Sigmatropic rearrangements transition structures

Fig. 6.14. Possible transition structures for [3,3]-sigmatropic rearrangement of 2-cyclohexenyl ester enol ethers. Adapted from J. Org. Chem., 68, 572 (2003), by permission of the American Chemical Society.

In contrast, applying frontier orbital theory to unimolecular reactions like electrocyclic reactions and sigmatropic rearrangements is inherently contrived, since we have artificially to treat a single molecule as having separate components, in order to have any frontier orbitals at all. Furthermore, frontier orbital theory does not explain why the barrier to forbidden reactions is so high—whenever it has been measured, the transition structure for the forbidden pathway has been 40 kJ mol-1 or more above that for the allowed pathway. Frontier orbital theory is much better at dealing with small differences in reactivity. 

This conclusion has been applied in the Ireland-Claisen rearrangement 5.56 —> 5.58, which is one of the most frequently used [3,3] sigmatropic rearrangements, because it sets up two usefully substituted stereogenic centres with high levels of predictability, stemming from the chair-like transition structure 5.57. 

Sigmatropic rearrangements, like the [2,3]-Wittig rearrangement 6.83 and its sulfur and aza analogues, are drawn for the general case in Fig. 6.13, where X=0, S, SR+, or NR2+. An envelope-shaped transition structure is almost always involved, because this allows the smooth development of head-on overlap... 

Treatment of a number of substituted allylic alcohols 102 with morpholino sulfenyl chloride 103 at low temperature led to the formation of amidosulfoxy-lates 104, which rearranged readily during warm up to room temperature. In accordance with the proposed transition-state structures for [2,3]-sigmatropic rearrangements [6,40], the double bond geometry in 105 was found to be E for all cases with = H, CH3 (with a decreased selectivity for some functionalized... 

Both these reactions are more or less diastereoselective and give more or less one diastereoi-somer of 70. To discover the stereochemistry of the major adduct, the silyl ester (R = SiMeZ-Bu) was hydrolysed to the free acid 71 (note the designer chemoselectivity) and this was treated with buffered iodine in MeCN to give a crystalline iodo-lactone 72. An X-ray of this compound revealed that the structure of 71 was as shown and that the [3,3]-sigmatropic rearrangement must have had a boat transition state. 

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