Cycloadditions Pt5

Question 1

The Diels alder [4+2] cycloaddition is one of the most basic cycloadditions reactions
What does it involve?

Answer 1

A favourable in-phase interaction betwen the HOMO of the diene and the LUMO of the dienophile

Question 2

Cyclopentadiene and maleic anhyride can react together in a…
What are the conditions required?

Answer 2

[2+2] cycloaddition
Performed under photochemical conditions

Question 3

Two ethanes can react together in a [2+2] cycloaddition to form a cyclobutane
Draw the HOMO and LUMO orbitals for Ethene

Answer 3

Question 4

Overlap between the HOMO and LUMO does not lead to productive overlap due to being out of phase - impossible to twist bonds to cause overlap - geometrically impossible
This is an example of a reaction that cannot be easily understood by looking at ground state orbital interactions
What does this mean for the reaction?

Answer 4

• This is why the reaction is undertaken under photochemical conditions (hv)
• If we excite one molecular orbital of ethene to produce the first excited state to produce the singly occupied molecular orbital - SOMO
• Results in a SOMO-LUMO interaction (note quite similar to 2 LUMO interactions)

Question 5

Predict which regioisomer is formed as the major product in the following photochemical [2+2] cycloadditions

Answer 5

Question 6

The following molecules are sterochemical permetations of the previous regioisomer
Describe their sterochemistry

Answer 6

• Number 1 and 2 are diasteroisomers
• Although 2 and 3 are symmetrical, the sterogenic centres are different - enantiomers