Sigmatropic rearrangement suprafacial migration

When an alkyl group migrates, there is an additional stereochemical feature to consider. The shift can occur with retention or inversion at the migrating center. The analysis of sigmatropic shifts of alkyl groups is illustrated in Fig. 11.7. The allowed processes include the suprafacial 1,3-shift with inversion and the suprafacial 1,5-shift with retention. Sigmatropic rearrangements of order [3,3] are very common ... 

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. 

Thus, as predicted by the orbital symmetry rules, this thermal suprafacial [1,3] sigmatropic reaction took place with complete inversion at C-7. Similar results have been obtained in a number of other cases.426 However, similar studies of the pyrolysis of the parent hydrocarbon of 103, labeled with D at C-6 and C-7, showed that while most of the product was formed with inversion at C-7, a significant fraction (11 to 29%) was formed with retention.427 Other cases of lack of complete inversion are also known.428 A diradical mechanism has been invoked to explain such cases.429 There is strong evidence for a radical mechanism for some [1,3] sigmatropic rearrangements.430 Photochemical suprafacial [1,3] migrations of carbon have been shown to proceed with retention, as predicted.431... 

A variety of sigmatropic rearrangements through six-electron transition states are known. In contrast to the rare [l,3]-migrations, [l,5]-shifts of hydrogen in dienes, suprafacially allowed, occur readily. The experiment outlined in Equation 12.88 confirms the predicted stereochemistry.139 The authors estimated the... 

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... 

However, the [1,5]-sigmatropic rearrangement is a concerted suprafacial reaction because the HOMO and LUMO (of migrating group, which is H, and of polyene component, pentadienyl) can interact in a suprafacial process (Fig. 8.53). The six electrons involved are considered to occupy the hydrogen HOMO (two) and the (two) and F2 (two) of pentadienyl. The LUMO of pentadienyl is Fs. 

Hydrogen is not the only group which can migrate, and there are therefore many other kinds of sigmatropic rearrangement. As usual, those involving a total of (4n + 2) electrons are allowed in the all-suprafacial mode, and these are the common reactions. Here are some examples.136-142... 

Figure 12.25 The suprafacial (same face) and antarafacial (spiral to migrate between opposite faces) sigmatropic rearrangement of a hydrogen atom across the ends of two different pi systems (pi LUMO ends in-phase on right, out-of-phase on left).

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