Sigmatropic migration suprafacial

The allene moiety of the products 70b, 72 and 75 is in each case chiral and, furthermore, an additional chiral center is created in 72a,b and 75b,e-g, thereby leading to the possible formation of diastereomers. However, the concerted nature of such sigmatropic processes should result in suprafacial migrations and formation of the racemate of only one diastereomer in each case, as shown for 74 — 75 in Scheme 7.10. High stereoselectivity can really be found for the reaction of (fc)-71a and 74b,e,f, but not for other examples of type 71 and for 74g, which lead to mixtures of diastereomers. 

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. 

In agreement with the theoretical prediction of suprafacial reaction, compounds 6-12 and 6-13 were produced rather than 6-14 and 6-15. Because of the favorable geometry for suprafacial migration, there are many examples of thermal [1,5] sigmatropic shifts, which occur with ease. 

Therefore, if we derive or remember one rule for a pericyclic reaction, then any time an MO phase change is added the rule will reverse. Two reversals cancel each other. For example, 4n face to face (supra-supra) cycloadditions are not thermally allowed. If we add two electrons, we fill the next highest MO, which has a phase reversal. This means An+2 cycloadditions are thermally favored. Thermal electrocyclic reactions of 4n species go conrotatory, whereas thermal 4n+2 electrocyclic reactions go disrotatory. Thermal sigmatropic reactions of 4n species go supra-inversion or antara-retention. Count arrows to tell whether the pericyclic reaction is 4n or 4n + 2. Phase reversals occur between retention/inversion at the migrating center, between antarafacial/suprafacial migration, with 4n vs. 4n+2 electrons, and between thermal and photochemically excited species. 

Today, it is believed that [2,3]-sigmatropic rearrangements of ammonium zwitterions such as 7 proceed through a five-membered pericyclic transition state 8 of a doubly suprafacial migration tScheme IS.31. The transition state is of the Hiickel t e, and since six electrons... 

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 ... 

These are suprafacial sigmatropic shifts of order [1,5] and should occur with retention of configuration at the migrating carbon. This stereochemical course has been established for the 1,5-alkyl shift that converts 16 to 17. The product which is isolated, 18, results from a subsequent 1,5-hydrogen shift, but this does not alter the stereochemistry at the migrating... 

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)... 

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... 

Thermolysis of 75 in solution177 can afford successively three dimers 321, 327, and 328. Their origin is the unstable diether 318, which at 80°C dimerizes to 321 by an unusual ( 8S + 6S) concerted cycloaddition. Above 110°C, 321 isomerizes to 327 by a concerted 1,5-suprafacial sigmatropic migration. 

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