Dynamic donor–acceptor [2]catenanes

Abstract

Donor-acceptor [2]catenanes based on cyclobis(paraquat-pphenylene) as the -acceptor ring have been used prominently in the construction of functional molecular devices. We report here their thermodynamically controlled synthesis from isolateddonor and -acceptor rings under the catalytic influence of tetrabutylammonium iodide. The initial nucleophilic attack of iodide ion, which opens up the -acceptor ring, is followed by complexation to the -donor ring and the subsequent catenation of the -donor ring by the -acceptor ring [2]catenane. The reaction is general in scope and proceeds in high yields, without giving rise to side-products.mechanically interlocked molecules ͉ nucleophilic catalysis ͉ self-assembly T opologically nontrivial molecules dubbed catenanes (1) contain two or more mutually interlocked, inseparable rings, arranged mechanically like the links in a chain.[2]Catenanes with several complementary recognition sites expressed as matching pairs between their two rings can be rendered bistable (2) or multistable (3) and hence endowed with switching properties (4), paving the way for molecular device applications, straddling diversity from unidirectional motors (5, 6) through reconfigurable switches (7,8) to components of electronic displays (9, 10).Although the stabilizing noncovalent interactions associated with the matching recognition sites are used routinely in a templating fashion to preorganize precursors in the kinetically controlled preparation of [2]catenanes, they have been used very rarely (11,12) as the thermodynamic driving force in their template-directed synthesis under equilibrium control. The vast majority of preparative strategies to access catenanes, and organic compounds in general, still relies on irreversible, kinetically controlled reactions. The distribution of competing products in kinetic reactions is controlled by the relative stabilities of the corresponding transition states. Kinetically controlled reactions remain central in synthesis on account of their relatively fast reaction times and high selectivities for the most part. Recently, however, dynamic covalent chemistry (DCC) (13,14) has begun to emerge as a powerful synthetic complement, if not an alternative, to the kinetic approach. Reactions involving DCC operate under thermodynamic control where covalent bonds are reversibly formed and broken until equilibrium is reached. At equilibrium, the distribution of products is dictated only by their relative thermodynamic stabilities. More stable products, in either a molecular or a supramolecular sense, profit from reversibility in this scenario, because a cascade of error-checking and proof-reading processes ultimately provides the energetically most favored compounds in superior yields. The equilibration products can be ''fixed'' to give isolable compounds, typically by an irreversible chemical transformation, e.g., reduction in dynamic imine chemistry, acidification in dynamic disulfide chemistry, etc.Molecular Borromean rings (15) and Solomon knots (16) are among the emin...

Synthesizing interlocked molecules dynamically

“…First and foremost, the discovery of new dynamic bonds and more efficient catalysts is paramount. The above examples67,68 of reversible nucleophilic substitution clearly illustrate how a new synthetic pathway can lead to novel and efficient syntheses of interlocked architectures. It is entirely reasonable to suggest that all of the discussed applications that involve one type of dynamic bond are translatable to the others.…”

“…The bimolecular nucleophilic substitution (S N 2) reaction, normally considered to be kinetically controlled, can be used in a dynamic sense if the attacking nucleophile is also of significant nucleofugality. It has been shown that this prospect can be used to create catenanes 67. The benzylic carbons on the cyclobis(paraquat‐ p ‐phenylene) ( CBPQT 4+ ) ring are susceptible to nucleophilic iodide attack (Scheme ) because the positively charged bipyridinium moieties are good leaving groups and the resulting linear molecule 51 •3PF 6 has considerably less ring strain than the parent CBPQT •4PF 6 .…”

An Introduction to the Mechanical Bond

“…The previously-developed 312 iodide-catalyzed self-assembly of donor-acceptor [2]catenanes has been applied to a polymerizable monomer containing a derivative of cyclobis(paraquat-p-phenylene) (CBPQT 4+ ) in 84% yield. When dimerized, the catenation proceeds in an unsurprising 49% yield.…”

“…See Chapter 3 for a review of MIM synthesis under thermodynamic control. Seminal examples 129,202,[309][310][311][312][313][314] of elegant thermodynamically controlled protocols for making mechanical bonds are illustrated in Figure 1.32. They include (in order of appearance) (i) slippage 309 (Section 3.1), which involves the passage of a ring over a size-complementary stopper that is sufficiently bulky to prevent deslipping, (ii) olefin metathesis 310 (Section 3.4), which involves a catalysttypically Grubbs' Ru alkylidene catalysts-scrambling the constituents of double bonds, (iii) disulfide exchange (Section 3.5.1), which involves the oxidative coupling of thiols and reductive cleavage of disulfides, (iv) kinetically labile metal-ligand bonds (Section 3.2)-a revolution led by Makoto Fujita,202 (v) reversible nucleophilic substitutions (Section 3.5.2), such competing S N 2 processes 312 in dynamic equilibrium, (vi) imine bonds 129,315 (Section 3.3.1) whose reversible nature facilitates the self-assembly of MIMs, (vii) boronic esters 313 (Section 3.3.3) formed reversibly from boronic acids and diols, and (viii) self-assembled monolayers 314 (SAMs) (Section 3.6).…”

“…202 A weakly nucleophilic leaving group (I -) competes with a nucleophilic macrocylization/catenation event in a reversible S N 2 reaction. 312 Reversible imine bonds facilitate the selection of MIMs from dynamic combinatorial libraries. 129 MIMs can be self-assembled from boronic acids and diols via the reversible formation of boronic esters.…”