Sigmatropic rearrangements suprafacial allowed

A thermal [1,3] sigmatropic rearrangement is allowed only if one component is antarafacial, but a photochemical [1,3] sigmatropic rearrangement is expected to be allowed when it proceeds suprafacially with respect to both components. The stereochemical requirement changes because under photochemical conditions, the HOMO of the three-atom component is tf/2 (symmetric), not i/q (antisymmetric). 

When the total number of electrons is a (4n+2) number, [m,n -sigmatropic rearrangements are allowed if both migrations are suprafacial or both antarafacial. 

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 ... 

Both suprafacial and antarafacial sigmatropic rearrangements are symmetry-allowed, but suprafacial rearrangements are often easier for geometric reasons. The rules for sigmatropic rearrangements are identical to those for cycJoaddition reactions (Table 30.3). 

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. 

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... 

Sigmatropic rearrangement is termed the 1,3-shift of an allylic atom pair ——Y with a concomitant 1,2-shift of an electron pair from Y to X (Scheme 1). It is symmetry allowed as a concerted suprafacial process. 

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... 

Let s look at the similar six-electron (three arrow) thermal 1,5 hydrogen sigmatropic rearrangement. We expect the ends of the pi system to change phase as we add two more electrons. This means the rules will reverse and the allowed process will be suprafacial. Figure 12.27 shows the HOMO-LUMO prediction of suprafacial. 

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