Orbital symmetry, conservation

POTENTIAL ENERGY SURFACES BARRIERS, MINIMA, AND FUNNELS 

Except in the case of very exothermic reactions (cf. Example 6.7), molecules that have traveled from the reactant geometry to the biradicaloid minimum will have little chance of reaching the product geometry on the same surface, because the return to Sg from the pericyclic minimum or a nearby conical intersection will be very efficient. 

---

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,... 

Molecular Orbital Symmetry Conservation in Transition Metal Catalysis Frank D. Mango... 

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... 

The stereochemistry of these cyclopropyl cleavages is governed by the principle of orbital symmetry conservation (for a discussion, see p. 1434). 

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 ... 

The principles of orbital symmetry conservation establish that concerted suprafacial [3, 3]-sigmatropic arrangements are allowed processes. Based on these principles and the results of numerous experiments their stereochemistry is highly predictable. 

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 rearrangement occurs at least partially with antarafacial allylie participation and was estimated to be concerted in the sense of orbital-symmetry conservation control. Transition-structures appropriate to concerted pathways were judged likely to have exorbitantly high energies as a result of poor orbital overlap and unfavorable steric interactions. 

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. 

Ogata has asked (in Japanese) the question What kind of learning is physical organic chemistry Its history, main themes, and research trends are said to be described.432 Bhatt has written on physical organic chemistry-retrospect and prospect.433 As Tetrahedron Perspective Number 1, a fascinating article by J A. Berson434 describes some of the historical background of the diene synthesis and the orbital symmetry conservation rules. 

---