[1]Rotaxanes and Pretzelanes: Synthesis, Chirality, and Absolute Configuration

Abstract

The synthesis of aliphatically bridged [1](n)rotaxanes and (n)pretzelanes in preparative yields and the dependency of their chiroptical properties on the length (n) of their bridge are reported. A cycloenantiomeric bis(sulphonamide)[2]rotaxane with a sulphonamide group in its axle and its wheel was intramolecularly dialkylated by homologous bifunctional oligomethylene reagents to form chiral [1](n)rotaxanes bearing bridges of different lengths (n) between the axle and the wheel. Intramolecular dialkylation by 1,omega-dibromoalkanes of a topologically chiral bis(sulphonamide)[2]catenane with a sulphonamide group in both of the macrolactam rings leads to pretzel shaped molecules ((n)pretzelanes) with homologous bridges between the two macrocycles. Their yields decrease with decreasing length of the bridge. The shortest bridge isolated so far in reasonable amounts consists of six methylene groups ((6)pretzelane). Remarkably, a covalent connection of axle and wheel in a [2]rotaxane was successful even with much shorter bridges-down to only three methylene groups ([1](3)rotaxane). The structural changes of the [1](n)rotaxanes with decreasing bridge length is expressed by an increasing high-field shift in the 1H NMR spectra. Enantiomeric resolution of the racemates of both series was achieved in seven cases for the [1](n)rotaxanes and two for the (n)pretzelanes by use of chiral HPLC columns. The circular dichrograms of both compound families show a strong dependency on the length of the bridge. However, the shortest bridges displayed some additional unexpected deviations. A new specification of the absolute configuration of supramolecules, such as [n]catenanes, [n]rotaxanes and (n)pretzelanes is introduced together with some nomenclature additions.

Deslipping of Rotaxanes with Axle Center Pieces of Different Lengths

“…Note that the two structures differ merely with respect to the orientation of the ester groups. these chemical shift differences, because the wheel moves along the axle quickly and the protons on time average feel the anisotropy to a lower extent compared to rotaxanes with shorter axles [14,15]. In addition, the axles of rotaxanes 1 -10 are floppier than those of other previous rotaxanes.…”

An Introduction to the Mechanical Bond

“…248 (iii) Kuroda's macrotricyclic homo [2]catenane of quadruply interlocked cages, 249 (iv) a macrobicyclic [3]catenane, also known as a bis [2]catenane, 250 (v) a branched [6]catenane, 135 (vi) a side-chain poly [2]catenane, 251 (vii) a main-chain poly [2]catenane, which was originally introduced by Geerts,252 (viii) a macrotricyclic hetero [2]catenane, introduced by Becher 253 (ix) a macrobicyclic hetero [4]catenane resulting from a collaboration between Vögtle and Sauvage, 254 (x) covalently bridged [2]catenanes, which originated 106 as precatena nes in Schill's directed syntheses, but later became known [255][256][257] as pretzelanes or [2]pseudocatenanes (xi) a doubly threaded macrobicyclic [2]catenane introduced by Becher 258 under the title of a [3]pseudocatenane, a topology also known [259][260][261] as a 'handcuff' catenane, (xii) a [4]pseudocatenane introduced by Chen,262 (xiii) a macrotricyclic hetero [7]catenane arising from a Nitschke-Sanders collaboration, 263 (xiv) Vögtle's cyclic bis [2]catenane, 264 and (xv) Chiu's cyclic tetrakis [2]catenane. 265 The structures of many beautiful MIMs have also been elucidated in the solid state by X-ray crystallography.…”

Hydrogen-Bond-Mediated Template Synthesis of Rotaxanes, Catenanes, and Knotanes

“…In the beginning, chiral rotaxanes were made which were stoppered by chiral groups such as sugar derivatives [183][184][185]. However, the more interesting challenge was the preparation of topologically chiral catenanes [127,186] and cycloenantiomeric rotaxanes [187][188][189][190]. There was not too much hope that a separation of the enantiomers of large and flexible molecules like rotaxanes or catenanes would be feasible, if they didn't contain chiral centers and thus differed only by marginal structural differences.…”