Kinetic Hydrogen Isotope Effects Research Articles

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Substituent effects in cyanoborohydride reductions of heterocyclic aromatic cations

First and second wave E½redn potentials have been measured by cyclic voltammetry for a set of fifteen 2,6-diphenyl-4-(X-phenyl)-pyrylium, -thiopyrylium and -N-methylpyridinium tetrafluoroborates, with X =p-CH3, -H, m-Cl, p-CF3 and m,m˙-(CF3)2. There are linear correlations between Hammett substituent constants and scaled potentials (E/0.059 15) giving slopes of ρ= 2.59 ± 0.06 (r2= 0.9984), 2.38 ± 0.07 (r2= 0.9973) and 3.10 ± 0.16 (r2= 0.9918), respectively, for the first wave potentials.Products, rates and kinetic hydrogen isotope effects for reductions of the cations with sodium and tetrabutylammonium cyanoborohydride in acetonitrile solution have been determined. Reaction of the thiopyrylium 2 (X = H) yields kinetically controlled 54 : 46 mixture of the 4H- and 2H-thiopyrans. With the pyrylium ion 1 (X = H) the 4H- to 2H- ratio is 24 : 76 with the 2H-pyran suffering electrocyclic ring opening to 1,3,5-triphenylpentadienal. Product ratios are weakly sensitive to substituents on the 4-phenyl, with electron withdrawing groups increasing the amount of 2H-product for both thiopyryliums and pyrylium cations. Reductions are first order in cation and cyanoborohydride. For sodium cyanoborohydride and 2,4,6-triphenylpyrylium, k(25 °C)= 65.6 dm3 mol–1 s–1 and for 2,4,6-triphenylthiopyrylium, k(25 °C)= 16.7 dm3 mol–1 s–1, with kinetic isotope effects (kBH3CN/kBD3CN) of 1.17 and 1.68, respectively. Logarithms of rate constants correlate with Hammett constants giving ρ= 1.46 ± 0.06 (r2= 0.9951) for the pyrylium series and ρ= 1.12 ± 0.08 (r2= 0.9831) for the thiopyryliums. The N-methylpyridinium ions do not react with cyanoborohydride.The X-ray crystal structure of 2,4,6-triphenyl-N-methylpyridinium tetrafluoroborate has been determined. Dihedral angles about bonds from the pyridinium ring to the 2- and 6-phenyls are larger than corresponding angles in 2,4,6-triphenylpyrylium and 2,4,6-triphenylthiopyryliums and are ascribed to relief of non-bonded interactions between the 2- and 6-phenyls and the N-methyl group.

Interpretations of cytochrome P450 mechanisms from kinetic studies

The catalytic mechanism of cytochrome P450 (P450) enzymes has generally been understood in terms of a classic cycle in which electron donation is often limiting and catalysis is understood in terms of hydrogen abstraction and rapid oxygen rebound. In the course of detailed investigations with kinetic hydrogen isotope effects we have studied two systems in which somewhat unusual isotope effects have been interpreted in terms of modifications of the general paradigm. The low isotope effects observed for N-demethylation reactions are in contrast to high values seen with P450-catalyzed C-hydroxylation and peroxidase-catalyzed N-demethylation and are consonant with a role for the P450 FeO 2+ entity in base-catalyzed deprotonation of an aminium radical. With P450 2E1, kinetic deuterium isotope effects are seen on the apparent K m for the substrate (increased) but not on V max. The results are interpreted in terms of a mechanism where CH bond cleavage is sensitive to deuterium substitution but a step following this is rate-limiting. This step may be product release.

Radical cation intermediates in N-dealkylation reactions.

1. A number of mechanistic possibilities exist for P450-catalysed N-dealkylation and have been considered over the years, including C- and N-hydroxylation and sequential electron transfer (SET). With peroxidases the evidence strongly favours SET and free radicals can be detected. Any mechanism must account for lack of incorporation of label from H218O into product by P450s and the high kinetic deuterium isotope effects that are seen in N-dealkylation reactions catalysed by peroxidases but not P450s. 2. Several lines of evidence support a role for SET in P450 amine oxidations, including Hammett analysis, products of dihydropyridine oxidations, and products of mechanism-based inhibition by strained cycloalkylamines. 3. The hypothesis was considered that the P450s act via base catalysis to deprotonate the aminium radical generated by SET, since the pKa has been estimated to be approximately 9. Dihydropyridine aminium radicals have low pKa (< 4) and are generally considered to have considerable kinetic acidity. None of the haemoproteins under consideration (including the peroxidases and haemoglobin) showed high kinetic hydrogen isotope effects for the oxidation of [4-2H]- or [4-3H]-labelled 1,4-dihydropyridines. These results are consonant with the view that P450s catalyse the deprotonation of N,N-dialkylaniline aminium radicals. 4. Since low isotope effects were seen with biomimetic metalloporphyrin models as well as P450s, the deprotonation is attributed to the (FeO)2+ entity, expected to be a strong base, and not the apoprotein. Thus, the FeO moiety of peroxidases is shielded, consistent with evidence by others that SET occurs through the porphyrin edge. Both P450s and peroxidases catalysed the oxidative N-demethylation of aminopyrine and N,N-dimethylaminothioanisole; however, only the peroxidases generated the stable coloured aminium radicals. 5. The rates of N-demethylation of variously para-substituted N,N-dimethylanilines can be used to undertake Hammett or Marcus analysis. The former yields rho = -0.6 and the latter an apparent E1/2 of approximately 1.8 for the formal (FeO)3+ entity of P4502B1. 6. Even in the oxidation of N,N-dialkylanilines, a finite rate of N-oxidation is seen (approximately 0.1% of N-dealkylation). The simplest paradigm has N-oxygenation and N-dealkylation both proceeding from a common aminium radical intermediate.

Light-catalysed reduction of tris(trimethylsilyl)methyl iodide by alcohols. A radical-chain process showing a large kinetic hydrogen isotope effect

The substituted-alkyl iodide (Me3Si)3Cl (Tsil) reacted with MeOH in the presence of AgO3SCF3, NaOMe, or (less readily) NaOAc under ordinary laboratory light to give (Me3Si)3CH (TsiH). Such reduction also took place readily in the presence of the corresponding alkoxide in EtOH or PriOH (in which a second organosilicon product was formed) and markedly less readily in ButOH (in which the second product dominated). In the presence of AgO3SCF3, under conditions in which MeOH and EtOH react readily, reaction was markedly slower with PriOH and did not take place with ButOH, ButCH2OH or PhCH2OH, and the (slow) reaction with CF3CH2OH gave a different product. The reaction in MeOH in the presence of AgO3SCF3, NaOMe, or NaOAc was inhibited by CCl4, PhMe or PhCH2OH. In CH3OD in the presence of NaOMe or AgO3SCF3 only undeuteriated TsiH was formed, and in CD3OD the reaction was very slow. Reactions in CD3OD–CH3OD mixtures showed that the kinetic isotope effect for this hydrogen atom abstraction was ca. 19 with NaOMe and ca. 22 with AgO3SCF3 present. The reactions with MeOH appear to involve a radical chain process in which Tsi˙ and I˙ are formed by homolytic cleavage of the Tsi–I bond, with the Tsi˙ radicals then abstracting H atoms from the CH3 group of CH3OH to give ˙CH2OH radicals, which abstract I˙ from Tsil to regenerate the Tsi˙ radicals. It is suggested that the function of NaOMe and AgO3CF3 is to remove I2, which otherwise acts as an effective radical trap.

Base catalysed photochemically induced hydrogen isotope exchange in o‐methylbenzophenone

AbstractPhotochemically induced hydrogen–deuterium and hydrogen–tritium exchange between o‐methylbenzophenone and labelled methanol is shown to be catalysed by sodium carbonate. Both the (E) and (Z)‐photoenols participate in the exchange reaction whereas in absence of the catalyst only the (E)‐photoenol is reactive. Analysis of the measured quantum yields for hydrogen isotope exchange reaction allowed the determination of the relative population of transient photoenols and to its comparison with that obtained from the flash photolysis experiments. Both hydrogen kinetic and solvent isotope effects are discussed.

ChemInform Abstract: Mechanism of the Reaction of Methylene with Benzene: A Study of Kinetic Hydrogen Isotope Effects and Theoretical Calculations

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Mechanism of the reaction of methylene with benzene: a study of kinetic hydrogen isotope effects and theoretical calculations

The reaction mechanism of singlet and triplet methylene with benzene and related aromatic compounds was investigated by kinetic isotope effects (KIEs), solvents effects, and product studies. The results are further rationalized by a series of ab initio calculations at MP2/6-31G * //RHF/6-31G * and UMP2/6-31G * //UHF/6-31G * levels of theory. The proposed 1c intermediate for the triplet reaction was found by means of the calculations, whereas no singlet analog 1 could be found

Evidence for specific base catalysis in N-dealkylation reactions catalyzed by cytochrome P450 and chloroperoxidase. Differences in rates of deprotonation of aminium radicals as an explanation for high kinetic hydrogen isotope effects observed with peroxidases.

Alkylamines are oxidized by a number of different types of enzymes, and the low oxidation potentials favor 1-electron transfer processes and aminium radicals. We studied the mechanism of N-dealkylation by hemoproteins using the prototypic substrate N,N-dimethylaniline (with isotopic substitution on the methyl groups), since there were considerable data available from kinetic deuterium isotope studies suggesting alternative mechanisms for different hemoproteins (Miwa, G.T., Walsh, J.S., Kedderis, G.L., and Hollenberg, P. F. (1983) J. Biol. Chem. 258, 14445-14449). Cytochrome P450 2B1 (P450), chloroperoxidase, and several biomimetic metalloporphyrin systems showed low kinetic hydrogen isotope effects (D(V/K)intra < or = 2.0, T(V/K) < or = 3.4), and hemoglobin, horseradish peroxidase, and prostaglandin H synthase yielded high isotope effects (D(V parallel K)intra > or = 3.5, T(V/K) > or = 8.1), in agreement with previous studies. Dinnocenzo and Banach (Dinnocenzo, J. P., and Banach, T. E. (1989) J. Am. Chem. Soc. 111, 8646-8653) have provided evidence that the pKa for an alpha-hydrogen of the N,N-dimethylaniline aminium radical is approximately 9 and also estimated that the pKa for the 4-hydrogen of a 1,4-dihydropyridine aminium radical is approximately 3.5. The oxidations of two model 1,4-dihydropyridines showed only low kinetic hydrogen isotope effects with all of the enzymes examined (D(V/K) < or = 2.3, T(V/K) < or = 2.8). Aminium radicals derived from aminopyrine and N,N-dimethylthioanisole accumulated only with those hemoprotein systems showing the high isotope effects with N,N-dimethylaniline. We conclude that specific base catalysis of alkyl hydrogen removal from aminium radicals by the (Fe.O)2+ complex is a feature of some hemoproteins, including P450s, and that the lack of such catalysis in other hemoproteins is the basis of their high kinetic hydrogen isotope effects.

Nitration and oxidation of substituted benzaldehydes in solutions of dinitrogen pentaoxide or nitronium trifluoromethanesulfonate in nitric acid

The competition between oxidation to the corresponding benzoic acid and nitration in the aromatic ring of some substituted benzaldehydes has been probed by kinetic and product studies. Nitrodeformylation was not detected. 4-Carboxybenzaldehyde is nitrated but nitro- and dinitro-substituted benzaldehydes are predominantly oxidised. Rapid reversible gem-dinitrate formation occurs in concentrated solutions of dinitrogen pentaoxide in nitric acid, and the equilibrium extent of formation of dinitrato-(4-nitrophenyl)methane from 4-nitrobenzaldehyde is reported. Both species are oxidised in processes in which α-hydrogen loss is at least partially rate determining, as shown by kinetic hydrogen isotope effects. There is evidence for the intrusion of radical mechanisms in concentrated solutions of dinitrogen pentaoxide.

Nitrosation by alkyl nitrites. Part 7. Comparison with thionitrites: reactions with phenols

Both isopentyl nitrite (lPN) and S-nitroso-N-acetylpenicillamine (SNAP) react with phenol in the pH range 2–9 to give overwhelmingly 4-nitrosophenol. For IPN the reaction is first order in both IPN and phenol and the measured rate constant is much reduced by the addition of isopentyl alcohol, suggesting that reaction occurs by prior hydrolysis of IPN to nitrous acid. When allowance is made for the nitrous acid ionisation we find no acid catalysis in the pH range 3.62–5.25 but acid catalysis at higher acidities. The corrected rate constant also increases with pH at pH values greater than 6. There is no bromide ion catalysis at pH 4 but a substantial kinetic primary hydrogen isotope effect (using [2H6] phenol) of 4.0 at pH 2.55 which decreases towards 1 as the pH is increased. All of these results are consistent with rapid and reversible hydrolysis of IPN, and phenol nitrosation results from the nitrous acid produced. All of the experimental features parallel those found in the nitrous acid nitrosation of phenol. The kinetic pattern for the reaction of SNAP with phenol is quite different, showing autocatalytic features. We have obtained an EPR spectrum from the photolysis of SNAP, characteristic of RS˙, and for the phenoxy radical derived from the reaction mixture of SNAP with 2,6-di-tert-butyl-4-methylphenol at pH 7. We propose a radical mechanism for the reaction of SNAP with phenol involving hydrogen-atom abstraction by the RS˙ radical and subsequent reaction of the phenoxy radical with nitric oxide formed by homolysis of SNAP.

Low kinetic hydrogen isotope effects in the dehydrogenation of 1,4-dihydro-2,6-dimethyl-4-(2-nitrophenyl)-3,5-pyridinedicarboxylic acid dimethyl ester (nifedipine) by cytochrome P-450 enzymes are consistent with an electron/proton/electron transfer mechanism

Cytochrome P-450 enzymes have been postulated to oxidize amines through a variety of mechanisms. One of the means of distinguishing among potential pathways involves the use of kinetic hydrogen isotope effects: low isotope effects are characteristic of aminium radical mechanisms while high values are consistent with hydrogen atom abstraction, a process documented in alkane hydroxylation. Nifedipine [1,4-dihydro-2,6-dimethyl-4-(2-nitrophenyl)-3,5-pyridinedicarboxylic+ ++ acid dimethyl ester] was prepared with isotopic substitution at the 4-position and utilized in determinations of the deuterium and tritium kinetic isotope effects. The noncompetitive intermolecular deuterium [DV, D(V/K)] and competitive intermolecular tritium [T(V/K)] isotope effects observed in the oxidation of nifedipine with liver microsomes prepared from untreated male rats were less than 2. Similar low deuterium and tritium isotope effects (less than 2) were also found with six other types of rat liver microsomal preparations and with three different human liver microsomal preparations, even though the rates of oxidation (expressed per nanomole of total cytochrome P-450) varied by an order of magnitude and other investigations indicate that several different cytochrome P-450 enzymes can catalyze the reaction. Similar low deuterium and tritium isotope effects were measured with a purified rat liver cytochrome P-450 preparation and horseradish peroxidase. These results are interpreted in terms of an electron/proton/electron transfer mechanism previously postulated for the oxidation of other dihydropyridines by cytochrome P-450 enzymes and model one-electron acceptors and argue against a mechanism involving hydrogen atom abstraction from nifedipine.

Rates of hydrogen exchange and kinetic isotope effects in the reaction of p‐nitrophenylnitromethane with amine bases in toluene solution: Absence of internal return

AbstractRates of loss of tritium from p‐nitrophenylnitromethane‐α‐t in toluene solution catalyzed by a series of amine bases show that hydrogen exchange in this system occurs at the same rate as ionization of the carbon acid, and that internal return is not taking place. This substantiates a previous suggestion that very large deuterium isotope effects reported for some of these reactions and attributed to extensive tunnelling were determined under conditions where the isotopic label was being lost and are therefore artifacts. This study provides isotope effects which are generally much smaller than those reported previously, but some tunnelling may still be taking place in some of these reactions.

Time-resolved electron energy loss spectroscopy study of water desorption from Ag(011)

We present time-resolved electron energy loss spectroscopy (EELS) results for the desorption of submonolayer coverages of H2O and D2O from Ag(011). The measurements were performed isothermally and effectively at constant coverage. At coverages above a few percent of a monolayer, we observe that the desorption is characterized by zero-order kinetics. That is, the desorption rate is independent of coverage. We hypothesize that this results from the formation of two-dimensional water islands on the surface at all coverages and temperatures measured. The desorption process is governed in part by the dissociation of water from the edges of these islands. Rapid diffusion on the surface maintains a quasiequilibrium between the molecules bound to islands and isolated adsorbed water molecules, thereby leading to pseudo-zero-order kinetics. This result is interpreted upon general thermodynamic grounds, and also using a simple kinetic model which leads to pseudo-zero-order desorption kinetics. An unusual hydrogen kinetic isotope effect is reported which lends support to our interpretation.

Lack of kinetic hydrogen isotope effect in acetylene pyrolysis

AbstractSecond order rate constants for C2H2 or C2D2 polymerizations into vinylacetylene and higher CnHn products have been measured in a static reactor by dynamic mass spectrometry between 770–980 K. They are nearly identical within experimental error (±50%). It is shown that these results are consistent with the participation of thermally equilibrated vinylidene H2C  C: as a reactive intermediate: equation image since this assumption only introduces a modest reverse equilibrium isotope effect (KiH/KiD ca. 0.48 in this range) into overall rate constants. At the same time they seem to discriminate in general against alternative mechanisms in which the required H‐atom transfers take place in rate determining steps. Present evidence, in conjunction with an updated analysis of relevant issues such as experimental and theoretical vs. termochemical estimates of the heat of formation of H2CC:, the nature of the transition states of singlet vinylidene addition reactions and the likelihood of discrete biradical intermediates in C2H2 dimerization, seems to lend further support to the notion that acetylene behaves as a singlet carbene at high temperatures.

ChemInform Abstract: Selective Cis‐Trans Isomerization of a Tetraene via an Allene Intermediate: Geometric Isomerism via an sp2 to sp2 Sigmatropic Hydrogen Shift and Kinetic Isotope Effect Studies.

AbstractThe kinetics of the (1,5) sigmatropic hydrogen shift (shs) of vinylallene (II) to trienes (III) and (V) (which interconvert with one another by an unknown mechanism) and of the (1,7) shs of dienallene (VII) to tetraenes (VIII) and (IX) are determined.

Kinetic isotope effect studies on aspartate aminotransferase: evidence for a concerted 1,3 prototropic shift mechanism for the cytoplasmic isozyme and L-aspartate and dichotomy in mechanism

The C alpha primary hydrogen kinetic isotope effects (C alpha-KIEs) for the reaction of the cytoplasmic isozyme of aspartate aminotransferase (cAATase) with [alpha-2H]-L-aspartate are small and only slightly affected by deuterium oxide solvent (DV = 1.43 +/- 0.03 and DV/KAsp = 1.36 +/- 0.04 in H2O; DV = 1.44 +/- 0.01 and DV/KAsp = 1.61 +/- 0.06 in D2O). The D2O solvent KIEs (SKIEs) are somewhat larger and are essentially independent of deuterium at C alpha (D2OV = 2.21 +/- 0.07 and D2OV/KAsp = 1.70 +/- 0.03 with [alpha-1H]-L-aspartate; D2OV = 2.34 +/- 0.12 and D2OV/KAsp = 1.82 +/- 0.06 with [alpha-2H]-L- aspartate). The C alpha-KIEs on V and on V/KAsp are independent of pH from pH 5.0 to pH 10.0. These results support a rate-determining concerted 1,3 prototropic shift mechanism by the multiple KIE criteria [Hermes, J. D., Roeske, C. A., O'Leary, M. H., & Cleland, W. W. (1982) Biochemistry 21, 5106]. The large C alpha-KIEs for the reaction of mitochondrial AATase (mAATase) with L-glutamate (DV = 1.88 +/- 0.13 and DV/KGlu = 3.80 +/- 0.43 in H2O; DV = 1.57 +/- 0.05 and DV/KGlu = 4.21 +/- 0.19 in D2O) coupled with the relatively small SKIEs (D2OV = 1.58 +/- 0.04 and D2OV/KGlu = 1.25 +/- 0.05 with [alpha-1H]-L-glutamate; D2OV = 1.46 +/- 0.06 and D2OV/KGlu = 1.16 +/- 0.05 with [alpha-2H]-L-glutamate) are most consistent with a two-step mechanism for the 1,3 prototropic shift for this isozyme-substrate pair.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective cis-trans isomerization of a tetraene via an allene intermediate. Geometric isomerism via an sp2 to sp2 sigmatropic hydrogen shift and kinetic isotope effect studies

A kinetic investigation in the unlabeled and deuterium-labeled series of the (1,5) sigmatropic hydrogen shift of vinylallene 5 to trienes 7 plus 8 and the (1,7) sigmatropic hydrogen shift of dienallene 6 to tetraenes 9 plus 10 has been completed. The results lead to the proposal that the pathway for geometric isomerization of the terminal double bond of tetraene 9 involves as an intermediate the allene 6. The allene 6 could in fact be isolated in the case of the isomerization of 9, and moreover, at long reaction times 6 and 9 are transformed to 10. For the isomerization of 5 to 7 plus 8, it was shown that 7 and 8 interconvert with one another, but it is unclear whether allene 5 is an intermediate in their interconversion. The temperature dependencies of the primary deuterium kinetic isotope effects observed for the rearrangement of 5 (via a (1,5) sigmatropic hydrogen shift) and 6 (via the corresponding (1,7)-shift) suggest a tunneling contribution.

Kinetics and mechanism of the cyclisation of 2′,6′-dihydroxychalcone and derivatives

pH–Rate profiles are reported for the cyclisation in water to 5-hydroxyflavanones of 2′,6′-dihydroxychalcone (1) and its 4-methoxy (2), 3,4-dimethoxy (3), 3,4,5-trimethoxy (4), 2,4,6-trimethoxy (5), 4-chloro (6), and 3,4,4′-trimethoxy (8) derivatives. As for the previously studied 2′,6′-dihydroxy-4,4′-dimethoxychalcone (7), rate coefficients are established for acid-catalysed cyclisation of neutral chalcone, for unimolecular cyclisation of the neutral, monoanionic, and dianionic chalcone, and for the base-catalysed reverse ring-opening reaction. Cyclisation of the monoanion of 2′,6′-dihydroxychalcone is almost 40 times faster than that of the monoanion of the 2′-hydroxy-6′-methoxychalcone (10) and is also estimated to be about ten times faster than that of the reactive monoanion of 2′,4′-dihydroxychalcone. These are the first calculations of the enhancement of rate of monoanion cyclisation by the 6′-OH group. The effect is only small, and is suggested to arise largely from stabilisation of the transition state for ketonisation by hydrogen bonding to enolate oxygen. Other reactivity differences amongst the chalcone monoanions are also discussed. Enthalpy and entropy of activation data are reported for monoanion cyclisation of (1), (2), and (4)–(6). Rate coefficients for the cyclisation of the chalcone monoanions are almost identical for (1)–(4) and (6) in water but not in deuterium oxide: kinetic hydrogen isotope effect (KIE) values are 3.4 (1), 5.7 (2), 4.9 (3), 3.0 (4), 7.5 (5), 2.9 (6), and 5.0 (8). For chalcones (2) and (7), the KIE values of which are both 5.7, the amounts of H versus D incorporation at the flavanone 3-carbon for monoanion cyclisation in H2O/D2O mixtures were established by mass spectroscopy. This gave product (or ‘discrimination’) isotope effect (PIE) values of 7.9 for (2) and 3.8 for (7), suggesting for (2) but not (7) an inverse isotope effect contribution to KIE from sources other than rate-limiting proton transfer to carbon. Monoanion cyclisation of (1) in D2O was established by 1H n.m.r. as involving almost equal amounts of anti and syn addition of 2′-O– to the enone double bond. Reactivity differences amongst the chalcones for reactions other than monoanion cyclisation are only briefly considered.

Primary and secondary kinetic hydrogen isotope effects in the solution and gas phase decomposition of t-alkoxides

Primary and secondary kinetic hydrogen isotope effects (k.i.e.s) in the elimination of toluene from the alkoxide of 1,2,3-triphenylpropan-2-ol have been determined and are consistent with rate limiting proton transfer in gas phase reaction and with rate limiting carbon–carbon bond cleavage in reaction in dimethyl sulphoxide (DMSO) solution.