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35348939171
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This is reasonable due to the exothermic character of Michael additions; namely, based on the Hammond postulate, the structure of the transition state for the interaction of 1a with 3 should not be very different from the reagent structures
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This is reasonable due to the exothermic character of Michael additions; namely, based on the Hammond postulate, the structure of the transition state for the interaction of 1a with 3 should not be very different from the reagent structures.
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35348997532
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Macro-model, Version 6.5, Department of Chemistry, Columbia University
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c) Macro-model, Version 6.5, Department of Chemistry, Columbia University.
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0000562558
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3 orbitale of, for example, O atoms see K. B. Wiberg, M. Marquez, H. Castejon, J. Org. Chem. 1994, 59, 6817-6822;
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3 orbitale of, for example, O atoms see K. B. Wiberg, M. Marquez, H. Castejon, J. Org. Chem. 1994, 59, 6817-6822;
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14
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35349019349
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-1 relative to the global minimum) which are different from T, T*, C, and C* only in the opposite configuration of the S asymmetry center; in other words, by an alternative (pseudoequatorial) orientation of the S-Ph substituent.
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-1 relative to the global minimum) which are different from T, T*, C, and C* only in the opposite configuration of the S asymmetry center; in other words, by an alternative (pseudoequatorial) orientation of the S-Ph substituent.
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15
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35349026029
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However, assessment of only T, T*, C, and C* is sufficient since the pseudoequatorial orientation of the S-Ph group in the four other structures brings this bulky substituent and the attacking crotonate too close and thus hinders the attack; c) With regard to the C- and N-pyramid of the anion, pairs T and T* as well as C and C* are invertomers. Obviously, there is no one-step interconversion between T and T* or C and C* invertomers. Diastereotopomerization of these forms occurs by a formal pathway bicycle T (or C, ring-opening, open chain form X1, carbanion inversion, open chain form X2, backbone rotation, open chain form X3, N-inversion, open chain form X4, ring-closure, bicycle T* or C*
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However, assessment of only T, T*, C, and C* is sufficient since the pseudoequatorial orientation of the S-Ph group in the four other structures brings this bulky substituent and the attacking crotonate too close and thus hinders the attack; c) With regard to the C- and N-pyramid of the anion, pairs T and T* as well as C and C* are invertomers. Obviously, there is no one-step interconversion between T and T* or C and C* invertomers. Diastereotopomerization of these forms occurs by a formal pathway bicycle T (or C) - ring-opening - open chain form X1 - carbanion inversion - open chain form X2 - backbone rotation - open chain form X3 - N-inversion - open chain form X4 - ring-closure - bicycle T* (or C*).
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16
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84990134356
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In other words, both the carbanion and the nitrogen are inverted in an open form of the backbone; d G. Boche, Angew. Chem. Int. Ed. Engl. 1989, 28, 277-297;
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In other words, both the carbanion and the nitrogen are inverted in an open form of the backbone; d) G. Boche, Angew. Chem. Int. Ed. Engl. 1989, 28, 277-297;
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g) E. D. Jemmis, G. Gopakumar In The Chemistry of Organolithium Compounds (Eds: Z. Rappoport, I. Marek), John Wiley & Sons, New York, 2004, pp. 1-45;
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A widespread primitive modeling of organic reaction stereochemistry is based on graphical alignment of reagents in their assumed approximate geometries (in other words, on drawing of perspective projections of common 3-D structures, Obviously, this is often highly speculative. On the other hand, an accurate estimation of relative rates of alternative stereochemical pathways is very laborious for bimolecular reactions of conformationally mobile molecules. In contrast to that of stable conformations, quantitative modeling of transition states using ab initio methods is laborious and computer time-consuming owing to difficulties in location of first-order transition states as well as their great number. Moreover, small differences (0.5-1.0 kcal mol-1) in free energy of activation for alternative pathways are sufficient to provide good stereoselectivity. The accuracy of conventional ab initio calculations is unfortunately of the same order of magnitude even for molecules in vac
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-1) in free energy of activation for alternative pathways are sufficient to provide good stereoselectivity. The accuracy of conventional ab initio calculations is unfortunately of the same order of magnitude even for molecules in vacuo. The inaccuracy may be higher for different solvation models. Thus, quantum mechanics ab initio modeling of transition states, in principle an "absolute" method, is in fact too complex to supply reliable analyses of stereoselectivity in reactions of flexible organic species in reasonable timescales. A combined approach, mechanistic consideration of computationally generated or experimentally derived geometries for starting compounds without quantitative modeling of transition states, has turned out to be successful in many cases. For selected examples see: a) A. E. Taggi, A. M. Hafez, H. Wack, B. Young, D. Ferraris, T. Lectka, J. Am. Chem. Soc. 2002, 124, 6626-6635;
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P. Garner, O. Şeşenoǧlu, H. Burgoon, Tetrahedron: Asymmetry 2003, 14, 2883-2887 as well as ref.[1d] For instance, in the case of allylic sulfone anions, this approach revealed different accessibility of exo and endo faces of 2-sulfonyl 2-norbornenyl Li anions upon electrophilic attack. h Thus, such a relatively simple modeling may be viewed as a fruitful practical compromise between the above mentioned approaches to explain or to predict stereochemistry of reaction products
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[1d] For instance, in the case of allylic sulfone anions, this approach revealed different accessibility of exo and endo faces of 2-sulfonyl 2-norbornenyl Li anions upon electrophilic attack. h) Thus, such a relatively simple modeling may be viewed as a fruitful practical compromise between the above mentioned approaches to explain or to predict stereochemistry of reaction products.
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29
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0037468201
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-1 for an optimal disposition of Li cation and a benzene ring (the Li atom is located about 2 Å above the geometrical center of the aromatic cycle; see: a) D. Kim, S. Hu, P. Tarakeshwar, K. S. Kim, J. Phys. Chem. A 2003, 107, 1228-1238;
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-1 for an optimal disposition of Li cation and a benzene ring (the Li atom is located about 2 Å above the geometrical center of the aromatic cycle; see: a) D. Kim, S. Hu, P. Tarakeshwar, K. S. Kim, J. Phys. Chem. A 2003, 107, 1228-1238;
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B. Goldfuss, M. Steigelman, T. Lyschmann, G. Schilling, F. Rominger, Chem. Eur. J. 2005, 11, 4019-4023. Therefore the energy of Li-Ph interaction for T3* is not negligible, albeit Li is above the plane of the phenyl ring at a 3 Å distance and shifted to the edge of the ring. Calculations at the MP2/6-31G(d,p) level (this work) showed that the energy of Li+-π interactions between the Li cation and benzene (a model system) is 13.0 kcal mol-1 when arranged as Li+ and the phenyl ring in conformer T3*. Hence, destroying this Li-Ph bonding in T3* should lead to an increase of the kinetic barrier of the addition. Obviously, this increase is probably much less than 13 kcal mol-1 since the Li-aryl bonding is not fully disrupted in the early transition state
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-1 since the Li-aryl bonding is not fully disrupted in the early transition state.
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