Di Stefano

Fast transimination in organic solvents in the absence of proton and metal catalysts. A key to imine metathesis catalyzed by primary amines under mild conditions

Amine–imine exchange reactions of sterically unhindered reactants were found to be surprisingly fast at room temperature in a variety of nonaqueous solvents in the absence of proton and metal catalysts. The reaction mechanism suggested by ab initio calculations in the gas phase involves nucleophilic addition to the CN bond in concert with proton transfer from the amine NH bond to the imine nitrogen via a highly imbalanced transition state. These very fast transimination reactions were utilized in the catalysis of imine metathesis. Imine metathesis, usually carried out in organic solvents at high temperature in the presence of metal catalysts, occurs smoothly at room temperature in the presence of primary amines under nonacidic conditions as a result of coupled transimination processes. Kinetic data fully consistent with the proposed reaction mechanism were obtained.

Toward an Artificial Acetylcholinesterase

The methanolysis of choline p-nitrophenylcarbonate in chloroform containing 1 % methanol is catalyzed with turnover by ditopic receptors 1 and 2, consisting of a calix[6]arene connected to a bicyclic guanidinium by means of a short spacer. The calix[6]arene subunit strongly binds to the trimethylammonium head group through cation–π interactions, whereas the guanidinium moiety is deputed to stabilize through hydrogen bonding reinforced by electrostatic attraction the anionic tetrahedral intermediate resulting from methoxide addition to the ester carbonyl. The observed cholinesterase activity had been anticipated on the basis of the ability of the ditopic receptors 1 and 2 to bind strongly to the choline phosphate DOPC, which is a transition state analogue for the BAc2-type cleavage of choline esters.

Effective catalysis of imine metathesis by means of fast transiminations between aromatic–aromatic or aromatic–aliphatic amines

This paper reports on a quantitative investigation of rates of amine-imine exchange reactions of primary amines with their benzylidene derivatives in organic solvents at room temperature. Exchange reactions involving aromatic-aromatic or aromatic-aliphatic amines were in all cases fast enough to allow their use in the effective catalysis of imine metathesis in the absence of acid and metal catalysis. Transiminations based on exchange between aromatic and aliphatic amines were retarded both by electron-donating and electron-withdrawing substituents in the para-position of the benzylidene moiety. This result was interpreted as arising from a change in the rate-determining step of the two-step transimination reaction.

A well-behaved dynamic library of cyclophane formaldehyde acetals incorporating diphenylmethane units

A new well-behaved dynamic library (DL) based on acetal cyclophanes incorporating diphenylmethane units is described. The effective molarities of the first two members of this library of cyclic oligomers C(2) and C(3) have been obtained from concentration profiles. Amplification of the dimer in the presence of a silver template, and cross-equilibration experiments are also reported.

Supramolecular catalysis of ester and amide cleavage by a dinuclear barium(II) complex

Five components spontaneously self-assemble to yield the productive complex 1, where one barium ion delivers an ethoxide to the carbonyl group of an amide substrate anchored by means of a distal carboxylate moiety to the other barium ion. High substrate specificity, fairly high reaction rates with catalytic turnover, and competitive inhibition by inert substrates are observed.

Target-induced amplification in a dynamic library of macrocycles. A quantitative study

The perturbation caused by a silver template on the composition of a DCL of cyclophane formaldehyde acetals was investigated as a function of monomer concentration ranging from dilute solutions to values approaching the critical monomer concentration. The target-induced amplification of the dimeric macrocycle was quantitatively analyzed in terms of effective molarity of the macrocycle in the absence and presence of the target.

Highly efficient intramolecular Cannizzaro reaction between 1,3-distal formyl groups at the upper rim of a cone-calix[4]arene

The 1,3-distal cone-calix[4]arene dialdehyde 1 undergoes Cannizzaro disproportionation in the presence of strong base, but its 1,2-vicinal regioisomer 3 and the analogous monoaldehyde 2 are unreactive under the same conditions. The high intramolecular reactivity of the 1,3-distal regioisomer 1 is measured and discussed in terms of Effective Molarity (EM).

Substituent effects on the catalytic activity of bipyrrolidine-based iron complexes

The catalytic activity and the selectivity of the new bipyrrolidine-based Fe(II) complexes 2·Fe(OTf)2 and 3·Fe(OTf)2 in the oxidation of a series of alkyl and alkenyl hydrocarbons as well as of an aromatic sulfide with H2O2 were tested and compared with the catalytic efficiency of Whites parent complex 1·Fe(OTf)2 in order to evaluate the sensitivity of the reaction to electronic effects.

Hydrocarbon oxidation catalyzed by a cheap nonheme imine-based iron(II) complex

Nonheme iron complex 1 is easily obtained by one-pot assembly of cheap and commercially available starting materials. This complex effectively catalyzes the oxidation of a number of non-activated C–H bonds by H2O2 with high turnover numbers and good selectivity.

One-shot preparation of an inherently chiral trifunctional calix[4]arene from an easily available cone-triformylcalix[4]arene

Via selective 1,3-distal intramolecular Cannizzaro disproportionation of an easily available cone-triformylcalix[4]arene, an inherently chiral trifunctional cone-calix[4]arene derivative has been prepared. The presence of three different functional groups (-CH2OH, -CHO and -COOH) at the upper rim of the calixarene scaffold makes this compound a versatile intermediate for the development of multifunctional devices. Interesting chiral discrimination of serine derivatives has been observed, presumably thanks to a multipoint-interaction involving the reversible imine bond formation and the hydrogen bonding of the hydroxyl group of the amino acid side-chain with the upper rim functional groups. Consistently, chiral discrimination was not observed with alanine and valine derivatives, lacking hydrogen bonding groups on the side-chain.