Fluorinated alkenes LUMOs

In the case of ether 231, the rearrangement can take place through two different vinyl termini. The authors predicted that the reaction would occur at the difluoro terminus, because the fluorines lower the energy of the alkene LUMO, closing the energy separation between the acceptor orbital and the carbanion HOMO, thus accelerating the reaction. The exclusive formation of product 241 confirms this hypothesis. 

Radicals with nucleophilic character add to electrophilic alkenes more rapidly than to nucleophilic alkenes whilst, conversely, the rate of addition of electrophilic radicals to electron-rich alkenes is greater than addition to electron-deficient alkenes. To be more sophisticated, we should refer to a high-SOMO (singly occupied molecular orbital nucleophilic) radical interacting favourably with a low-LUMO (lowest unoccupied molecular orbital electrophilic) alkene but, for simplicity and brevity, we will continue to use these short-hand terms [177]. For instance, compare rates of addition of perfluor-oalkyl radicals to ethene and various fluorinated derivatives with their rates of H-atom abstraction from heptane (Table 7.9) [186]. Broadly, reactivity of the alkene decreases with fluorine content, with trifluoromethyl having a large effect. 

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