The experimental manifestations of corner-cutting tunneling

Kim, Yong-Ho; Kreevoy, Maurice M.

doi:10.1021/ja00044a024

Cited by 88 publications

(120 citation statements)

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“…Table 2 shows some important features of the reaction. The KIE predicted by CVT is only 2.9-3.9 over 0-80°C, because in this bent TS the difference between H and D ZPE loss at the TS is only 3.1 kJ·mol −1 , less than the 5 kJ·mol −1 expected for a linear TS (10,11). The contribution of SCT tunneling to the rate can be estimated as (κ SCT -1)/κ SCT .…”

Section: Computational Results and Discussionmentioning

confidence: 75%

Evidence for tunneling in base-catalyzed isomerization of glyceraldehyde to dihydroxyacetone by hydride shift under formose conditions

Cheng

Doubleday

Breslow

2015

Proc. Natl. Acad. Sci. U.S.A.

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Hydrogen atom transfer reactions between the aldose and ketose are key mechanistic features in formose chemistry by which formaldehyde is converted to higher sugars under credible prebiotic conditions. For one of these transformations, we have investigated whether hydrogen tunneling makes a significant contribution to the mechanism by examining the deuterium kinetic isotope effect associated with the hydrogen transfer during the isomerization of glyceraldehyde to the corresponding dihydroxyacetone. To do this, we developed a quantitative HPLC assay that allowed us to measure the apparent large intrinsic kinetic isotope effect. From the Arrhenius plot of the kinetic isotope effect, the ratio of the preexponential factors A H /A D was 0.28 and the difference in activation energies E a(D) − E a(H) was 9.1 kJ·mol −1 . All these results imply a significant quantum-mechanical tunneling component in the isomerization mechanism. This is supported by multidimensional tunneling calculations using POLYRATE with small curvature tunneling.formose reaction | quantum tunneling | hydride shift | prebiotic reactions W e have described a new mechanism for the formose reaction ( Fig. 1) (1), essentially the same as we had proposed earlier (2) except that the isomerizations of aldose to ketose and the reversal involve a hydride shift rather than an enolization (3-7). Our evidence came from the finding that 2-deuteroglyceraldehyde was converted to 1-deuterodihydroxyacetone under conditions of the formose reaction, with catalysis by Ca 2+ at pH 12. In 2001 Nagorski and Richard had reported an extensive study of the interconversion of glyceraldehyde and dihydroxyacetone by either enolization or hydride shift and had seen that with Zn 2+ the hydride shift mechanism was the exclusive process, by a mechanism closely related to our more recent one (8). We also showed that the isomerization of the ketose erythrulose to the aldose aldotetrose in D 2 O did not lead to deuterium incorporation, as it would have in an enolization process, so it also uses the hydride shift mechanism (1). The first study of the formose reaction in D 2 O was performed by Benner, who saw that no deuterium was incorporated in the intermediates; for some reason he did not invoke hydride shift mechanisms (6).It should be mentioned that we saw that the presence of formaldehyde in the formose reaction in D 2 O led to trapping of any enols formed; without formaldehyde, as in our previous study, there is subsequent deuterium incorporation in dihydroxyacetone, but not significant in glyceraldehyde. We saw that the glyceraldehyde was present almost entirely as its hydrate, a gem diol, but dihydroxyacetone was present mainly as the ketone. This reverses the normally accepted enolization relative rates. Materials and MethodsIn a hydride shift the distance traveled by the proton is small, comparable to the range of its wave character, so we have investigated the possibility that there is a quantum-mechanical tunneling process involved (9). We find that there is indeed tunnelin...

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Section: Computational Results and Discussionmentioning

confidence: 75%

Evidence for tunneling in base-catalyzed isomerization of glyceraldehyde to dihydroxyacetone by hydride shift under formose conditions

Cheng

Doubleday

Breslow

2015

Proc. Natl. Acad. Sci. U.S.A.

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“…26, 27 In contrast, our small isotope effects provide no evidence for tunneling in the hydride transfers from Cp*(CO) 2 OsH, as much larger values of k H /k D would be expected if tunneling were involved. In addition, the temperaturedependent data provide an additional useful criterion for an assessment of the possible role of tunneling.…”

Section: This Equation Incorporates Several Assumptions and Is Based mentioning

confidence: 61%

“…Experimentally determined values of E a D -E a H that significantly exceed the zero-point energy differences are generally taken as evidence of tunneling. 27 The rate of hydride transfer from the ruthenium hydride is much higher than that found for either the Fe or Os analogues, and a complete study of the kinetics of hydride transfer from Cp*(CO) 2 RuH to Ph 3 C + BF 4 -could not be carried out. The reaction was studied at two concentrations of [Cp* (CO) 2 RuH] at -55°C, but at higher concentrations or higher temperatures, the rate of the reaction was too fast to be reliably determined using our stopped-flow equipment.…”

Section: This Equation Incorporates Several Assumptions and Is Based mentioning

confidence: 98%

Hydride Transfer from (η⁵-C₅Me₅)(CO)₂MH (M = Fe, Ru, Os) to Trityl Cation: Different Products from Different Metals and the Kinetics of Hydride Transfer

Cheng

Bullock

2002

Organometallics

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Reactions of the metal hydride complexes Cp*(CO) 2 MH (M ) Fe, Ru, Os) with Ph 3 C + BF 4 -in CH 2 Cl 2 were studied. Hydride transfer from Cp*(CO) 2 FeH to Ph 3 C + BF 4 -gives Cp*(CO) 2 -FeFBF 3 . Hydride transfer from the ruthenium hydride Cp*(CO) 2 RuH to Ph 3 C + BF 4 -produces the bridging hydride complex {[Cp*(CO) 2 Ru] 2 (µ-H)} + BF 4 -, indicating that Cp*(CO) 2 RuH exhibits significant nucleophilicity in addition to hydridic reactivity of the Ru-H bond. The osmium hydride Cp*(CO) 2 OsH reacts with Ph 3 C + BF 4 -in CH 2 Cl 2 to give a mixture of Cp*-(CO) 2 OsFBF 3 and [Cp*(CO) 2 Os(ClCH 2 Cl)] + BF 4 -. The kinetics of these hydride transfer reactions were monitored by stopped-flow methods, leading to the second-order rate law. The temperature dependence of the rate constants was determined for the iron hydride, the osmium hydride, and the osmium deuteride Cp*-(CO) 2 OsD. Activation parameters for hydride transfer from Cp*(CO) 2 FeH are ∆H q ) 2.6 ( 0.1 kcal mol -1 and ∆S q ) -22.1 ( 0.4 cal K -1 mol -1 ; for Cp*(CO) 2 OsH the activation parameters are ∆H q ) 4.9 ( 0.1 kcal mol -1 and ∆S q ) -16.8 ( 0.5 cal K -1 mol -1 . A kinetic isotope effect (k H /k D ) 1.6 at 0°C) was found for the reaction of Cp*(CO) 2 OsD. The order of kinetic hydricity is HRu > HFe > HOs. Second-order rate constants (extrapolated to 25°C from data collected at lower temperatures) are k ) 3.2 × 10 5 M -1 s -1 for Cp*(CO) 2 OsH and k ) 1.1 × 10 6 M -1 s -1 for Cp*(CO) 2 FeH; for Cp*(CO) 2 RuH, k > 5 × 10 6 M -1 s -1 is estimated at 25°C. Rate constants were also determined for hydride transfer to Ph 2 (p-MeOC 6 H 4 )C + at 25°C: k ) 1.5 × 10 5 M -1 s -1 for Cp*(CO) 2 RuH, k ) 4.1 × 10 4 M -1 s -1 for Cp*(CO) 2 FeH, and k ) 3.2 × 10 3 M -1 s -1 for Cp*(CO) 2 OsH.Transition-metal hydrides are critically important in homogeneous catalysis and are involved in more than one step of many catalytic cycles. The delivery of hydrogen to an organic substrate in hydrogenations, hydroformylations, and other catalytic reactions is accomplished by reactions with metal-hydride bonds (M-H). An understanding of the factors influencing the rates of cleavage of M-H bonds will be useful in the rational design of new catalysts and the improvement of known catalysts. Some metal hydrides exhibit diverse modes of M-H bond cleavage in reactions with different substrates. Neutral metal carbonyl hydrides can undergo cleavage of their M-H bonds by proton transfer, 1 by homolytic cleavage resulting in hydrogen atom transfer, 2 and by hydride transfer. 3 We have been studying ionic hydrogenations, in which organic substrates are hydrogenated by sequential delivery of a proton and a hydride. These reactions require hydride transfer from a metal to carbon, as has been found for stoichiometric ionic hydrogenations of alkenes, 4 alkynes, 5 and ketones. 6 Similar hydride transfer reactions are involved in the formation of ether complexes of tungsten from the reaction of acetals with acid and metal hydrides 7 and in the preparation of ketone complexes of tungsten f...

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“…Other criteria, such as Swain-Schaad exponents, [21] despite being frequently used, [10,[22][23][24][25][26] were shown to not always be suitable as indicators for tunneling. [20,27,28] Te chniques to measure KIEs, especially in biochemical systems,are reviewed elsewhere. [24] All these can, of course,o nly serve as indications,s ince no reaction will proceed exclusively by atom tunneling or exclusively without it.…”

Section: Methods To Determine Atom Tunnelingmentioning

confidence: 99%

Atom Tunneling in Chemistry

2016

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Quantum mechanical tunneling of atoms is increasingly found to play an important role in many chemical transformations. Experimentally, atom tunneling can be indirectly detected by temperature-independent rate constants at low temperature or by enhanced kinetic isotope effects. In contrast, the influence of tunneling on the reaction rates can be monitored directly through computational investigations. The tunnel effect, for example, changes reaction paths and branching ratios, enables chemical reactions in an astrochemical environment that would be impossible by thermal transition, and influences biochemical processes.

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The experimental manifestations of corner-cutting tunneling

Cited by 88 publications

References 2 publications

Evidence for tunneling in base-catalyzed isomerization of glyceraldehyde to dihydroxyacetone by hydride shift under formose conditions

Evidence for tunneling in base-catalyzed isomerization of glyceraldehyde to dihydroxyacetone by hydride shift under formose conditions

Hydride Transfer from (η⁵-C₅Me₅)(CO)₂MH (M = Fe, Ru, Os) to Trityl Cation: Different Products from Different Metals and the Kinetics of Hydride Transfer

Atom Tunneling in Chemistry

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The experimental manifestations of corner-cutting tunneling

Cited by 88 publications

References 2 publications

Evidence for tunneling in base-catalyzed isomerization of glyceraldehyde to dihydroxyacetone by hydride shift under formose conditions

Evidence for tunneling in base-catalyzed isomerization of glyceraldehyde to dihydroxyacetone by hydride shift under formose conditions

Hydride Transfer from (η5-C5Me5)(CO)2MH (M = Fe, Ru, Os) to Trityl Cation: Different Products from Different Metals and the Kinetics of Hydride Transfer

Atom Tunneling in Chemistry

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Hydride Transfer from (η⁵-C₅Me₅)(CO)₂MH (M = Fe, Ru, Os) to Trityl Cation: Different Products from Different Metals and the Kinetics of Hydride Transfer