Pericyclic reactions disrotatory process

The closed and open forms, 4 and 5, respectively, represent the formal starting and end points of an electrocyclic reaction. In terms of this pericyclic reaction, the transition state 6 can be analysed with respect to its configurational and electronic properties as either a stabilized or destabilized Huckel or Mobius transition state. Where 4 and 5 are linked by a thermally allowed disrotatory process, then 6 will have a Hiickel-type configuration. Where the process involves (4q + 2) electrons, the electrocyclic reaction is thermally allowed and 6 can be considered to be homoaromatic. In those instances where the 4/5 interconversion is a 4q process, then 6 is formally an homoantiaromatic molecule or ion. 

Finally, the last entry in Table 6.5 is, like the first, another example of a cheletropic reaction. This time, in contrast to the first, a diene (rather than a monoene) and sulfur (IV) dioxide (rather than the more reactive carbene) are undergoing an orbital symmetry allowed (A + t[2s) pericyclic process. This allowed reaction occurs with retention of symmetry since (although not shown in Table 6.5) the geometry of the substituted diene is retained in the product. Thus, as shown in Equation 6.57, the diene, (2E,4E)-hexadiene (c Figure 4.42), enters the reaction in a suprafacial fashion and, as expected for a disrotatory process (Chapter 4), produces the d5 -2,5-dihydrothiophene-l,l-dioxide ... 

Figure 15.18 shows several examples of electrocyclic processes. Since the reactions are always allowed in either a conrotatory or disrotatory manner, the key issue is the control of stereochemistry. Electrocyclic reactions provide a good example of the power of pericyclic reactions in this regard. In all cases, the reaction proceeds as predicted from the various theoretical approaches. The restrictions placed by the orbital analysis on the reaction pathway are nicely demonstrated by examples D and E in Figure 15.18 only the stereochemistry given is found. An instructive example of the fact that it is the number of electrons that controls the process, not the number of atoms or orbitals, is the conrotatory ring closure of the four-electron pentadienyl cation prepared by protonation of a divinyl ketone (example G). 

The cis relationship between methyl and ring junction methine hydrogen atom in 7 indicates a disrotatory process. Spirotricyclic product 8 was also isolated in 17% yield from this reaction, presumably as a consequence of a cascade of two pericyclic reactions. A thermally allowed [1,7]-hydrogen shift leads from 6 to triene 9 that undergoes 6ti-disrotatory ring closure to 8. In both photochemical as well as thermal electrocyclizations, there may be multiple... 

The stereospecific ring-opening reactions of cyclopropyl derivatives have played a key role in establishing selection rules for pericyclic processes . Extensive and conclusive evidence has been presented in support of DePuy s initial postulation that disrotatory ringopening and C-X bond heterolysis are synchronous processes, and all kinetic , stereochemical and theoretical findings lend credence to the DePuy-Hoffmann-Wood ward rule Substituents on the same side of the 3-membered ring as... 

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