Preparation of aldehydes
Reduction of nitrile with a less powerful reducing reagent, e.g. DIBAH,
produces aldehyde. The reaction is carried out at low temperatures (À78
C)
in toluene.
R C N
R C H
O
Aldehyde
Nitrile
ii. H 2 O
i. DIBAH
5.8 Pericyclic reactions
Pericyclic reactions are concerted reactions that take place in a single step
without any intermediates, and involve a cyclic redistribution of bonding
electrons. The concerted nature of these reactions gives fine stereochemical
control over the generation of the product. The best-known examples of this
reaction are the Diels–Alder reaction (cyclo-addition) and sigmatropic
rearrangement.
5.8.1 Diels–Alder reaction
In the Diels–Alder reaction, a conjugated diene reacts with an a,bunsaturated carbonyl compound, generally called a dienophile. A dienophile is a reactant that loves a diene. The most reactive dienophiles
usually have a carbonyl group, but it may also have another electronwithdrawing group, e.g. a cyano, nitro, haloalkene or sulphone group
conjugated with a carbon–carbon double bond.
CN
NO 2
Cl
O
O
S
O
O
Dienophiles other than carbonyl group directly linked to the conjugated system
The Diels–Alder reaction is in fact a [4 + 2] cycloaddition reaction, where
C-1 and C-4 of the conjugated diene system become attached to the doublebonded carbons of the dienophile to form a six-membered ring. For
example, 1,3-butadiene reacts with maleic anhydride to produce tetrahydrophthalic anhydride on heating.
O
O
O
O
O
O
H
H
+
1
4
1, 3-Butadiene
A conjugated diene
Maleic anhydride
A dienophile
Benzene
Heat
Tetrahydrophthalic anhydride
95%
278
CH5 ORGANIC REACTIONS
Reduction of nitrile with a less powerful reducing reagent, e.g. DIBAH,
produces aldehyde. The reaction is carried out at low temperatures (À78
C)
in toluene.
R C N
R C H
O
Aldehyde
Nitrile
ii. H 2 O
i. DIBAH
5.8 Pericyclic reactions
Pericyclic reactions are concerted reactions that take place in a single step
without any intermediates, and involve a cyclic redistribution of bonding
electrons. The concerted nature of these reactions gives fine stereochemical
control over the generation of the product. The best-known examples of this
reaction are the Diels–Alder reaction (cyclo-addition) and sigmatropic
rearrangement.
5.8.1 Diels–Alder reaction
In the Diels–Alder reaction, a conjugated diene reacts with an a,bunsaturated carbonyl compound, generally called a dienophile. A dienophile is a reactant that loves a diene. The most reactive dienophiles
usually have a carbonyl group, but it may also have another electronwithdrawing group, e.g. a cyano, nitro, haloalkene or sulphone group
conjugated with a carbon–carbon double bond.
CN
NO 2
Cl
O
O
S
O
O
Dienophiles other than carbonyl group directly linked to the conjugated system
The Diels–Alder reaction is in fact a [4 + 2] cycloaddition reaction, where
C-1 and C-4 of the conjugated diene system become attached to the doublebonded carbons of the dienophile to form a six-membered ring. For
example, 1,3-butadiene reacts with maleic anhydride to produce tetrahydrophthalic anhydride on heating.
O
O
O
O
O
O
H
H
+
1
4
1, 3-Butadiene
A conjugated diene
Maleic anhydride
A dienophile
Benzene
Heat
Tetrahydrophthalic anhydride
95%
278
CH5 ORGANIC REACTIONS