Substrate Selective Reactions in the Presence of Lewis Acids
19
dienophile controlled and that electron-withdrawing substituents lower the LUMO
energies of dienophiles, accelerating the reaction rate.
Ph
CF 3
O
Ph
Me
O
COCF 3
Ph
H
H
9 (90%)
7
8
better dienophile
Scheme 3.10
The reversal of the selectivity during the reaction in the presence of a Lewis
acid could be interpreted (as we have discussed previously), in terms of the different stabilities of the Lewis acid-carbonyl complexes formed in each case.
The Lewis acid-catalyzed Diels-Alder reactions have been studied in depth,
particularly when DE-unsaturated aldehydes and ketones are involved as dienophiles. It has been established that the effect of the catalyst is to lower the LUMO
energy of the dienophile, by coordination with the carbonyl group. It is evident
that the more stable complex will cause a larger lowering in the LUMO energy. In
this case, in the presence of the Lewis acid, complexes 21 and 22 should be
formed. However, as we have discussed above, the coordination ability of more
electrophilic carbonyl groups to a Lewis acid is weaker than that of their less electrophilic analogues. In fact, we have commented previously that trifluoroacetophenone-BF 3 complex 17 has not been detected by NMR. Consequently, it is reasonable to consider that in the presence of a Lewis acid only 22 is formed in the
reaction medium (Scheme 3.11).
Therefore, in the presence of BF 3 ·OEt 2 , compound 8 would form a coordination
complex with the Lewis acid 22, which is a better dienophile (lower LUMO) than
7. The cycloaddition between 22 and cyclopentadiene affords the observed reacf f
tion product (Scheme 3.11).
CF 3
O
Ph
Me
O
Ph
BF 3 .OEt 2
CF 3
O
BF 3
Ph
Me
O
BF 3
Ph
COMe
Ph
H
H
10
7
8
reaction product
21
22
not formed
Scheme 3.11
19
dienophile controlled and that electron-withdrawing substituents lower the LUMO
energies of dienophiles, accelerating the reaction rate.
Ph
CF 3
O
Ph
Me
O
COCF 3
Ph
H
H
9 (90%)
7
8
better dienophile
Scheme 3.10
The reversal of the selectivity during the reaction in the presence of a Lewis
acid could be interpreted (as we have discussed previously), in terms of the different stabilities of the Lewis acid-carbonyl complexes formed in each case.
The Lewis acid-catalyzed Diels-Alder reactions have been studied in depth,
particularly when DE-unsaturated aldehydes and ketones are involved as dienophiles. It has been established that the effect of the catalyst is to lower the LUMO
energy of the dienophile, by coordination with the carbonyl group. It is evident
that the more stable complex will cause a larger lowering in the LUMO energy. In
this case, in the presence of the Lewis acid, complexes 21 and 22 should be
formed. However, as we have discussed above, the coordination ability of more
electrophilic carbonyl groups to a Lewis acid is weaker than that of their less electrophilic analogues. In fact, we have commented previously that trifluoroacetophenone-BF 3 complex 17 has not been detected by NMR. Consequently, it is reasonable to consider that in the presence of a Lewis acid only 22 is formed in the
reaction medium (Scheme 3.11).
Therefore, in the presence of BF 3 ·OEt 2 , compound 8 would form a coordination
complex with the Lewis acid 22, which is a better dienophile (lower LUMO) than
7. The cycloaddition between 22 and cyclopentadiene affords the observed reacf f
tion product (Scheme 3.11).
CF 3
O
Ph
Me
O
Ph
BF 3 .OEt 2
CF 3
O
BF 3
Ph
Me
O
BF 3
Ph
COMe
Ph
H
H
10
7
8
reaction product
21
22
not formed
Scheme 3.11