Ivan Vatsouro

Extraction of americium and europium by CMPO-substituted adamantylcalixarenes

Summary Eight p-adamantylcalix[4]arene derivatives, bearing four CMPO-like functions [(CH2)nNHC(O)CH2- P(O)Ph2] at the wide (4a,b, n = 0, 1) or narrow (5a–c and 6a–c, n = 2–4) rims were synthesized for the first time. Studies of the extraction of americium(III) and europium(III) from 3 M HNO3 solutions to organic phases (dichloromethane, m-nitro-trifluoromethylbenzene) showed: (i) The extraction ability for all the adamantylcalixarene ligands is much better than for their monomeric analogues –N-(1-adamantyl)-, N-(1-adamantylmethyl)- and N,N-(dibutyl)carbamoylmethyldiphenylphosphine oxides 7a, 7b, 8; (ii) The extraction percentage increases strongly with increasing length of the spacer for all types of ligands 4–6, and best extraction results were found for 4b (n = 1) and 5c (n = 4); (iii) The separation coefficient DAm/DEu for the investigated compounds did not exceed 2, which is close to the narrow rim CMPO calixarenes, studied earlier; (iv) Variation of the spacer length between CMPO groups attached to the 1,3- and 2,4-positions of the calixarene platform in 6 did not lead to appreciably improved extractants, neither with respect to the extraction abilities (D) nor to the selectivities (DAm/DEu).

Copper(I)-Catalyzed Cycloaddition of Azides to Multiple Alkynes: A Selectivity Study Using a Calixarene Framework

Copper(I)-catalyzed addition of limited amounts of azides to multiple alkynes, which led to statistical mixtures of triazole/acetylene derivatives or, in other cases, resulted in preferred formation of multiple triazoles, was studied at pre-organizable calixarene platforms bearing up to four propargyl groups. Depending on calixarene structures and reaction conditions, the unprecedented specific or selective formation of exhaustively triazolated calixarenes or a complete loss of the selectivity were observed. Both autocatalytic copper activation and a local copper(I) concentration increase due to copper-triazole complexation were thoroughly studied as the most expected reasons for the selectivity and both were disproved. Mixed triazolated/propargylated calixarenes and their copper(I) complexes proved not to be involved in the cascade-like process that was modeled to be driven by an intramolecular transfer of two copper(I) ions from a just-formed binuclear copper intermediate to the adjacent acetylene unit.

Calix[4]tubes: An Approach to Functionalization

Adamantylcalix[4]arenes decorated with ester groups and calix[4]arene tetratosylates were used to prepare a series of calix[4]tubes bearing 3-methoxycarbonyl- and 3-methoxycarbonylmethyl-1-adamantyl units (up to eight) in good yield. These compounds were subjected to further chemical transformations giving a wide set of novel ester-, acid-, hydroxy-, amine-, and urea-functionalized calix[4]tubes. Introduction of urea groups into the calixtube core led not only to anion-targeted receptors, but also provided heteroditopic behavior of the hosts, which enriches the well-established potassium-uptake ability of calix[4]tubes.

Chiral Heteroditopic Baskets Designed from Triazolated Calixarenes and Short Peptides

Cone calix[4]arenes and calix[6]arenes bearing two, three, and four short peptide units each having two chiral carbon atoms were prepared. The syntheses were performed by using an efficient modular approach that includes the Ugi preparation of the azido-peptide followed by its reactions with the propargylated calixarenes under CuAAC (Cu(I) -catalyzed azide-alkyne cycloaddition) conditions. The three novel multitopic hosts were probed for their ability to bind metal ions by UV titration, and showed the highest complexation efficiency towards copper(II) and lead(II). These two cations possessed quite different complexation modes with copper(II) bound predominantly by multiple-triazole sites, in contrast to lead(II), which is stabilized mainly by multiple interactions with amide groups of the peptide units. Circular dichroism data for the free chiral hosts, their equimolar mixtures with copper(II) perchlorate and lead(II) perchlorate, and for tertiary mixtures of all three compounds showed the formation of mono- and binuclear complexes, or a switching behavior, depending on the structure of the host and the addition order of the cations.

Tuning conformations of calix[4]tubes by weak intramolecular interactions

The conformational distribution in distally functionalized adamantylated calix[4]tubes was analyzed with NMR and quantum-chemical calculations. Without any intramolecular interactions, ester-, acid-, amino-, alcohol-, isocyanate-, phthalimide- and urea-substituted calixtubes preferred less sterically hindered flattened cone conformers in solution. In contrast, in a non-polar medium (CDCl3) for calixtubes bearing two 3-carboxymethyl-1-adamantyl or 3-(2-ureidoethyl)-1-adamantyl units, the less sterically preferable flattened cone conformation with bulky substituents connected by intramolecular hydrogen bonds was found to be more favorable. On the other hand, for a calixtube with two positively charged units, only the conformer with distanced substituents was exclusively detected. Thus, the conformational equilibrium in functionalized calix[4]tubes can be controlled by intramolecular hydrogen bonding between appropriately arranged carboxylic groups or urea moieties, or by electrostatic repulsion of positively charged substituents.

Domino construction of a bullataketal core via double bond cleavage in activated dihydrofurans

A novel approach to the tricyclic core of bullataketals A and B, natural benzannulated 2,7-dioxabicyclo[3.2.1]octanes, was developed via a domino reaction including a formal decarbonylation of 3-acyl-4-(o-hydroxyaryl)-substituted 4,5-dihydrofurans followed by spontaneous assembly of the methanobenzodioxepine skeleton. This operationally simple method provides ample opportunities for variation of substituents in the fused benzene moiety as well as at the bridgehead ketal C9 atom of the final products. The successful conversion of both 3-keto- and 3-ester-substituted dihydrofurans into functionalized methanobenzodioxepines pointed toward a substrate-controlled mechanism. This was supported experimentally and by computational studies. While 3-ester-4,5-dihydrofurans are transformed via ring opening/ring closure with further decarboxylation of the generated γ-lactone, 3-keto-4,5-dihydrofurans undergo stepwise CC bond cleavage including intramolecular hemiketalization and retro-aldol reaction as the key steps.

Synthesis of polyfunctional phosphorus-containing calixarenes in cycloaddition reactions of azides to alkynes

Mono- and dibromo derivatives of calix[4]arene were used for the first time in phosphorylation, alkylation, and azide-alkyne cycloaddition sequence in order to synthesize polyfunctional macrocycles containing phosphine oxide groups at the upper rim and 1,4-disubstituted triazole rings at the lower rim. The effect of reaction conditions onto the selectivity of the lower-rim alkylation of phosphoruscontaining calixarenes and the ratio of spatial isomers in the exaustively alkylated products was studied. The reduction of phosphine oxide groups and modification of functional groups in the triazole rings were used for the synthesis of bulky hydrophilic phosphinecontaining ligands. The catalytic activity of in situ obtained rhodium complexes of phosphine-containing calixarenes was preliminarily studied by 1-octene hydroformylation in ethanol.