Sigmatropic Rearrangement of Propylene

This reaction will be analysed in somewhat greater detail than has been done up to now. As noted in Section 7.1.1.1, a common assumption made whenever reaction path computations are carried out is that the same atomic orbital basis set is adequate for following the reaction path. If the reactive system is made up entirely of carbon and hydrogen atoms, the electron and orbital count is particularly simple Each C atom brings four electrons and each H atom provides one the minimal basis set comprises a I5 AO for each H atom and a set of four - one 2s and three 2p AOs - for each C atom. The number of independent linear combinations that can be constructed from a system with Nc carbon and A h hydrogen atoms is therefore iNc + iVu), half of which will be occupied and half unoccupied in the closed shell ground-state of all the molecular species concerned. 

Orbitals in minimal basis set 18 Valence electrons 18 Symmetry species of the atomic and group orbitals in 02  

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  

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Figure8.6. Correspondence diagram for [l,3]-sigmatropic rearrangement of propylene...

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