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1
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33846446636
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In certain cases, an alternative to degenerate rearrangement can consist of the existence of stable intermediate species with nonclassical bonds (such as the bridged cation in the case of Figure 1a, the symmetrical intermediate in the case of Figure 1b, or the structure with equalized CO and OH bonds in the case of Figure 1c). In fact, the choice between these two possibilities is not always trivial and may require additional experiments.
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In certain cases, an alternative to degenerate rearrangement can consist of the existence of stable intermediate species with nonclassical bonds (such as the bridged cation in the case of Figure 1a, the symmetrical intermediate in the case of Figure 1b, or the structure with equalized CO and OH bonds in the case of Figure 1c). In fact, the choice between these two possibilities is not always trivial and may require additional experiments.
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3
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0006633502
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J. S. McKennis, L. Brener, J. S. Ward, and R. Pettit, J. Am. Chem. Soc. 93 (1971) 4957-4958.
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McKennis, J.S.1
Brener, L.2
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Pettit, R.4
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4
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0010002975
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Reaction Graphs
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D. Bonchev and O. Mekenyan Eds, Kluwer Acad. Publ, Dordrecht
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A. T. Balaban, Reaction Graphs, in: D. Bonchev and O. Mekenyan (Eds.), Graph Theoretical Approaches to Chemical Reactivity, Kluwer Acad. Publ., Dordrecht, 1994, pp. 137-180.
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Graph Theoretical Approaches to Chemical Reactivity
, pp. 137-180
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Balaban, A.T.1
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G. A. Jones and E. K. Lloyd, Automorphism Groups of Some Chemical Graphs, in: R. B. King (Ed.), Chemical Applications of Topology and Graph Theory, Stud. Phys. Theor. Chem., 28, Elsevier, Amsterdam, 1983, pp. 252-267.
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G. A. Jones and E. K. Lloyd, Automorphism Groups of Some Chemical Graphs, in: R. B. King (Ed.), Chemical Applications of Topology and Graph Theory, Stud. Phys. Theor. Chem., Vol. 28, Elsevier, Amsterdam, 1983, pp. 252-267.
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19
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0003831071
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McGraw-Hill, New York, 713 pp
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J. Brocas, M. Gielen, and R.Willem, The Permutational Approach to Dynamic Stereochemistry, McGraw-Hill, New York, 1983, 713 pp.
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(1983)
The Permutational Approach to Dynamic Stereochemistry
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Brocas, J.1
Gielen, M.2
Willem, R.3
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20
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0010084081
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Perspectives in Theoretical Stereochemistry
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Springer-Verlag, Berlin, 247 pp
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I. Ugi, J. Dugundji, R. Kopp, and D. Marquarding, Perspectives in Theoretical Stereochemistry, Lecture Notes in Chemistry, Vol. 36, Springer-Verlag, Berlin, 1984, 247 pp.
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Lecture Notes in Chemistry
, vol.36
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Ugi, I.1
Dugundji, J.2
Kopp, R.3
Marquarding, D.4
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24
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33846458205
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Just as in the case of nondegenerate reactions, many degenerate processes invented by a chemist or designed by a computer cannot be experimentally accomplished. Estimation of the feasibility of new DTs may require special investigations such as calculation of thermodynamic and kinetic parameters, modeling of the stereochemistry for transition states, consideration of the possible mechanisms for multistage processes, etc.
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Just as in the case of nondegenerate reactions, many degenerate processes invented by a chemist or designed by a computer cannot be experimentally accomplished. Estimation of the feasibility of new DTs may require special investigations such as calculation of thermodynamic and kinetic parameters, modeling of the stereochemistry for transition states, consideration of the possible mechanisms for multistage processes, etc.
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28
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33846444642
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30 In this rearrangement, just like in the process shown in Figure 1e, multiple repetition of the [3,3]-sigmatropic shift (the Cope reaction) leads to averaging of all carbons and all hydrogens in the molecule. However, the number of structures rearranging into each other is much greater in the case of bullvalene (1,209,600) than in the case of hypostrophene (five; see the graph in Figure 1f).
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30 In this rearrangement, just like in the process shown in Figure 1e, multiple repetition of the [3,3]-sigmatropic shift (the Cope reaction) leads to " averaging" of all carbons and all hydrogens in the molecule. However, the number of structures rearranging into each other is much greater in the case of bullvalene (1,209,600) than in the case of hypostrophene (five; see the graph in Figure 1f).
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30
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84868357103
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G. Schröder, Chem. Ber. 97 (1964) 3140-3149.
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(1964)
Chem. Ber
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, pp. 3140-3149
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Schröder, G.1
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33
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84961469044
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M. Baudler, H. Ternberger, W. Faber, and J. Hahn, Z. Naturforsch. 34b (1979) 1690-1697.
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Z. Naturforsch
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, pp. 1690-1697
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Baudler, M.1
Ternberger, H.2
Faber, W.3
Hahn, J.4
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38
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33846433440
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J. C. Barborak, J. Daub, D. M. Follweiler, and P. v. R. Schleyer, J. Am. Chem. Soc. 91 (1969) 7760-7761.
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(1969)
J. Am. Chem. Soc
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, pp. 7760-7761
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Barborak, J.C.1
Daub, J.2
Follweiler, D.M.3
Schleyer, P.V.R.4
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41
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33846448444
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40,42
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40,42
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44
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37049129078
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A thorough search unexpectedly showed that, in contrast to simple 1,4-dioxa-Cope reactions, at least two such [3,3]-shifts have been observed in complex polycyclic systems. Rearrangement of the α-tocopherol spirodimer (see M. Chauhan, F. M. Dean, K. Hindley, and M. Robinson, Chem. Commun. (1971) 1141-1143) is an impressive example; this multistep process is acid-catalyzed and probably proceeds via the intermediate phenoxylium ion.
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A thorough search unexpectedly showed that, in contrast to simple "1,4-dioxa-Cope" reactions, at least two such [3,3]-shifts have been observed in complex polycyclic systems. Rearrangement of the α-tocopherol spirodimer (see M. Chauhan, F. M. Dean, K. Hindley, and M. Robinson, Chem. Commun. (1971) 1141-1143) is an impressive example; this multistep process is acid-catalyzed and probably proceeds via the intermediate phenoxylium ion.
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47
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0001343855
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R. Curci, V. Lucchini, G. Modena, P. J. Kocienski, and G. Ciabattoni, J. Org. Chem. 38 (1973) 3149-3153.
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(1973)
J. Org. Chem
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, pp. 3149-3153
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Curci, R.1
Lucchini, V.2
Modena, G.3
Kocienski, P.J.4
Ciabattoni, G.5
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57
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33846463720
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In the general case, classification of atoms into reaction centers, structural centers, and substituents depends on the choice of resonance structures for reagents and products; the distinction of structural centers and substituents can also be nontrivial see ref. 53 for details, Here is a simple example: carbon atoms of the phenyl group are not structural centers in most cases; the entire group C6H5 is typically regarded as a univalent substituent of a special kind
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5 is typically regarded as a univalent substituent of a special kind.
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58
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33846409387
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As far as we are aware, no attempts to elaborate a hierarchical classification of degenerate bond redistributions have been made. The very notion of type has been systematically applied to DTs in only one paper: in Ref. 13, all DTs characterized by the same reaction equation were attributed to the same type. It is important that designations of reactions often used by organic chemists, i,j]-sigmatropic shift, i,j]-migration in cations, etc, should not serve as the basis for classification either: these designations are not universal, can be just as well applied to nondegenerate transformations, and sometimes fail to distinguish between completely dissimilar processes see examples of [3,3]-shifts in Figures 3a-h and examples of [1,2, and [1,3]-shifts in carbonium ions in Figures 6a-d
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As far as we are aware, no attempts to elaborate a hierarchical classification of degenerate bond redistributions have been made. The very notion of type has been systematically applied to DTs in only one paper: in Ref. 13, all DTs characterized by the same reaction equation were attributed to the same type. It is important that designations of reactions often used by organic chemists ([i,j]-sigmatropic shift, [i,j]-migration in cations, etc.) should not serve as the basis for classification either: these designations are not universal, can be just as well applied to nondegenerate transformations, and sometimes fail to distinguish between completely dissimilar processes (see examples of [3,3]-shifts in Figures 3a-h and examples of [1,2]- and [1,3]-shifts in carbonium ions in Figures 6a-d).
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60
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33846410981
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The resulting transformation evidently represents a certain structural type of degeneracy, which is unusual in the sense that no topological degeneracy type corresponds to it, Otherwise, the process characterized by the symbolic equation in Figure 7c would also be degenerate, Thus, in the general case, the interrelationship between topological types/subtypes and structural types/subtypes of degeneracy is not as trivial as it may seem at first sight
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The resulting transformation evidently represents a certain structural type of degeneracy, which is unusual in the sense that no topological degeneracy type corresponds to it. (Otherwise, the process characterized by the symbolic equation in Figure 7c would also be degenerate.) Thus, in the general case, the interrelationship between topological types/subtypes and structural types/subtypes of degeneracy is not as trivial as it may seem at first sight.
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61
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0037676268
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Similar degenerate [1,2]-shifts in the dodecahedryl radical and anion seem to be unknown as well. The possible stabilization of anionic analog of Figure 7g owing to the rolling charge effect was considered as far back as 40 years ago; see H. P. Schultz, J. Org. Chem. 30 (1965) 1361-1364.
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Similar degenerate [1,2]-shifts in the dodecahedryl radical and anion seem to be unknown as well. The possible stabilization of anionic analog of Figure 7g owing to the "rolling charge" effect was considered as far back as 40 years ago; see H. P. Schultz, J. Org. Chem. 30 (1965) 1361-1364.
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69
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33846406837
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For unsigned centers, the valence (or reaction number, in the more strict terminology)48,50 is allowed to change here only by two units (see centers X in Figures 9a,f, the sum of valence changes over all atoms cannot exceed 4. In principle, charged centers can change their valence by one (such centers are denoted by • and •+ if the charge corresponds to the lower valence of the center, or by X and X+ if the charge corresponds to its higher valence, by 3 (denoted by X and X+ if the charge corresponds to the lower valence, or by Y and Y+ if the charge corresponds to the higher valence, or by 5 units. The program makes it possible to distinguish underlined (referred to as pseudospecific) centers from non-underlined ones. In the example under consideration, equations with centers whose valence is changed by 5 units and equations with centers Y are not generated
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+ if the charge corresponds to the higher valence), or by 5 units. The program makes it possible to distinguish underlined (referred to as pseudospecific) centers from non-underlined ones. In the example under consideration, equations with centers whose valence is changed by 5 units and equations with centers Y are not generated.
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70
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33845471185
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R. P. Lutz, Chem. Rev. 84 (1984) 205-247.
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(1984)
Chem. Rev
, vol.84
, pp. 205-247
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Lutz, R.P.1
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71
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33846412541
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2), cyanamide and carbodiimide (N≡C-NR- and -N=C=NR), or cyanate and isocyanate (N≡C-O- and -N=C=O) groups.
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2), cyanamide and carbodiimide (N≡C-NR- and -N=C=NR), or cyanate and isocyanate (N≡C-O- and -N=C=O) groups.
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72
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33846448671
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Moscow, in Russian
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S. S. Tratch, M. S. Molchanova, and N. S. Zefirov, in: Third All-Russian Conference on Molecular Modeling, Moscow, 2003, p. 123 [in Russian].
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(2003)
Third All-Russian Conference on Molecular Modeling
, pp. 123
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Tratch, S.S.1
Molchanova, M.S.2
Zefirov, N.S.3
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