Novel Multifarenes and their Metal Binding Properties

The most common cavitands, such as calixarenes, resorcinarenes, calixpyrroles, pillararenes, cucurbiturils and cyclodextrins, are formed by cyclooligomerization of a single monomeric building block, which dictates particular binding properties, substitution patterns, functionalization and solubility. Therefore, modular cavitands that are constructed from multiple building blocks could offer shape- and size-controlled supramolecular architectures along with unique binding properties. For instance,

We have previously reported on a new class of modular cavitands with alternating building blocks, multifarenes, which are conveniently accessible by three complementary synthetic approaches.^[1] These first generation multifarenes were found to be quite flexible molecules, exhibiting temperature-dependent dynamic properties with modest binding constants due to the high entropy toll. To achieve higher binding efficiency and selectivity, we considered several rigidification approaches, including linking side chain functionalities, fusion to aromatic subunits, and covalently capping the portals by various strategies. We reasoned that fusion of aromatic systems to the diamine subunit would not only rigidify the resultant multifarene, but could also create an “aromatic box” for specific binding of hydrophobic guest molecules and organic cations by p-charge interactions. We used Schiff base formation from dialdehyde and diamine, as a convenient platform for the synthesis of discrete imine and amine macrocycles.

Aromatic multifarenes were easily prepared using a single-step cyclization.^[2] The family of [2+2], including their characterization and functional isomerization, set the stage for the design and synthesis of larger members of this family of multifarenes. Metal binding studies (by NMR, ITC and HRMS) showed that the new rigid multifarenes coordinate metal ions (Ag^I, Pd^II, Cu^II, Zn^II and Hg^II) strongly and selectively, depending on the nature of functional groups attached to multifarene portals.

References:

1. Parvari, G.; Annamalai, S.; Borovoi, I.; Chechik, H.; Botoshansky, M.; Pappob, D.; Keinan, E. Commun. 2014, 50, 2494-2497.
2. Swamy P. C. A.; Solel, E.; Reany, O.; Keinan, E. submitted.