Sigmatropic shifts forbidden, antarafacial

A similar analysis of the 1,5-sigmatropic shift of hydrogen leads to the opposite conclusion. The relevant frontier orbitals in this case are the hydrogen Is orbital and ij/j of the pentadienyl radical. The suprafacial mode is allowed whereas the antarafacial mode is forbidden. The suprafacial shift corresponds to a favorable six-membered ring. 

Sigmatropic shifts represent another important class of pericyclic reactions to which the Woodward-Hoffmann rules apply. The selection rules for these reactions are best discussed by means of the Dewar-Evans-Zimmerman rules. It is then easy to see that a suprafacial [1,3]-hydrogen shift is forbidden in the ground state but allowed in the excited state, since the transition state is isoelectronic with an antiaromatic 4N-HQckel system (with n = 1), in which the signs of the 4N AOs can be chosen such that all overlaps are positive. The antarafacial reaction, on the other hand, is thermally allowed, inasmuch as the transition state may be considered as a Mobius system with just one change in phase. 

Concerted 1,3-sigmatropic shift is feasible only if it proceeds antarafacially or suprafacially with inversion at migration centre. Suprafacial 1, 3-migration of hydrogen is thermally forbidden as there is no question of inversion at this atom which is bonded to carbon through spherically symmetrical IS orbital. Antarafacial 1, 3-concerted shift is forbidden because of steric inhibition of such a process. 

An alternative analysis of sigmatropic reactions involves drawing the basis set atomic orbitals and classifying the resulting system as Htickel or Mobius in character. When this classification has been done, the electrons involved in the process are counted to determine if the TS is aromatic or antiaromatic. The conclusions reached are the same as for the frontier orbital approach. The suprafacial 1,3-shift of hydrogen is forbidden but the suprafacial 1,5-shift is allowed. Analysis of a 1,7-shift of hydrogen shows that the antarafacial shift is allowed. This analysis is illustrated in Figure 10.31. These conclusions based on orbital symmetry considerations are supported by HF/6-31G calculations, which conclude that 1,5-shifts should be suprafacial, whereas... 

In thermal reaction, bonding interaction is maintained in the suprafacial mode of 1,5-shift and hence this process is symmetry allowed, while the antarafacial shift is symmetry forbidden. The suprafacial shift also corresponds to a favorable six-electron Huckel-type transition state in thermal reaction, whereas Huckel-type TS for suprafacial [l,3]-sigmatropic hydrogen shift is antiaromatic and is a forbidden process (Fig. 4.2) [1, 2]. Photochemically, [l,5]-hydrogen shift in the suprafacial mode is a symmetry forbidden process, but antarafacial shift is a symmetry allowed process (Fig. 4.3). 

A bonding interaction can be maintained only in the antarafacial mode therefore, the 1,3-sigmatropic suprafacial hydrogen shift is considered forbidden. Since the geometry required for the orbital symmetry-allowed antarafacial shift is very contorted, this shift, too, is of high energy, and the concerted process in unlikely under conditions of thermal activation. 

Analysis of sigmatropic rearrangement is also possible by PMO method and conclusion about their feasibility are same as by other methods. For instance, 1, 3-sigmatropic suprafacial shift occurs via transition state with 0 nodes and 4 electrons (antiaromatic) and hence it is thermally forbidden. But [1, 3]-antarafacial sigma migration occurs through a transition state with 1 node and four electrons (aromatic T.S.) and is therefore thermally allowed. 

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