Suprafacial sigmatropic shift

Hiickel-type systems (such as Hilcfcel pericyclic reactions and suprafacial sigmatropic shifts) obey the same rules as for sigma electron. The rationale for this observation is clear If the overlap between adjacent p-electron orbitals is positive along the reaction coordinate, only the peraiutational mechanism can... 

A similar analysis of the 1,5-sigmatropic shift of hydrogen leads to the opposite conclusion. The relevant frontier orbitals in this case are the hydrogen Is orbital and ij/j of the pentadienyl radical. The suprafacial mode is allowed whereas the antarafacial mode is forbidden. The suprafacial shift corresponds to a favorable six-membered ring. 

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 3//-azepines obtained by cycloaddition of azirines to cyclopentadienones (see Section 3.1.1.1.2.) are thought to arise from the initially formed 2/7-azepines by [1,5]-H suprafacial sigmatropic shifts.31-108 In contrast, 1/Z-azepine 9 results from the thermal rearrangement of the nonisolable 2//-azepine-2-carboxylate 8.13 Presumably, the 1 //-azepine is stabilized, relative to the 3//-isomer, by intramolecular hydrogen bonding between the NH and the adjacent ester group. 

When dimethyl-2,2-dioxo-5-methyl-177,377-pyrrolo[l,2-r-][l,3]thiazole-6,7-dicarboxylate was heated under flash vacuum pyrolysis at 700°C/10 3mmHg, sulfur dioxide was eliminated and the vinylpyrrole 391 was obtained, which can be explained by allowed suprafacial [1,8]H shifts in the 871 1,7-dipolar system 390. Concerted sigmatropic shifts can only occur when the methyl groups adopt an inward (Z)-conformation (Scheme 57) <2002J(P1)1795, 2004TL3889, 2005JOC6629>. 

The outcome of all this for photochemical sigmatropic shifts is that those most commonly observed are of order (1.3) or (1.7) these involve 4 or 8 electrons, respectively, and occur in a suprafacial manner. Examples of photochemical 1.3-shifts of hydrogen are found for monoalkenes (2.25) and for conjugated dienes 2.26). In the case of dienes a 1,3-shift is favoured over a 1,5-shift, because for the latter to occur photochemically it would have to take place in an antarafacial manner. Note that in both examples the direction of... 

Thiazole itself reacts with DMAD at room temperature in DMF, the initially formed adduct (88) rearranging either via a concerted suprafacial 1,5-sigmatropic shift or by a non-concerted pathway proceeding via zwitterion (89) to (90 R = R = H Scheme 50) (79HC(34-i)i, p. 97). The three monomethylthiazoles and 2,5-dimethylthiazole undergo the same type of cycloaddition with rearrangement when condensed with DMAD in DMF (Scheme 50 R,R = Me, H H, Me-4 H, Me-5 Me, Me-5). 

---