Stepwise, Protecting Group Free Synthesis of [4]Rotaxanes

Abstract

Despite significant advances in the last three decades towards high yielding syntheses of rotaxanes, the preparation of systems constructed from more than two components remains a challenge. Herein we build upon our previous report of an active template copper-catalyzed azide-alkyne cycloaddition (CuAAC) rotaxane synthesis with a diyne in which, following the formation of the first mechanical bond, the steric bulk of the macrocycle tempers the reactivity of the second alkyne unit. We have now extended this approach to the use of 1,3,5-triethynylbenzene in order to successively prepare [2]-, [3]- and [4]rotaxanes without the need for protecting group chemistry. Whilst the first two iterations proceeded in good yield, the steric shielding that affords this selectivity also significantly reduces the efficacy of the active template (AT)-CuAAC reaction of the third alkyne towards the preparation of [4]rotaxanes, resulting in severely diminished yields.

Porphyrinoid rotaxanes: building a mechanical picket fence

“…1 ) of [2]rotaxane 4 ⊂ 3 with non-interlocked thread 4 and macrocycle 3 further confirmed the formation of the mechanical bond; although many of the resonances associated with the axle remain unaffected by mechanical bond formation (H n , H o , H p , H q , and protons associated with Ar 1 and Ar 2 ), which is in keeping with their location away from the threaded region of the axle, triazole proton H k is shifted considerably to lower field (Δ δ ∼ 1.8 ppm). This is consistent with previous observations of C–H·N hydrogen bonding between the polarised triazole-H k and the Lewis basic pyridine nitrogen donors 19 and suggests that the macrocycle is largely localised over the triazole unit. Conversely, benzylic protons H j appear at higher field in the interlocked structure (Δ δ ∼ 1.2 ppm) due to the close proximity of the induced magnetic field of the electron rich aromatic units of the macrocycle.…”

“… 18 Furthermore, by reducing the size of the macrocycle employed, we have demonstrated that Leigh's active template modification of the CuAAC reaction (AT-CuAAC), 15 in which a copper centre bound in the cavity of a bipyridine macrocycle mediates the formation of the triazole, is a general approach to functionalised and functional rotaxanes in excellent yield. 19 Thus, although it has yet to be applied in this context, 20 the AT-CuAAC reaction appears particularly appropriate for the synthesis of triazole functionalised porphyrin rotaxanes without altering their otherwise desirable properties.…”

Using the Mechanical Bond to Tune the Performance of a Thermally Activated Delayed Fluorescence Emitter**

“…Them ost striking structural difference between 1&2 and 1&2 2 is that the Tz units of the latter adopt a syn-syn orientation whereas those of 1&2 preferentially adopt a syn-anti arrangement, the syn oriented unit being that encircled by the macrocycle. [33] This difference can be explained by observing that the syn conformation of Tz units minimises steric repulsion between an encircling macrocycle and the Cz unit. A syn-anti preference was also observed in the case of axle 1 as this minimises repulsion between the dipoles associated with the Tz rings.…”

Efficient Multicomponent Active Template Synthesis of Catenanes

“…We recently optimized our small macrocycle 13 modification of Leigh’s AT Cu-mediated azide–alkyne cycloaddition (AT-CuAAC) 14 reaction for the rapid and efficient iterative synthesis of oligorotaxanes. 15 This suggested an opportunity as, were these advantages to be maintained in the formation of catenanes, these conditions might allow efficient pseudo-high dilution reactions. Here we report that this approach is extremely successful, producing catenanes in good to excellent yield with short reaction times.…”

“…The yield of the reactions presented is particularly striking as selectivity for both mechanical bond formation and macrocyclization over uncontrolled oligomerization is required for efficient catenane synthesis. Our original hypothesis was that this could be achieved by optimizing the bipyridine-mediated AT-CuAAC reaction, which is highly selective for interlocked products 10b , 11 , 13 , 15 as the alkyne and azide reactive functional groups are projected on opposite faces of the ring in the key bond forming step, 29e to proceed rapidly under pseudo-high dilution conditions in order to maintain a low instantaneous concentration of the substrates. However, it remained unclear whether secondary interactions between the bipyridine macrocycle and the nascent triazole macrocycle also played a significant role in preorganizing the reaction intermediates to lead to a single major catenane product, although the broad substrate scope observed suggested this was not the case.…”