Transition-state structure and the temperature dependence of the kinetic isotope effect

Anhede, Bo; Bergman, N.-A.

doi:10.1021/ja00336a061

Cited by 17 publications

(9 citation statements)

References 2 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…For instance, a bent transition state 53 for H-transfer is characterized by near temperature independence of KIE ( E H − E D ≈ 0) and an unusually high ratio of A H / A D (≥1.4). It has to be noted that the validity of these correlations has been questioned . Especially high values of A H / A D have been suspected 55 to conceal mechanistic complexity rather than characterize an elementary step…”

Section: Resultsmentioning

confidence: 99%

“…It has to be noted that the validity of these correlations has been questioned. 55 Especially high values of A H /A D have been suspected 55 to conceal mechanistic complexity rather than characterize an elementary step. KIE values have been determined for the intermolecular carbenoid Si-H(D) insertion into PhMe 2 Si-H/ PhMe 2 Si-D (1:1) at five different temperatures (-40, -30, -10, 0, +25 °C).…”

Section: E) Control Experiments To Assess Possible Si-h Bond Cleavage...mentioning

confidence: 99%

See 1 more Smart Citation

Speciation and Mechanistic Studies of Chiral Copper(I) Schiff Base Precursors Mediating Asymmetric Carbenoid Insertion Reactions of Diazoacetates into the Si−H Bond of Silanes

et al. 2000

View full text Add to dashboard Cite

Cu(CH 3 CN) 4 ](PF 6 ) and the chiral C 2 -symmetric diimine ligand (1R,2R)-bis((2,6-dichlorobenzylidene)diamino)cyclohexane (R,R-1) (1.2 equiv) mediate asymmetric carbenoid insertion of aryl diazoesters into the Si-H bond of silanes in good to high yields and levels of enantiocontrol. Dichloromethane solutions of [Cu(CH 3 CN) which feature three distinct copper complexes in the crystal lattice. 1 H NMR and electrospray ionization MS (ESI-MS) studies establish that only [Cu I (R,R-1)(CH 3 CN)] + is present in solution in high yields. Upon addition of stoichiometric PhC(N 2 )CO 2 Me, cations [Cu I (R,R-1)(C(CO 2 Me)Ph)] + and [Cu I (R,R-1)(C(CO 2 Me)Ph)(CH 3 CN)] + are detected by ESI-MS, consistent with the presence of a copper-carbenoid moiety. The catalytically active precursor is most likely a mononuclear unit of the type [Cu I (R,R-1)(CH 3 CN) n ] + , as suggested by the linearity of plots relating the enantiomeric excess (ee) of product to that of the ligand (Kagan's analysis). Hammett plots correlate enhanced catalytic reactivities with stabilization of a sizable positive charge on the carbenoid carbon and a smaller positive charge on the silicon atom, but the corresponding enantioselectivities are insensitive to these electronic properties. The kinetic isotope effect for carbenoid insertion into PhMe 2 Si-H(D) varies from 2.12 (-40 °C) to 1.08 (25 °C), in agreement with other small KIE values observed for processes in which Si-H activation is involved in the turnover-limiting step. A linear Eyring plot of ln(k(major enantiomer)/k(minor enantiomer)) over a range of 80 K supports the notion of a single step controlling the levels of enantioselection. No H/D scrambling is observed in competitive carbenoid insertions into PhMe 2 Si-D/Ph 2 MeSi-H, indicating that the insertion reaction takes place in a concerted fashion. These results are discussed in light of an early transition state, characterized by hydrogen-first penetration of the Si-H bond into the copper-carbenoid cavity, which is assumed to impart high levels of enantioselectivity due to intrinsic preorganization under the influence of the specific ligand and aryl diazoesters employed.

show abstract

Section: Resultsmentioning

confidence: 99%

Section: E) Control Experiments To Assess Possible Si-h Bond Cleavage...mentioning

confidence: 99%

Speciation and Mechanistic Studies of Chiral Copper(I) Schiff Base Precursors Mediating Asymmetric Carbenoid Insertion Reactions of Diazoacetates into the Si−H Bond of Silanes

et al. 2000

View full text Add to dashboard Cite

show abstract

“…However, the nonlinear transition‐state hypothesis was soon shown to be wrong by McLennan et al 139, 140. for the H‐shift in pentadiene and by Anhede et al 141. for the intramolecular proton transfer in mono‐protonated methylenediamine.…”

Section: Experimental Examplesmentioning

confidence: 99%

Heavy atom motions and tunneling in hydrogen transfer reactions: the importance of the pre‐tunneling state

Limbach

Schowen

2010

J of Physical Organic Chem

View full text Add to dashboard Cite

Arrhenius curves of selected hydrogen transfer reactions in organic molecules and enzymes are reviewed with the focus on systems exhibiting temperature‐independent kinetic isotope effects. The latter can be rationalized in terms of a ‘pre‐tunneling state’ which is formed from the reactants by heavy atom motions and which represents a suitable molecular configuration for tunneling to occur. Within the Bell–Limbach tunneling model, formation of the pre‐tunneling state dominates the Arrhenius curves of the H and the D transfer even at higher temperatures if a large energy Em is required to reach the pre‐tunneling state. Tunneling from higher vibrational levels and the over‐barrier reaction via the transition state which lead to temperature‐dependent kinetic isotope effects dominate the Arrhenius curves only if Em is small compared to the energy of the transition state. Using published data on several hydrogen transfer systems, the type of motions leading to the pre‐tunneling state is explored. Among the phenomena which lead to large energies of the pre‐tunneling state are (i) cleavage of hydrogen bonds or coordination bonds of the donor or acceptor atoms to molecules or molecular groups in order to allow the formation of the pre‐tunneling state, (ii) the occurrence of an energetic intermediate on the reaction pathway within which tunneling takes place, and (iii) major reorganization of a molecular skeleton, requiring the excitation of specific vibrations in order to reach the pre‐tunneling state. This model suggests a solution to the puzzle of Kwart's findings of temperature‐independent kinetic isotope effects for hydrogen transfer in small organic molecules. Copyright © 2010 John Wiley & Sons, Ltd.

show abstract

“…The observed kinetic isotope effect k D /k H ≈ 0.3 is consistent with that typically observed for C-D vs C-H bond cleavage for relatively linear transition states. 33,34 An obvious effect of reduction in the amount of the complex carrying three deuterium atoms (i.e., the complex resulted from the nucleophilic attack on hydrogen of the R group of the aldehydes; thus, the -CD 3 group was retained) in the series acetaldehyde-propanal-hexanal was observed (Table S2, Supporting Information). This effect apparently results from nu- cleophilic attack on the hydrogens of different groups (-CH 3 , -C 2 H 5 , and -C 5 H 9 ).…”

Section: Resultsmentioning

confidence: 99%

Gas-Phase Reactions of Bis(η⁵-cyclopentadienyl)methylzirconium Cations with Ketones and Aldehydes

Aksenov¹,

Contreras²,

Richardson³

et al. 2006

Organometallics

View full text Add to dashboard Cite

The reactions of the bis(η 5 -cyclopentadienyl)methylzirconium cation (1) with ketones and aldehydes in the gas phase have been studied by Fourier transform ion cyclotron resonance mass spectrometry. Reactions of 1 with a majority of the ketones studied resulted in consecutive addition of one and two substrate molecules and/or elimination of alkanes. Deuterium-labeled substrates and methylzirconocene were used to investigate the mechanistic details. On the basis of DFT calculations, the key products of the elimination reaction(s) were identified as η 3 -enolate complexes formed via an insertion/elimination mechanism. Similar product ion structures are postulated for the reaction of 1 with aldehydes in cases where these complexes are either the only or the major reaction product. A multiple-step mechanism is proposed, which involves migratory insertion of an aldehyde molecule into the methylzirconocene cation, followed by β-H elimination, and, via a six-membered cyclic transition state, formation of the resulting enolate complex. When a β-H elimination pathway is not available for ketones, the reaction is instead proposed to proceed via direct nucleophilic attack of the metal-bound alkyl preceded by a fast migratory insertion equilibrium.

show abstract

Transition-state structure and the temperature dependence of the kinetic isotope effect

Cited by 17 publications

References 2 publications

Speciation and Mechanistic Studies of Chiral Copper(I) Schiff Base Precursors Mediating Asymmetric Carbenoid Insertion Reactions of Diazoacetates into the Si−H Bond of Silanes

Speciation and Mechanistic Studies of Chiral Copper(I) Schiff Base Precursors Mediating Asymmetric Carbenoid Insertion Reactions of Diazoacetates into the Si−H Bond of Silanes

Heavy atom motions and tunneling in hydrogen transfer reactions: the importance of the pre‐tunneling state

Gas-Phase Reactions of Bis(η⁵-cyclopentadienyl)methylzirconium Cations with Ketones and Aldehydes

Contact Info

Product

Resources

About

Transition-state structure and the temperature dependence of the kinetic isotope effect

Cited by 17 publications

References 2 publications

Speciation and Mechanistic Studies of Chiral Copper(I) Schiff Base Precursors Mediating Asymmetric Carbenoid Insertion Reactions of Diazoacetates into the Si−H Bond of Silanes

Speciation and Mechanistic Studies of Chiral Copper(I) Schiff Base Precursors Mediating Asymmetric Carbenoid Insertion Reactions of Diazoacetates into the Si−H Bond of Silanes

Heavy atom motions and tunneling in hydrogen transfer reactions: the importance of the pre‐tunneling state

Gas-Phase Reactions of Bis(η5-cyclopentadienyl)methylzirconium Cations with Ketones and Aldehydes

Contact Info

Product

Resources

About

Gas-Phase Reactions of Bis(η⁵-cyclopentadienyl)methylzirconium Cations with Ketones and Aldehydes