Formation of six-membered rings - Diels-Alder reactions

2 Formation of six-membered rings - Diels-Alder Reactions [245, 246] 

In this section we will consider [4 + 2] cycloaddition reactions in which the fluoroalkene acts as the dienophile. For related reactions involving fluorinated dienes, see Section IIG. 

We have seen (Table 7.13) that in reactions between fluoroalkenes and dienes, [2 + 2] cycloaddition as opposed to [4 + 2] cycloaddition is the dominant reaction [246, 247], and systematic studies performed on reactions of this type, such as that between 1,1-dichlorodifluoroethene and isoprene, provide a strong case for the intermediacy of 

The thermal addition of trifluoroethene to cyclopentadiene at and below 122° C yields a 1,4-cycloadduct, with less than 0.1% of 1,2-cycloadduct, whereas a photosensitised reaction between these two reactants (a di-radical process) leads to a product consisting of 87% of the 1,2-cycloadduct and 13% of the 1,4-cycloadduct [250]. These contrasting results led to the conclusion that the thermally induced Diels-Alder product arises from a normal concerted process and it is probable that, in general, whilst the 1,2-cycloadditions and 1,4-Diels-Alder additions are competing processes, they are mechanistically unrelated. 

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