Ramalingam Natarajan

Water Chains in Hydrophobic Crystal Channels: Nanoporous Materials as Supramolecular Analogues of Carbon Nanotubes

The behavior of water in narrow apolar pores has attracted much recent interest. Although it might seem that such channels should repel water, it transpires that they can be hydrated and moreover that their “hydrophobic” nature promotes rapid water flow. The phenomenon is observed in biology, where the pores of aquaporins (water-transporting proteins) are composed largely of hydrophobic amino acids. It is also seen for carbon nanotubes (CNTs) which, according to both theoretical and experimental studies, allow rapid passage of water molecules. Water flow through CNTs can be selective and controllable, suggesting applications in water-purification and nanofluidic devices. To understand these systems, it is important to gather structural information on water in hydrophobic environments. Especially relevant is the “water wire”, a one-dimensional hydrogen-bonded chain of water molecules. The water in aquaporin channels adopts this motif, as does the water in the narrowest and best-understood CNTs. However, while there are many crystal structures which show water molecules in single file, there is limited information on water wires in purely hydrophobic environments. In particular there seem to be no structures in which water wires are surrounded exclusively by p systems. We now report the crystal engineering of channels bounded by nonpolar aromatic units, and the structural characterization of linear chains of water molecules within these pores. We have previously shown that steroidal bisphenylureas 1 (Figure 1a) crystallize as monohydrates to form structures with hexagonal P61 symmetry, containing one-dimensional channels of unusually large diameter (Figure 1b,c). Groups R and R are directed into the channels, so that in principle they can be varied substantially without disrupting the crystal packing. Potentially, this should allow a broad scope for tuning of channel size and properties. Indeed, our original report described three structures 1a–c, in which group R was altered to give pore diameters of 14.3, 12.3, and 11.6 , respectively. Having shown that R could be varied, we sought to confirm that ester group CO2R 2 could also be used to tune the properties of the crystals. Included among our targets were steroids 2–4, featuring large aromatic groups linked to C24 O via two-carbon tethers. Modeling suggested that this arrangement would allow the aromatic groups to lie roughly parallel to the channel axis, lining the walls and reducing the diameters to about 6 to 7 . We could thus create an environment not dissimilar to that inside a CNT. Esters 2–4 Figure 1. a) Previously described steroidal bis-phenylureas forming nanoporous crystals. b) The structure of 1b in the crystal, showing the bound water molecule. c) Packing diagram for 1b viewed along the axis of the channels. A single steroid is highlighted in the centre of the diagram, showing how the ester CH3 groups form part of the surface of one channel while the C3-terminal NHPh groups protrude into another. The ester CH3 and C3 NHPh groups are shown in space-filling mode. Also highlighted are the water molecules bound to each unit of 1b.

Cucurbit[7]uril: a high-affinity host for encapsulation of amino saccharides and supramolecular stabilization of their α-anomers in water.

Cucurbit[7]uril (CB[7]), an uncharged and water-soluble macrocyclic host, binds protonated amino saccharides (D-glucosamine, D-galactosamine, D-mannosamine and 6-amino-6-deoxy-D-glucose) with excellent affinity (Ka =10(3) to 10(4)  M(-1) ). The host-guest complexation was confirmed by NMR spectroscopy, isothermal titration calorimetry (ITC), and MALDI-TOF mass spectral analyses. NMR analyses revealed that the amino saccharides, except D-mannosamine, are bound as α-anomers within the CB[7] cavity. ITC analyses reveal that CB[7] has excellent affinity for binding amino saccharides in water. The maximum affinity was observed for D-galactosamine hydrochloride (Ka =1.6×10(4)  M(-1) ). Such a strong affinity for any saccharide in water using a synthetic receptor is unprecedented, as is the supramolecular stabilization of an α-anomer by the host.

Tunable Porous Organic Crystals: Structural Scope and Adsorption Properties of Nanoporous Steroidal Ureas

Previous work has shown that certain steroidal bis-(N-phenyl)ureas, derived from cholic acid, form crystals in the P61 space group with unusually wide unidimensional pores. A key feature of the nanoporous steroidal urea (NPSU) structure is that groups at either end of the steroid are directed into the channels and may in principle be altered without disturbing the crystal packing. Herein we report an expanded study of this system, which increases the structural variety of NPSUs and also examines their inclusion properties. Nineteen new NPSU crystal structures are described, to add to the six which were previously reported. The materials show wide variations in channel size, shape, and chemical nature. Minimum pore diameters vary from ∼0 up to 13.1 Å, while some of the interior surfaces are markedly corrugated. Several variants possess functional groups positioned in the channels with potential to interact with guest molecules. Inclusion studies were performed using a relatively accessible tris-(N-phenyl)urea. Solvent removal was possible without crystal degradation, and gas adsorption could be demonstrated. Organic molecules ranging from simple aromatics (e.g., aniline and chlorobenzene) to the much larger squalene (Mw = 411) could be adsorbed from the liquid state, while several dyes were taken up from solutions in ether. Some dyes gave dichroic complexes, implying alignment of the chromophores in the NPSU channels. Notably, these complexes were formed by direct adsorption rather than cocrystallization, emphasizing the unusually robust nature of these organic molecular hosts.

Hollow nanotubular toroidal polymer microrings

Despite the remarkable progress made in the self-assembly of nano- and microscale architectures with well-defined sizes and shapes, a self-organization-based synthesis of hollow toroids has, so far, proved to be elusive. Here, we report the synthesis of polymer microrings made from rectangular, flat and rigid-core monomers with anisotropically predisposed alkene groups, which are crosslinked with each other by dithiol linkers using thiol-ene photopolymerization. The resulting hollow toroidal structures are shape-persistent and mechanically robust in solution. In addition, their size can be tuned by controlling the initial monomer concentrations, an observation that is supported by a theoretical analysis. These hollow microrings can encapsulate guest molecules in the intratoroidal nanospace, and their peripheries can act as templates for circular arrays of metal nanoparticles.

ZnI2-Catalyzed Diastereoselective [4 + 2] Cycloadditions of β,γ-Unsaturated α-Ketothioesters with Olefins.

The potential of β,γ-unsaturated α-ketothioesters participating in hetero-Diels-Alder reaction has remained unexplored. We report herein the first study of a ZnI2-catalyzed highly diastereoselective inverse electron demand hetero-Diels-Alder reaction of β,γ-unsaturated α-ketothioesters with olefins to access highly substituted 3,4-dihydro-2H-pyrans. All the reactions proceed with cis-selectivity in moderate to excellent yields. Under similar reaction conditions, terminal alkynes undergo direct conjugate 1,4-addition to yield δ,ε-acetylenic α-ketothioesters. Furthermore, the utility of these cycloadducts has been demonstrated by an NBS-MeOH mediated stereospecific efficient access to fully substituted pyran rings. The product bromoethers undergo E2 elimination with DBU, resulting in substituted 3,6-dihydro-2H-pyrans. In addition, the thioester moiety of the products has been used for further transformations, such as amidations and Fukuyama coupling reactions.

Synthesis of Bis-pyrrolizidine-Fused Dispiro-oxindole Analogues of Curcumin via One-Pot Azomethine Ylide Cycloaddition: Experimental and Computational Approach toward Regio- and Diastereoselection.

Curcumin has been transformed to racemic curcuminoids via an azomethine ylide cycloaddition reaction using isatin/acenaphthoquinone and proline as the reagents. The products were characterized by extensive 1D/2D NMR analysis and single-crystal X-ray crystallographic studies. The enantiomers of one racemic product were separated by HPLC on a Chiralcel OD-H column and were indeed confirmed by the CD spectra of the separated enantiomers.

Polymorphism of an o-anisaldehyde: a novel example of channel-type organization sustained by weak C-H···O and C-H···N hydrogen bonds

The aldehyde 1 exhibits dimorphic behaviour; one of the modifications results from a novel helical self-assembly of the aldehyde into a channel-type organization sustained entirely by C–H⋯O and C–H⋯N hydrogen bonds.

Cofacial Organic Click Cage to Intercalate Polycyclic Aromatic Hydrocarbons

The synthesis of a 3-fold symmetric cofacial organic cage (COC) through Cu(I)-catalyzed azide-alkyne cycloaddition is reported. The COC can function as an efficient receptor for carcinogenic polycyclic aromatic hydrocarbons to intercalate them in its intrinsic cavity through donor-acceptor and π···π stacking interactions. The association constants (Ka) are in the range of 3.7 × 10(4) to 1.3 × 10(6) M(-1). X-ray diffraction analysis authenticated that the polycyclic aromatic hydrocarbons (PAHs) are intercalated in the interior of the COC.

Phthalate tethered strategy: carbohydrate nitrile oxide cycloaddition to 12–15 member chiral macrocycles with alkenyl chain length controlled orientation of bridged isoxazolines

The highly selective synthesis of phthalate templated bridged isoxazoline macrocyclic lactones is demonstrated using readily accessible carbohydrate precursors via intramolecular nitrile oxide–alkene cycloaddition. The structures of the macrocycles were established by 2D NMR as well as X-ray diffraction study. The orientation of the bridged isoxazoline ring is dependent on the distance between the 1,3-dipole and dipolarophile. The macrocycles are amenable to further transformations to higher amino sugars by sequential removal of the phthalate template and reductive cleavage of the isoxazoline moiety.

D–A–D Structured Bis-acylhydrazone Exhibiting Aggregation-Induced Emission, Mechanochromic Luminescence, and Al(III) Detection

A readily accessible D-A-D triad molecule 1 was synthesized through acylhydrazone bond formation using carefully chosen building blocks. The molecule 1 exhibits emission through charge-coupled proton transfer and enhanced emission induced through aggregation and mechanochromic luminescence. Further, it detects Al(III) selectively among other cations in an efficient manner.