Addition reactions transition state symmetry

How do orbital symmetry requirements relate to [4tc - - 2tc] and other cycloaddition reactions Let us constmct a correlation diagram for the addition of butadiene and ethylene to give cyclohexene. For concerted addition to occur, the diene must adopt an s-cis conformation. Because the electrons that are involved are the n electrons in both the diene and dienophile, it is expected that the reaction must occur via a face-to-face rather than edge-to-edge orientation. When this orientation of the reacting complex and transition state is adopted, it can be seen that a plane of symmetry perpendicular to the planes of the... 

In addition to the use of symmetry to rationalize the structures and force fields of stable molecules and transition states, there has in recent years been considerable interest in its use for predicting the course of chemical reactions. These predictions are based upon the correlation of the electronic states or molecular orbitals of reactants and products through possible intermediate nuclear configurations (49,53-55). Two of the standard examples will illustrate the power of the arguments. A third example will show the limitations. 

The Woodward-Hoffmann rules also allow the prediction of the stereochemistry of pericyclic reactions. The Diels-Alder reaction is an example of (re4s + re2s) cycloaddition. The subscript s, meaning suprafacial, indicates that both elements of the addition take place on the same side of the re-system. Addition to opposite sides is termed antarafacial. The Woodward-Hoffmann rules apply only to concerted reactions and are derived from the symmetry properties of the orbitals involved in the transition state. These rules may be summarised as shown in Table 7.1. 

Variations in interatomic distances for the complexes and transition states on going from E = Si to Ge are rather small and of the same magnitude as those found in the products. Energies of stationary points for reactions 9 and 10 are presented in Table 24. An additional feature of the complex Cl is its C symmetry, and the existence as left (Cll) or right (Clr) handed forms, which are separated by the very low ( ia = 0.4 Kcalmol-1) rotational transition state TSO. A similar situation was found for transition state TS2. It also has a C symmetry, and possesses left (TS21) and right (TS2r) handed forms divided by a low rotational maximum. 

In addition, the reader may realize that axis of rotation can still be present in some chiral Cp-metal complexes (e.g., a C2 axis in the enantiomeric forms in 22 and 23, a C5 axis in 24). With rotation axes present the systems are not asymmetric, only dissymmetric (i.e., lacking mirror symmetry). This is, however, sufficient to induce the existence of enantiomeric forms (218). Moreover, it is known from numerous examples that chiral ligands with C2 symmetry can provide for a higher stereoselectivity in (transition metal-catalyzed) reactions than comparable chiral ligands with a total lack of symmetry. The effect is explained by means of a reduced number of possible competing diastereomeric transition states (218). Hence, rotational symmetry elements may be advantageous for developing useful Cp-metal-based catalytic systems. 

The four-membered cyclic transition state is not allowed by orbital symmetry theory and parity rules. It requires inversion of configuration at the a-carbon and trans addition to the alkene by a conrotatory process, which is sterically impossible [261,263]. The six-membered transition state is allowed by parity rules, but the relative contributions of this pathway and that by unimolecular ionization depends on their relative rate constants and therefore their free energies of activation. Since the transition state of electrophilic addition to alkenes proceeds with a very late transition state requiring an electrophile with a highly developed charge, covalent species are not sufficiently polarized to react directly with alkenes. Thus, the reaction should occur in two steps rather than by a concerted addition [264],... 

In addition, Dewar demonstrated that the classification of pericyclic reactions has nothing to do with symmetry. The nature of the reaction is defined by the AO overlap topology in a pericyclic transition state and not by the MO symmetry. 

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