Orbital symmetry conservation reactions

The Woodward-Hoffmann method [52], which assumes conservation of orbital symmetry, is another variant of the same idea. In it, the emphasis is put on the symmetries of molecular orbitals. Longuet-Higgins and Abramson [53] noted the necessity of state-to-state correlation, rather than the orbital correlation, which is not rigorously justified (see also, [30,44]). However, the orbital symmetry conservation rules appear to be very useful for most themial reactions. 

Which of the following reactions are allowed according to the orbital symmetry conservation rules Explain,... 

For the discussion of the rearrangements in section III only stepwise reaction sequences will be considered. We must note, however, that many of these transformations may just as well result from concerted processes in accordance with the rules for orbital symmetry conservation,... 

Keywords historical background, discovery of Diels-Alder reaction, discovery of orbital symmetry, conservation rule... 

Diels-Alder Reaction Dienophiles Difluoroallene Dioxoindoles Dioxolanyliums Discovery of Diels-Alder reaction Discovery of orbital symmetry conservation rule Diterpenes Domino Drtmane sesquiterpenes Compilation 0031, 9804, 9742, 9741. 9737, 9705, 9410, 9406, 9306, 9218, 9118 9126 9705 9833, 9604 9222 9222 0003 0033, 0024, 9835, 9014 9116... 

If a four-membered ring peroxide (1.2-dioxetane) is involved in a reaction, its concerted bond cleavage into two carbonyl moieties should yield one of these in its excited electronic state on the basis of the orbital symmetry conservation rules of R. B. Woodward and R. Hoffmann ... 

Theoretical analyses of the reaction path of photocyclization point to the same conclusion. Thus the qualitative state correlation procedure clearly indicates that photocyclization takes place by a conrotatory process in the Orbital Symmetry Conservation sense requiring a C2 molecular symmetry in 7 and in its symmetric congeners. The same conclusion were reached in the subsequent numerical analysis of the photocyclization of 7 and of 44 The detailed molecular structures of these two molecules and of 61 have been calculated by semi-empirical energy minimization procedures (cf also Ref. ). 

These reactions are characterized by the phenomenon that the frontier orbitals of the reactants maintain a defined stereochemical orientation throughout the w hole reaction. Most noteworthy in this respect, is the principle of orbital symmetry conservation ( Woodward-Hoffmann rules la), but the phenomenon is much more general, as shown by the following examples of Self-Immolative Stereoconversion or Chirality Transfer . This term describes processes by which a stereocenter in the starting material is sacrificed to generate a stereocenter in the product in an unambiguous fashion. This is, of course, the case in classical SN2-displacements. 

Unlike thermal [2 + 2] cycloadditions which normally do not proceed readily unless certain structural features are present (see Section 1.3.1.1.), metal-catalyzed [2 + 2] cycloadditions should be allowed according to orbital symmetry conservation rules. There is now evidence that most metal-catalyzed [2 + 2] cycloadditions proceed stepwise via metallacycloalkanes as intermediates and both their formation and transformation are believed to occur by concerted processes. In many instances such reactions occur with high regioselectivity. Another mode for [2 + 2] cyclodimerization and cycloadditions involves radical cation intermediates (hole-catalyzed) obtained from oxidation of alkcnes by strong electron acceptors such as triarylammini-um radical cation salts.1 These reactions are similar to photochemical electron transfer (PET) initiated [2 + 2] cyclodimerization and cycloadditions in which an electron acceptor is used in the irradiation process.2 Because of the reversibility of these processes there is very little stereoselectivity observed in the cyclobutanes formed. 

Although thermal [2 + 2] cycloadditions are forbidden as concerted reactions by the orbital symmetry conservation rules the same structural features which promote intermolecular cy-cioadditions will also promote intramolecular reactions. In addition, the proximity between two alkene moieties dictated by the tether length and rigidity would make these processes entropically favorable. A few reports have documented thermal intramolecular cycloadditions to cyclopropenes and activated alkenes. The thermal Cope rearrangement of allylcyclopropenes apparently proceeds by a two-step mechanism in which intramolecular [2 + 2] adducts have been observed.72-73... 

The rules of orbital symmetry conservation apply only to concerted reactions in photochemical processes these are usually those of singlet excited states, since the triplet states often lead to long-lived biradical intermediates. 

Orbital Symmetry Conservation in Bimolecular Cycloadditions. The cycloaddition reactions of carbonyl compounds to form oxetanes with ethylenes, as well as those of enones and their derivatives to form cyclobutanes, are examples of reactions which originate from triplet excited states and lead in the first step to biradical intermediates. Such reactions are of course not concerted, and they show little or no stereo-specificity. 

Predictions can be made about the suitability of different system trajectories on the basis of orbital symmetry conservation rules (207). The most suitable trajectory is an approximation to the reaction path of the reaction under study. The rules can also yield information about the possible structure of the activated complex. The correlation diagram technique has been improved in a series of books by Epiotis et al. (214-216). The method is based on self-consistent field-configuration interaction or valence bond (SCF-CI or VB) (including ionic structures) wave functions. Applications on reactions in the ground states as well as in the excited electronic states are impressive however, the price to be paid for the predictions seems to be rather high. 

The highest six occupied molecular orbitals and the LUMO of 89 and 90 are displayed in Figure 4.22. If a planar transition state connects these two molecules, then the MOs along the reaction path are rigorously separated into those that are symmetric and antisymmetric with respect to the molecular plane. The symmetric orbitals cannot mix with the antisymmetric orbitals and therefore the electron occupancy of each set of MOs will not change. The reaction is allowed because of the orbital symmetry conservation. 

To appreciate the conformational implications of orbital-symmetry conservation, we consider these two reactions with groups Ri and R2 replacing two of the terminal hydrogens. (As per convention, the remaining hydrogens are not drawn.) If the reactions are carried out under thermal conditions, they proceed as follows ... 

Orbital symmetry conservation extends simple molecular orbital theory into reaction chemistry. Its principal power is its simplicity in application. Orbital symmetry descriptions of transforming systems focus on the molecular orbitals and their symmetries relative to common elements of symmetry (i.e., the symmetry elements of the transition state) maintained throughout a concerted reaction. 

Molecular orbital symmetry conservation requires that molecular orbitals maintain their symmetry about the common elements across the reaction coordinate. This gives the orbital correlations outlined in Fig. 5. The olefin AS n combination, for example, transforms into the... 

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