Stereoselective Synthesis of Molecular Square and Granny Knots

Abstract

We report on the stereoselective synthesis of both molecular granny and square knots through the use of lanthanide-complexed overhand knots of specific handedness as three-crossing “entanglement synthons”. The composite knots are assembled by combining two entanglement synthons (of the same chirality for a granny knot; of opposite handedness for a square knot) in three synthetic steps: first, a CuAAC reaction joins together one end of each overhand knot. Ring-closing olefin metathesis (RCM) then affords the closed-loop knot, locking the topology. This allows the lanthanide ions necessary for stabilizing the entangled conformation of the synthons to subsequently be removed. The composite knots were characterized by 1H and 13C NMR spectroscopy and mass spectrometry and the chirality of the knot stereoisomers compared by circular dichroism. The synthetic strategy of combining building blocks of defined stereochemistry (here overhand knots of Λ- or Δ-handed entanglement) is reminiscent of the chiron approach of using minimalist chiral synthons in the stereoselective synthesis of molecules with multiple asymmetric centers.

Isogonal weavings on the sphere: knots, links, polycatenanes

“…Weavings, knots, links and related geometrical structures are currently of considerable interest in materials chemistry and biochemistry, as testified in recent reviews (Lim & Jackson, 2015;Flapan, 2015;Bruns & Stoddart, 2016;Pieters et al, 2016;Horner et al, 2016;Fielden et al, 2017) and in other recent reports (Danon et al, 2017;Wu et al, 2017;Kim et al, 2018;Zhang et al, 2018;Leigh et al, 2019;Sawada, Saito et al, 2019;Sawada, Inomata et al, 2019;Inomata et al, 2020). Suggested knots most suitable for self-assembly have recently been identified (Marenda et al, 2018).…”

Lanthanide‐Containing Rotaxanes, Catenanes, and Knots

“…Significant progress has been made in the preparation and chemical utilization of lanthanide templated knots, as reported by Leigh and co-workers. [33][34][35][36][37] This laboratory's first example of a lanthanide templated knot involved cyclizing three equivalents of a 2,6-pyridinedicarboxamide ligand around a Ln(III) cation (Figure 13). [33] In their system, careful design of the ligand introduced inter-ligand π-π stacking in the precursor complex to arrange the end-groups such that cyclization would lead to the knot rather than unknotted macrocycle.…”

“…The Leigh group has also reported upon the preparation of square and granny knots. [37] These were assembled by use of lanthanide complexed overhand knots of specific handedness, termed "entanglement synthons". To prepare the target knots, appropriate entanglement synthons were first joined at one end by use of the CuAAC "click" reaction, then at the other end by Grubbs catalyzed RCM.…”

Template‐Free Self‐Assembly of Molecular Trefoil Knots and Double Trefoil Knots Featuring Cp*Rh Building Blocks

“…Unfortunately, such strategies are not extendable to many more complicated knot topologies, due to the limitations of the metal templates imposed by their preferred coordination numbers and geometries . Alternative synthetic strategies will need to be developed to access the majority of knot topologies at the molecular level …”