Page 236 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN011A-543 February 12, 2002 12:40

Organic Macrocycles 523

TABLE V Stability Constants (log K ) for Reac- TABLE VI Stability Constants (log K ) for Reac-
tion in Methanol at 25 C of Arenediazonium and tion in Anion with Macrocyclic Ligand 10
◦
Anilinium Cations with 18-Crown-6
Log K, reaction with
Cation Log K Anion ML 10 (protonated)
−
PhNH + 3.80 Cl (in water) >4.0
3
−
2-CH 3 C 6 H 4 NH + 2.86 Br (in water) <1.0
3
−
2,6-(CH 3 ) 2 C 6 H 3 NH + 2.00 Br (in 90% methanol) 1.75
3
PhNN + 2.50
2-CH 3 C 6 H 4 NN + a
two types of anion binding are known. The first involves
2,6-(CH 3 ) 2 C 6 H 3 NN + a
macrocyclic structures containing basic sites that provide
a No measurable reaction. positively charged binding sites when protonated. Exam-
ples of these are compounds 10 (in protonated form) and
shows that complex stability is a regular function of the 14 in Fig. 1, and binding constants with typical anions are
+
electron density in the N moiety. As electron density found in Table VI. Structure 14 is based on cyclodextrin,
2
increases, stability declines. which is a large cyclic polysaccharide.
The binding of organoammonium-type cations to Cyclodextrins are able to accommodate neutral mole-
macrocycles is the subject of intense interest due to the cules as well as anions. The large cavity contains numer-
ability of such systems to mimic enzymes. Specifically, ous hydrogen-bonding sites if needed. In general, the cav-
it is possible to add functionalities to the macrocyclic ity simply provides a comfortable microenvironment for
structure to permit chiral recognition in substrates. Chiral many neutral species, especially when the solvent envi-
macrocycle 12 in Fig. 1, in the (S,S)-form, for example, ronment is less than ideal.
binds one enantiomer of α-(1-naphthyl)ethylammonium
perchlorate more strongly than the other isomer [log K = IV. APPLICATIONS OF MACROCYCLIC
2.47 ± 0.01 for the (R)-ammonium salt and 2.06 ± 0.01 LIGANDS
for the (S)-salt]. The difference in binding constants re-
sults from a greater steric hindrance to the approach of the
A. Extractants
substrate for one isomer due to the presence of the bulky
functionalities. Oneofthefirstidentifiedusesforcrownetherandcryptand
ligands was as phase transfer agents in catalyzing syn-
thetic organic reactions. The macrocycle can be used to
III. COMPLEXATION OF ANIONS solubilize salts having oxidizing or reducing anions into
AND NEUTRAL MOLECULES hydrophobic solvents. For example, KMnO 4 can be solu-
bilized into benzene by 18-crown-6. The “naked” MnO −
4
+
Considerably less attention has been given to the binding ion that accompanies the K –18-crown-6 complex in so-
of anions to macrocycles than to that of cations. Basically lution is a very powerful oxidizing agent in this medium.
Use of naked anions of this type has provided a method to
enhance the efficiency of many synthetic reactions.
Macrocycles have also been proposed as metal ion ex-
tractants in separation processes. It has been shown by
J. McDowell and his co-workers at Oak Ridge National
Laboratory that synergistic effects occur when crown
ethers are used as coextractants with traditional extrac-
tants like diethyl hexylphosphoric acid (HDEHP). Specif-
ically, the degree of metal ion extraction is greater when
both crown and HDEHP are used together than the sum
of extraction efficiencies when each is used separately.
Furthermore, by using the crown, the selectivity of ex-
traction processes can be altered in this manner.

B. Selective Ion Separations
◦
FIGURE 7 Plot of log K for formation in methanol at 25 C of the
+
18-crown-6 complex of p-RC 6 H 4 NN versus Hammett σ p values Macrocyclic crown ethers can be attached to silica gel
+
of R. through stable C Si and Si O Si bonds (see Fig. 8) by