Symmetry rules, sigmatropic shifts

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... 

Application of symmetry rules also yields important results for the class of reactions known as sigmatropic shifts, in which a double bond switches position as an atom or a group migrates ... 

Table 7.4 Orbital Symmetry Rules for Thermal (A) and Photochemical (/iv) Sigmatropic [/, J] Shifts...

It is of special interest that the orbital symmetry rules can be applied to sigmatropic migrations within ionic species. E.g. the suprafacial 1,2 shift within a carbonium ion is symmetry-allowed ... 

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. 

The rearrangement step is a ground-state thermal process, and may be classified as a [l,4]-sigmatropic shift of carbon across the face of a 2-oxybutenyl cation. The Woodward-Hoffmann rules require a sigmatropic shift in this system to proceed with inversion of configuration. The symmetry properties for 1,4-shifts with inversion and retention are shown in Fig. 11.6. 

Experimentally found selection rules for the borotropic migrations correspond to the sigmatropic shifts with inversion on the migrating atom according to the orbital symmetry principles [43]. This in turn corresponds to the electronic configuration of the boron atom ideal for the formation of a Moebius type transition state. 

With these descriptors in hand, we can look at the generalized orbital symmetry rule. There is a definite binary nature to the theory of pericyclic reactions. For cycloadditions, [2-f2] is forbidden (all suprafacial), whereas [4-F2] is allowed (all suprafacial). Continuing with the series, [6+2] is forbidden, and [8+2] is allowed. We will also encounter patterns in the other kinds of pericyclic reactions presented electrocyclic reactions, sigmatropic shifts, etc. Based on patterns such as these. Woodward and Hoffmann proposed the following rule for all pericyclic reactions ... 

Lastly, we can examine all these different shifts using the generalized orbital symmetry rule. Various examples of allowed combinations are shown in Figure 15.21 D. The [3,3] shift is best viewed as a three-component reaction, where all three act in a suprafacial manner in order to be allowed. Generalizing all these approaches for two-component reactions yields Table 15.3, which summarizes sigmatropic shifts. 

We noted in Chapter 15 that, for the most part, the orbital symmetry rules are not directly applicable to photochemistry. However, some photochemical reactions of simple tt systems do give products that are consistent with expectations based on orbital symmetry, although this does not prove that these are concerted, pericyclic processes, The photochemical selection rules for pericyclic reactions are opposite of those for thermal pericyclic reactions. For example, there are many examples of [1,3] and [1,7] sigmatropic shifts that appear to go by the photochemically "allowed" suprafacial-suprafacial pathway Eqs. 16.22 and 16.23 show two (recall that the thermal reactions would be suprafacial-antarafacial). These reactions occur upon direct irradation, while sensitized photolysis produces products more consistent with biradical-type reactions. 

Orbital symmetry rules apply to sigmatropic reactions within ionic species also. For this reason, [1, 2]-shift in a carbonium ion is symmetry allowed [1, 4]-shift occurs with inversion of configuration, e.g., in but-2-en-l-yl cation [1, 6]-shift in hexa-2,4-diene-l-yl cation is expected through suprafacial transition state. 

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