Diels-Alder Reaction, 1.2 and 1,4 addition, MO Theory

Diels-Alder reaction and 1,2 and 1,4 addition are siblings concepts. They are related to diene structure that yield out different products becuase diene has cis and trans. Cis diene will give diels-alder reaction; Trans diene will give 1,2 addition and 1,4 addition - the kinetics and thermodynamics products. ( Fast or slow products)

Trans diene                          Cis diene

When solving this type of reaction, the first and the most important thing is to number each carbon on diene and dienephile. Although it seem trivial, rather, the sequence of numbering comes to be very handy while solving the problems.

Step1 - number 1 to 4 at diene, 5 and 6 on dienephile. The arrow always worth some partial credits and it mostly move at clockwise direction as it is shown below. This is true ONLY if the dienephile is symmetry. What if it is not? Some more examples will show later. But bear in mind that if the dienephile is not symmetry, you will have to move around the direction of dienephile on your own. You cannot use what it was drawn on the exam.  We will get back to this again. For the simple case ...

Another example is if the diene now is a close ring, this will make the drawing more 3D like.

 Still, first step is to number 1 to 4 on diene and 5, 6 on dienephile. Though you may wonder how about the ring. See example below... the peak of the diene is drawn upwards in the end.

1,2 addition and 1,4 addition