1,3-Suprafacial migration

As expected, the Mobius-Hiickel method leads to the same predictions. Here we look at the basis set of orbitals shown in G and H for [1,3] and [1,5] rearrangements, respectively, A [1,3] shift involves four electrons, so an allowed thermal pericyclic reaction must be a Mobius system (p. 1070) with one or an odd number of sign inversions. As can be seen in G, only an antarafacial migration can achieve this. A [1,5] shift, with six electrons, is allowed thermally only when it is a Hiickel system with zero or an even number of sign inversions hence it requires a suprafacial migration. 

Thermal 1,5-hydrogen shifts are thus allowed and, because of the symmetry of the T.S. (39), the H atom in the product (37, x = 1) will be on the same side of the common plane of the polyene s carbon atoms as it was in the starting material (36, x = 1) this is described as a suprafacial shift. This latter point would not be experimentally verifiable in the above example, but that thermal 1,5-shifts (which are quite common) do involve strictly suprafacial migration has been demonstrated in the compound (40). This is found, on heating, to yield a mixture of (41) and (42), which are produced by suprafacial shifts in the alternative conformations (40a) and (406), respectively ... 

Another example of suprafacial migration with inversion is the rearrangement of bicyclo octenone to give again a ketone and reported by J.A. Berson (1972). 

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. 

If j is of the form 4n 1, suprafacial migrations are photochemically allowed. For antarafacial migrations, the restrictions are reversed. See also Antarafacial Suprafacial Reactions... 

Artemisyl, Santolinyl, Lavandulyl, and Chrysanthemyl Derivatives.— The presence of (41) in lavender oil has been reported earlier. Poulter has published the full details of his work (Vol. 5, p. 14) on synthetic and stereochemical aspects of chrysanthemyl ester and alkoxypyridinium salt solvolyses (Vol. 3, pp. 20—22) and discussed its biosynthetic implications. Over 98% of the solvolysis products are now reported to be artemisyl derivatives which are formed from the primary cyclopropylcarbinyl ion (93) which results from predominant (86%) ionization of the antiperiplanar conformation of (21)-)V-methyl-4-pyridinium iodide the tail-to-tail product (96 0.01%) may then result from the suprafacial migration of the cyclopropane ring bond as shown stereochemically in Scheme 3. This is consistent with earlier work (Vol. 7, p. 20, ref, 214) reporting the efficient rearrangement of the cyclobutyl cation (94) to (96) and its allylic isomer, via the tertiary cyclopropylcarbinyl cation (95). ... 

The degenerate rearrangement of methylenecyclobutane occurs with inversion at C2 and suprafacial migration across one side of the allylie unit. However, the rearrangement is 65% antarafacial with respect to the allylie component.82... 

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. 

Photolysis of 4,4-diphenyI-4//-pyran (280) in tert-butyl alcohol gave a mixture of unsaturated aldehyde 619 and compounds 620a,b.296,453 Surprisingly, the formation of aldehyde 619 apparently proceeded via intermediate 617 and not by the alternative pathway via isomeric 2H-pyran 618 (Scheme 38). As mentioned in Sections V,E,4b and V,E,4c, 4-benzyI-2,4,6-triaryl-4//-pyrans and their thia analogs photoisomerize to 2-benzyI-2//species236,392,400,414 416 by a concerted 1,3-suprafacial migration of... 

Another example of suprafacial migration with inversion is the rearrangement of bicycio octenone... 

In an excited state (i.e. photochemically) the HOMO is now ij 4 and it is antisymmet-rical. Therefore, [1,5]-suprafacial migration of H is no longer possible i.e. it is symmetry forbidden. The photochemical reaction will follow the antarafacial route, which is... 

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. 

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