Antarafacial migration sigmatropic

A similar analysis of [1,5] sigmatropic rearrangements shows that in this case the thermal reaction must be suprafacial and the photochemical process antarafacial. For the general case, with odd-numbered /, we can say that [1,/] suprafacial migrations are allowed thermally when j is of the form 4n + 1, and photochemically when j has the form An - 1 the opposite is true for antarafacial migrations. 

The actual reported results bear out this analysis. Thus a thermal [1,3] migration is allowed to take place only antarafacially, but such a transition state would be extremely strained, and thermal [1,3] sigmatropic migrations of hydrogen are unknown." On the other hand, the photochemical pathway allows suprafacial [1,3] shifts, and a few such reactions are known, an example being " ... 

In general four types of transition states can be envisaged for sigmatropic migrations of the order (/, /), two in which the migrating group moves suprafacially (a) and (b) and two in which it means antarafacially (c) and (d)... 

The rules for [m,n -sigmatropic rearrangements, where m / 1 and n / I, are more complex still. The bond can migrate suprafacially or antarafacially on either component, with the great majority of known reactions being... 

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