Managing Tight Binding Receptors for New Separations Technologies Page: 3 of 3
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SWITCH-BINDING AND RELEASE: Previously, we described the synthesis of a photo-
cleavable, switch-release polyether cryptand 5-(2-nitrophenyl)-4,7,13,16,21,24-hexaoxo-1,10-
diazabicyclo[8,8,8]hexacosane-2,9-dione (1), its ability to capture Ca+2 and other ions and their
photorelease by UV light through the "switch/release mechanistic" sequence. Additional details
have been learned. In a separate "catch and release" protocol, the metal ion participates in the
template closure of the N-substituted 7,16-diazacyclo[8.8] macrocycle (2) forming the identical
cryptate of 1, but proceeding through the complex of a "lariat macrocycle" as an intermediate.
Work on this latter reaction sequence has now been completed.
Synthesis of the Lariat macrocycle. The Lariat macrocycle, 16-{2-[2-
ethoxycarbonylmethoxy-1-(2-nitrophenyl)-ethoxy]-acetyl}-1,4,10,13-tetraoxa-7,16-
diazacyclooctadecane (2) was synthesized using the same nitrophenyl ethan-1,2-diol used for 1.
Selective protection of the 10 alcohol was followed by allylation of the 20 alcohol allyl ether in
quantitative yield. Removal of the silyl protecting group followed by etherification with ethyl
iodoacetate yielded an allyl ester which was converted with ozone at -78 C to an acid-ester
introducing dissymmetry to the bridging arm. The acid group was coupled to 1,4,10,13-tetraoxa-
7,16-diaza-cyclooctadecane to yield the lariat macrocycle 2.
Structural features of the cryptand-cryptate system. There are, in principle, eight
stereoisomeric cryptands having the structure 1 which derive from the existence of a stereogenic
center at the phenyl group attachment and the four conformational orientations of the amide
carbonyl. All four diastereoisomeric pairs are 1H NMR observable and are inseparable by
conventional LC and column chromatography methods. Raising the temperature causes the
isomers to interconvert more rapidly and at 100 C the signals coalesce. The mixture returns to
the original upon cooling.
Effects of metal on the structure and chemistry. When one equivalent of Ca (BF4)2 is stirred
in the presence of the cryptand, 95% of the Ca+2 is incorporated rapidly into the cryptand and the
mixture of isomeric cryptands collapses to a single structure (1H NMR). That conformer has
both carbonyls oriented away or "out" relative to the metal ion.
Capture of Ca+2 by the N-substituted 7,16-diazacyclo[8.8] macrocycle 2 (the Lariat
macrocycle) was also examined. When 2 was dissolved in D20 containing two equivalents of
Ca2+ the product cryptate appear slowly requiring 7 days for complete conversion (1H-NMR,
13C-NMR and mass spectral analysis). In the absence of Ca2+ cyclization was extremely sluggish,
taking months to form any measurable quantities of the cryptate.
Photochemistry. Photolysis of the Ca+2 cryptates in water results in only 40% release of the
Ca+2 based on the disappearance of the cryptate. The remaining 60% of Ca+2 is bound to the
opened macrocyclic crown ether. This finding presents a challenge to the concept of the catch
and release strategy since the complete release of ion is the goal of this research.
The quantum efficiencies for the disappearance of the cryptate and cryptand are solvent
dependent (disappearance of cryptate: MeOH, 0.18; 0.099 in H20) whereas the quantum
efficiency for appearance of the opened macrocycle in MeOH is 0.060. On the other hand, the
efficiencies were the same for both the cryptate and cryptand under otherwise identical
conditions. Thus, the metal ion does not appear to influence the efficiency of ring opening.
INFORMATION ACCESS: M.M. Hassan, C. Zhang, J-I. Lee, K.M. Bushan, A.
McCasland, R.S. Givens, D.H. Busch, "Dynamics of Switch-binding by a linear ligand that
transforms to a macrocycle upon chelation: synthesis, kinetics and equilibria," ACS, Symposium
Volume, Submitted 2003; J-I Lee, K.M. Bushan, C. Zhang, D.H. Busch*, and R.S. Givens*, "A
novel separations methodology using tight-binding photolabile cryptands." in preparation.
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Busch, Daryl H. & Givens, Richard S. Managing Tight Binding Receptors for New Separations Technologies, report, June 1, 2004; United States. (https://digital.library.unt.edu/ark:/67531/metadc779403/m1/3/?rotate=270: accessed April 26, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.