Dinuclear Barium(II) Complexes Based on a Calix[4]arene Scaffold as Catalysts of Acyl Transfer

A K+-promoted Diels-Alder reaction by using a self-assembled macrocyclic boronic ester containing two crown ether moieties

A K+-promoted Diels-Alder reaction of 1,4,9,10-anthradiquinone with various dienes is achieved in the presence of a self-assembled macrocyclic boronic ester [2+2]crown containing two crown ether moieties. The reaction rate is remarkably accelerated (up to 206-fold) compared to that in the absence of the promoter. Furthermore, the reaction proceeds regioselectively to yield an internal adduct. The self-assembly protocol was also demonstrated.

Straightforward synthesis and catalytic applications of rigid N,O-type calixarene ligands

Here, we report a simple one-step access to new rigid N,O-calixarene ligands which is based on copper-catalized amination at the lower rim. We also present their main group and late transition metal complexes which show superior catalytic activity, in several organic transformations, compared with known metal calixarene complexes.

Supramolecular catalysis. Part 2: artificial enzyme mimics

The design of artificial catalysts able to compete with the catalytic proficiency of enzymes is an intense subject of research. Non-covalent interactions are thought to be involved in several properties of enzymatic catalysis, notably (i) the confinement of the substrates and the active site within a catalytic pocket, (ii) the creation of a hydrophobic pocket in water, (iii) self-replication properties and (iv) allosteric properties. The origins of the enhanced rates and high catalytic selectivities associated with these properties are still a matter of debate. Stabilisation of the transition state and favourable conformations of the active site and the product(s) are probably part of the answer. We present here artificial catalysts and biomacromolecule hybrid catalysts which constitute good models towards the development of truly competitive artificial enzymes.

Upper rim guanidinocalix[4]arenes as artificial phosphodiesterases

Calix[4]arene derivatives, blocked in the cone conformation and functionalized with two to four guanidinium units at the upper rim were synthesized and investigated as catalysts in the cleavage of the RNA model compound 2-hydroxypropyl p-nitrophenyl phosphate. When compared with the behavior of a monofunctional model compound, the catalytic superiority of the calix[4]arene derivatives points to a high level of cooperation between catalytic groups. Combination of acidity measurements with the pH dependence of catalytic rates unequivocally shows that a necessary requisite for effective catalysis is the simultaneous presence, on the same molecular framework, of a neutral guanidine acting as a general base and a protonated guanidine acting as an electrophilic activator. The additional guanidinium (guanidine) group in the diprotonated (monoprotonated) trifunctional calix[4]arene acts as a more or less innocent spectator. This is not the case with the tetrasubstituted calix[4]arene, whose mono-, di-, and triprotonated forms are slightly less effective than the corresponding di- and triguanidinocalix[4]arene derivatives, most likely on account of a steric interference with HPNP caused by overcrowding.

Di- and Trinuclear Zn2+ Complexes of Calix[4]arene Based Ligands as Catalysts of Acyl and Phosphoryl Transfer Reactions

The calix[4]arene scaffold, blocked in the cone conformation by proper alkylation of the lower rim hydroxyls, was used as a convenient molecular platform for the design of bi- and trimetallic Zn2+ catalysts. The catalytic activity of the Zn2+ complexes of calix[4]arenes decorated at the 1,2-, 1,3-, and 1,2,3-positions of the upper rim with 2,6-bis[(dimethylamino)methyl]pyridine units were investigated in the cleavage of ester 6 and of the RNA model compound HPNP. High rate enhancements, up to 4 orders of magnitude, were observed in a number of catalyst-substrate combinations. Interestingly the order of catalytic efficiency among regioisomeric dinuclear complexes in the cleavage of ester 6 is 1,2-vicinal >> 1,3-distal, but it is reversed in the reaction of HPNP. The higher efficiency of trinuclear compared to dinuclear complexes provides an indication of the cooperation of three Zn2+ ions in the catalytic mechanism.