Sigmatropic rearrangements antarafacial processes

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. 

Several cases of photochemical reactions, for which the thermal equivalents were forbidden, are shown below. In some cases the reactions simply did not occur thermally, like the [2 +2] and [4 +4] cycloadditions, and the 1,3- and 1,7-suprafacial sigmatropic rearrangements. In others, the photochemical reactions show different stereochemistry, as in the antarafacial cheletropic extrusion of sulfur dioxide, and in the electrocyclic reactions, where the 4-electron processes are now disrotatory and the 6-electron processes conrotatory. In each case,... 

Sigmatropic rearrangements involving a total of 4n electrons are much less common—one of the components must undergo an antarafacial process, and the geometrical requirements for this are often, though not always, severe. Here are some examples.143-145... 

The [5,5] sigmatropic rearrangement of (3Z,7Z)-l,3,7,9-decatetraene (equation 11.9) could in theory take place through either a [5a + 5a] or a [5s -I- 5s] pathway. Houk and co-workers calculated a 9 kcal/mol preference for the suprafacial-suprafacial pathway because of geometric strain in the transition structure for the antarafacial-antarafacial process. ... 

Analysis of these sigmatropic processes using Woodward-Hoffmann rules leads to selection rules for sigmatropic rearrangements (Table 4.1) [1, 2]. For sigmatropic rearrangements of order [i, j], where i > 1, the suprafacial or antarafacial nature of migration for both the components should be specified. 

The HOMO of an allyl radical is antisymmetric. A [1,3] sigmatropic rearrangement cannot occur in a suprafacial process. An antarafacial rearrangement is symmetry allowed, but cannot occur because of geometric restrictions. 

According to the Woodward-Hoffmann rales, five concerted transition states are possible for the Claisen as well as the closely related Cope rearrangements chair, boat, twist, cross and plane (Table 6). Only the chair and boat TS have to be considered, as twist, cross and plane are antarafacial-anta-rafacial processes and require highly elevated temperatures. - For the correct prediction of product stereochemistry it is nevertheless crucial to know the preference for chair- or boat-like transition state in the actual 3,3-sigmatropic shift. 

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