Lewisacid Catalysis In Water

Lewis-acid catalysis of Diels-Alder reactions (Figure 7.5) in organic solvents leads to an enhancement of the reaction rate, because of the lowering in energy for the lowest unoccupied molecular orbital (LUMO) of the dienophile, and an improvement in the selectivity with specific ligands.

Experiments aimed at examining the possibility of catalyzing Diels-Alder reaction in water gave rise to interesting results. For the reaction of bidentate die-nophiles (1) with cyclopentadiene,11 shown in Figure 7.6, the reaction in water was less accelerated by the catalyst than the one in acetonitrile, probably because

Endo Exo

Figure 7.5 Diels-Alder reaction catalyzed by a Lewis acid (LA).

Endo Exo

Figure 7.5 Diels-Alder reaction catalyzed by a Lewis acid (LA).

Figure 7.6 First example of Lewis-acid catalyzed Diels-Alder reaction in water. M2+ =

CH2S03_ Na+ CH2N+(CH3)3Br

Figure 7.6 First example of Lewis-acid catalyzed Diels-Alder reaction in water. M2+ =

reaction in water.
TABLE 7.2 Effects of the Lewis Acid in Different Solvents for Reaction (1)

Solvent

Catalyst

Half-Life

Endo/Exo

k(rel)

Acetonitrile

None

1.5 years

67/33

1

Ethanol

7 months

77/23

2.7

Water

48 hours

84/16

288

Acetonitrile

Cu2+

5 minutes

94/6

158.000

Ethanol

(10 mM)

15 minutes

96/4

54.900

Water

4 minutes

93/7

691.000

TABLE 7.3 Solvent Effect on Enantioselectivity; Catalysis by Copper (L-abrine) as the Lewis Acid

Solvent

Time (days)

Enantiomeric Excess (ee) (%)

Water

3

74

Ethanol

10

39

Acetonitrile

7

17

THF

8

24

Chloroform

11

44

the hydrophobic effects are present but reduced, and the hydrogen bonding effect is diminished.

The reaction with the compounds shown in Figure 7.7 showed that there was no influence of Cu 2+ on the rate of the reaction with cyclopentadiene, confirming that bidentate complexation is essential for efficient catalysis.

Lewis-acid catalysis (Table 7.2) was observed for Cu2+, Ni2+, Co2+, and Zn2+, with Cu2+ being the best catalyst (both strongest binding and most efficient in accelerating the reaction with CPD). Varying the solvent (Table 7.3) in the catalysis by Cu2+ ions with L-abrine as a ligand illustrates a large increase in enantioselectivity in water. So water promotes enantioselectivity.12

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Responses

  • bungo
    Why is dielsalder reaction in water green?
    9 years ago
  • fulvus
    What are the effects of lewis acids in a green diels alder reaction?
    8 years ago

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