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Transition state

About: Transition state is a research topic. Over the lifetime, 4978 publications have been published within this topic receiving 117965 citations. The topic is also known as: transition state of elementary reaction.


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Journal ArticleDOI
TL;DR: This work is exploring the possibility of employing zeolites as a media for achieving chiral induction during photoreactions, and results on photoelectrocylization of tropolone alkyl ethers are discussed at length.
Abstract: Chiral induction of chemical reactions continues to be one of the main concerns of chemists. While basic rules of chiral induction of thermal reactions have been reasonably established, the same is not true of photochemical reactions. Short excited state lifetime and low activation energies for reactions in the excited state(s) leave very little room for manipulating the diastereomeric transition states. Yet impressive chiral induction of photochemical reactions in the solid state has been achieved. On the other hand, chiral induction of photoreactions of organic molecules in solution continues to be inefficient at ambient conditions. We are exploring the possibility of employing zeolites as a media for achieving chiral induction during photoreactions. The motivating force for such an attempt is the fact that chiral chemistry in the solid state is not completely general due to the fact that not all molecules crystallize. To achieve chiral induction one needs a chiral perturber. Zeolites are not chiral and therefore the perturber is added to the medium. Thus the medium for a photoreaction is a chirally modified zeolite. Of the several reactions investigated, results on photoelectrocylization of tropolone alkyl ethers are discussed at length. The confined space offered by the zeolite supercage forces a reactant and the chiral inductor to interact intimately to yield enantiomerically enriched product. Due to the transitory nature of the reaction cavity in solution such close interactions are less likely in isotropic solvent media. The examples discussed herein show negligible chiral induction in solution, whereas in a zeolite one obtains induction as high as 90%.

68 citations

Journal ArticleDOI
TL;DR: In this article, the activation of CH4 by models of Cp*2LnZ (Z = H, Me) have been carried out for the entire lanthanide series.
Abstract: DFT(B3PW91) calculations of the activation of CH4 by models (Cl2LnZ) of Cp*2LnZ (Z = H, Me) have been carried out for the entire lanthanide series. Cl2LnZ appears to be a good model for Cp*2LnZ. It reproduces well the coordination around the lanthanide. The energetics of the transformation X2LnH + CH4 → X2LnCH3 + H2 are fairly close for X = Cl and Cp and the difference in behavior can be attributed to the stronger electron donating ability of Cp. Formation of the lanthanide hydride complex is calculated to be exothermic in agreement with experimental evidence. The energy profiles of the reactions Cl2LnH + CH4 → Cl2LnCH3 + H2; Cl2LnH* + CH4 → Cl2LnH + H*CH3; Cl2LnCH*3 + CH4 → Cl2LnCH3 + H–CH*3 have been calculated. The transition states for the first and third transformations are energetically accessible, in good agreement with the known experimental data. The second reaction has a transition state of very high energy indicating an unfeasible reaction. The geometry of the transition stuctures are suggestive of a proton transfer between two anionic species (Z and CH3−; Z = H− and CH3−) in the field of the lanthanide fragment.

68 citations

Journal ArticleDOI
TL;DR: In this paper, a density functional study on the mechanism of the asymmetric alpha-alkylation of aldehydes has been conducted and the authors provide an explanation for the remarkable and unexpected increase in enantioselectivity that is observed when using 2-methylproline instead of proline as the catalyst.
Abstract: [structure: see text] An organocatalytic asymmetric alpha-alkylation of aldehydes has recently been shown to provide cyclic aldehydes in high yields and enantioselectivities upon treating substituted acyclic halo-aldehydes with a catalytic amount of 2-methylproline in the presence of 1 equiv of triethylamine. Here, we report a density functional study on the mechanism of this reaction. The crucial step is an intramolecular nucleophilic substitution in the enamine intermediate. The added base accelerates the reaction through the electrostatic activation of the leaving group and affects the stereoselectivity by stabilizing anti and syn transition states to a different extent. On the basis of the computed barriers and transition states, we provide an explanation for the remarkable and unexpected increase in enantioselectivity that is observed when using 2-methylproline instead of proline as the catalyst. Calculated and observed enantiomeric excess values are in good agreement.

68 citations

Journal ArticleDOI
TL;DR: In this paper, transition state scaling (TSS) linear free energy relations are extended to C-H, O-H and C-C bond breaking reactions occurring on the ring and the functional groups of furan.
Abstract: Bronsted–Evans–Polanyi (BEP) and transition state scaling (TSS) linear free energy relations are extended to C–H, O–H, C–O and C–C bond breaking reactions occurring on the ring and the functional groups of furan (hydrofuran, dihydrofuran, trihydrofuran, and tetrahydrofuran) and furfural derivatives (e.g., furfural, furfuryl alcohol, methyl furan, etc.) on Pd(111). The relations perform statistically as well as those for small molecules reported previously. Hydrogenation/dehydrogenation reactions have smaller deviations compared to C–C and C–O bond breaking ones. This is in line with the degree of structural change during reaction and agrees with observations in previous works. We conclude that BEP relations developed for small molecules are not statistically different from those developed for furanics. A universal BEP relation is not statistically different from most of the BEP relations developed for furanics, with the exception of C–O, O–H and C–H scission reactions at the functional group, for which on...

67 citations

Journal ArticleDOI
TL;DR: A linear transformation is proposed that allows viewing a complex nonlinear chemical reaction system via decoupled dynamic variables, each one associated with a particular phenomenon such as a single chemical reaction, a specific mass transfer or heat transfer.

67 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202364
2022136
2021148
2020155
2019145
2018147