Size-selective catalysis of ester and anilide cleavage by the dinuclear barium(II) complexes of cis- and trans-stilbenobis(18-crown-6)

Controlling the liberation rate of the in situ release of a chemical fuel for the operationally autonomous motions of molecular machines

Second-order rate constants of the aminolysis of 2-cyano-2-phenylpropanoic anhydride 3 by a series of N-methylanilines differently substituted in the aromatic moiety (4a-d) were measured in dichloromethane. The common reaction product of aminolysis is 2-cyano-2-phenylpropanoic acid 1, which is known to be an effective fuel for acid-base driven molecular machines, but cannot be used in molar excess with respect to the machine. The motivation behind the kinetic study has been the prospect of using the aminolysis of 3 to supply the machine with fuel at a rate that is never so high as to overfeed the system, thus avoiding the malfunction of the machine with concomitant waste of fuel. Knowledge of the kinetic parameters dictated the choice of 4c as the best nucleophile in the lot for feeding acid 1 into a catenane-based molecular machine at a rate that ensured a correct operation.

In Situ Switching of Site-Selectivity with Light in the Acetylation of Sugars with Azopeptide Catalysts

We present a novel concept for the in situ control of site-selectivity of catalytic acetylations of partially protected sugars using light as external stimulus and oligopeptide catalysts equipped with an azobenzene moiety. The isomerizable azobenzene-peptide backbone defines the size and shape of the catalytic pocket, while the π-methyl-l-histidine (Pmh) moiety transfers the electrophile. Photoisomerization of the E- to the Z-azobenzene catalyst (monitored via NMR) with an LED (λ = 365 nm) drastically changes the chemical environment around the catalytically active Pmh moiety, so that the light-induced change in the catalyst shape alters site-selectivity. As a proof of principle, we employed (4,6-O-benzylidene)methyl-α-d-pyranosides, which provide a change in regioselectivity from 2:1 (E) to 1:5 (Z) for the monoacetylated products at room temperature. The validity of this new catalyst-design concept is further demonstrated with the regioselective acetylation of the natural product quercetin. In situ irradiation NMR spectroscopy was used to quantify photostationary states under continuous irradiation with UV light.

Photoswitchable catalysis using organometallic complexes

Photoswitchable catalysis using organometallic complexes: a ligand design perspective.

Photoswitchable catalysis based on the isomerisation of double bonds

Photoswitchable catalysis is a young but rapidly evolving field that offers great potential for non-invasive dynamic control of both activity and selectivity in catalysis. This Feature Article summarises the key developments accomplished over the past years through the incorporation of photoswitchable double bonds into the structure of catalytically competent molecules.

Rapid conversion of hindered arylsulfonates to alkyl chlorides with retention of configuration

Arylsulfonates of hindered secondary alcohols are converted to the corresponding alkyl chlorides very rapidly and in good yields in the presence of titanium tetrachloride at low temperatures. These reactions proceed with exclusive retention of configuration.

Arylsulfonate-based nucleophile assisting leaving groups

[Chemical reaction: See text] The synthesis and unique reactivity of a series of arylsulfonate-based nucleophile assisting leaving groups (NALG) containing oligomeric ether units (including crown ethers) attached to the arylsulfonyl ring in the ortho orientation are described. The reactions of a variety of these ether-containing alkyl sulfonates with metal halides proceeded at substantially greater rates than electronically similar sulfonates. These ether-containing leaving groups also displayed marked selectivity for lithium halides relative to the corresponding sodium and potassium salts in nucleophilic displacement reactions.

Calix[4]arene-Based Zn2+ Complexes as Shape- and Size-Selective Catalysts of Ester Cleavage

[structures: see text] The kinetics of methanolysis of a number of esters endowed with a carboxylate anchoring group have been investigated in the presence of di- and trinuclear Zn2+ complexes of calix[4]arenes functionalized at the upper rim with nitrogen ligands. The results (i) emphasize the importance of a good match between ester size and intermetal distance, (ii) reveal a substrate independent superiority of the 1,2-vicinal dinuclear catalyst 1-Zn2 to its 1,3-distal regioisomer 2-Zn2, and (iii) provide further evidence for the concurrence of the three metal ions of 3-Zn3 in the catalytic mechanism.

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.

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

Two novel regioisomeric calix[4]arene derivatives (2 and 3), decorated with two aza[18]crown-6 units at vicinal (1,2) or diagonal (1,3) positions of the upper rim, were synthesized. The catalytic activities of their dinuclear Ba2+ complexes were investigated in the ethanolysis of esters 8-11, endowed with a carboxylate anchoring group. Major results are as follows: 1) the two metal ions in the dinuclear catalysts work together in a cooperative fashion; 2) the vicinal calix[4]arene catalyst 2 is far superior to its diagonal regioisomer 3 in the reactions of all of the investigated esters; and 3) the distance between the carboxylate and ester carbonyl, which increases regularly from 8 to 11, influences reactivity of catalytic ester cleavage in a way that is decidedly suggestive of the importance of a good match between ester size and metal-to-metal distance. However, the superiority of the vicinal catalyst 2 relative to 3 cannot be explained on the basis of the putative match of ester size to intermetal distance, thus providing an indication that additional, still poorly understood effects may contribute significantly to catalytic efficiency.

The bis-barium complex of a butterfly crown ether as a phototunable supramolecular catalyst

Reversible phototuning of the catalytic efficiency of the bis-barium complex of azobis(benzo-18-crown-6) in the basic ethanolysis of anilide derivatives has been achieved by light-induced cis right arrow over left arrow trans interconversion of the azobenzene spacer unit of the catalyst. The geometry of the productive catalyst-substrate complex is more favorable when the concave cis form of the catalyst is involved. Continuous photoregulation of the catalytic activity at any intermediate value between the "HIGH" and "LOW" levels was achieved by proper adjustment of the excitation wavelength or the irradiation time. The complete and relatively fast interconvertibility of photostationary states allowed the activity of the catalyst to be repeatedly photoswitched "HIGH" and "LOW" in the course of the same run.