Template‐Directed Syntheses of Rigid Oligorotaxanes under Thermodynamic Control

Abstract

The application of mechanically interlocked molecules (MIMs) as molecular switches for nanoelectronics, [1] as molecular actuators for constructing artificial muscles, [2] and in controlling the release of molecules stored in mesoporous silica nanoparticles [3] remains an area of intense activity in research. Dynamic covalent chemistry [4] (DCC), a process whereby covalent bonds are formed reversibly under thermodynamic control, hence possessing an element of "proofreading" or "error-checking", has emerged as a valuable tool in the construction of MIMs, such as rotaxanes, [5] catenanes, [6] suitanes, [7] trefoil knots, [8] Borromean rings, [9] and Solomon knots.[10] The main attribute of DCC is that it operates under equilibrium such that the formation of undesired, kinetically competitive intermediates will eventually make their way back to the thermodynamically most stable product.Oligorotaxanes are commonly prepared employing a "threading-followed-by-stoppering" approach [11] ( Figure 1, Strategy A), wherein preformed rings locate around wellchosen recognition units in a thread that is stoppered subsequently. This approach suffers from the fact that it does not provide complete control over the number of threaded rings-which often do not encircle every recognition unit-and, moreover, stoppering conditions must be mild and chosen carefully. Alternatively, the "clipping" approach [12] ( Figure 1, Strategy B) takes advantage of noncovalent bonding interactions to control ring formation by templation, starting from acyclic precursors and using every single recognition site. With adequate system design, equilibrating reactions including olefin metathesis, [13] disulfide [5c, 14] and imine bond formation [5b, 15] have all been utilized extensively in the syntheses of MIMs. For example, it has been demonstrated [12f] that this "clipping" approach is effective in the near quantitative synthesis of oligorotaxanes in a one-pot, multicomponent self-assembly process from 1) a dumbbell component containing n -CH 2 NH 2 + CH 2 -recognition sites and 2) n equivalents each of 2,6-pyridinedicarboxaldehyde (5) and tetraethyleneglycol bis(2-aminophenyl)ether (6). The [2]rotaxane 1 R·PF 6 composed of bis(3,5-dimethoxybenzyl)-ammonium hexafluorophosphate and the [24]crown-8 derivative (red ring in Figure 2) is the quantitative outcome of (1) plus (2) in CD 3 CN within 5 min of mixing the starting materials. In this self-assembly process, involving -CH 2 NH 2 + CH 2 -activation/recognition sites, the clipping reaction is driven by a combination of DCC and noncovalent bonding interactions [15b] Whereas much research has been devoted to the syntheses and applications of such MIMs, very little research has been dedicated to controlling their overall conformations. From computational investigations, [16] the above-mentioned oligorotaxanes were predicted to assume flexible conformations. We suspected that [p···p] interactions between adjacent encircling rings would not only provide added stability to the oligorotaxanes, but...

An Introduction to the Mechanical Bond

“…We emphasize here that thermodynamically controlled reactions-especially olefin metathesis, metal coordination, and imine bonding-have been indispensible in orchestrating the self-assembly of many of the most exotic, beautiful, and challenging topologies and architectures including molecular necklaces, 184,186,190,247,316,317 handcuff catenanes, 259 331 and very large discrete rigid-rod oligorotaxanes. 138,300 3. Active Template Synthesis: A step beyond the template is the 'active template,' introduced in 2006 independently by the groups of Saito 332 and Leigh.…”

“…In Figure 1.16, we simply present a few examples that we find noteworthy. [134][135][136][137][138][139] In Section 1.3, see Figures 1.25, 1.27, and 1.29, as well as Scheme 1.3 to find more beautiful mechanically interlocked crystal structures.…”

Catalyzing Excellence: In a Competitive High-Stakes Research Arena, Can We Be Our Own Catalysts for Success?

“…The first draft of my NSF proposal described the dynamic covalent synthesis of oligorotaxanes, a class of one-dimensional (1D) MIMs, and it did so pretty well. I went into detail describing a general ''clipping'' protocol 2,3 for preparing oligorotaxanes in high (>90%) yields and high (>97%) conversion efficiencies by taking advantage of reversible imine chemistry. Aryl-substituted 2,6-diformylpyridines and bis(2-aminophenyl)ethers are essentially condensed into rings organized along oligoammonium ion templates under thermodynamic control.…”