Unimolecular reactions electrocyclic

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... 

Electrocyclic reaction (Section 30.3) A unimolecular peri-cyclic reaction in which a ring is formed or broken by a concerted reorganization of electrons through a cyclic transition state. For example, the cyciization of 1,3.5-hexatriene to yield 1,3-cyclohexadiene is an electrocyclic reaction. 

Nitrosoimines can undergo thermal reaction, a unimolecular, two-step mechanism has been proposed, as shown in Scheme 3.22 [193]. In this mechanism, a concerted electrocyclization is envisioned to form the strained four-membered ring in 41, followed by a presumably forbidden, but highly exothermic, deazetization to give 41. The electrocyclic ring closure is, at first glance, a 4-electron process, analogous to the cyclization of butadiene [194] or acrolein [194, 195]. This would be expected to involve rotation around the C=N bond coupled with C-O bond formation. 

Whereas cycloadditions are characterized by two components coming together to form two new a-bonds, electrocyclic reactions are invariably unimolecular. They are characterized by the creation of a ring from an open-chain conjugated system, with a a-bond forming across the ends of the conjugated system, and with the conjugated system becomes shorter by one p-orbital at each end. 

In an electrocyclic reaction, a cyclic system (ring closure) is formed through the formation of a a-bond from an open-chain conjugated polyene system at the cost of a multiple bond and vice versa (ring opening). These reactions are unimolecular in nature as the rate of reactions depends upon the... 

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. 

It has been found possible to extend and amplify QMOT procedures so that they apply to chemical reactions. One of the most striking examples of this was application to unimolecular cyclization of an open conjugated molecule (e.g., cw-1,3-butadiene, closing to cyclobutene). This type of reaction is called an electrocyclic reaction. The details of the electrocyclic closure of cw-1,3-butadiene are indicated in Fig. 14-17. 

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