Dario Pasini

Macrocycles as Precursors for Organic Nanotubes

The use of cyclic structures, which organize themselves in a predicted manner to form tubular structures, is an area of great active interest. This review focuses on the design and characterization of cyclic, covalent compounds as building blocks for tubular aggregates and their behavior as multifunctional, synthetic, defined nanoobjects. The birth of host-guest chemistry (1) is generally associated with the discovery of crown ethers and their template syntheses mediated by alkaline ions; since then, synthesis of cyclic, covalent compounds able to selectively recognize specific guests by means of noncovalent interactions has been an exciting area of research which has found applications in organic synthesis, catalysis, materials science and separation science. In some cases, not only could the guest be recognized by the cyclic structure, but could also largely favor the cyclization reaction in the host synthesis. Carbon-based nanotubes, on the other hand, have emerged in the past decade as a striking example of nanospaces with confined dimensions. Their relatively straightforward synthesis, their availability using mid or high pressure arc volatilization techniques starting from graphitic substrates are making them promising candidates for a variety of applications in materials science (2). Their functionalization, obtained in various ways, and their solubilization using appropriate unsaturated polymers are elegant steps towards their possible utilization and manipulation as organic nanocontainers (3).

The Click Reaction as an Efficient Tool for the Construction of Macrocyclic Structures

The Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC, known as the click reaction) is an established tool used for the construction of complex molecular architectures. Given its efficiency it has been widely applied for bioconjugation, polymer and dendrimer synthesis. More recently, this reaction has been utilized for the efficient formation of rigid or shape-persistent, preorganized macrocyclic species. This strategy also allows the installment of useful functionalities, in the form of polar and function-rich 1,2,3-triazole moieties, directly embedded in the macrocyclic structures. This review analyzes the state of the art in this context, and provides some elements of perspective for future applications.

Cyclophanes and [2]catenanes as ligands for transition metal complexes: Synthesis, structure, absorption spectra, and excited state and electrochemical properties

Two tetracationic cyclophanes and two [2]catenanes containing paraquat-type units together with one or two transition metal chelating 2,2′-bipyridyl moieties have been synthesized. The metal-binding units have been used to generate novel mono- and binuclear ruthenium(II), rhenium(I), silver(I) (right), and copper(I) complexes.

Polymorphism-dependent aggregation induced emission of a push-pull dye and its multi-stimuli responsive behavior†

A comprehensive optical investigation of 1,1-dicyano-2,2-bis(4-dimethylaminophenyl)ethylene (1) is presented. The compound crystallizes in four different forms all displaying AIE behavior. The crystalline forms A and B are yellow-orange-emissive, while C and D are green-emissive. On the basis of X-ray structural analysis, the weak intermolecular interactions account for restricted internal rotations, leading to fluorescence enhancement in the crystals; however, the difference in emission color is ascribed to the various conformations of the molecules in the four crystalline forms. In addition, the emission color of crystals of A can be tuned by heating and grinding, that of B by grinding only, while crystals of C show chronochromic behavior. An explanation for such a rich variety of luminescence behavior is formulated here through the use of steady state and time resolved photoluminescence, X-ray diffraction analysis and DFT and TDDFT calculations. The involved chromic mechanism appears to be mainly associated with surface defects induced by the external stimuli rather than an amorphization process, as frequently observed for other stimuli responsive compounds.

Knockout of pgdS and ggt genes improves γ-PGA yield in B. subtilis.

One of the emerging biopolymers that are currently under active investigation is bacterial poly(γ‐glutamic acid) (γ‐PGA). However, before its full industrial exploitation, a substantial increase in microbial productivity is required. γ‐PGA obtained from the Bacillus subtilis laboratory strain 168 offers the advantage of a producer characterized by a well defined genetic framework and simple manipulation techniques. In this strain, the knockout of genes for the major γ‐PGA degrading enzymes, pgdS and ggt, leads to a considerable improvement in polymer yield, which attains levels analogous to the top wild γ‐PGA producer strains. This study highlights the convenience of using the laboratory strain of B. subtilis over wild isolates in designing strain improvement strategies aimed at increasing γ‐PGA productivity. Biotechnol. Bioeng. 2013; 110: 2006–2012.

A soluble polymer-bound Evans’ chiral auxiliary: synthesis, characterization and use in cycloaddition reactions

Abstract The synthesis of novel soluble polymer-supported optically-active oxazolidinone (Evans’ chiral auxiliary) with different chiral monomer/styrene ratios is described. The polymer was obtained in high yields and then functionalized with trans -crotonic anhydride to give a high loading polymer (2.42 mmol/g). The reactivity was tested in the 1,3-dipolar cycloaddition with diphenylnitrone under catalyzed and uncatalyzed conditions. The cycloadducts could be obtained in high purities and fair yields after reductive cleavage. The stereoselectivity is parallel to that obtained with the model substrate under classical solution conditions.

Structurally-variable, rigid and optically-active D2 and D3 macrocycles possessing recognition properties towards C60.

The reactivity of aromatic dicarboxylic acids, in combination with an axially-chiral, suitable dibenzylic alcohol, derived from BINOL, has been exploited in one-pot esterification reactions for the direct formation of several rigid, homochiral macrocycles. Cyclic adducts possessing, respectively, average molecular D(2) and D(3) symmetries, have been characterized in pure forms, with isolated yields and selectivities which are substantially different from those rising from purely statistical arguments. NMR and CD spectroscopies detect the structural and shape variability in the scaffolds, reflected both in terms of large changes in chemical shifts of selected proton resonances, and in terms of the variation of the CD signature related to the dihedral angle defined by the binaphthyl units embedded in the rigid cyclic skeleton. The crystal structure of two homochiral D(2) macrocycles show the formation of ordered nanostructures in the solid state, with the naphthyl rings of the binaphthyl units packing intermolecularly in an eclipsed-like conformation to yield nonhelical tubular arrangements. The larger D(3) cyclic adducts are able to afford stable complexes with C(60) in toluene solution, with comparable binding strengths, yet whose stoichiometries are dependent on small variations in the spacing units and therefore in the shapes of the internal cavities of the cyclic structures.

Site-selective supramolecular synthesis of halogen-bonded cocrystals incorporating the photoactive azo group

The halogen-bonding-driven self-assembly of α,ω-diiodoperfluoroalkanes with trans-4,4′-azobipyridine gives rise to infinite 1D networks in the crystalline state. The four nitrogen atoms of the azobispyridine module could allow for the formation of various isomeric products. Interestingly, a site-selective supramolecular synthesis occurs, since only the pyridyl nitrogen atoms are involved in the recognition process. This selectivity can be rationalized on the basis of a higher basicity and steric accessibility of the pyridine nitrogens, with respect to the azo nitrogens, and with the formation of an architecture where the hydrocarbon and perfluorocarbon modules segregate.

Diastereoselective Self‐Assembly of [2]Catenanes

Two chiral π-electron-rich crown ethers incorporating either a binaphthol or two D-mannitol units have been synthesized and their abilities to bind bipyridinium guests demonstrated. Both crown ethers could be interlocked mechanically with cyclobis(paraquat-p-phenylene) to afford two chiral [2]catenanes. Furthermore, these crown ethers were also mechanically interlocked with a tetracationic cyclophane incorporating a 2,2′-dihydroxy-1,1′-binaphthyl spacer to afford mixtures of diastereoisomeric [2]catenanes. The composition of these mixtures was determined by 1H-NMR-spectroscopic and HPLC analyses which revealed that modest diastereoselection (56:44–67:33) occurs during the kinetically controlled self-assembly of the catenanes. The free energy barriers (12.8–16.8 kcal mol–1) associated with the circumrotation of one macrocyclic component through the cavity of the other and vice versa were determined by variable-temperature 1H-NMR spectroscopy. In addition, another dynamic process involving the “rocking” of the mean planes of the mechanically interlocked macrocycles with respect to each other was also identified and the associated free energy barriers (10.3–10.4 kcal mol–1) determined.

Solvent effect as the result of frontier molecular orbital interaction. VII. The retro-diels-alder reaction.☆

Abstract The solvent effect on the retro-Diels-Alder (R.D.A.) reaction of 1,4,4a,9a-tetrahydro-4a-methyl-(1α,4α,4aα,9aα) -1,4-methanoanthracene-9,10-dione to 2-methyl-1,4-naphthoquinone and cyclopentadiene was investigated kinetically in 16 solvents. The hyperbolic relationship between the kinetic data and the Acceptor Number of the solvent is strong evidence that the solvent acts as an electrophile which lowers the activation energy of the reaction. Furthermore when these rate constants are plotted vs those of the previously investigated Diels-Alder (D.A.) reaction of 1,4-naphthoquinone and 2,3-dimenthylbutadiene, a linear relationships is obtained. The linearity of the graph is a good indication that the nature of the solvent effect is the same in both D.A. and R.D.A. reactions. The above relationships and the thermodynamic parameters strongly suggest that the R.D.A. reaction is a “late transition state” pericyclic reaction whose solvent effect derives from a specific interaction between the solvent and the product.