Highly selective synthesis of a 1,3,5-tris-protected calix[6]arene-type molecular platform through coordination and host-guest chemistry

An elegant methodology based on the synergistic combination of coordination and host-guest chemistry led to the highly efficient synthesis of a unique C(3v)-symmetrical, calix[6]arene-based molecular platform with three protected amino arms in alternating positions. The key step involves the formation of a stable supramolecular host-guest Zn(II) complex from a C(6v)-symmetrical calix[6]hexaamine. Indeed, in the presence of a polar neutral guest and a strong donor that acts as an exogenous ligand, three alternating amino groups of this calix[6]hexaamine are selectively coordinated to the Zn(II) ion while the three others remain free and are thus much more reactive toward chemical reagents. In addition to this protective role, the metal centre preorganises the C(3v)-symmetrical complex in such a way that the uncoordinated NH(2) groups are directed toward the outside of the cavity; they are then accessible for a chemical transformation. Hence, reaction of these alternating free amino groups with a protective reagent (i.e., Boc(2)O) followed by zinc decoordination quantitatively and selectively yielded the 1,3,5-tris-Boc-protected calixarene derivative on a gram scale. It was shown that the presence of all the partners of the key intermediate Zn complex (i.e., the metal centre, the exogenous ligand and the included guest) is crucial for a high selectivity. Finally, a two step sequence that led to a C(3v)-symmetrical 1,3,5-tris-acetylated calix[6]hexaamine through the removal of the Boc groups illustrates that the 1,3,5-tris-protected calix[6]hexaamine is a promising molecular platform. Examples of such readily available C(3v)-symmetrical calixarene-based building blocks are extremely rare in the literature.     

Synthesis and host–guest properties of a calix[6]arene based receptor closed by an internal ion-paired cap

The straightforward synthesis of a C_s symmetrical calix[6]arene possessing carboxylic acid groups as well as an ammonium arm is described. This calixarene can encapsulate ammonium ions through a highly selective recognition process thanks to the presence of an internal ion-paired cap that preorganizes the cavity and constitutes an efficient binding site.     

Simple locally compact groups acting on trees and their germs of automorphisms

Automorphism groups of locally finite trees provide a large class of examples of simple totally disconnected locally compact groups. It is desirable to understand the connections between the global and local structure of such a group. Topologically, the local structure is given by the commensurability class of a vertex stabiliser; on the other hand, the action on the tree suggests that the local structure should correspond to the local action of a stabiliser of a vertex on its neighbours.     

Optically pure calix[6]tris-ammoniums: syntheses and host-guest properties toward neutral guests

[reaction: see text] Optically pure calix[6]arenes bearing chiral amino arms 4, 7, and 10 have been synthesized in high yields from the known symmetrically 1,3,5-trismethylated calix[6]arene. For both compounds 7 and 10, the key step consists of an efficient selective alkylation on the narrow rim of the calix[6]arene with Ba(OH)2 as the base. All of these chiral calix[6]tris-amines possess a similar flattened cone conformation with the cavity occupied by the methoxy groups. In contrast to 4 and 7, upon protonation, the enantiopure calix[6]arene 10 can switch to the opposite flattened cone conformation through self-assembly of its ammonium arms in an ion-paired cap which closes the cavity. As shown by NMR host-guest studies and an X-ray structure, the obtained polarized host (10 x 3H+) behaves as a remarkable endo-receptor for small polar neutral molecules. Thanks to the tris-cationic chiral binding site of 10 x 3H+, it was shown that the guests experience a chiral environment upon inclusion. Finally, the first example of enantioselective molecular recognition inside the cavity of a calix[6]arene has been evidenced with a racemic 1,2-diol guest.     

Kinetic and Thermodynamic Stabilization of Metal Complexes by Introverted Coordination in a Calix[6]azacryptand

The Huisgen thermal reaction between an organic azide and an acetylene was employed for the selective monofunctionalization of a X₆‐azacryptand ligand bearing a tren coordinating unit [X₆ stands for calix[6]arene and tren for tris(2‐aminoethyl)amine]. Supramolecular assistance, originating from the formation of a host–guest inclusion complex between the reactants, greatly accelerates the reaction while self‐inhibition affords a remarkable selectivity. The new ligand possesses a single amino‐leg appended at the large rim of the calixarene core and the corresponding Zn²⁺ complex was characterized both in solution and in the solid state. The coordination of Zn²⁺ not only involves the tren cap but also the introverted amino‐leg, which locks the metal ion in the cavity. Compared with the parent ligand deprived of the amino‐leg, the affinity of the new monofunctionalized X₆tren ligand 6 for Zn²⁺ is found to have a 10‐fold increase in DMSO, which is a very competitive solvent, and with an enhancement of at least three orders of magnitude in CDCl₃/CD₃OD (1:1, v/v). In strong contrast with the fast binding kinetics, decoordination of Zn²⁺ as well as transmetallation appeared to be very slow processes. The monofunctionalized X₆tren ligand 6 fully protects the metal ion from the external medium thanks to the combination of a cavity and a closed coordination sphere, leading to greater thermodynamic and kinetic stabilities.     

A novel receptor based on a C3v-symmetrical PN3-calix[6]cryptand

A C3v-symmetrical PN3-calix[6]cryptand was prepared in six steps from the known 1,3,5-tris-methylated calix[6]arene through a remarkably efficient [1 + 1] macrocyclization reaction. A 1H NMR study showed that the P,N-crypto cap rigidifies the whole edifice in a cone conformation ideal for molecular recognition applications. The ability of this new receptor to perform selective endo-complexation is illustrated with ammonium guests.     

A lattice in more than two Kac-Moody groups is arithmetic

Let Γ < G ₁ × … × G _n be an irreducible lattice in a product of infinite irreducible complete Kac-Moody groups of simply laced type over finite fields. We show that if n ≥ 3, then each G _i is a simple algebraic group over a local field and Γ is an S-arithmetic lattice. This relies on the following alternative which is satisfied by any irreducible lattice provided n ≥ 2: either Γ is an S-arithmetic (hence linear) group, or Γ is not residually finite. In that case, it is even virtually simple when the ground field is large enough. More general CAT(0) groups are also considered throughout.     

The study of electron transfer in advanced materials (electrodes for Li-ion batteries or catalysts)

Mössbauer Spectroscopy (MS) is really suitable to study local electronic structures. Its hyperfine parameters, isomer shift (IS) and quadrupole splitting (QS) allow to characterize the oxidation state and coordination of the probed element. So, the capabilities of this powerful technique have been highlighted for the study of electron transfer occurring during electrochemical or catalytic processes. Several examples illustrate how MS can be used for the determination of reaction mechanisms in new electrode materials of Li-ion batteries (Sb or Sn-based, Ti oxides) or reforming catalysts (supported bimetallic PtSn catalysts). Deeper insight into the mechanisms determining the electrochemical or catalytic performances can be expected.     

Ipso-Nitration of calix[6]azacryptands: intriguing effect of the small rim capping pattern on the large rim substitution selectivity

The ipso-nitration of calix[6]arene-based molecular receptors is a important synthetic pathway for the elaboration of more sophisticated systems. This reaction has been studied for a variety of capped calixarenes, and a general trend for the regioselective nitration of three aromatic units out of six in moderate to high yield has been observed. This selectivity is, in part, attributed to the electronic connection between the protonated cap at the small rim and the reactive sites at the large rim. In addition, this work highlights the fact that subtle conformational properties can drastically influence the outcome of this reaction.     

Alkoxyamine-mediated radical synthesis of indolinones and indolines

Thermolysis of alpha-alkoxyamino propionanilides produces indolinones, whereas thermal reaction of N-allylaniline derivatives with various Tordo-type alkoxyamines results in formation of indolines in the radical regime. [reaction: see text]