Polycatenation under Thermodynamic Control

Abstract

Molecular switches based on mechanically interlocked molecules [1,2] have been shown to operate just as effectively in condensed phases [3] as in solution. Thus, we have witnessed these bistable donor-acceptor catenanes and rotaxanes [2] serving as the switchable components [4] in molecular electronic devices, [4] nanoelectromechanical systems, [5] plasmonic devices, [6] and mechanized nanoparticles. [7,8] To date, fabrication has relied on the formation of self-assembled monolayers [9] of one sort or another. In an effort to improve upon the processibility of switchable bistable catenanes, they have been incorporated into polymeric scaffolds [10] in the form of side-chain poly[2]catenanes. [11,12] The weakness of this approach to the localizing of bistable catenanes on the sides of polymer chains is that their implantation relies upon kinetic control with its inherent absence of proof-reading and error-checking. [13] An attractive alternative, which is potentially much more modular and open to (complex) structural variation, is one in which multiple catenations are carried out under templation [14] on a reactive polymer itself under thermodynamic control. [15] Here, we describe the synthesis of a side-chain poly[2]catenane using a synthetic protocoltested first of all on monomeric and dimeric analogues as models-in which nucleophilic substitutions, rendered dynamic under catalytic control, [16] are responsible for multiple catenations occurring all along the polymer chain, entirely driven in a synergistic fashion to completion by the intra-and intermolecular side-chain [p···p] stacking interactions of contiguous and interdigitated catenanes.Donor-acceptor [2]catenanes comprised of the tetracationic cyclophane, cyclobis(paraquat-p-phenylene) [17] (CBPQT 4+ ), as the p-electron accepting ring interlocked mechanically with a p-electron-donating crown ether are typically formed by a clipping procedure [18] to make the mechanical bond with the simultaneous formation of the [2]catenane. This route, although much employed, suffers from the weakness of being irreversible, i.e., should reactive sites come together by mistake, the product is committed effectively, to becoming contaminated with constitutional defects. Circumventing kinetic control requires a process in which proof-reading and error-checking ultimately lead to the designed transformations-and avoid introducing constitutional anomalies in the case of chemically modified polymers-under equilibrium control. This process relies upon dynamic covalent chemistry, [13] wherein covalent bonds are formed and broken reversibly until the thermodynamically most stable outcome is obtained.Recently, we have reported [16] the synthesis of donoracceptor [2]-and [3]catenanes using an iodide-catalyzed thermodynamically reversible nucleophilic substitution to open and close one of the rings, namely the tetracationic cyclophane. In theory at least, the same approach (Scheme 1) should be applicable to the formation of polycatenanes [19] of the main-chain, [20] pendant, [21] ...

Stereochemistry of Molecular Figures‐of‐Eight

“…Modification of CBPQT 4+ with a single functional handle on one of the phenylene rings is possible and does not alter significantly the binding capabilities of the cyclophane. Although this approach has been used to synthesize complex molecules, such as pretzelanes,8, 13 polymers,14 and linked nanoparticles,15 further modification of CBPQT 4+ with multiple functional handles has remained an elusive target until recently, though it does provide access to new topologies that have been unattainable thus far. Although a two‐armed self‐complexing rotaxane7b has been synthesized containing a bisfunctionalized cyclophane formed through an intramolecular template approach, an uncomplexed bisfunctional cyclophane has not been isolated and characterized until very recently.…”