Unimolecular reactions sigmatropic rearrangements

The ratios of nucleophilic substitution versus [3,3]-sigmatropic rearrangement for the collapse of allenyl(aryl)iodine(III), generated from the reaction of aryliodanes with propargylsilanes in the presence of Bp3.0Et2 in alcohols, have been determined. The mechanism proposed by the authors ° involves the generation of propargyl cations from the allenyliodine (III) via a unimolecular pathway. 

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. 

Applying frontier orbital theory to unimolecular reactions like electrocyclic ring closures and sigmatropic rearrangements is inherently contrived, since we are looking at only one orbital. To set up an interaction between frontier orbitals, we... 

A sigmatropic rearrangement is a unimolecular reaction in which a a bond moves from one position in the molecule (20) to another (21). Thus (20) - (21) is a [3,3] sigmatropic reaction. The numbers (see 20) give the position of the new cJ bond relative to the old. 

Sigmatropic rearrangements are the unimolecular isomerization reactions in which a a-bond moves from one position to another over an unsaturated system. In such reactions, rearrangement of the Tt-bonds takes place to accommodate the new a-bond, but the total number of Tt-bonds remains the same. 

In order to apply the FMO approach in unimolecular pericyclic reactions like electrocyclic reactions and sigmatropic rearrangements, we have to treat a single molecule as having separate components. In such a case, only HOMO of the component has to be considered to predict the feasibility of the reaction under given conditions. Furthermore, this theory does not teU why the energy barrier to forbidden reactions is so high. 

Our interest in this area began with chorismate mutase, which is responsible for the only sigmatropic rearrangement known in a primary metabolic pathway (Figure 6). It is unique also for the fact that the reaction it catalyzes is unimolecular, and occurs at a measurable rate in aqueous solution by a mechanism that is substantially the same as that... 

The major reaction in the thermal decomposition of 2,3-dihydrofuran (9) is a unimolecular isomerization to cyclopropanecarboxaldehyde (89JPC-1139). In an analogous [1,3] sigmatropic reaction, the isomerization of 2-methyl-4,5-dihydrofuran (10) leads to acetylcyclopropane, which can rearrange to 3-penten-2-one (94JPC2341). The latter product may also be formed directly from 10. 

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