- Heterogeneous 1H and 13C Parahydrogen-Induced Polarization of Acetate and Pyruvate Esters
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Magnetic resonance imaging of [1-13C]hyperpolarized carboxylates (most notably, [1-13C]pyruvate) allows one to visualize abnormal metabolism in tumors and other pathologies. Herein, we investigate the efficiency of 1H and 13C hyperpolarization of acetate and pyruvate esters with ethyl, propyl and allyl alcoholic moieties using heterogeneous hydrogenation of corresponding vinyl, allyl and propargyl precursors in isotopically unlabeled and 1-13C-enriched forms with parahydrogen over Rh/TiO2 catalysts in methanol-d4 and in D2O. The maximum obtained 1H polarization was 0.6±0.2 % (for propyl acetate in CD3OD), while the highest 13C polarization was 0.10±0.03 % (for ethyl acetate in CD3OD). Hyperpolarization of acetate esters surpassed that of pyruvates, while esters with a triple carbon-carbon bond in unsaturated alcoholic moiety were less efficient as parahydrogen-induced polarization precursors than esters with a double bond. Among the compounds studied, the maximum 1H and 13C NMR signal intensities were observed for propyl acetate. Ethyl acetate yielded slightly less intense NMR signals which were dramatically greater than those of other esters under study.
- Salnikov, Oleg G.,Chukanov, Nikita V.,Kovtunova, Larisa M.,Bukhtiyarov, Valerii I.,Kovtunov, Kirill V.,Shchepin, Roman V.,Koptyug, Igor V.,Chekmenev, Eduard Y.
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p. 1389 - 1396
(2021/05/31)
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- Study of the mechanism for the hydrolysis of alkoxy(aryl)(phenyl)-λ6- sulfanenitriles, ArPhS(OR)(?N)
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The hydrolysis of alkoxy(aryl)(phenyl)-λ6-sulfanenitriles in several buffer solutions was found to follow a good pseudo-first-order kinetic equation, giving the corresponding sulfoximides and alcohols (for the case of the hydrolysis of neopentyloxy-λ6-sulfanenitrile, giving a rearranged product, 2-methyl-2-butanol). The dependence of the rate of hydrolysis on the structure of the alkyl group showed the opposite trend to the usual S(N)2 character, i.e. Me +] at pH more than 6.08, and trends to saturate at low pH. According to these kinetic results, a two-step reaction mechanism was proposed which involves a pre-equilibrium protonation on the nitrogen atom of the alkoxy-λ6- sulfanenitriles, followed by a rate-determining C-O bond cleavage via an S(N)2 or S(N)1 mechanism on the alkyl carbon atom depending on the structure of the alkyl group. From a double-reciprocal plot of 1/k(obs) vs. 1/[H+], the pK(a) value and the rate constant of the second reaction of neopentyloxy(diphenyl)-λ6-sulfanenitrile were estimated to be 5.02 and 7.02x10-3 s-1, respectively. The substituent effects on the phenyl group of neopentyloxy(diphenyl)-λ6-sulfanenitrile afforded a large negative p- value (-1.88) for pK(a) and positive one (+1.66) for the second reaction at 25.2 °C. The small negative p-values observed at pH 6.27 for diphenyl(propoxy)-λ6-sulfanenitrile (-0.42) and neopentyloxy(diphenyl)- λ6-sulfanenitrile (-0.26) were found to be the results of a cancellation of those for the opposite trend of the reactions of the pre-equilibrium and the second step. The activation parameters for both the pre-equilibrium and the subsequent reactions were also estimated based on the parameters for the hydrolysis of neopentyloxy(diphenyl)-λ6-sulfanenitrile at pH 6.22 and 2.99. The buffer effect is due to a nucleophilic attack of the buffer base to the alkyl carbon atom of the protonated alkoxy-λ6-sulfanenitriles. The sulfoximide moiety in the protonated λ6-sulfanenitrile is revealed to be a very good leaving group.
- Yoshimura, Toshiaki,Dong, Tiaoling,Fujii, Takayoshi,Ohkubo, Masanori,Sakuta, Mikiko,Wakai, Youko,Ono, Shin,Morita, Hiroyuki,Shimasaki, Choichiro
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p. 957 - 965
(2007/10/03)
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- Surface-Mediated Isomerization and Oxidation of Allyl Alcohol on Cu(110)
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Allyl alcohol reacts with clean and oxygen-covered Cu(110) surfaces to produce propanal, acrolein, n-propyl alcohol, and hydrogen under ultrahigh-vacuum conditions.Very small amounts of propylene and water are also formed.This pattern of reactivity contrasts sharply to the selective oxidation to acrolein observed on Ag(110).On the clean Cu(110) surface allyl alcohol undergoes O-H cleavage to form the surface alkoxide CH2=CHCH2O(a) and H(a).The results suggest that the olefin in this species undergoes partial hydrogenation to the surface-bound oxametallacycles (-CH2CH2CH2O-)(a) and (-CH-(CH3)CH2O-)(a) and complete hydrogenation to CH3CH2CH2O(a).Propanal forms at 320 K via further reaction of these oxametallacycles.Evidence for a ?-bonded allyl oxide CH2=CHCH2O(a), which is more stable than n-propoxide (CH3CH2CH2O(a)) toward β-hydride elimination, is presented.This allyl oxide decomposes at 370 K to form acrolein.The interaction of the double bond with the surface apparently restricts the interaction of the β-C-H bond with the surface and increases the stability of this species.Propanal, acrolein and H2 are formed at 435 K by a process thought to involve the thermal decomposition of (-CH2CH2CH2O-)(a) by a β-hydride elimination pathway.This dehydrogenation pathway exhibits an activation energy 8 kcal/mol greater than for acyclic alkoxides.The conversion of allyl alcohol to propanal and propyl alcohol obviously involves the hydrogenation of the double bond which, by comparison, does not occur for propylene coadsorbed with hydrogen under similar conditions on this surface.Clearly, the hydroxyl group in allyl alcohol facilitates the hydrogenation of its olefin group by tethering the doublebond to the surface at temperatures higher than the normal desorption temperature of olefins.
- Brainard, Robert L.,Peterson, Cynthia G.,Madix, Robert J.
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p. 4553 - 4561
(2007/10/02)
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- KINETIC ISOTOPE EFFECT IN THE OXIDATION OF CYCLIC AND LINEAR ACETALS WITH OZONE AND MOLECULAR OXYGEN
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Substitution of hydrogen atoms at the acetal carbon by deuterium in 1,3-dioxolane, 1,3-dioxane, and dipropoxymethane leads to a decrease in the rate constant of ozonization and oxidation.The magnitude of the kinetic isotope effect lies in the range of 2-3 and is somewhat lower than the known values (4-6) for alcohols, ethers, and carboxylic acids.This is evidently due to the smaller extension of the C-H bond in the transition state of the reaction of acetals with ozone molecules and peroxide radicals.
- Kuramshin, E. M.,Sadaeva, R. Kh.,Gumerova, V. K.,Zlot-skii, S. S.,Rakhmankulov, D. L.
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p. 136 - 139
(2007/10/02)
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