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The Neighboring Component Effect in a Tristable [2]Rotaxane Yuping Wang, Tao Cheng, Junling Sun, Zhichang Liu, Marco Frasconi, William A

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The Neighboring Component Effect in a Tristable [2]Rotaxane Yuping Wang, Tao Cheng, Junling Sun, Zhichang Liu, Marco Frasconi, William A Subscriber access provided by Caltech Library Article The Neighboring Component Effect in a Tristable [2]Rotaxane Yuping Wang, Tao Cheng, Junling Sun, Zhichang Liu, Marco Frasconi, William A. Goddard, and J. Fraser Stoddart J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08519 • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts. is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Page 1 of 27 Journal of the American Chemical Society 1 2 3 4 5 6 7 8 The Neighboring Component Effect in a Tristable [2]Rotaxane 9 10 † ‡ † 11 Yuping Wang, Tao Cheng, Junling Sun, Zhichang Liu,∇ Marco Frasconi,§ 12 ‡ ,†,⊥,║ 13 William A. Goddard III, and J. Fraser Stoddart* 14 15 † 16 Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA ‡ 17 Materials and Process Simulation Center, California Institute of Technology, 1200 East California Boulevard, 18 Pasadena, California 91125, USA 19 ∇School of Science, Westlake University, 18 Shilongshan Road, Hangzhou 310024, China § 20 Department of Chemical Sciences, University of Padova, Via Marzolo 1, Padova 35131, Italy. 21 ⊥Institute for Molecular Design and Synthesis, Tianjin University, 92 Weijin Road, Nankai District, Tianjin 300072, P.R. China 22 ║School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia 23 24 25 26 *E-mail: [email protected] 27 28 29 30 31 32 33 34 35 MAIN TEXT 36 37 38 39 40 41 42 *Correspondence Address 43 Professor J Fraser Stoddart 44 45 Department of Chemistry 46 47 Northwestern University 48 49 2145 Sheridan Road 50 Evanston, IL 60208-3113 (USA) 51 52 Tel: (+1)-847-491-3793 53 54 E-Mail: [email protected] 55 56 57 58 59 60 ACS Paragon Plus Environment Journal of the American Chemical Society Page 2 of 27 1 2 3 Abstract: The redox properties of cyclobis(paraquat-p-phenylene) cyclophane (CBPQT4+) renders 4 5 6 it a uniquely variable source of recognition in the context of mechanically interlocked molecules, 7 8 through aromatic donor-acceptor interactions in its fully oxidized state (CPBQT4+) and radical- 9 10 pairing interactions in its partially reduced state (CBPQT2(•+)). Although it is expected that the fully 11 12 (0) 13 reduced neutral state (CBPQT ) might behave as a p-donating recognition unit, resulting in a 14 15 dramatic change in its binding properties when compared with the other two redox states, its role 16 17 in rotaxanes has not yet been investigated. To address this challenge, we report herein the synthesis 18 19 4+ 20 of a tristable [2]rotaxane in which a CBPQT ring is mechanically interlocked with a dumbbell 21 22 component containing five recognition sites¾(i) one, a bipyridinium radical cation (BIPY(•+)) 23 24 located centrally along the axis of the dumbbell, straddled by (ii) two tetrafluorophenylene units 25 26 4+ 27 linked to (iii) two triazole rings. In addition to the selective recognition between (iv) the CBPQT 28 29 ring and the triazole units, and (v) the CBPQT2(•+) ring and the reduced BIPY(•+) unit in the 30 31 dumbbell component, investigations in solution have now confirmed the presence of additional 32 33 (0) 34 noncovalent bonding interactions between the CBPQT ring, acting as a donor in its neutral state 35 36 towards the two tetrafluorophenylene acceptors in the dumbbell component. The unveiling of this 37 38 piece of molecular recognition in a [2]rotaxane is reminiscent of the existence in much simpler, 39 40 covalently linked, organic molecules of neighboring group participation (anchimeric assistance 41 42 43 giving way to transannular interactions) in small-, medium-, and large-membered rings. 44 45 46 47 Introduction 48 49 1-2 50 The neighboring group effect, alias anchimeric assistance, occupies a special place in the annuls 51 52 of physical organic chemistry and organic stereochemistry. The intramolecular nature of the effect 53 54 which, not only leads to the speeding up of reactions3-8 when they are potentially operative, but 55 56 57 58 59 2 60 ACS Paragon Plus Environment Page 3 of 27 Journal of the American Chemical Society 1 2 3 also often dictates9-11 regio- and stereospecifically the constitutions and configurations of the 4 5 6 products obtained under tight stereochemical control in what are geometrically constrained 7 8 environments. This intramolecular effect, which was explored in acyclic as well as cyclic 9 10 molecules, is one of the keystones of what became known as conformational analysis.12 Already 11 12 13 13 back in 2012, we argued in a review that the thermodynamic parameters associated with model 14 15 host-guest complexes provide a good starting point to rationalize the ratio of ground-state and 16 17 metastable co-conformations in bistable[2]rotaxanes at equilibrium. We demonstrated that these 18 19 [2]rotaxanes exhibit a strong correlation between their ground-state distribution constants and the 20 21 22 relative free energies of closely related model complexes. We pointed out in the review that the 23 24 semiquantitative treatment of the ground-state distribution of co-conformations (translational 25 26 isomers)14 is reminiscent of the type of conformational analysis12 that can be carried out on 27 28 29 multiply substituted cyclohexane rings once the conformational free energies¾also referred to as 30 31 A-values12¾between equatorial and axial substituents on monosubstituted cyclohexane rings are 32 33 12 34 known. Although caution has to be exercised in applying the principle of the additivity of A- 35 36 values, the approach is generally one that can be used in a predictive manner. Now, let us assume 37 38 that we introduce into a rotaxane, a recognition site between the ring and dumbbell components 39 40 that is known to be an extremely weak one, at least in the context of a host-guest complex.15 Will 41 42 43 we witness an intramolecular neighboring component effect in the rotaxane that is analogous to 44 45 the neighboring group effect in molecules that only contain covalent bonds? 46 47 Recently, we reported16 that, although a 1:1 inclusion complex between the fully reduced p- 48 49 (0) 50 electron-rich cyclobis(paraquet-p-phenylene) (CBPQT ) ring and the p-electron-deficient guest 51 52 1,4-dicyanobenzene (DCB) can be isolated as a solid-state superstructure, the binding constant for 53 54 55 this host-guest complex in (toluene) solution is too small to be measured by the usual techniques. 56 57 58 59 3 60 ACS Paragon Plus Environment Journal of the American Chemical Society Page 4 of 27 1 2 3 Now, what if this 1:1 host-guest complex was to be transplanted, so to speak, into a rotaxane, 4 5 (0) 6 would the DCB unit constitute a recognition site for the CBPQT ring? In this paper, we set out 7 8 to answer this question by designing a tristable [2]rotaxane, which (i) allows us to explore the 9 10 interactions of the CBPQT(0) ring with recognition sites that reflects the electronic properties of 11 12 13 DCB and, at the same time, (ii) makes it possible for us to understand more about the CBPQT ring 14 (0) 2(•+) 4+ 15 in its different redox states, i.e., CBPQT , CBPQT and CBPQT interacting with three 16 17 different recognition sites on the dumbbell component¾namely, p-electron-deficient, radical and 18 19 20 p-electron-rich sites, respectively¾which have been chosen for incorporation into the dumbbell 21 22 component of a tristable [2]rotaxane. A bipyridinium (BIPY2+) unit was introduced into the 23 24 dumbbell as the radical recognition site thanks to the well-established17-20 recognition between the 25 26 2(•+) •+ 27 CBPQT ring and the bipyridinium radical cation (BIPY ). By contrast, the choice of the p- 28 29 electron-rich recognition sites for CBPQT4+ is subtle, since the ideal candidate should be able to 30 31 interact with the CBPQT4+ ring, while not competing with the desire of the p-electron-rich 32 33 34 CBPQT(0) to interact with the p-electron-deficient recognition sites on the dumbbell component. 35 36 In order to meet these two requirements, we have introduced triazole units which act (i) as the p- 37 38 4+ 39 electron-rich recognition sites for CBPQT and, at the same time, (ii) as the means of templating 40 41 the synthesis of the tristable [2]rotaxane by the copper(I) catalyzed azide-alkyne cycloaddition21 42 43 (CuAAC).
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