Analysis and Improvement of an Anion-Templated Rotaxane Synthesis

Abstract

A series of new rotaxanes with axles different in length was prepared. Following the synthetic protocol utilizing a known anion template effect (Scheme 1), surprisingly low yields in the order of 2 ± 5% were obtained (Scheme 3), which furthermore significantly depended on the nature of the stopper (Fig. 1). Variations in the synthetic procedures and computational results from Monte Carlo simulations allowed us to analyze the origin of these findings: The rotaxane wheel 3 acts as a noncovalently bound −protecting group× for the stopper nucleophile. The protection of the nucleophilic phenolate O-atom depends much on the steric demands of the stoppers (see 2 vs. 10) which induce different conformations of the wheel. Based on this model, an improved synthetic scheme is suggested. 1. Introduction. ± The synthesis of rotaxanes, catenanes, and other types of mechanically interlocked molecules [1] strongly relies on the operation of efficient template effects [2]. There exist different apparoches including inter alia those based on the tetrahedral [3] or octahedral [4] coordination geometry of metal ions, p-donor/pacceptor interactions [5], and H-bonding involving ammoniums ions [6] or neutral amides [7]. The latter is believed to play a crucial role in the formation of a trefoil knot with twelve amide bonds [8]. Recently, Vˆgtle and co-workers reported [9] the highyield rotaxane synthesis shown in Scheme 1, which is based on the recognition of phenolate stopper 2 À within the macrocyclic rotaxane wheel 3. Two H-bonds bind the trityl phenolate with a surprisingly high binding constant of K > 10 5 m À1 [9] [10] so that the equilibrium is shifted far to the side of the stopper-wheel complex 2 À ¥ 3. If the axlecenter piece 1 is added to the reaction mixture, the semi-axle 4 is formed ± either in a direct reaction of 1 with free 2 À , or in a reaction of 1 and 2 À ¥ 3 followed by deslipping that occurs due to the much lower strength of the H-bonds formed between the wheel and the semi-axle ether O-atom. Finally, the semi-axle 4 reacts with 2 À ¥ 3 to yield the rotaxane 5 ¥ 3 in up to 95% yield.For deslipping experiments [11], we attempted to synthesize rotaxanes with axlecenter pieces of different lengths, i.e., 9a ± j (obtained from 6 and 7a ± j via 8a ± j, see Scheme 2), and stoppers of intermediate size such as 10 (Scheme 3). Surprisingly, the rotaxane yields decreased dramatically for the rotaxanes discussed here, even to below 5%. Instead, large amounts of the free axle were isolated as the by-product. Three questions arise from these findings: i) Why does the yield of rotaxane depend so much on the nature of the stopper? ii) If this is due to an unfavorable competition between axle and rotaxane formation, why is the free axle formed so much faster than the rotaxane if 10 is applied as the stopper instead of 2? iii) What is the influence of the center piece?

Binuclear titanocenes linked by the bridge combination of rigid and flexible segment: Synthesis and their use as catalysts for ethylene polymerization

“…The α,ω-bis (4-bromomethylphenoxy)alkanes 6-10 [23] were prepared according to Scheme 1 starting with p-cresole via the α,ω-bis (4-methylphenoxy)alkanes 1-4 [24][25][26] and 5. They react with cyclopentadienyl sodium and butyl lithium with formation of the air-and moisture-sensitive dilithium salts 11-15 (Scheme 2), which were isolated as pale yellow powders.…”

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

“…These complexes promise to have some potential for the generation of larger, more complex structures through self-assembly in the future. The first problems with the new anion-templated rotaxane synthesis were encountered when the preparation of rotaxanes with smaller 3,5-di-tert-butyl phenol stoppers was attempted [248]. These rotaxanes were intended for the investigation of their deslipping behavior (see below) which should provide insight into the size complementarity of the stopper and the wheel cavity.…”

Mass spectrometric evidence for catenanes and rotaxanes from negative-ESI FT-ICR tandem-MS-experiments

“…The syntheses, the purification, and the characterization of all compounds used in this study have been described before [14,15,17,18,[20][21][22]26,27]. The N-deuterated macrocycle D 4 -1 was prepared by stirring 1 in deuterated methanol/dichloromethane 9:1 for 48 h. An 1 H NMR spectrum and the parent ion region in the ESI mass spectra confirm the exchange of nearly all amide protons against deuterium (>96% of all NH protons exchanged).…”

“…Another important difference is that axles 19 and 20 do not have any groups that are prone to forming strong hydrogen bonds with any of the macrocycles, while the phenolate formed upon deprotonation of 21 is capable of strong hydrogen bonding to two amide groups of the macrocycles. In dichloromethane, a binding constant of K > 10 5 M −1 was determined The strong binding of phenolates and other anions to tetralactam macrocycles in solution has been utilized for designing template effects for efficient rotaxane synthesis [20][21][22][28][29][30][31][32][33]. In the gas phase, one might expect the binding to be even stronger because there is no competition with the solvent anymore.…”

“…The largest structure is the trefoil dodecaamide knot 13, which in contrast to the catenanes is a mono-macrocycle, although it has an intertwined topology [18,19]. Finally, the rotaxanes 14-18 [20][21][22] are included here with stopper groups large enough to prevent deslippage even at higher internal energies [23][24][25]. The axles 19-21 serve for comparison and are needed to prepare mixtures of axle and wheel in order to be able to investigate the behavior of non-covalent complexes of these two compounds.…”