Sigmatropic rearrangements suprafacial process

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 ... 

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. 

This thermal sigmatropic rearrangement is a suprafacial process since five electron pairs are involved in the reaction. 

Several cases of photochemical reactions, for which the thermal equivalents were forbidden, are shown below. In some cases the reactions simply did not occur thermally, like the [2 +2] and [4 +4] cycloadditions, and the 1,3- and 1,7-suprafacial sigmatropic rearrangements. In others, the photochemical reactions show different stereochemistry, as in the antarafacial cheletropic extrusion of sulfur dioxide, and in the electrocyclic reactions, where the 4-electron processes are now disrotatory and the 6-electron processes conrotatory. In each case,... 

However, the [1,5]-sigmatropic rearrangement is a concerted suprafacial reaction because the HOMO and LUMO (of migrating group, which is H, and of polyene component, pentadienyl) can interact in a suprafacial process (Fig. 8.53). The six electrons involved are considered to occupy the hydrogen HOMO (two) and the (two) and F2 (two) of pentadienyl. The LUMO of pentadienyl is Fs. 

Sigmatropic rearrangement is termed the 1,3-shift of an allylic atom pair ——Y with a concomitant 1,2-shift of an electron pair from Y to X (Scheme 1). It is symmetry allowed as a concerted suprafacial process. 

Let s look at the similar six-electron (three arrow) thermal 1,5 hydrogen sigmatropic rearrangement. We expect the ends of the pi system to change phase as we add two more electrons. This means the rules will reverse and the allowed process will be suprafacial. Figure 12.27 shows the HOMO-LUMO prediction of suprafacial. 

The TS for [3,3]-sigmatropic rearrangements can be considered to be two interacting allyl fragments. When the process is suprafacial in both groups, an aromatic orbital array results and the process is thermally allowed. Usually a chairlike TS is involved but a boatlike conformation is also possible. ... 

The [5,5] sigmatropic rearrangement of (3Z,7Z)-l,3,7,9-decatetraene (equation 11.9) could in theory take place through either a [5a + 5a] or a [5s -I- 5s] pathway. Houk and co-workers calculated a 9 kcal/mol preference for the suprafacial-suprafacial pathway because of geometric strain in the transition structure for the antarafacial-antarafacial process. ... 

Full details have appeared of the behaviour, inter alia of 2,6-disubstituted aryl 2-nitrophenyl sulphides with triethyl phosphite and of 2,6-disubstituted aryl 2-azidophenyl sulphides on thermolysis, following preliminary reports (see Volume 1, p. 470). In both cases the reaction involves a novel heteroaromatic rearrangement that is interpreted in terms of the intervention of nitrene intermediates. The process is exemplified by the behaviour of 2-nitrophenyl 2,6-dimethylphenyl sulphide (40) and 2-azidophenyl 2,6-dimethylphenyl sulphide (41) (Scheme 6). The sigmatropic rearrangement of the postulated intermediate (42), which is necessary to produce the [Z>,e]azepine (44), is not allowed in the suprafacial sense hence the intervention of a biradical species (43) is considered to be more likely. 

In [2,3]-sigmatropic rearrangement, the interactions between the orbitals of allylic radical and migrating group take place in a suprafacial manner to produce a TS of Huckel-type topology consisting of 6e. TS is aromatic in nature and symmetry allowed process (Tig. 4.7). 

Analysis of these sigmatropic processes using Woodward-Hoffmann rules leads to selection rules for sigmatropic rearrangements (Table 4.1) [1, 2]. For sigmatropic rearrangements of order [i, j], where i > 1, the suprafacial or antarafacial nature of migration for both the components should be specified. 

Sigmatropic shift is thermally allowed suprafacial process of Site and 2oe. The thermal rearrangements of aryl ethers 154 to 155, l,2-(l,3-butadienyl) cyclohexyl enolates 156 to 157 and hydrazobenzene 158 to p-benzidine 159 are illustrative... 

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