The Long and the Short of It: Regiospecific Syntheses of Isomers of Dicarbomethoxydibenzo-27-crown-9 and Binding Abilities of Their Pyridyl Cryptands

Abstract

The two isomers 6 and 9 of cis(4,4'-)-dicarbomethoxydibenzo-27-crown-9 with tri- and tetra-(ethyleneoxy) linkages transposed were synthesized regiospecifically in high yields (94 and 92%, respectively) by the Wang-Pederson-Wessels (WPW) protocol and were converted via the corresponding diols 7 and 10 to the corresponding pyridyl cryptands 3 and 4 by reaction with pyridine-2,6-dicarbonyl chloride. As expected from Corey-Pauling-Koltun (CPK) models, the cryptand with the tri(ethyleneoxy) arm para to the ester linkages, "short-armed" cryptand 3, did display a higher binding constant (K = 2.4 × 10 M) with paraquats than the analogous dibenzo-30-crown-10-based cryptand previously studied; however, the effect was only twofold. Its binding to diquat was reduced by an order of magnitude (1.5 × 10 M), as expected on the basis of its narrower cavity. Also as expected, the cryptand with the tetra(ethyleneoxy) arm para to the ester linkages, "long-armed" cryptand 4, possessed diminished binding with both paraquats and diquat relative to the 30-crown-based analogue; in these systems, 2:1 H:G complexes were also detected by mass spectrometry. A crystal structure is reported for 3·DQ(PF).

Improved complexation of paraquats with crown ether‐based pyridyl cryptands

“…Through analysis of CPK models and crystal structures, host molecules have been advanced to highly evolved crown‐based pyridyl cryptands, for example 5 and 6 . However, little improvement has been offered in way of the paraquat guest, the standard being dimethyl paraquat as the PF 6 salt ( 7 ) which displays reasonable association constants with various crown ethers[2–5] and analogous cryptands [4] …”

“…Alterations to the paraquat salt were investigated in terms of the N‐alkyl group and the counter anion. Focus was directed toward the association constants of cryptand 5 , given that its precursor crown ether 2 is produced through a high yielding templation process called the WPW process after its three key developers (Wang, Pederson, Wessels) . And diol 2 is converted to cryptand 5 in 89% yield via a template process …”

Synthesis, Structure and Supramolecular Properties of a Novel C3 Cryptand with Pyridine Units in the Bridges

“…Many aromatic tripodand units were employed to access this type of cryptand. Thus, bicyclocyclophanes exhibiting 1,3,5-trisubstituted benzene [ 10 , 11 , 12 , 13 , 14 ], 2,4,6-trisubstituted-1,3,5-triazine [ 15 , 16 , 17 ], tertiary amines [ 18 , 19 ], or phosphines [ 20 , 21 ], as well as cyclotribenzylene [ 22 , 23 , 24 ] as central units, connected by various bridges, such as oligoethyleneoxide [ 25 , 26 ], di-yne [ 17 , 27 , 28 ], pyridine [ 29 , 30 , 31 ], or triazole [ 32 , 33 , 34 , 35 ], in agreement with the employed macrocyclization procedure, have been reported and showed good binding abilities for various guests.…”

“Reverse” pyridyl cryptands as hosts for viologens

“…There is an ongoing quest in host‐guest chemistry to discover new and stronger complexations, higher yielding syntheses of components, and novel molecular recognition motifs [1]. These efforts lead to better synthetic methods, new materials, and opportunities for superior structural control and unusual and useful properties.…”

“…The crown ether hosts provided modest binding strengths ( K a = 10 2 to 10 3 mol −1 ) with cationic guests, eg, secondary ammonium and bipyridinium salts,[2] but these were supplanted by the higher binding ( K a ≥ 10 5 mol −1 with bipyridinium guests) cryptand hosts [3]. Over time, highly efficient syntheses have been developed for the crown ethers[1g],[3d],[4] and the derived cryptands [3h]…”