Sigmatropic rearrangement 1, 3-shift

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 ... 

Under acidic conditions, the first step involves protonation of the imine nitrogen followed by tautomerization to form an ene-hydrazine intermediate (7). After the tautomerization, a [3,3]-sigmatropic rearrangement occurs, which provides intermediate 8. Rearomatization then occurs via a proton shift to form the imine 9 which cyclizes to form the 5-membered ring 10. Finally, loss of ammonia from 11 generates the indole nucleus in 12. 

Because a [1,5] sigmatropic rearrangement involves three electron pairs (two ir bonds and one cr bond), the orbital-symmetry rules in Table 30.3 predict a suprafacial reaction. In fact, the 1,5] suprafacial shift of a hydrogen atom across... 

Thia-[2,3]-Wittig sigmatropic rearrangement of lithiated carbanions 47, obtained by deprotonation of the S-allylic sulfides 46, affords the thiols 48 or their alkylated derivatives 49. The corresponding sulfonium ylides 51, prepared by deprotonation of the sulfonium salts 50 also undergoes a [2,3]-sigmatropic shift leading to the same sulfides 49 [36,38] (Scheme 13). As far as stereochemistry is concerned, with crotyl (R R =H,R =Me) and cinnamyl (R, R =H,R =Ph) derivatives, it has been shown that the diastereoselectivity depends on the nature of the R substituent and on the use of a carbanion or an ylide as intermediate. 

Ammonium ylides can isomerize to (1,2) rearrangement products (Stevens rearrangement) or to (2,3) shift products (Sommelet-Hauser sigmatropic rearrangement) when allyl or benzyl are located on the nitrogen atom. A strong microwave effect is noticed (Eq. 66) [116]. 

This is called (1, 3) shift because the original o bond attached at carbon 1 has been broken and it has attached itself at the carbon 3. But in a petadiene of the type given below (1,5) sigmatropic rearrangement can take place. 

Labelling experiments using both 15N and 2H indicate that the rearrangement is intramolecular. Reactions are also acid-catalysed and are believed to occur via the Wheland intermediates 74 and 75. The most likely interpretation is that the rearrangement occurs within the Wheland intermediate by a direct 1,3-shift rather than by consecutive 1,2-shifts, and that the process can be regarded as a typical [l,5]-sigmatropic rearrangement. 

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