Effect of Dienophile Substituents on Chemoselectivity

SCHEME 4.3 Observed products associated with the dimerization of dienes 8a-d. 

As shown in Table 4.1, formation of the mixed adduct is favored over homodimerization of 8a with the simple styrene 13a, but this selectivity is inverted for the case of the more bulky dienophile tra 5-[3-methylstyrene 13b, presumably due to steric effects. Although the overall reaction is highly exothermic on the radical cation surface, the reaction is not insensitive to steric effects. Chemoselectivity in the radical cation cycloaddition is largely a consequence of a substrate s ability to stabilize the radical cation of the oxidized species through the formation of a weakly bound ion-molecule complex. Such complexes have been known for a long time in gas-phase 

Entry Reagent Sensitizer Solvent Yield (%) Mixed Product/8a Dimer 

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