Correspondence diagram for -sigmatropic rearrangement of propylene

There are no surprises The pathway chosen by the correspondence diagram is indeed the one that goes through the antarafacial transition state, as predicted by the WH-Rules. To be sure, there is very little experimental evidence for the occurrence of thermal [l,3]-sigmatropic rearrangements, but their rarity has been reasonably ascribed to the high steric strain [14, p. 1016] and poor orbital overlap [30, p. 99] in the allowed antarafacial transition state. 

This reaction can be analysed by means of a simple extension of the analysis of the [l,3]-shift just discussed. Pentadiene has two carbon and two hydrogen atoms more than propylene the total number of orbitals is thus raised by ten to twenty-eight, as is the number of valence electrons. However, after the four acc and three fixed ctch bonds are combined to form seven localized molecular orbitals and their antibonding counterparts to seven more, we are left with 14 mobile electrons in 7 doubly occupied MOs. There is no need to draw a new correspondence diagram for the reaction to a slight amplification of Fig. 8.6 is all that is required. 

The only change that has to be made in the hypothetical STS at the extreme right of Fig. 8.6 is the replacement of the non-bonding Py b2) orbital by three allyl 7T orbitals a bonding 62 a non-bonding 02 and an antibonding 62 orbital  

The fact that AH is so much lower than that for the corresponding rearrangement of pentadiene, for which the suprafacial pathway is geometrically convenient, supports our conclusion that the suprafacial rearrangement of the latter, though it is undoubtedly the preferred pathway at elevated temperatures - and is probably concerted though perhaps not synchronous - is inconsistent with orbital symmetry conservation. 

Note first that the shift of a substituent from atom j to atom 1 - or equivalently to atom j — 1), can be described as either a [1,2]- or a [l,j]-shift it is universally regarded to be the latter. Moreover, unless the ring is extremely large, only the suprafacial pathway need be considered. The same Rules are conventionally applied to it as to the linear [l,j]-rearrangement, so the thermal rearrangement of a single migratory atom is forbidden when j = 3,7. (4n — 1), 

---