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Diastereomeric Transition States (diastereomeric + transition_states)

Distribution by Scientific Domains

Chemistry	100%

Distribution within Chemistry

General & Introductory Chemistry	50%

Selected Abstracts

Diastereoselective Reactions of the Tiglic Acid Functionality Mediated by Oxazolidine Chiral Auxiliaries: A Mechanistic Comparison of DMD andm -CPBA Epoxidations versus Singlet Oxygen and PTAD Ene Reactions

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 14 2005
Aurelia Pastor
Abstract 2,2-Dimethyloxazolidines have been utilized as chiral auxiliaries for the diastereoselective functionalization of the optically active tiglic acid derivatives (S)- 1 by means of epoxidation with DMD or m -CPBA and ene reactions with 1O2 or PTAD. In the DMD and m -CPBA epoxidations, high diastereoselectivities but opposite senses of diastereomer selection were observed. In contrast, the stereochemistry of the 1O2 and PTAD ene reactions depended on the size of the attacking enophile: whereas essentially perfect diastereoselectivity was obtained with PTAD, much lower stereoselection was observed with 1O2. The stereochemical results for the DMD and m -CPBA epoxidations and the PTAD ene reaction are explained in terms of the energy differences for the corresponding diastereomeric transition states, dictated by steric and electronic effects. The PTAD ene reaction for these tiglic acids (S)- 1 provides, after removal of the chiral auxiliaries, an attractive synthetic route for optically active ,-amino acid derivatives.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]

Origin of Enantioselectivity in the Organocatalytic Reductive Amination of ,-Branched Aldehydes

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 4 2009
Tommaso Marcelli
Abstract The reason for enantioselectivity in the reductive amination of ,-branched aldehydes was investigated. The relative energies of all the diastereomeric transition states for hydride transfer of a suitable computational model were calculated at the B3LYP/6-311+G(2d,2p) level of theory. Our calculations successfully reproduce and rationalize the experimentally observed stereochemical outcome of the reaction. [source]

Mechanistic Investigation of Chiral Phosphoric Acid Catalyzed Asymmetric Baeyer,Villiger Reaction of 3-Substituted Cyclobutanones with H2O2 as the Oxidant

CHEMISTRY - A EUROPEAN JOURNAL, Issue 10 2010
Senmiao Xu Dr.
Abstract The mechanism of the chiral phosphoric acid catalyzed Baeyer,Villiger (B,V) reaction of cyclobutanones with hydrogen peroxide was investigated by using a combination of experimental and theoretical methods. Of the two pathways that have been proposed for the present reaction, the pathway involving a peroxyphosphate intermediate is not viable. The reaction progress kinetic analysis indicates that the reaction is partially inhibited by the ,-lactone product. Initial rate measurements suggest that the reaction follows Michaelis,Menten-type kinetics consistent with a bifunctional mechanism in which the catalyst is actively involved in both carbonyl addition and the subsequent rearrangement steps through hydrogen-bonding interactions with the reactants or the intermediate. High-level quantum chemical calculations strongly support a two-step concerted mechanism in which the phosphoric acid activates the reactants or the intermediate in a synergistic manner through partial proton transfer. The catalyst simultaneously acts as a general acid, by increasing the electrophilicity of the carbonyl carbon, increases the nucleophilicity of hydrogen peroxide as a Lewis base in the addition step, and facilitates the dissociation of the OH group from the Criegee intermediate in the rearrangement step. The overall reaction is highly exothermic, and the rearrangement of the Criegee intermediate is the rate-determining step. The observed reactivity of this catalytic B,V reaction also results, in part, from the ring strain in cyclobutanones. The sense of chiral induction is rationalized by the analysis of the relative energies of the competing diastereomeric transition states, in which the steric repulsion between the 3-substituent of the cyclobutanone and the 3- and 3,-substituents of the catalyst, as well as the entropy and solvent effects, are found to be critically important. [source]

Origin of the ,-Facial Stereoselectivity in the Addition of Nucleophilic Reagents to Chiral Aliphatic Ketones as Evidenced by High-Level Ab,Initio Molecular-Orbital Calculations

CHEMISTRY - AN ASIAN JOURNAL, Issue 6 2006
Osamu Takahashi Dr.
Abstract Ab,initio molecular-orbital (MO) calculations were carried out, at the MP2/6-311++G(d,p)//MP2/6-31G(d) level, to investigate the conformational Gibbs energy of alkyl 1-cyclohexylethyl ketones, cyclo- C6H11CHCH3COR (R=Me, Et, iPr, and tBu). In each case, one of the equatorial conformations was shown to be the most stable. Conformers with the axial CHCH3COR group were also shown to be present in an appreciable concentration. Short CH,,,CO and CH,,,OC distances were found in each stable conformation. The result was interpreted on the grounds of CH,,,,(CO) and CH,,,O hydrogen bonds, which stabilize the geometry of the molecule. The ratio of the diastereomeric secondary alcohols produced in the nucleophilic addition to cyclo- C6H11CHCH3COR was estimated on the basis of the conformer distribution. The calculated result was consistent with the experimental data previously reported: the gradual increase in the product ratio (major/minor) along the series was followed by a drop at R=tBu. The energy of the diastereomeric transition states in the addition of LiH to cyclo- C6H11CHCH3COR was also calculated for R=Me and tBu. The product ratio did not differ significantly in going from R=Me to tBu in the case of the aliphatic ketones. This is compatible with the above result calculated on the basis of the conformer distribution. Thus, the mechanism of the ,-facial selection can be explained in terms of the simple premise that the geometry of the transition state resembles the ground-state conformation of the substrates and that the nucleophilic reagent approaches from the less-hindered side of the carbonyl , face. [source]