Sigmatropic shifts allowed, suprafacial

The results were interpreted in terms of an allowed thermal 1,3-sigmatropic shift with suprafacial-mvQxsiow stereochemistry, and the lower specificity in and higher temperature necessary for the rearrangement of syn material was argued to be due to steric inhibition of the inversion pathway. 

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 ... 

Both these 1,51 hydrogen shifts occur by a symmetry-allowed suprafacial rearrangement, as illustrated in Figure 30.12. In contrast with these thermal [L,51 sigmatropic hydrogen shifts, however, thermal [1,3 hydrogen shifts are unknown. Were they to occur, they would have to proceed by a strained antarafacial reaction pathway. 

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 " ... 

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>. 

Thermal sigmatropic 1,5 hydrogen shifts are quite common in certain allene and diene systems, cis- 1,3-Dienes have the proper geometric arrangement to undergo a thermally allowed suprafacial hydrogen shift, cis-1,3-Hexadiene, for instance, gives... 

Furthermore, a rather bold but chemically more feasible mechanism can account for the proposed migration of the acetoxy group. This is double, symmetry allowed, suprafacial-suprafacial, Claisen-type, 3,3-sigmatropic shift. 

This operation is in essence an incomplete, symmetry allowed suprafacial 1,2-sigmatropic shift, where the low electron density that develops progressively at the lip of the cyclopropane ring is compensated by the favorably located dicyanocarbanion. This prevents the transformation of IX into the hypothetical cation X, a potential precursor of divergent reactions. 

The [1,5] shift would be a suprafacial reaction. The [1,7] shift must be antarafacial in the v component to be thermally allowed. The proton, located at carbon A in 6-20, moves from the bottom side of the six-mem-bered ring to the top side of the w system at the other end. In general, the antarafacial process, necessary for a thermal [1,7] sigmatropic shift, occurs with ease, especially when compared to the antarafacial process that would be required for a thermal [1,3] sigmatropic shift. 

Sigmatropic shifts represent another important class of pericyclic reactions to which the Woodward-Hoffmann rules apply. The selection rules for these reactions are best discussed by means of the Dewar-Evans-Zimmerman rules. It is then easy to see that a suprafacial [1,3]-hydrogen shift is forbidden in the ground state but allowed in the excited state, since the transition state is isoelectronic with an antiaromatic 4N-HQckel system (with n = 1), in which the signs of the 4N AOs can be chosen such that all overlaps are positive. The antarafacial reaction, on the other hand, is thermally allowed, inasmuch as the transition state may be considered as a Mobius system with just one change in phase. 

Suprafacial [1, 5]-sigmatropic shifts of hydrogen and silyl substituents are allowed, generally facile, and proceed without inversion of stereochemistry on the migrating atom. A well-known example is the 1,5-shift of an allylsilyl group in silylcyclopentadienes, which proceeds with retention of configuration281. 

II.B above). As is apparent from the experimental yields, the different xylene isomers afford different proportions of products, signifying that complete equilibration does not take place during the lifetime of the ion. These experimental results present one of the earliest demonstrations of orbital symmetry-allowed suprafacial sigmatropic shifts of methyl groups. 

According to the rules of orbital symmetry, vicinal hydride shift in cations is a thermally allowed suprafacial 1,2-sigmatropic shift. Migration of a more distant hydrogen can occur by successive 1,2-shifts, via a bridged structure (such as the cyclooctyl cation in Scheme 7 ), or by corner-to-corner transfer in a protonated cyclopropane (which give net 1,3-shifts). An EBFlow experiment has been able to measure the competition between 1,2- and 1,3-shift in the 1-fluoropropyl system. 

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