198 research outputs found
Chemo-, regio-, and diastereoselectivity preferences in the reaction of a sulfur ylide with a dienal and an enone
No full textMechanistic insights into an interesting class of reaction between sulfur ylides with (i) a dienal, and (ii) an enone, obtained by using density functional theory, is reported. The kinetic and thermodynamic factors responsible for chemo-, regio-, and diastereoselectivities are established by identifying all key transition states and intermediates along the reaction pathway for 1,2-, 1,4-, and 1,6-modes of attack of dimethylsulfonium benzylide to 5-phenylpenta-2,4-dienal. The reaction profiles for 1,2- and 1,4-modes of addition are also evaluated for the reaction between dimethylsulfonium benzylide and pent-3-en-2-one. Our results show that the final outcome of the reaction with both these substrates would be decided by the interplay between kinetic and thermodynamic factors. It is found that the addition of a semi-stabilized ylide to conjugated carbonyl compounds prefers to proceed through a 1,4( conjugate) pathway under thermodynamic conditions, which is in accordance with the available experimental reports. However, the formation of epoxides via a 1,2-(direct) addition pathway is computed to be equally competitive, which could be the favored pathway under kinetic conditions. Even though the lower barrier for the initial addition step is kinetically advantageous for the direct (or 1,2-) addition pathway, the higher energy of the betaine intermediates-as well as the reversibility of the accompanying elementary step-may disfavor product formation in this route. Thus, high diastereoselectivity in favor of 2,3-trans cyclopropanecarbaldehyde is predicted in the case of the dienal, using the most favored conjugate addition (1,4-addition) pathway. Along similar lines, ylide addition to the enone is identified to exhibit a preference toward conjugate addition over direct (1,2-) addition. The importance of transition state analysis in delineating the controlling factors towards product distribution and diastereoselectivity is established
Computational chemistry and molecular modeling : principles and applications / K.I. Ramachandran, G. Deepa, K. Namboori.
No full text"An exclusive URL (http://www.amrita.edu/cen/ccmm/) for this book with the required support materials has been provided for readers ..."--Preface.pharmacy bookfair2015Includes bibliographical references and index.xxi, 397 pages
Nonheme Iron Oxidant Formed in the Presence of H2O2 and Acetic Acid Is the Cyclic Ferric Peracetate Complex, Not a Perferryloxo Complex
No full textOxidative C-H bond activation, is a transformation of fundamental and practical interest, particularly if it can be carried out with high regio- and enantioselectivity. With nonheme iron oxygenases as inspiration (e.g., the Rieske oxygenases), a family of biomimetic nonheme iron complexes has been found to catalyze hydrocarbon oxidations by H2O2 via a postulated Fe-V(O)(OH) oxidant. Of particular interest is the Fe(S,S-PDP) catalyst discovered by White that, in the presence of acetic acid as an additive, performs selective C-H bond activation, even in complex organic molecules. The corresponding Fe-V(O)(OAc) species has been suggested as the key oxidant. We have carried out DFT studies to assess the viability of such an oxidant and discovered an alternative formulation. Theory reveals that the barrier for the formation of the putative Fe-V(O)(OAc) oxidant is too high for it to be feasible. Instead, a much lower barrier is found for the formation of a [(S,S-PDP)Fe-III(kappa(2)-peracetate)] species. In the course of C-H activation, this complex undergoes O-O bond homolysis to become a transient [(S,S-PDP)Fe-IV(O)(AcO center dot)] species that performs the efficient hydroxylation of alkanes. Thus, the acetic acid additive alters completely the nature of the high-valent oxidant, which remains disguised in the cyclic structure. This new mechanism can rationalize the many experimental observations associated with the oxidant formed in the presence of acetic acid, including the S = 1/2 EPR signal associated with the oxidant. These results further underscore the rich multioxidant scenario found in the mechanistic landscape for nonheme iron catalysts
Micro-raman spectroscopy of caries lesion formation in dental enamel
No full textCaries lesions form by a complex process of chemical interactions between dental enamel and its environment. They can cause cavities and pain, and are expensive to fix. Lesions form by slow demineralization over many months, even years. It is hard to characterize in vivo as a result of environmental factors and remineralization by ions in the oral cavity. In this thesis the process of demineralization was carried out in vitro and micro-Raman spectroscopy used to investigate and characterize the lesion's chemistry. Demineralization occurs by diffusion across the depth of the lesion of mineral ions via interstitial spaces in the dental enamel. Hydroxyl ions are initially lost by acidic attack, which increases the interstitial space. The demineralization is retarded by diffusion processes in the opposite direction, and a balance in the charges of the ions must be maintained. Having multiple ions diffusing simultaneously is termed “coupled diffusion”. A subsurface highly demineralized region is formed, but this can be remineralized.
Micro-Raman spectroscopy is a powerful tool for studying material composition by exciting chemical bonds in the sample. Using micro-Raman to characterize the chemical composition of lesions may help in developing preventative measures to stop their formation. Raman (λ=785 nm) was used to characterize lesions grown over 5, 7, 9, 11 and 14 days. The amide I peak at ~1605 cm-1, which has not been observed previously, was seen in the maturing lesions. The extreme demineralization in these lesions enables the organic peaks to be seen rather than the normally stronger mineral peaks. Analysis of crystallinity shows that there is always a reduction in mineral content with distance below the enamel surface, but this becomes magnified as the lesion matures. Type B carbonate substitution for phosphate ions can also be examined with Raman. Correcting for crystallinity shows that both carbonate and phosphate ions are lost at the same rate during demineralization.
In summary, micro-Raman is an effective and relatively easy tool to use in lesion characterization. It also has the advantage that it can be used to identify changes in both the mineral and protein phases of enamel.M.S.Includes bibliographical references (p. 53-55)
Computational investigations on the general reaction profile and diastereoselectivity in sulfur ylide promoted aziridination
No full textMechanism and diastereoselectivity of sulfur ylide promoted aziridination reactions were studied by density functional theory with inclusion of solvent effects through the continuum solvation model. The general reaction pathway was modeled for the addition of substituted sulfur ylides (Me2S+CH−R) to an aldimine ((E)-methyl ethylidenecarbamate, MeHC=NCO2Me). The nature of the substituents on the ylidic carbon atom substantially affects the reaction profile. The stabilized (R=COMe) and semistabilized (R=Ph) ylides follow a cisoidaddition mode leading to trans aziridines via anti betaine intermediates. The simplest model ylide (unstabilized, R=H) underwent cisoid addition in a similar fashion. In the case of stabilized ylides product diastereoselectivity is controlled by the barriers of the elimination step leading to the 2,3-trans aziridine, whereas it is decided in the addition step in the case of semistabilized ylides. The importance of steric and electronic factors in diastereoselective addition (2 and 5) and elimination (5) transition states was established. Comparison of results obtained with the gas-phase optimized geometries and with the fully optimized solvent-phase geometries reveals that the inclusion of solvent effects does not bring about any dramatic changes in the reaction profiles for all three kinds of ylides. In particular, diastereoselectivity for both kinds of ylides was found to be nearly the same in both these approaches
Density functional theory investigations on sulfur ylide promoted cyclopropanation reactions: insights on mechanism and diastereoselection issues
No full text[graphics] The mechanism and diastereoselectivity of synthetically useful sulfur ylide promoted cyclopropanation reactions have been studied using the density functional theory method. Addition of different substituted ylides (Me2S+CH-R) to enone ((E)-pent-3-en-2-one, MeHCCH-COMe) has been investigated. The nature of the substituent on the ylidic carbon brings about subtle changes in the reaction profile. The stabilized (R = COMe) and semistabilized (R = Ph) ylides follow a cisoid addition mode, leading to 1,2-trans and 1,2-cis cyclopropanes, respectively, via syn and anti betaine intermediates. The simplest and highly reactive model ylide (R = H) prefers a transoid addition mode. Diastereoselectivity is controlled by the barrier for cisoid-transoid rotation in the case of stabilized ylides, whereas the initial electrophilic addition is found to be the diastereoselectivity-determining step for semistabilized ylides. High selectivity toward trans cyclopropanes with stabilized ylides are predicted on the basis of the relative activation energies of diastereomeric torsional transition states. The energy differences between these transition states could be rationalized with the help of weak intramolecular as well as other stereoelectronic interactions
Enantio- and Diastereoselectivities in Chiral Sulfur Ylide Promoted Asymmetric Aziridination Reactions
No full textDensity functional theory investigation on the factors controlling enantio- and diastereoselection in asymmetric aziridination reaction by the addition of chiral bicyclic sulfur ylides to substituted aldimines is presented. Hi-fi levels of enantioselection are predicted toward the formation of (2S,3S)-cis and (2R,3S)-trans aziridines by the addition of stabilized ylide (R = COMe) respectively to SO(2)Me and CO(2)Me protected aldimines. Similarly, high %ee is predicted for the formation of (2S,3R)-cis aziridines from semistabilized (R = Ph) ylide. Moderate to high levels of diastereoselectivity is noticed as well. The present study highlights that a correct prediction oil extent of enantioselection requires the knowledge of the activation barriers for elementary steps beyond the initial addition step. In the case of stabilized ylides the ring-closure (or elimination Of Sulfur compound) is found to be crucial in controlling enantio- and diastereoselection. A cumulative effect of electronic as well as other weak interactions is identified as factors contributing to the relative energies of transition states leading to enantio- and diastereomeric products for the stabilized ylide addition to aldimines. On the contrary, steric control appears quite dominant With semistabilized ylide addition. With the smallest substituent on ylide (R = Me), high enantioselectivity is predicted for the formation of (2R,3R)-trans aziridines although the %de in this case is found to be very low
ChemInform Abstract: Exchange-Enhanced Reactivity in Bond Activation by Metal-Oxo Enzymes and Synthetic Reagents
No full textDoes the TauD Enzyme Always Hydroxylate Alkanes, While an Analogous Synthetic Non-Heme Reagent Always Desaturates Them?
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