Suprafacial and Antrafacial Addition
™
Suprafacial is addition to lobes on the same side of
the π system
H2n
Cn
supra, supra
symmetry-allowed
Cn
H2n
™
Antrafacial is addition to lobes on the opposite side
of the π system
H2n
Cn
supra, antara
symmetry-allowed
Cn
H2n
1
™ A supra,antara is impossible in [2 + 2] and [4 + 2]
due to geometric grounds.
™ If the ring being formed is big enough, both
supra,supra and supra,antara process are
geometrically possible.
™ Cycloadditions are reversible. These cycloreversions
(retro-Diels-Alder rxn.) follow the same symmetry
rules as cycloadditions, since they occur via the same
transition states.
2
1
™ Diels-Alder reactions are reversible.
™ Reaction conditions can often be chosen so as to favor
reactants or products.
™ For example, cyclopentadiene is normally obtained by
the reverse Diels-Alder reaction of dicyclopentadiene.
2
Cyclopentadiene
(bp 41oC)
Dicyclopentadiene
(bp 170oC)
• This Diels-Alder dimer, a viscous liquid, slowly forms from
cyclopentadiene at RT.
• Cyclopentadiene, a volatile liquid, is distilled from the dimer on heating.
3
retro- Diels-Alder reaction of dicyclopentadiene
+
Diels-Alder reaction of dicyclopentadiene
[4 + 2]
+
RT
4
2
Woodward – Hoffmann Rules for [i + j] cycloaddition
i+j
Thermal
Photochemical
4n
i.e. [2 + 2]
supra,antara
antara,supra
supra,supra
antara,antara
4n + 2
i.e. [4 + 2]
supra,supra
antara,antara
supra,antara
antara,supra
5
[6+4] thermal cycloaddition
• Six is not the only (4n + 2) number and there are a few
cycloadditions involving 10 electrons.
• These are mostly diene + triene, that is, 4π + 6π cycloaddition.
O
O
=
O
There is an endo relationship between the carbonyl group
and the back of the diene – this product is formed in 100% yield.
6
3
Photochemical Cycloaddition
Photochemical cycloaddition of alkenes to form four-membered
rings [2+2] has been used in many synthesis sequences.
Some of the reactions require a photosensitizer; others do not.
O
+
O
hν
O
O
(C6H5)2CO
O
O
Even heterocycles can be generated when one of the multiple
bonds includes a heteroatom.
(C6H5)2
hν
(C6H5 )2C=O +
(C6 H5 )2
O
O
+
7
Intramolecular Diels-Alder cyclizations can often accomplish
formation of rather complex ring systems in a single step
and with high stereoselectivity.
For example, B was prepared by heating A as the key step
in preparing a microbial metabolic product.
O
O
H3 C
O
O
O
O
o
O
H3C
Diene
180 C
O
H 3C
Dienophile
A
C6H5
C6H5
CH3
B
8
4
A process related to the Diels-Alder reaction is 1,3-dipolar addition.
Reactions involve the combination of a 1,3-dipolar compound
with a dipolarophile.
C6H5 N
N
N
C6H5
+
C6H5 N
N
C2H5O2CC
C6H6
CCO2C2H5
N
C2H5O2C
Diethyl butynedioate
(Diethyl acetylenedicarboxylate)
N
N
N
CO2C2H5
4,5-Dicarboethoxy-3-phenyl
-1,2,3-triazole 87%
Phenyl azide
1,3-Dipolar additions are often used to prepare heterocyclic cpds.
9
Sigmatropic Reaction
™ A concerted reaction of the type,
G
C
G
(C
C)n
(C
C)n
C
in which a group migrates with its σ bond within a π
framework, an ene or polyene.
™ The migration is accompanied by a shift in π bonds.
G
C
G
G
C
C
C
C
C
C
C
C
H
[1,3] – sigmatropic reaction
10
5
[1,3] – sigmatropic reaction
G
G
G
C
C
C
C
1
C
C
C
C
C
H
3
[1,5] – sigmatropic reaction
G
G
C
C C
1
C
C
C
5
G
C
C
C
C
C
C
C
C
C
™ In the designations [1,3] and [1,5] the “3” and “5” refer to
the number of the carbon to which group G is migrating
(the migrating terminus)
™ The “1” specifies that in both reactant and product
bonding is to the same atom in the migrating group.
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The important Cope rearrangement of hexa-1,5-dienes,
is a [3,3] sigmatropic reaction.
2
1
C
C
C
1
C
2
C
3
C
C
C
C
3
C
C
C
C
C
C
C
C
C
There is a change in position of attachment in G
as well as in the π framework.
In the transition state of a sigmatropic rxn., the migrating gr.
is bonded to both the migration source (origin) and
the migration terminus.
12
6
™ The bonding in the TS# for sigmatropic rxns. arise from
overlap between an orbital of an atom or free radical (G)
and an orbital of an allylic free radical (the π framework).
™ In the TS#, there is overlap between the HOMO of one
component and the HOMO of the other.
™ Each HOMO is singly occupied, and together they
provide a pair of electrons.
13
™ The HOMO of an allylic radical depends on the number
of carbons in the π framework.
™ The migrating gr. is passed from one end of the allylic
radical to the other, and it is the end carbons that we
are concerned with the symmetry requirements.
.
.
HOMO of pentadienyl
HOMO of allyl propenyl
.
.
.
.
HOMO of heptatrienyl
14
7
Stereochemically Hydrogen Migration
™ Suprafacial Sigmatropic shift
H
H
™ Antarafacial Sigmatropic shift
H
H
[1,3] or [1,5] antara shift is difficult due to the geometry,
since the π framework to be twisted far from the planarity
that requires for delocalization of electrons.
15
™ [1,3] and [1,5] sigmatropic rxn. seem to be limited to
supra shifts.
™ A [1,3] supra shift of H is symmetry-forbidden.
Since the s orbital of H would have to overlap p
lopes of opposite phase.
H cannot be bonded simultaneously to both carbons.
[1,3] symmetry forbidden
™ A [1,5] supra shift of H is symmetry-allowed.
16
8
For larger π frameworks, both supra and antara shifts
should be possible on geometric grounds.
™ [1,7]-H shift, should be antara, a [1,9]-H shift, supra, and so on.
For photochemical reactions, as before, predictions are
exactly reversed.
The facts agree with the above predictions: [1,3] sigmatropic
shifts of H are not known, whereas [1,5] shifts are well known.
CH3
[1,5]-H
CH2
heat
CD2
CHD2
17
[1,5] - H shifts
™ [1,5] Shifts of hydrogen atoms, usually at temperatures of
200oC and above, are common reactions.
™ In small cyclic systems, the hydrogen migrations must almost
certainly be suprafacial.
H3 C
H3 C
H
H
CH3
CH3
H
H
18
9