Cycloadditions: the Diels-Alder reaction (cont'd)

Substituent effect on product formation.

We treat the two most common combinations in Diels-Alder reactions:

1. an electron donor at C1 in the diene, and an electron withdrawing group in the dienophile. In this case, two products are possible, containing either a 3,4-cyclohexene ring ('ortho product')

   

   or the 3,5-product ('meta product').

   

   We saw before that in 1-methoxycyclopentadiene the largest HOMO coefficient is at C4. In the LUMO of vinyl aldehyde it is the non-substituted carbon that has the highest coefficient. Between those two atoms the first sigma bond is formed relatively fast.
   If we rotate the vinyl aldehyde 180 degrees, the coefficients do not match, and the reaction is slower.
   So the 3,4 product dominates.

2. an electron donor at C2 in the diene, and an electron withdrawing group in the dienophile. We can expect either a 1,5- ('meta type')

   
   

   or a ('para') 1,4-subsituted cyclohexene skeleton.

   

   In 2-methoxycyclopentadiene, it is C1 that has the largest HOMO coefficient, (-0.675 vs. 0.408 at C4), and it is the 1,4 product that is formed fastest.