Symmetry-forbidden pathway

Notice that a symmetry-allowed pathway is one in which in-phase orbitals overlap a symmetry-forbidden pathway is one in which out-of-phase orbitals would overlap. A synunetry-aUowed reaction can take place under relatively mild conditions. If a reaction is symmetry-forbidden, it caimot take place by a concerted pathway. If a symmetry-forbidden reaction takes place at all, it must do so by a nonconcerted mechanism. 

Assuming a reactive oxonium ylide 147 (or its metalated form) as the central intermediate in the above transformations, the symmetry-allowed [2,3] rearrangement would account for all or part of 148. The symmetry-forbidden [1,2] rearrangement product 150 could result from a dissociative process such as 147 - 149. Both as a radical pair and an ion pair, 149 would be stabilized by the respective substituents recombination would produce both [1,2] and additional [2,3] rearrangement product. Furthermore, the ROH-insertion product 146 could arise from 149. For the allyl halide reactions, the [1,2] pathway was envisaged as occurring via allyl metal complexes (Scheme 24) rather than an ion or radical pair such as 149. The remarkable dependence of the yield of [1,2] product 150 on the allyl acetal substituents seems, however, to justify a metal-free precursor with an allyl cation or allyl radical moiety. 

Symmetry requirements for concerted reductive elimination of dialkyls have been considered (314), and for trialkylgold(III) species reductive elimination from a trigonal, three-coordinate intermediate was found to be symmetry forbidden. Solvent participation or the involvement of T-shaped species, however, was suggested as possible. Charge transfer to the high-oxidation state gold(III) center and reductive elimination from such a charge transfer state was proposed as an alternative reaction pathway. 

Additional evidence for a stepwise pathway is provided by the fact that a two-step Diels-Alder reaction is observed, in which a formal [2 + 2] reaction gives a vinylcyclobutane (64) which then rearranges to the formal [4 + 2] product (Scheme 41, An = P-CH3OC6H4)118. It has been shown that orbital symmetry control does not operate in these reactions Symmetry-allowed and symmetry-forbidden reactions may take place with equal facility depending upon the conditions119. It has also been shown that the obtention of formally [4 + 2] or [2 + 2] products depends on many factors, including solvent and whether it is the diene or the dienophile which is ionized120. 

A characteristic feature of [2 + 2] cycloaddition reactions is that the symmetry properties of the frontier orbitals of the reactants make them formally symmetry forbidden [4] through a symmetric pathway. As a result, the reactants must overcome an activation barrier which makes the process very slow for homogeneous reactants. In organic chemistry, photoexcitation can be used to change the nature of the frontier orbital occupation, breaking the 7r bond and hence facilitating the reaction, but otherwise high heat and other extreme conditions are required in order to make this type of reaction proceed. For reactions with silicon(lOO), chemisorption is observed to be facile for most alkenes even at room temperature [16], contrary to naive expectations based on the cycloaddition model. 

The so-called least-motion path has been popular for many years. In the original formulation, the elementary reactions that involve the least change in the atomic and electronic configurations are favored (160,161). This hypothesis has found numerous applications in organic and inorganic chemistry (162-167). However, it is necessary to admit that there exist rather numerous exceptions, primarily due to the fact that the non-least-motion pathway is symmetry allowed while the least-motion path is symmetry forbidden (168,169). Dimerization of singlet carbenes is a typical example. 

This chapter includes additional problems related to the material in Chapters 3-6. Some of the mechanisms are mixed for example, there might be a pericyclic reaction followed by a hydrolysis in either acid or base. If a reaction appears to be pericyclic, be sure to determine whether the reaction is symmetry-allowed or symmetry-forbidden under the reaction conditions. If it is symmetry-forbidden, a nonconcerted reaction pathway through radical or charged intermediates will be the most likely mechanism. 

---