Rotaxanes hydrogen-bonding

Schalley CA, WeUandt T, Briiggemann J,V6gtle F (2004) Hydrogen-Bond-Mediated Template Synthesis of Rotaxanes, Catenanes, and Knotanes. 248 141-200 Scheer M,see Balazs G (2003) 232 1-23... 

Figure B. Pictorial representation of the self-assembly of pseudorotaxa-nes based on (a) charge-transfer and C-H—O hydrogen-bonding interactions between 1,1 -diben-zyl-4,4 -bipyridinium dication and 1,5-dinaphtho[38] crown-10 (1/5DN38C10), and (b) hydrogen-bonding interactions between dibenzyl ammonium ion and dibenzo[24]crown-8 (DB24C8). A possible route towards the synthesis of rotaxanes and catenanes is also schematized.

As will be discussed later, ammonium salts have also been used as hydrogen bond-donor templates to favour the formation of macrocycles (yielding pseu-dorotaxanes and rotaxanes). 

The strong hydrogen bonding interactions observed between the oxygen atoms of crown ethers and the N-H groups of ammonium groups can be successfully employed to prepare pseudorotaxanes and rotaxanes by templated processes. This approach has been extensively utilised by Stoddart, Busch and others to obtain a wide range of interlocked species. 

Scheme 14 [3]Rotaxane 23 can be prepared by hydrogen-bonding templated synthesis employing the threading-followed-by-capping methodology...

Scheme 16 Synthesis of [3]rotaxane 27 by hydrogen-bonding templated synthesis... 

The rotaxanes presented so far in this section have been prepared by the thread-ing-followed-by-capping methodology. However, hydrogen bond-donor templates... 

Vogtle has developed this approach further and employed a series of anionic templates to prepare rotaxanes (instead of the neutral template in the above reaction) [65-67]. In this approach a phenolate, thiophenolate or sulfonamide anion is non-covalently bound to the tetralactam macrocycle (46) forming a host-guest complex via hydrogen bonding (see Scheme 21). 

With this separation of the hydrogen bond-acceptor template and the sites for the stopper attachment, the steric effects due to shielding by the wheel are avoided making the synthesis of the rotaxane 48 more efficient (in some of the systems the yields of rotaxane formation increase from 5 to 30% when the functionalised centre-piece is added). 

Assembly of Rotaxanes by an "Independent" Hydrogen-Bonding Template... 

Loeb has reported a series of pseudorotaxanes [84,85] and rotaxanes [86,87] where C-H- 0 hydrogen bonding interactions (together with N+- -O attractive forces) play an important contribution in templating the formation of the interlocked species. In particular, the formation of a pseudorotaxane was observed when equimolar amounts of [pyCH2CH2py]2+ and the crown ether 20 were mixed. The structural characterization of the resulting host-guest complex... 

Scheme 33 Loeb has reported the templated syntheses of a series of pseudorotaxanes and rotaxanes which is based on C-H---0 hydrogen bonding interactions as the main directing force... 

Further studies by the same authors have led to the formation of [2]rotaxanes, [3]rotaxanes and pseudo-polyrotaxanes [85-87]. In all these interlocked species, in spite of the presence of aromatic rings in the axle and wheel, tt-ti interactions do not seem to play a role in the templating process. This highlights once again the importance of C-H---0 hydrogen bonding in the assembly of interlocked species. 

Schalley CA, Weilandt T, Briiggemann J, Vogtle F (2004) Hydrogen-bond-mediated template synthesis of rotaxanes, catenanes, and knotanes. Top Curr Chem 248 141-200... 

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

Hydrogen bonding is thought to be responsible for the formation of such molecules as the catenane 37 [17], rotaxane 396 [18] and even knot 397 [19]. 

Especially, the subdivision in different hydrogen bond acceptor atom sets improves the performance of the SEN approach while a subdivision depending on the hydrogen bond donor atom showed only a minor improvement compared to the general fit of Reiher et al. Thus, the SEN approach has proven as a tool to investigate hydrogen bonds of, e.g., transition metal compounds (171,174-177), peptides (178), enzymes (179), DNA and RNA (173), molecular switches (180), ionic liquids (181,182), and rotaxanes (183). However, the SEN approach is not solely restricted to hydrogen bond detection. This approach can also be apphed to determine the covalent interaction between metal atoms (184) or phosphorus atoms (162,185). Therefore, it is suitable for different kind of interactions. 

Fullerene stoppers have also been introduced in rotaxanes as a way to probe the motion of the ring thanks to their well-defined photophysical and electrochemical properties.54 In the rotaxane shown in Scheme 9.12, the hydrogen bonding station (a glycylglycine template) was placed far away from the fullerene by a triethylene glycol spacer.55... 

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