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Kirill I. Zamaraev - One of the best experts on this subject based on the ideXlab platform.
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Carbenium Ion properties of octene 1 adsorbed on zeolite h zsm 5
Catalysis Letters, 1994Co-Authors: Alexander G. Stepanov, V. N. Romannikov, Mikhail V Luzgin, Kirill I. ZamaraevAbstract:It is shown that octene-1 adsorbed on zeolite H-ZSM-5 at ambient temperature exhibits Carbenium Ion properties. Namely: (1) According to2H NMR, the proton of the acidic ≡Al-OH-Si≡ group of the zeolite is transferred into the CH2= group of the octene-1 molecule. (2) According to13C NMR the13C label inserted into the terminal CH2= group of the octene-1 molecule is scrambled over its hydrocarbon skeleton. Thermodynamic and kinetic parameters for carbon scrambling are measured within the temperature range 290–343 K. The zeolite framework is shown to favour the formatIon of the linear rather than branched CarbeniumIon.
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^13C solid state NMR evidence for the existence of isobutyl Carbenium Ion in the reactIon of isobutyl alcohol dehydratIon in H-ZSM-5 zeolite
Catalysis Letters, 1993Co-Authors: Alexander G. Stepanov, Kirill I. ZamaraevAbstract:Using two-dimensIonal J -resolved and CP/MAS^13C NMR, the pathway for the transfer of the^13C label from the CH_2 group of isobutyl alcohol into the hydrocarbon skeleton of butene oligomers has been elucidated in the course of isobutyl alcohol dehydratIon inside H-ZSM-5 zeolite. First, the label is transferred selectively into the CH_2 group of the isobutyl silyl ether reactIon intermediate (IBSE), and then into the CH and CH_3 groups of the isobutyl fragment (-CH_2CH(CH_3)_2) of IBSE and/or butene oligomers. Finally, it is scrambled over the carbon skeleton of the oligomers. The obtained data suggest that isobutyl Carbenium Ion is formed as a reactIon intermediate or transitIon state during the transformatIon of isobutyl silyl ether into butene oligomers.
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13c cp mas nmr study of isobutyl alcohol dehydratIon on h zsm 5 zeolite evidence for the formatIon of stable isobutyl silyl ether intermediate
Catalysis Letters, 1992Co-Authors: Alexander G. Stepanov, V. N. Romannikov, Kirill I. ZamaraevAbstract:DehydratIon of isobutyl alcohol selectively labelled with a13C nucleus in the CH2 group (i-BuOH[1−13C]) has been studied on H-ZSM-5 zeolite within the temperature range 296–448 K using13C CP/MAS NMR. The formatIon of the isobutyl silyl ether intermediate (IBSE) has been detected. It is stable below 398 K. Within the temperature range 398–423 K IBSE decomposes gradually to produce first a butene dimer, probably 2,5-dimethyl-l-hexene and then other butene dimers and oligomers. AtT > 423 K scrambling of the selectively labelled carbon of the initial dimeric product over various positIons in the carbon skeleton of the final dimers (oligomers) is observed. This is explained in terms of the formatIon of Carbenium Ion as the reactIon intermediate.
James F Haw - One of the best experts on this subject based on the ideXlab platform.
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a persistent Carbenium Ion on the methanol to olefin catalyst hsapo 34 acetone shows the way
Journal of Physical Chemistry B, 2001Co-Authors: Weiguo Song, John Nicholas B And, James F HawAbstract:In situ 13C NMR with both CAVERN and pulse−quench methods was used to understand the chemistry of acetone on the silico-aluminophosphate catalyst HSAPO-34. The isotropic shift of [2-13C]acetone shows that most of the acid sites on this catalyst are weaker than those on aluminosilicate zeolites, but a minority site was resolved with a shift higher than that on zeolite HZSM-5. At elevated temperatures, acetone dimerized to diacetone alcohol and dehydrated to mesityl oxide. Mesityl oxide cracked in the presence of water to acetic acid and isobutylene. TrimerizatIon of butenes formed a significant amount of a persistent Carbenium Ion with unusual spectroscopic properties, and this is proposed to be the heptamethylcyclopentenyl catIon, the first observatIon of a persistent Carbenium Ion on a SAPO catalyst. Chemical shift calculatIons at the GIAO-MP2 level revealed a discrepancy for one signal of the proposed catIon but ruled out an alternative assignment. Methylaromatic formatIon coincided with cyclopentenyl c...
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acid base chemistry of a Carbenium Ion in a zeolite under equilibrium conditIons verificatIon of a theoretical explanatIon of Carbenium Ion stability
Journal of the American Chemical Society, 2001Co-Authors: Weiguo Song, John Nicholas B And, James F HawAbstract:The 1,3-dimethylcyclopentenyl Carbenium Ion (C7H11+) was reproducibly prepared on zeolite HZSM-5 using a pulse-quench reactor, and then each of a number of bases was coadsorbed into the catalyst channels to either compete with the catIon for protonatIon or to possibly react with it as a nucleophile. For seven bases with proton affinities (PA) between 142 and 212.1 kcal/mol, there was no reactIon with C7H11+. CoadsorptIon of smaller amounts of dimethylacetamide (PA = 217 kcal/mol) also produced no reactIon, but with a higher loading, a proton was transferred from the Carbenium Ion to the base to leave 1,3-dimethylcyclopenta-1,3-diene in the zeolite as a neutral olefin. DeprotonatIon was the primary reactIon with coadsorptIon of either pyridine (PA = 222 kcal/mol) or trimethylphosphine (PA = 229.2 kcal/mol). The estimated experimental deprotonatIon enthalpy for C7H11+, ∼217 kcal/mol in the zeolite, is in excellent agreement with MP4/6-311G* gas-phase value of 215.6 kcal/mol. CoadsorptIon of either NH3 (PA =...
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nmr observatIon of indanyl Carbenium Ion intermediates in the reactIons of hydrocarbons on acidic zeolites
Journal of the American Chemical Society, 1994Co-Authors: James F HawAbstract:In situ MAS-NMR was used to study the reactIons of styrene and methylstyrene on the acidic zeolites HZSM-5, HY, and H-mordenite. The results are introduced and discussed in the context of our recently proposed reclassificatIon of typical zeolites as strong acids rather than superacids. We also show that the observatIon of indanyl catIons but not simply phenylCarbenium Ions as persistent species in high concentratIon is an expected consequence of the relative acid strengths of the zeolites and Carbenium Ions. 55 refs., 8 figs., 2 tabs.
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cyclopentenyl Carbenium Ion formatIon in acidic zeolites an in situ nmr study of cyclic precursors
Journal of the American Chemical Society, 1994Co-Authors: James F HawAbstract:Previous in situ NMR studies of the reactIons of propene and ethylene on zeolites have claimed the formatIon of small amounts of cyclopentenyl Carbenium Ions. The present investigatIon clearly demonstrates the formatIon of these Ions in high yield from cyclic precursors on acidic zeolites. In situ {sup 13}C solid-state NMR experiments are reported for the reactIons of cyclopentene, cyclopentanol, cyclohexanol, and cyclohexane on a variety of zeolites of differing acid strength. The reactIon sequence for the alcohols was dehydratIon, double-bond migratIon, oligomerizatIon, C{sub 6} to C{sub 5} ring equilibratIon, and accumulatIon of cyclopentenyl Ions. In some of the experiments up to one-third of the C{sub 5} ring units of the oligomers were positively charged. A {sup 13}C chemical shift anisotropy measurement verified that the downfield signals are due to Carbenium Ions. Ex situ identificatIon of the neutral extractable oligomers combined with comparison to model Carbenium Ion shifts permitted a plausible identificatIon of the specific Ions formed. The formatIon of products containing five-membered rings from cyclohexanol was modeled by the cyclohexane-methylcyclopentane skeletal rearrangements reactIon. This investigatIon firmly establishes the formatIon of cyclopentenyl Ions in zeolites and considers their significance in regard to reactIon mechanisms. 61 refs., 7 figs.
Alexander G. Stepanov - One of the best experts on this subject based on the ideXlab platform.
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Carbenium Ion properties of octene 1 adsorbed on zeolite h zsm 5
Catalysis Letters, 1994Co-Authors: Alexander G. Stepanov, V. N. Romannikov, Mikhail V Luzgin, Kirill I. ZamaraevAbstract:It is shown that octene-1 adsorbed on zeolite H-ZSM-5 at ambient temperature exhibits Carbenium Ion properties. Namely: (1) According to2H NMR, the proton of the acidic ≡Al-OH-Si≡ group of the zeolite is transferred into the CH2= group of the octene-1 molecule. (2) According to13C NMR the13C label inserted into the terminal CH2= group of the octene-1 molecule is scrambled over its hydrocarbon skeleton. Thermodynamic and kinetic parameters for carbon scrambling are measured within the temperature range 290–343 K. The zeolite framework is shown to favour the formatIon of the linear rather than branched CarbeniumIon.
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^13C solid state NMR evidence for the existence of isobutyl Carbenium Ion in the reactIon of isobutyl alcohol dehydratIon in H-ZSM-5 zeolite
Catalysis Letters, 1993Co-Authors: Alexander G. Stepanov, Kirill I. ZamaraevAbstract:Using two-dimensIonal J -resolved and CP/MAS^13C NMR, the pathway for the transfer of the^13C label from the CH_2 group of isobutyl alcohol into the hydrocarbon skeleton of butene oligomers has been elucidated in the course of isobutyl alcohol dehydratIon inside H-ZSM-5 zeolite. First, the label is transferred selectively into the CH_2 group of the isobutyl silyl ether reactIon intermediate (IBSE), and then into the CH and CH_3 groups of the isobutyl fragment (-CH_2CH(CH_3)_2) of IBSE and/or butene oligomers. Finally, it is scrambled over the carbon skeleton of the oligomers. The obtained data suggest that isobutyl Carbenium Ion is formed as a reactIon intermediate or transitIon state during the transformatIon of isobutyl silyl ether into butene oligomers.
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13c cp mas nmr study of isobutyl alcohol dehydratIon on h zsm 5 zeolite evidence for the formatIon of stable isobutyl silyl ether intermediate
Catalysis Letters, 1992Co-Authors: Alexander G. Stepanov, V. N. Romannikov, Kirill I. ZamaraevAbstract:DehydratIon of isobutyl alcohol selectively labelled with a13C nucleus in the CH2 group (i-BuOH[1−13C]) has been studied on H-ZSM-5 zeolite within the temperature range 296–448 K using13C CP/MAS NMR. The formatIon of the isobutyl silyl ether intermediate (IBSE) has been detected. It is stable below 398 K. Within the temperature range 398–423 K IBSE decomposes gradually to produce first a butene dimer, probably 2,5-dimethyl-l-hexene and then other butene dimers and oligomers. AtT > 423 K scrambling of the selectively labelled carbon of the initial dimeric product over various positIons in the carbon skeleton of the final dimers (oligomers) is observed. This is explained in terms of the formatIon of Carbenium Ion as the reactIon intermediate.
Norman J Oppenheimer - One of the best experts on this subject based on the ideXlab platform.
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reactIons of charged substrates 6 the methoxymethyl Carbenium Ion problem 1 a semiempirical study of the kinetic and thermodynamic stabilities of linear and cyclic oxo and thioCarbenium Ions generated from pyridinium and dimethylanilinium Ions
Journal of Organic Chemistry, 1996Co-Authors: Neil Buckley, Norman J OppenheimerAbstract:AM1-calculated energy profiles for dissociatIon of (methoxymethyl)pyridinium and dimethylanilinium Ion substrates show that the methoxymethyl Carbenium Ion is not sufficiently stable to exist as an intermediate on the reactIon coordinate for this model reactIon. [(Thiomethoxy)methyl]pyridinium Ion, however, has a distinct transitIon state because of the stability of the resulting Ion−neutral complex. The complete potential energy surfaces for water displacement on the methoxymethyl substrate with either pyridine or dimethylaniline as the leaving group show distinct transitIon states and very flat surfaces for the Ion−neutral complexes in which interactIon of the Carbenium Ion with both leaving group and nucleophile is stabilizing. Secondary systems studied, including linear methoxy and thiomethoxy substrates, 5- and 6-membered cyclic oxo and thio substrates, and ribosyl-, xylopyranosyl-, and glucopyranosylpyridinium Ions yield Ion−neutral complexes with sufficient intrinsic stability to exist as intermedi...
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reactIons of charged substrates 6 the methoxymethyl Carbenium Ion problem 1 a semiempirical study of the kinetic and thermodynamic stabilities of linear and cyclic oxo and thioCarbenium Ions generated from pyridinium and dimethylanilinium Ions
Journal of Organic Chemistry, 1996Co-Authors: Neil Buckley, Norman J OppenheimerAbstract:AM1-calculated energy profiles for dissociatIon of (methoxymethyl)pyridinium and dimethylanilinium Ion substrates show that the methoxymethyl Carbenium Ion is not sufficiently stable to exist as an intermediate on the reactIon coordinate for this model reactIon. [(Thiomethoxy)methyl]pyridinium Ion, however, has a distinct transitIon state because of the stability of the resulting Ion-neutral complex. The complete potential energy surfaces for water displacement on the methoxymethyl substrate with either pyridine or dimethylaniline as the leaving group show distinct transitIon states and very flat surfaces for the Ion-neutral complexes in which interactIon of the Carbenium Ion with both leaving group and nucleophile is stabilizing. Secondary systems studied, including linear methoxy and thiomethoxy substrates, 5- and 6-membered cyclic oxo and thio substrates, and ribosyl-, xylopyranosyl-, and glucopyranosylpyridinium Ions yield Ion-neutral complexes with sufficient intrinsic stability to exist as intermediates. Comparison with solutIon data, primarily activatIon entropy and Bronsted coefficients, suggests that the sugar oxoCarbenium Ions, either as distinct, solvent-equilibrated intermediates or elements of Ion-neutral complexes, are formed by unimolecular dissociatIon of the respective substrates in solutIon.
Daniel D. Oprian - One of the best experts on this subject based on the ideXlab platform.
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Mechanism Underlying Anti-Markovnikov AdditIon in the ReactIon of Pentalenene Synthase.
Biochemistry, 2020Co-Authors: Jason O. Matos, Ramasamy P. Kumar, Alison C., Mackenzie Patterson, Isaac J. Krauss, Daniel D. OprianAbstract:Most terpene synthase reactIons follow Markovnikov rules for formatIon of high-energy Carbenium Ion intermediates. However, there are notable exceptIons. For example, pentalenene synthase (PS) unde...
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structural characterizatIon of early michaelis complexes in the reactIon catalyzed by limonene synthase from citrus sinensis using fluorinated substrate analogues
Biochemistry, 2017Co-Authors: Ramasamy P. Kumar, Jason O. Matos, Isaac J. Krauss, B R Morehouse, Karan Malik, Hongkun Lin, Daniel D. OprianAbstract:The stereochemical course of monoterpene synthase reactIons is thought to be determined early in the reactIon sequence by selective binding of distinct conformatIons of the geranyl diphosphate (GPP) substrate. We explore here formatIon of early Michaelis complexes of the (+)-limonene synthase [(+)-LS] from Citrus sinensis using monofluorinated substrate analogues 2-fluoro-GPP (FGPP) and 2-fluoroneryl diphosphate (FNPP). Both are competitive inhibitors for (+)-LS with KI values of 2.4 ± 0.5 and 39.5 ± 5.2 μM, respectively. The KI values are similar to the KM for the respective nonfluorinated substrates, indicating that fluorine does not significantly perturb binding of the ligand to the enzyme. FGPP and FNPP are also substrates, but with dramatically reduced rates (kcat values of 0.00054 ± 0.00005 and 0.00024 ± 0.00002 s–1, respectively). These data are consistent with a stepwise mechanism for (+)-LS involving IonizatIon of the allylic GPP substrate to generate a resonance-stabilized Carbenium Ion in the r...