Synthetic applications of the Cannizzaro reaction

The Cannizzaro reaction has emerged as a versatile synthetic tool for the construction of functionalized molecules. Dating back to the 19th century, this reaction, though initially used for the synthesis of an alcohol and acid functionality from aldehydes, has henceforth proven useful to generate diverse molecular entities using both intermolecular and intramolecular synthetic strategies. Immense applications in the synthesis of hydroxy acids and esters, heterocycles, fused carbocycles, natural products, and others with broad substrate scope have raised profound interest from methodological and synthetic standpoints. The ongoing development of reagents, solvents, and technologies for the Cannizzaro reaction reflects the broader trend in organic synthesis towards more sustainable and efficient practices. The focus of this review is to highlight some recent advances in synthetic strategies and applications of the Cannizzaro reaction towards the synthesis of potentially useful molecules.

Dissipative control of the fluorescence of a 1,3-dipyrenyl calix[4]arene in the cone conformation

The temporal control (ON/OFF/ON) of the fluorescence of a dichloromethane/acetonitrile 1 : 1 solution of calixarene 3 decorated with two pyrenyl moieties at the upper rim is attained by the addition of CCl₃CO₂H used as a convenient chemical fuel.

Time Programmable Locking/Unlocking of the Calix[4]arene Scaffold by Means of Chemical Fuels

In this work, we report that 2-cyano-2-phenylpropanoic acid and its p-Cl, p-CH₃ and p-OCH₃ derivatives can be used as chemical fuels to control the geometry of the calix[4]arene scaffold in its cone conformation. It is shown that, under the action of the fuel, the cone calix[4]arene platform assumes a "locked" shape with two opposite aromatic rings strongly convergent and the other two strongly divergent ("pinched cone" conformation). Only when the fuel is exhausted, the cone calix[4]arene scaffold returns to its resting, "unlocked" shape. Remarkably, the duration of the "locked" state can be controlled at will by varying the fuel structure or amount. A kinetic study of the process shows that the consume of the fuel is catalyzed by the "unlocked" calixarene that behaves as an autocatalyst for its own production. A mechanism is proposed for the reaction of fuel consumption.

The canonical behavior of the entropic component of thermodynamic effective molarity. An attempt at unifying covalent and noncovalent cyclizations

The statistically corrected entropic component of effective molarity (EM_S*) complies with the “canonical” values expressed by the log plot of EM_S* vs. the number n of single bonds in the ring product.

A calix[4]arene with acylguanidine units as an efficient catalyst for phosphodiester bond cleavage in RNA and DNA model compounds

Conjugated carbonyl units in a calixarene scaffold provide the right amount of flexibility for catalysis with a minimum entropic cost.

Inherently chiral cone-calix[4]arenes via a subsequent upper rim ring-closing/opening methodology

Access to chiral calix[4]arenes can unlock novel supramolecular architectures for enantioselective catalysis and molecular recognition. However, accessibility to these structures has been significantly hindered so far. We report herein the synthesis and characterization of di- and trifunctionalized cone-calix[4]arenes featuring a lactone moiety spanning the distal positions at the upper rim. The lactones force the whole skeleton to assume pinched-cone conformations. The ring-closure is favored by the high conformational flexibility of the calixarene scaffold. The new lactones are remarkably stable in the solid state, while a quick hydrolysis to restore the parent carboxylic acids occurs in solution under acidic/basic conditions. Slow aminolyses of lactones 2-3 yield inherently chiral products featuring three different functionalities at the upper rim, at room temperature. The subsequent ring-closing/opening methodology presented here highlights the versatility of these lactones as powerful synthons for the preparation of a variety of threefold upper rim functionalized, inherently chiral calix[4]arenes fixed in the cone structure.

Applications of dynamic combinatorial chemistry for the determination of effective molarity

A new strategy for determining thermodynamic effective molarities (EM) for macrocylisation reactions using dynamic combinatorial chemistry under dilute conditions is presented. At low concentrations, below the critical value, Dynamic Libraries (DLs) of bifunctional building blocks contain only cyclic species, so it is not possible to quantify the equilibria between linear and cyclic species. However, addition of a monofunctional chain stopper can be used to promote the formation of linear oligomers allowing measurement of EM for all cyclic species present in the DL. The effectiveness of this approach was demonstrated for DLs generated from mixtures of 1,3-diimine calix[4]arenes, linear diaminoalkanes and monoaminoalkanes. For macrocycles deriving from one bifunctional calixarene and one diamine, there is an alternating pattern of EM values with the number of methylene units in the diamine: odd numbers give significantly higher EMs than even numbers. For odd numbers of methylene units, the alkyl chain can adopt an extended all anti conformation, whereas for even numbers of methylene units, gauche conformations are required for cyclisation, and the associated strain reduces EM. The value of EM for the five-carbon linker indicates that this macrocycle is a strainless ring.

ATP cleavage by cone tetraguanidinocalix[4]arene

The upper rim cone tetraguanidinocalix[4]arene 1 is a highly effective catalyst of ATP hydrolysis. The catalytically most active species is the triprotonated form of the catalyst. The three protonated guanidinium groups provide the electrostatic driving force for substrate binding and activation, while the neutral guanidine most likely acts as a nucleophilic catalyst.