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Author: Zhihong Wang
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Languages : en
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Several organic reactions that could potentially involve coarctate transition states were investigated by a combination of experimental and theoretical studies. In the thermal fragmentation of [delta]-1,3,4-oxadiazolines, the mechanism supported by kinetic isotope effects and theoretical calculations is a three-step process that does not demonstrate any special stabilization in coarctate transition states. Rather than undergoing a direct coarctate conversion to product, the mechanism avoids coarctate steps. The last step is a concerted coarctate reaction, but being concerted may be viewed as being enforced by the necessity to avoid high-energy intermediates. In the deoxygenation of epoxides with dichlorocarbene, the stabilization from the transition state aromaticity is not great enough to compete with the preference for asynchronous bonding changes. KIEs and calculations suggested that the reaction occurs in a concerted manner but with a highly asynchronous early transition state with much more C[alpha]-O bond breaking than C[beta]-O bond breaking. In the Shi epoxidation, a large [beta]-olefinic 13C isotope effect and small [alpha-carbon isotope effect indicated an asynchronous transition state with more advanced formation of the C-O bond to the [beta]-olefinic carbon. The calculated lowest-energy transition structures are generally those in which the differential formation of the incipient C-O bonds, the "asynchronicity," resembles that of an unhindered model, and the imposition of greater or less asynchronicity leads to higher barriers. In reactions of cis-disubstituted and terminal alkenes using Shi's oxazolidinone catalyst, the asynchronicity of the epoxidation transition state leads to increased steric interaction with the oxazolidinone when a [pi]-conjugating substituent is distal to the oxazolidinone but decreased steric interaction when the [pi]-conjugating substituent is proximal to the oxazolidinone. Dynamic effects were studied in Diels-Alder reaction between acrolein and methyl vinyl ketone. This reaction yields two products in a ratio of 3.0 " 0.5. Theoretical studies shows that only one transition structure is involved in the formation of both. Quasiclassical trajectory calculations on an MP2 surface give a prediction of a product ratio of 45:14 (3.2:1), which is in good agreement with the experimental observation.
Author: Zhihong Wang
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
Several organic reactions that could potentially involve coarctate transition states were investigated by a combination of experimental and theoretical studies. In the thermal fragmentation of [delta]-1,3,4-oxadiazolines, the mechanism supported by kinetic isotope effects and theoretical calculations is a three-step process that does not demonstrate any special stabilization in coarctate transition states. Rather than undergoing a direct coarctate conversion to product, the mechanism avoids coarctate steps. The last step is a concerted coarctate reaction, but being concerted may be viewed as being enforced by the necessity to avoid high-energy intermediates. In the deoxygenation of epoxides with dichlorocarbene, the stabilization from the transition state aromaticity is not great enough to compete with the preference for asynchronous bonding changes. KIEs and calculations suggested that the reaction occurs in a concerted manner but with a highly asynchronous early transition state with much more C[alpha]-O bond breaking than C[beta]-O bond breaking. In the Shi epoxidation, a large [beta]-olefinic 13C isotope effect and small [alpha-carbon isotope effect indicated an asynchronous transition state with more advanced formation of the C-O bond to the [beta]-olefinic carbon. The calculated lowest-energy transition structures are generally those in which the differential formation of the incipient C-O bonds, the "asynchronicity," resembles that of an unhindered model, and the imposition of greater or less asynchronicity leads to higher barriers. In reactions of cis-disubstituted and terminal alkenes using Shi's oxazolidinone catalyst, the asynchronicity of the epoxidation transition state leads to increased steric interaction with the oxazolidinone when a [pi]-conjugating substituent is distal to the oxazolidinone but decreased steric interaction when the [pi]-conjugating substituent is proximal to the oxazolidinone. Dynamic effects were studied in Diels-Alder reaction between acrolein and methyl vinyl ketone. This reaction yields two products in a ratio of 3.0 " 0.5. Theoretical studies shows that only one transition structure is involved in the formation of both. Quasiclassical trajectory calculations on an MP2 surface give a prediction of a product ratio of 45:14 (3.2:1), which is in good agreement with the experimental observation.
Author: Jacqueline Besinaiz Thomas
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Languages : en
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Book Description
Detailed mechanistic studies were conducted on several organic reactions that exhibit product selectivity (regio-, peri-, or enantioselectivity). The organic reactions studied were electrophilic aromatic substitutions, Diels-Alder cycloadditions of 1,3- dienes with cyclopentadieneone, Lewis acid catalyzed ene reactions with olefins, chlorinations of alkynes, and the enantioselective intramolecular Stetter reaction. Analyses of these systems were conducted by measurement of kinetic isotope effects, standard theoretical calculations, and in some cases dynamic trajectories. Mechanistic studies of electrophilic aromatic substitution, Lewis acid catalyzed ene reaction with olefins, the chlorination of alkynes, and the Diels-Alder cycloadditions of 1,3-dienes with cyclopentadienones, suggest that the origin of selectivity is not always a result of selectivity result from a kinetic competition between two closely related pathways to form distinct products. All of these systems involve one transition state on a potential energy surface that bifurcates and leads to two distinct products. In these systems, experimental kinetic isotope effects measured using natural abundance methodology, theoretical modeling of the potential energy surfaces, and trajectory analyses suggests that selectivites (regio- and periselectivities) are a result of influences by momenta and steepest-descent paths on the energy surface. The work here has shown that in order to understand selectivity on bifurcating surfaces, transition state theory is not applicable. In place of transition state energetics, the guiding principles must be those of Newtonian dynamics. In the mechanistic studies for the enantioselective intramolecular Stetter reaction, the origin of selectivity is a result of multiple transition states and their relative energies. Experimental H/D kinetic isotopes effects had lead to the conclusion that two different mechanisms were operating for reactions where carbenes were generated in situ versus reactions using free carbenes. However, 13C kinetic isotope effects and theoretical modeling of the reaction profile provide evidence for one mechanism operating in both cases.
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Publisher: Academic Press
ISBN: 0128122749
Category : Medical
Languages : en
Pages : 634
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Book Description
Experimental Analysis of Enzyme Mechanism Using Isotope Effects, Volume 596, the latest release in the Methods in Enzymology series, continues the legacy of this premier serial with quality chapters authored by leaders in the field. Chapters in this comprehensive update include Measurement of enzyme binding isotope effects, Chemical ligation and isotope labeling to locate dynamic effects, Measurement of heavy enzyme isotope effects, Extracting kinetic isotope effects from a global analysis of reaction progress curves, KIE of metabolic flux and enzymes, Solvent and Primary KIE on Flavin Enzymes, and The Rapid Determination of Primary Deuterium Isotope Effects on Enzyme-Catalyzed Proton Transfer at Carbon in 50/50 HOH/DOD. Readers who are interested in applying or understanding this research will find useful methods currently used for measuring isotope effects on solution and enzyme reactions. Written by pioneers of modern isotope effect research Is the only collection of modern kinetic isotope effect methods currently available
![Isotope Effects, Dynamic Effects and Mechanisms of [2+2] Cycloadditions PDF](https://books.google.com/books/content/images/frontcover/pOnboAEACAAJ?fife=w80-h100)
Author: Zhuo Chen
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Category :
Languages : en
Pages :
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In this dissertation, experimental probes and theoretical calculations have been applied to delineate mechanisms of various [2+2] cycloadditions. Besides common experimental observations and transition state theories, this dissertation focuses on the application of kinetic isotope effects (KIEs) and dynamic effects for a better understanding of the mechanisms. The studies of dimerization of allene and [2+2] cycloaddition between 1,1-dimethyl allene and dimethyl maleate showed significant intramolecular KIEs and important reaction intermediates. These experimental observations strongly support a stepwise mechanism via a diradical intermediate. Based on this proposal, theoretical calculations gave excellent predictions of experimental observations. The controversy from previous literatures was well resolved by carefully analyzing the experimental observations. Research on Lewis acid catalyzed [2+2] cycloaddition between allene and alkenes were conducted via a combination of product studies, experimental kinetic isotope effects, common theoretical calculations and quasiclassical trajectory simulations. The results identified two important dynamic effects in these reactions, the bifurcating energy surface and the non-statistical recrossing. These dynamic effects explained the inverse KIEs in the reaction between allene and tetramethylethylene and the regioselectivity in the reaction between allene and isopropylidenecyclohexane. In stabilized Wittig olefination, the betaine was proposed to be an intermediate in the formation of the oxaphophetane as opposed to modern undertandings of a concerted mechanism. Experimental KIEs were consisted with a two-step mechanism and theoretical calculations located an intermediate along the reaction pathway. Trajectory simulations also showed significant amount of recrossing at the transition state and a possible hidden entropic intermediate in the reaction. These results provided a unique angle to understand the mechanism and the selectivity in the stabilized Wittig olefinations. Dynamic effects not only play important roles in common organic reactions, but also in complicated enzynamic reactions, such as the transannular Diels-Alder reaction catalyzed by the corresponding "Diels-Alderase" SpnF. This reaction includes the role of a [6+4] cycloaddition, a bispericyclic transition state, a bifurcating energy surface, a dynamically stepwise cycloaddition, an entropically-delineated intermediate, and transition state recrossing in the mechanism. The reaction is not its caricature from classical mechanistic analysis and it is not well described by either concerted or stepwise labels. Instead, the mechanism is richer and can only be understood by consideration of dynamics. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/151885
Author: Paul F. Cook
Publisher: CRC Press
ISBN: 9780849353123
Category : Medical
Languages : en
Pages : 656
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Book Description
Isotope effects have become one of the most powerful tools available to the enzymologist for probing enzymic mechanisms. Enzyme Mechanism from Isotope Effects presents the basic theory underlying isotope effects, including the latest findings on proton tunneling and coupled atomic notions. Specific theoretical applications are emphasized in regard to the types of information that can be obtained using isotope effects. The book also examines recent theoretical treatments of the product dependence of deuterium isotope effects, multiple isotope effects and isotope effects on intermediate partitioning. Other topics include a complete discussion of methods for measuring isotope effects, including a detailed description of the use of the isotope ratio mass spectrometer to obtain isotope effects, and a review of the literature regarding mechanistic information obtained from isotope effects for individual classes of enzyme-catalyzed reactions. Enzyme Mechanism from Isotope Effects is an excellent reference source for investigators using isotope effects in their research. The book is also valuable for reference libraries and instructors teaching courses in enzyme mechanism.
Author: Brian E. Schulmeier
Publisher:
ISBN:
Category :
Languages : en
Pages : 86
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Author: Marvin J. Stern
Publisher:
ISBN:
Category : Chemical kinetics
Languages : en
Pages : 44
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Author: C.L. Collins
Publisher:
ISBN: 9780841202894
Category :
Languages : en
Pages : 435
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Author: Amnon Kohen
Publisher: CRC Press
ISBN: 1420028022
Category : Medical
Languages : en
Pages : 1092
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Book Description
The field of isotope effects has expanded exponentially in the last decade, and researchers are finding isotopes increasingly useful in their studies. Bringing literature on the subject up to date, Isotope Effects in Chemistry and Biology covers current principles, methods, and a broad range of applications of isotope effects in the physical, biolo
Author: Valentin P. Ananikov
Publisher: John Wiley & Sons
ISBN: 3527678220
Category : Science
Languages : en
Pages : 483
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Book Description
Exploring and highlighting the new horizons in the studies of reaction mechanisms that open joint application of experimental studies and theoretical calculations is the goal of this book. The latest insights and developments in the mechanistic studies of organometallic reactions and catalytic processes are presented and reviewed. The book adopts a unique approach, exemplifying how to use experiments, spectroscopy measurements, and computational methods to reveal reaction pathways and molecular structures of catalysts, rather than concentrating solely on one discipline. The result is a deeper understanding of the underlying reaction mechanism and correlation between molecular structure and reactivity. The contributions represent a wealth of first-hand information from renowned experts working in these disciplines, covering such topics as activation of small molecules, C-C and C-Heteroatom bonds formation, cross-coupling reactions, carbon dioxide converison, homogeneous and heterogeneous transition metal catalysis and metal-graphene systems. With the knowledge gained, the reader will be able to improve existing reaction protocols and rationally design more efficient catalysts or selective reactions. An indispensable source of information for synthetic, analytical, and theoretical chemists in academia and industry.
