Pericyclic reactions antarafacial process

Predict the product of the following pericyclic reaction. Is this [5,5) shift a suprafacial or an antarafacial process ... 

We may further extend the analysis of pericyclic reactions by considering that a single p orbital, denoted by the symbol m, can be a participant in a pericyclic reaction. In this analysis, one lobe of the p orbital makes up the top face of a one-atom n system, while the other lobe makes up the bottom face. The participation of a single p orbital is suprafacial if both cycloaddition processes involve only one of the two lobes of the p orbital, and it is antarafacial if the cycloaddition involves both. We may thus predict that the conrotatory opening of the cyclopropyl anion to an allyl anion (Figure 11.72) should take place via an -F 2 ] pathway. Conversely, the opening of the cation would be a -F 2 ] process, giving the opposite stereochemistry in the product." ... 

We noted in Chapter 15 that, for the most part, the orbital symmetry rules are not directly applicable to photochemistry. However, some photochemical reactions of simple tt systems do give products that are consistent with expectations based on orbital symmetry, although this does not prove that these are concerted, pericyclic processes, The photochemical selection rules for pericyclic reactions are opposite of those for thermal pericyclic reactions. For example, there are many examples of [1,3] and [1,7] sigmatropic shifts that appear to go by the photochemically "allowed" suprafacial-suprafacial pathway Eqs. 16.22 and 16.23 show two (recall that the thermal reactions would be suprafacial-antarafacial). These reactions occur upon direct irradation, while sensitized photolysis produces products more consistent with biradical-type reactions. 

Transformation of a carbon-carbon triple bond to a carbon-carbon double bond (equation 1) can proceed through a variety of mechanisms. If the reaction takes place in one step (equation 2), the reaction is called pericyclic and, because in the product the atoms or groups X and Y are always on the same side of the double bond, the addition is a suprafacial one. In principle, it is also possible that X and Y are donated by different molecules (equation 3). Transition state a leads to a suprafacial addition product and transition state b to an antarafacial one. These termolecular processes would require the simultaneous encounter of three reacting species, which is not likely to happen. In most cases the addition... 

Similar to the previous pericyclic processes, the sigmatropic reactions can also proceed by two alternative mechanisms. The first of them, where the migrating atom or group remains on the same side of the nodal plane of the n system during the whole process, is called suprafacial, whereas for the other, antarafacial mechanism, the migration of the group from one side of the nodal plane to the other is typical. 

Occasionally, though, you will run across a more exotic pericyclic process, and will want to decide if it is allowed. In a complex case, a reaction that is not a simple electrocyclic ringopening or cycloaddition, often the basic orbital symmetry rules or FMO analyses are not easily applied. In contrast, aromatic transition state theory and the generalized orbital symmetry rule are easy to apply to any reaction. With aromatic transition state theory, we simply draw the cyclic array of orbitals, establish whether we have a Mobius or Hiickel topology, and then count electrons. Also, the generalized orbital symmetry rule is easy to apply. We simply break the reaction into two or more components and analyze the number of electrons and the ability of the components to react in a suprafacial or antarafacial manner. 

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