1,3-Elimination of two heteroatoms

Based on the diene substrates and the reaction conditions, two distinct mechanisms can be followed to induce alkene activation and consequent cycloisomerization. The first mechanism invokes the hydrometallation of an alkene, followed by its subsequent carbometallation into the second one, terminated by a p-hydride elimination. The second mechanism implies the formation of a it-allylic complex either by its prior or later to the cycloisomerization formation (Scheme 7.35). Dienes possessing heteroatom functionality in the allylic position is able to provide it-allylic complex with a metal prior its cyclization. On the other hand, polyenes bearing 1,3-dienes in their system are commonly producing a 1,3-metal complex, which is readily cycloisomerize to give a ii-allyl metal complex after the formation of the new carbon—carbon bonds. 

As our computational results presented above demonstrate, it is highly unlikely that heterocycles would be good dienes for Diels-Alder reactions if formation of one or two C-N bonds were involved in the course of the reaction. This automatically eliminates some tautomeric forms of five-membered heterocycles with heteroatoms in 1 and 2 positions as well as five-membered heterocycles with heteroatoms in 1,2,3 and 1,2,5 positions. A major reason for the low reactivity of the heterocycles is because of their high aromaticity. It is obvious that diminishing or eliminating the aromaticity in these heterocycles would make them better dienophiles for Diels-Alder reactions. 

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