Symmetry-disallowed reaction

Symmetry-allowed reactions often occur under relatively mild conditions, but symmetry-disallowed reactions can t occur by conceited paths. Either they take place by nonconcerted, high-energy pathways, or they don t take place at all. 

Symmetry-allowed, symmetry-disallowed (Section 30.2) A symmetry-allowed reaction is a pericyclic process that lias a favorable orbital symmetry for reaction through a concerted pathway. A symmetry-disallowed reaction is one that does not have favorable orbital symmetry for reaction through a concerted pathway. 

Reviews of general interest this year have concerned force-field calculations (by Altona), Bredt s rule, adamantane rearrangements," triptycene chemistry, the relation between mass-spectral behaviour and thermolytic and photolytic reactivity, the removal of orbital symmetry restrictions to organic reactions, catalysis by transition metals of symmetry-disallowed reactions, and [2,2]paracyclophanes, many of whose reactions lead to bridged or cage molecules. 

If the symmetries of reactant and product orbitals match up, or correlate, the reaction is said to be symmetry-allowed. If the symmetries of reactant and product orbitals don t correlate, the reaction is symmetry-disallowed. 

The initial dimerization presumably leads to a [2.2.2]propellane derivative that undergoes thermolysis to the diene that is the observed product. The dimerization occurs in dilute solutions, but at higher concentrations, polymers are found. This finding is in accord with the proposal that these reactions occur via initial formation of a 1,4-diyl, which closes to form the cyclobutane ring. In this way, the reaction avoids an orbital symmetry disallowed pathway. The diyl could react with the alkene at higher concentrations to give a polymer. 

Reactions that are symmetry-disallowed either don t take place or occur by a higher-energy nonconcerted pathway. 

Just because a reaction is symmetry-disallowed doesn t mean that it can t occur. Reactions that are symmetry-allowed occur by relatively low-energy, concerted pathways. Reactions that are symmetry-disallowed must take place by higher energy, nonconcerted routes. 

A cycloaddition reaction is one in which two unsaturated molecules add to one another, ieJding a C3 clic product. As with electrocydic reactions, cycloadditions are controlled by the orbital symmetry of the reactants. Symmetry-allowed processes often take place readily, but symmetry-disallowed processes take place with great difficulty, if at all, and then only by non-concerted pathways. Let s look at two examples to see how they differ. 

Why aren t the [2 + 2] cycloaddition and retro-cycloaddition of the Wittig reaction disallowed The Woodward-Hoffmann rules state that when the symmetries of the MOs of the reactants are mismatched in ways that we have discussed, the TS for the reaction is raised very high in energy, and the reaction is therefore disallowed. In principle, even a disallowed reaction can proceed at sufficiently high temperatures the reason this approach usually doesn t work is that other reactions usually take place at temperatures far below those required for the disal-... 

Baldwin, John E. 1972. "Orbital-Symmetry-Disallowed Energetically Concerted Reactions." Accounts of Chemical Research 5 402-406. 

If the symmetries of the orbitals of the reactants and products match, a concerted symmetry-allowed reaction occurs. If the symmetries of the reactants and products do not match, the reaction is symmetry forbidden (or sometimes, symmetry disallowed ). Symmetry-allowed reactions occur under reasonably mild conditions because the molecular orbitals can be smoothly transformed from reactant to product. Symmetry-forbidden reactions may actually occur, but they require much higher energies than symmetry-allowed reactions, and they occur by a different mechanism. They are not concerted, usually generate intermediates in multistep reactions, and are not stereospecific. 

Note that this substituent is absent in (71) and hence the radical mechanism would be discouraged in favor of a concerted reverse cycloaddition. The final product (69) could then arise by two-step cycloaddition of (76) to another mole of (67). The above suggested mechanism has the advantage of avoiding symmetry problems associated with the simpler mechanism (73—<-79— 69). If the latter mechanism is concerted according to the FMO theory, the overlapping lobes of the HOMO orbital of one reactant and the LUMO orbital of the other reactant should have the same phase. The coefficients of the HOMOs and LUMOSs of (67) and (78) reveal a mismatch for suprafacial attack and therefore the reaction is disallowed. Similarly, suprafacial extrusion of N2 from (73) is disallowed. 

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