Organic Chemistry

1008 CHAPTER 24 Catalysis

In cases A and B, the metal ion exerts the same kind of catalytic effect as a pro-

ton does. In a reaction in which the metal ion has the same catalytic effect as a

proton (increasing the electrophilicity of a reaction center or decreasing the basici-

ty of a leaving group), the metal ion is often called an electrophilic catalyst.

In case C, metal-ion complexation of water increases its nucleophilicity by con-

verting it to metal bound hydroxide ion. The of water is 15.7. When a metal

ion complexes with water, it increases its tendency to lose a proton: The of

metal-bound water depends on the metal atom (Table 24.1). When metal-bound

water loses a proton, metal-bound hydroxide ion is formed. Metal bound hydrox-

ide ion, while not as good a nucleophile as hydroxide ion, is a better nucleophile

than water.

pKa

pKa

Table 24.1 The pKaof Metal-Bound Water

12.7 8.9

11.8 8.7

11.6 7.2

10.6 6.8

Ni^2 + 9.4 Be^2 + 5.7

Mn^2 + Cu^2 +

Cd^2 + Fe^2 +

Mg^2 + Zn^2 +

Ca^2 + Co^2 +

M^2 + pKa M^2 + pKa

Now we will look at some examples of metal-ion-catalyzed reactions. catalyzes

the condensation of two molecules of the ethyl ester of glycine to form the ethyl ester of

glycylglycine. The actual catalyst is a cobalt complex,

Coordination of the metal ion with the carbonyl oxygen makes the carbonyl group

more susceptible to nucleophilic attack by stabilizing the developing negative charge

on the oxygen in the transition state.

The decarboxylation of dimethyloxaloacetate can be catalyzed by either or

In this reaction, the metal ion complexes with two oxygen atoms of the reactant. Com-

plexation increases the rate of decarboxylation by making the carbonyl group more

susceptible to receiving the electrons left behind when CO 2 is eliminated.

Cu^2 + or Al^3 +

dimethyloxaloacetate

+

O

C

O

C

CH 3

CH 3

C

O

−O C O−

O

C

O

C CHCH 3

CH 3

−O CO 2

Cu^2 + Al^3 +.

+

+

+

OCH 2 CH 3 H 2 NCH 2 COCH 2 CH 3

C O

O

Co

CH 2

NH 2

Co^2 +

2 H 2 NCH 2 COCH 2 CH 3 +

O

H 2 NCH 2 CNHCH 2 COCH 2 CH 3 CH 3 CH 2 OH

OO

[Co(ethylenediamine) 2 ]^2 +.

Co^2 +