Positive Cooperativity in the Template-Directed Synthesis of Monodisperse Macromolecules
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- 作者：Matthew E. Belowich ; Cory Valente ; Ronald A. Smaldone ; Douglas C. Friedman ; Johannes Thiel ; Leroy Cronin ; J. Fraser Stoddart
- 刊名：The Journal of the American Chemical Society
- 出版年：2012
- 出版时间：March 21, 2012
- 年：2012
- 卷：134
- 期：11
- 页码：5243-5261
- 全文大小：864K
- 年卷期：v.134,no.11(March 21, 2012)
- ISSN：1520-5126
文摘
Two series of oligorotaxanes R and R鈥?/b> that contain 鈭扖H2NH2+CH2鈥?recognition sites in their dumbbell components have been synthesized employing template-directed protocols. [24]Crown-8 rings self-assemble by a clipping strategy around each and every recognition site using equimolar amounts of 2,6-pyridinedicarboxaldehyde and tetraethyleneglycol bis(2-aminophenyl) ether to efficiently provide up to a [20]rotaxane. In the R series, the 鈭扤H2+鈥?recognition sites are separated by trismethylene bridges, whereas in the R鈥?/b> series the spacers are p-phenylene linkers. The underpinning idea here is that in the former series, the recognition sites are strategically positioned 3.5 脜 apart from one another so as to facilitate efficient [蟺路路路蟺] stacking between the aromatic residues in contiguous rings in the rotaxanes and consequently, a discrete rigid and rod-like conformation is realized; these noncovalent interactions are absent in the latter series rendering them conformationally flexible/nondiscrete. Although in the R鈥?/b> series, the [3]-, [4]-, [8]-, and [12]rotaxanes were isolated after reaction times of <5鈥?0 min in yields of 72鈥?5%, in the R series, the [3]-, [4]-, [5]-, [8]-, [12]-, [16]-, and [20]rotaxanes were isolated in <5 min to 14 h in 88鈥?8% yields. It follows that while in the R鈥?/b> series the higher order oligorotaxanes are formed in lower yields more rapidly, in the R series, the higher order oligorotaxanes are formed in higher yields more slowly. In the R series, the high percentage yields are sustained throughout, despite the fact that up to 39 components are participating in the template-directed self-assembly process. Simple arithmetic reveals that the conversion efficiency for each imine bond formation peaks at 99.9% in the R series and 99.3% in the R鈥?/b> series. This maintenance of reaction efficiency in the R series can be ascribed to positive cooperativity, that is, when one ring is formed it aids and abets the formation of subsequent rings presumably because of stabilizing extended [蟺路路路蟺] stacking interactions between the arene units. Experiments have been performed wherein the dumbbell is starved of the macrocyclic components, and up to five times more of the fully saturated rotaxane is formed than is predicted based on a purely statistical outcome, providing a clear indication that positive cooperativity is operative. Moreover, it would appear that as the R series is traversed from the [3]- to the [4]- to the [5]rotaxane, the cooperativity becomes increasingly positive. This kind of cooperative behavior is not observed for the analogous oligorotaxanes in the R鈥?/b> series. The conventional bevy of analytical techniques (e.g., HR-MS (ESI) and both 1H and 13C NMR spectroscopy) help establish the fact that all the oligorotaxanes are pure and monodisperse. Evidence of efficient [蟺路路路蟺] stacking between contiguous arene units in the rings in the R series is revealed by 1H NMR spectroscopy. Ion-mobility mass spectrometry performed on the R and R鈥?/b> series yielded the collisional cross sections (CCSs), confirming the rigidity of the R oligorotaxanes and the flexibility of the R鈥?/b> ones. The extended [蟺路路路蟺] stacking interactions are found to be present in the solid-state structures of the [3]- and [4]rotaxanes in the R series and also on the basis of molecular mechanics calculations performed on the entire series of oligomers. The collective data presented herein supports our original design in that the extended [蟺路路路蟺] stacking between contiguous arene units in the rings of the R series of oligorotaxanes facilitate an essentially rigid rod-like conformation with evidence that positive cooperativity improves the efficiency of their formation. This situation stands in sharp contrast to the conformationally flexible R鈥?/b> series where the oligorotaxanes form with no cooperativity.