydroxypropane (Zn(2)(L(2)O)). We present a binding mode in which each terminal phosphoryl oxygen atom binds to one zinc center, respectively, and the nucleophilic 2-hydroxypropyl group coordinates to one of the zinc ions, while the hydroxide from deprotonation of a water molecule coordinates to the other zinc ion. Our calculations found a concerted mechanism for the HpPNP cleavage with a 16.5 kcal/mol reaction barrier. An alternative proposed stepwise mechanism through a pentavalent oxyphosphorane dianion reaction intermediate for the HpPNP cleavage was found to be less feasible with a significantly higher energy barrier. In this stepwise mechanism, the deprotonation of the nucleophilic 2-hydroxypropyl group is accompanied with nucleophilic attack in the rate-determining step. Calculations of the nucleophile (18)O kinetic isotope effect (KIE) and leaving (18)O KIE for the concerted mechanism are in reasonably good agreement with the experimental values. Our results indicate a specific-base catalysis mechanism takes place in which the deprotonation of the nucleophilic 2-hydroxypropyl group occurs in a pre-equilibrium step followed by a nucleophilic attack on the phosphorus center. Detailed comparison of the geometric and electronic structure for the HpPNP cleavage reaction mechanisms in the presence/absence of catalyst revealed that the catalyst significantly altered the determining-step transition state to become far more associative or tight, that is, bond formation to the nucleophile was remarkably more advanced than leaving group bond fission in the catalyzed mechanism. Our results are consistent with and provide a reliable interpretation for the experimental observations that suggest the reaction occurs by a concerted mechanism (see Humphry, T.; Iyer, S.; Iranzo, O.; Morrow, J. R.; Richard, J. P.; Paneth, P.; Hengge, A. C. J. Am. Chem. Soc. 2008, 130, 17858-17866) and has a specific-base catalysis character (see Yang, M.-Y.; Iranzo, O.; Richard, J. P.; Morrow, J. R. J. Am. Chem. Soc. 2005, 127, 1064-1065).