Complexation by N-(3,6,9-trioxadecyl)monoaza-12-crown-4 lariat ether: a “calabash” complex of a potassium cation by a synthetic macrocycle containing a total of only seven donor atoms
✍ Scribed by Banita D. White; Kristin A. Arnold; Frank R. Fronczek; Richard D. Gandour; George E. Gokel
- Publisher
- Elsevier Science
- Year
- 1985
- Tongue
- French
- Weight
- 215 KB
- Volume
- 26
- Category
- Article
- ISSN
- 0040-4039
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✦ Synopsis
Structural and cation binding data for the title compound demonstrate that the large value of KS results from complete K+ cation encapsulation by one nitrogen and six oxygen atoms despite the presence of a twelve-membered macroring.
In a previous study,' the myth of a hole-size relationship was dispelled, at least for the series of flexible mono-macrocycles 12-crown-4 through 24-crown-8 with the cations Nat, Kt, NH4+, and Ca2+. The prevailing view seems to be that a twelve-membered ring is "too small" to accommodate either a sodium or potassium cation. Were this so, 12-crown-4 should show selectivity for Lit over either Nat or Kt. Popov has reported that log KS for Lit with 12crown-4 in anhydrous MeOH solution is * 0.2 This contrasts with 12-crown-4 to Nat and Kf binding constants (log KS, MeOH) of 1.7 for both cases.' lj-(3,6,9-Trioxadecyl)monoaza-12-crown-4, 1 complexes Nat and Kt with homogeneous stability constants (log KS> of 3.973 and 3.84, respectively, in anhydrous MeOH. In marked contrast, the stability constants for Na+ with 12crown-4 or 15-crown-5 in MeOH are 1.7 and 3.24, respectively. Indeed, in the simple macrocycle series, only binding by 18-crown-6 (log KS = 4.35) exceeds that for 1. If a hole-size relationship of any sort operates, one might expect binding for the larger K+ ion to be greatly diminished relative to that for the smaller Nat ion. We report here that not only is Kt cation binding by 1 substantial and similar in magnitude to Nat binding, the cation is completely enveloped by the donor groups present in both ring and sidearm. Compound 1 was prepared by alkylation of monoaza-12-crown-4 and the structure was ultimately confirmed by X-ray analysis. 4 Monoaza-12-crown-4 was obtained by hydrogenolysis5 of I-benzylmonoaza-12-crown-4 which was, in turn, prepared as reported by Dale and Calverley from benzylamine and l,ll-diiodo-3,6,9_trioxaundecane (tetraethylene glycol diiodide).6 Monoaza-12crown-4 (l.Og, 0.006 mol) was dissolved in MeCN (25 mL) containing Na2C03 (anhydrous, 0.7g,