Oxygen exchange as a function of racemization in 1-phenyl-1-ethanol. Kinetic evidence for ion-dipole pair intermediates

Merritt, Margaret V.; Bell, Stephanie J.; Cheon, H.-J.; Darlington, Jeanne A.; Dugger, T. L.; Elliott, Nancy B.; Fairbrother, Genevieve; Melendez, C. S.; Smith, Esther V.; Schwartz, P. L.

doi:10.1021/ja00165a047

Cited by 17 publications

(12 citation statements)

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“…In the preceding paper of this series, the kinetics and the dynamics of the unimolecular racemization and regioisomerization of O-protonated ( S )- trans -4-hexen-3-ol (the IS anti / IS syn mixture in Scheme ) have been investigated in the dilute gas state, i.e., in a reaction environment entirely free from the interference of solvation, ion-pairing, etc., that normally complicates analogous studies in solution. − The results are consistent with a gas-phase intramolecular racemization and regioisomerization involving the intermediacy of the hydrogen-bonded complexes B and C in Scheme , In the particular case of B and C , the moving H 2 O molecule is coplanarly coordinated to the in-plane hydrogens of the allyl cation moiety.…”

Section: Introductionmentioning

confidence: 63%

Intracomplex CH₃OH Walking around Optically Active 1-Methyl-3-ethylallyl Cations. A Gas-Phase Kinetic Study

Troiani

Speranza

1998

J. Org. Chem.

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The kinetics and the mechanism of the racemization and regioisomerization of O-methylated (S)trans-4-hexen-3-ol (IS′) or (R)-trans-3-hexen-2-ol (IIR′) have been investigated in the gas phase at 720 Torr and in the 40-120 °C temperature range. The starting oxonium intermediates were generated in the gas phase by the reaction of (CH 3 ) 2 Cl + ions, formed by stationary γ-radiolysis of bulk CH 3 Cl, on the corresponding optically active alcohols. The rate constant of the gas-phase regioisomerization of IS′ ((3.4-16.0) × 10 6 s -1 ) was found to exceed that of its racemization ((1.9-9.8) × 10 6 s -1 ) over the entire temperature range. Similar differences were observed for the regioisomerization ((2.9-15.0) × 10 6 s -1 ) and the racemization of IIR′ ((1.8-9.6) × 10 6 s -1 ). By analogy with previous experimental and theoretical evidence, these results are consistent with intramolecular racemization and regioisomerization processes involving the intermediacy of two distinct hydrogen-bonded complexes, wherein the CH 3 OH molecule is coplanarly coordinated to the in-plane hydrogens of the 1-methyl-3-ethylallyl moiety. The activation parameters for their formation from the IS′ and IIR′ were evaluated and compared with those concerning the racemization and regioisomerization of O-protonated (S)-trans-4-hexen-3-ol (IS), previously measured in the gas phase under similar experimental conditions. The comparison reveals that gas-phase racemization and regioisomerization of O-protonated (S)-trans-4-hexen-3-ol (1S′) (AOHdH 2 O) involve transition structures located early along the reaction coordinate, whereas the transition structures involved in the rearrangement of O-methylated (S)-trans-4-hexen-3-ol (1S′) and (R)trans-3-hexen-2-ol (IIR′) (AOH ) CH 3 OH) are placed later along the reaction coordinate and are characterized by a strong coordination of the moving CH 3 OH molecule with the hydrogens of the allylic moiety.

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Section: Introductionmentioning

confidence: 63%

Intracomplex CH₃OH Walking around Optically Active 1-Methyl-3-ethylallyl Cations. A Gas-Phase Kinetic Study

Troiani

Speranza

1998

J. Org. Chem.

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“…As olefin hydration hardly occurred under the applied conditions on HBEA and H 3 PO 4 , the E2-like pathways alone, with concerted C–O and C–H bond scissions, cannot explain the significant 18 O incorporation (9–17%) into cyclohexanol. With the S N 2 path for oxygen exchange between water and secondary/tertiary alcohols also ruled out 13 14 15 16 , the only possible pathway for this level of 18 O incorporation would be recombination between 18 O water and an intermediate, which is formed on C–O bond cleavage and which precedes the C β –H bond cleavage TS. This, in turn, makes the E1-type path the dominating mechanism for dehydration of cyclohexanol, regardless of whether the hydronium ion exists in homogeneous solution or localized in a pore.…”

Section: Resultsmentioning

confidence: 99%

Enhancing the catalytic activity of hydronium ions through constrained environments

et al. 2017

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The dehydration of alcohols is involved in many organic conversions but has to overcome high free-energy barriers in water. Here we demonstrate that hydronium ions confined in the nanopores of zeolite HBEA catalyse aqueous phase dehydration of cyclohexanol at a rate significantly higher than hydronium ions in water. This rate enhancement is not related to a shift in mechanism; for both cases, the dehydration of cyclohexanol occurs via an E1 mechanism with the cleavage of Cβ–H bond being rate determining. The higher activity of hydronium ions in zeolites is caused by the enhanced association between the hydronium ion and the alcohol, as well as a higher intrinsic rate constant in the constrained environments compared with water. The higher rate constant is caused by a greater entropy of activation rather than a lower enthalpy of activation. These insights should allow us to understand and predict similar processes in confined spaces.

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“…The results of reactions with unlabelled cyclohexanol and H 2 18 O as solvent ( Table 2 ) and the fact that hydration of olefin hardly occurred under the applied conditions ( Supplementary Notes 1, 2 and 4 ) allow us to conclude that an E2 pathway alone, with concerted C–O and C–H bond scissions, cannot account for the significant 18 O incorporation (10–32%) into cyclohexanol. With the S N 2 path for 16 O– 18 O exchange between water and secondary/tertiary alcohols also ruled out 28 29 30 31 32 , the only possible pathway for this level of 18 O incorporation would be the recombination between H 2 18 O and an intermediate that is formed upon the C–O bond cleavage and precedes the C β –H bond cleavage transition state (TS). Therefore, we conclude that dehydration of cyclohexanol in aqueous phase proceeds along the E1 path.…”

Section: Resultsmentioning

confidence: 99%

Tailoring nanoscopic confines to maximize catalytic activity of hydronium ions

Shi

Eckstein²,

Vjunov

et al. 2017

Nat Commun

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Acid catalysis by hydronium ions is ubiquitous in aqueous-phase organic reactions. Here we show that hydronium ion catalysis, exemplified by intramolecular dehydration of cyclohexanol, is markedly influenced by steric constraints, yielding turnover rates that increase by up to two orders of magnitude in tight confines relative to an aqueous solution of a Brønsted acid. The higher activities in zeolites BEA and FAU than in water are caused by more positive activation entropies that more than offset higher activation enthalpies. The higher activity in zeolite MFI with pores smaller than BEA and FAU is caused by a lower activation enthalpy in the tighter confines that more than offsets a less positive activation entropy. Molecularly sized pores significantly enhance the association between hydronium ions and alcohols in a steric environment resembling the constraints in pockets of enzymes stabilizing active sites.

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Oxygen exchange as a function of racemization in 1-phenyl-1-ethanol. Kinetic evidence for ion-dipole pair intermediates

Cited by 17 publications

References 0 publications

Intracomplex CH₃OH Walking around Optically Active 1-Methyl-3-ethylallyl Cations. A Gas-Phase Kinetic Study

Intracomplex CH₃OH Walking around Optically Active 1-Methyl-3-ethylallyl Cations. A Gas-Phase Kinetic Study

Enhancing the catalytic activity of hydronium ions through constrained environments

Tailoring nanoscopic confines to maximize catalytic activity of hydronium ions

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Oxygen exchange as a function of racemization in 1-phenyl-1-ethanol. Kinetic evidence for ion-dipole pair intermediates

Cited by 17 publications

References 0 publications

Intracomplex CH3OH Walking around Optically Active 1-Methyl-3-ethylallyl Cations. A Gas-Phase Kinetic Study

Intracomplex CH3OH Walking around Optically Active 1-Methyl-3-ethylallyl Cations. A Gas-Phase Kinetic Study

Enhancing the catalytic activity of hydronium ions through constrained environments

Tailoring nanoscopic confines to maximize catalytic activity of hydronium ions

Contact Info

Product

Resources

About

Intracomplex CH₃OH Walking around Optically Active 1-Methyl-3-ethylallyl Cations. A Gas-Phase Kinetic Study

Intracomplex CH₃OH Walking around Optically Active 1-Methyl-3-ethylallyl Cations. A Gas-Phase Kinetic Study