Q8P - Organic Chemistry (Mcmurry)

Question

Which of the following alkenes would you expect to be good Diels–Alder dienophiles?

Step-by-Step Solution

Answer

Compound (a) Acryloyl chloride is a good dienophile.

Compound (b) methyl pent-4-enoate  is a not good dienophile.

Compound (c) cyclohexene is a not good dienophile.

Compound (d) cyclohex-2-enone is a good dienophile.

Compound (e) cyclo-3-enone is a not good dienophile.

Dienophile

Dienophiles react with diene through 4+2 cycloaddition, forming the cyclic adduct product.

Dienophile reactivity depends on the following factor:

• Electron-withdrawing group 
• Weak pie bond
• Release ring strain

Compound (a) Acryloyl chloride dienophile

Good dienophiles have an electron-withdrawing group (EWG) conjugated with an alkene double bond.

Compound (a)   Acryloyl chloride has the carbonyl group, the electron-withdrawing group. It is also in conjugation with a double bond of the alkene part; therefore, it fulfills both conditions of the good dienophile.

Thus Compound (a) is a good dienophile

Compound (b) Methyl pent-4-enoate dienophile

Good dienophiles have an electron-withdrawing group (EWG) conjugated with an alkene double bond.

Compound (b) methyl pent-4-enoate  has the carbonyl group, the electron-withdrawing group, but the carbonyl group is not conjugation with a double bond of the alkene part; therefore, it does not fulfill both conditions of the good dienophile.

Thus Compound (b) is not a good dienophile.

Compound (c) Cyclohexene dienophile

Good dienophiles have an electron-withdrawing group (EWG) conjugated with an alkene double bond.

Compound (c) cyclohexene doesn't have any electron-withdrawing group and no conjugation present in the Compound. It does not fulfill any condition of a good dienophile.

Thus Compound (c) is not a good dienophile.

Compound (d) Cyclohex-2-enone dienophile

Good dienophiles have an electron-withdrawing group (EWG) conjugated with an alkene double bond.

Compound (d) cyclohex-2-enone has the carbonyl group, the electron-withdrawing group, and the carbonyl group is conjugated with a double bond of the alkene part; it fulfills both conditions of the good dienophile.

Thus Compound (d) is a good dienophile.

Compound (e) Cyclo-3-enone dienophile

Good dienophiles have an electron-withdrawing group (EWG) conjugated with an alkene double bond.

Compound (e) cyclo-3-enone has the carbonyl cyclo-3-enone has a carbonyl electron-withdrawing group, but the carbonyl group is not conjugation with a double bond of the alkene part; therefore, it does not fulfill both conditions of the good dienophile.

Thus Compound (e) is not a good dienophile.