Synthesis and Protonation Behavior of 26-Membered Oxaaza and Polyaza Macrocycles Containing Two Heteroaromatic Units of 3,5-Disubstituted Pyrazole or 1-Benzylpyrazole. A Potentiometric and¹H and¹³C NMR Study

Abstract

The synthesis and acid−base behavior of two series of 26-membered dioxatetraamine and hexaamine heterocyclophanes containing two nuclei of either pyrazole (4a and 6a) or 1-benzylpyrazole (4b and 6b), respectively, are reported. Dipodal (2 + 2) condensations of 3,5-pyrazoledicarbaldehyde 2a or its 1-benzyl derivative 2b with 1,5-diamino-3-oxapentane afford in both cases the stable Schiff bases 3a,b in 90% yield, which after reduction with NaBH4 gave 4a,b in 75% and 84% yield, respectively. Condensation of 2a with diethylenetriamine leads to a complex mixture containing imidazolidine isomers, which was reduced in situ to afford 6a in 30% yield. Condensation of 2b with the same amine gave the stable diimidazolidine derivative 5b, which after crystallization was isolated as a pure compound in 80% yield and fully identified from analytical and 1H and 13C NMR data as a constitutional isomer with both imidazolidine rings located at the side of the pyrazole closer to the benzylic substituents. Reduction of 5b with NaBH4 afforded the polyamine 6b in 86% yield. Protonation constants of 4a,b and 6a,b have been determined by potentiometric methods in the pH 2−11 range, and their protonation sequences were established by a 1H and 13C NMR study in D2O at variable pH. For each compound, the number of protonation constants equals the number of nitrogens in the side chains. In the pH range studied, the pyrazole rings are not involved in protonation or deprotonation processes.

CuII and ZnII Coordination Chemistry of Pyrazole‐Containing Polyamine Receptors − Influence of the Hydrocarbon Side Chain Length on the Metal Coordination

“…[9] As can be derived from the basicity constants, the two first protonations of L 4 occur in the pH range of 11Ϫ8. Thus, in the NMR spectra of 4 recorded at pH ϭ 8.0 (Table 1), the carbon signal shifted the most upfield is that of carbon atoms C-9 (∆δ ϭ Ϫ3.61 ppm), whereas the signal of carbon atoms C-8 and that of the quaternary carbon atoms C-3,5 do not experience significant shifts in this pH region.…”

“…These variations are of a similar order to those previously reported for L 1 . [9] It is important to point out that such variations do not reflect any particular implication of the pyrazole subunits in the protonation process, since it is well known that β-and γ-aromatic carbon atoms belonging to polyamine-bridged cyclophanes containing benzene units also bear similar chemical shifts upon protonation. [12,13] Finally, we also obtained the disodium dipyrazolate salt 4Ј [Na 2 (H Ϫ2 L 4 )] as a pure solid (m.p.…”

“…Such behaviour contrasts with that of the free ligands, for which neither deprotonation nor protonation processes occur at the pyrazole rings throughout the pH range of 2Ϫ11. [9,11] On the other hand, all the dinuclear Cu II complexes are EPR-silent.…”

Pyrazolate‐Based Dinuclear α‐Diimine‐Type Palladium(II) and Nickel(II) Complexes – a Bimetallic Approach in Olefin Polymerisation

“…The pyrazole derivatives I and II with carbonyl substituents in the 3-and 5-positions of the heterocycle were prepared with slight modifications of the procedures reported in the literature [13,14] and were used in the subsequent Schiff base formation (Scheme 2). Proper conditions turned out to be critical in order to achieve reasonable yields in the final condensation step and to assure formation of the difunctionalised products.…”

“…Compounds I, [13] II [14] and HL 3 [7] were synthesised according to published procedures. In order to improve the conversion yield in the last step of the synthesis of II, a mixture of water and ethanol (1 : 4) instead of pure water was used as the solvent.…”