Synthesis of molecular trefoil knots 31 by

Mathematical Types, Theory

Making use of the Alexander Briggs notation, the trefoil knot (31) is definitely the simplest nontrivial knot as well as the most responsive to chemical synthesis. Many different synthetic ways to prepare trefoil knots had been investigated. Especially, the passive metal template-directed methodology along with RCM has become successfully utilized to mechanically restrict the relative positions of molecular components. Sauvage et approach. reported a canonical sort of the covalent post set up modification simply by describing the preparation of the molecular trefoil knot 238 using olefin RCM reaction to capture the metal-assembled progenitor. The dinuclear Cu(I) intricate 237 was quantitatively made by treatment of the 1, 10-phenanthroline derived ligand 236 with [Cu(MeCN)4]PF6 intricate. Notably, the metal dexterity reorganized the terminal olefin into close proximity favoring, in the existence of G1ST catalyst, a double RCM and the creation of trefoil knot 238 (74% yield as a combination of E/Z isomers). Finally, catalytic hydrogenation with Pd/C (5 mol% Pd) afforded quantitatively the knot 239.

As an extension of the copper mineral template approach, Sauvage and co-workers applied the Fe(II) template-directed assembly of trefoil knots. The diiron(II) dual helix 240, obtained in good deliver by responding the terpyridine ligands with an aqueous Fe(II) sulfate solution was submitted to RCM cyclization (G2ST since catalyst) affording the trefoil knot 241 in twenty percent yield, being a mixture of E (55%) and Z (45%) isomers (scheme 58). In such a case, the low deliver can be the result of the negative conformation that ligands adopt around the metal during the engagement ring closure method.

The stereoselective prep of a topological chiral trefoil knot was reported by Von Zelewsky and Sauvage. The synthetic strategy took advantage of both Cu(I) template impact on RCM ligation and conversion from traditional to topological chirality. Stereoselective reaction of the diolefinic twine 243 with [Cu(MeCN)4]PF6 complicated afforded speedily and quantitatively the dinuclear Cu(I) sophisticated 244, which in turn possesses a C2-symmetric layout. Further RCM reaction making use of the G1ST catalyst afforded 245 in 74% overall yield as a mixture of three isomers. Notably, the enantiopure strings induced chirality at each material centre of the double-stranded helix, favouring as a result the activity of a sole enantiomer of trefoil knot. Quantitative catalytic hydrogenation gave the saturated compound 246, featuring precisely the same C2 symmetry, and final demetalation with KCN afforded the corresponding chiral trefoil knot in quantitative yield.

Hunter ou al. developed a Zn(II) template synthesis of a molecular trefoil knot by the folding-threading-RCM strategy. Specifically, treatment of the flexible ligand strands 248a, b, made up of three bipyridine units and two alkenyl chains, with Zn(ClO4)2 ends in the spontaneous and quantitative folding and threading in a stable open-knot conformation. The alkene-functionalized open-knot complexes 248a, b inside the presence of G1ST catalyst underwent trapping by olefin ring closure, leading to the formation of unsatured closed-knot processes 249a, b, respectively. Additional conversion in to the satured Zn(II) trefoil knots 250a, n was allowed by catalytic hydrogenation. The demetalation procedure, achieved by Li2S, occurred simply for the intermediate 243b.

Another approach, involving the Lanthanide template activity of achiral and chiral molecular trefoil knots, was described by the Leigh’s group. In the first case, the synthesis was based on the capacity of 2, 6-dicarbonylpyridyl motif to behave as a tridentate ligand pertaining to lanthanide ion in a well-defined 3: 1 ligand/metal-ion proportion (scheme 68). In particular, take care of ligand 252 with a option of Ln(CF3SO3)3 salt (Ln = Eu, Lu), accompanied by precipitation in dichloromethane, offered the corresponding processes 253a and 253b in 85% (Eu) and 90% (Lu) yields, respectively.

The lanthanide complexes 253a, b underwent to RCM reaction in the presence of HG2ND catalyst, affording the 81-atom-loop trefoil knots 254a, b in 58% yield along with the topologically trivial unknot macrocycle isomers (17%). Remarkably, the ПЂ-ПЂ stacking communications of lanthanide complex demonstrated able to reorganize the fatal alkene moieties of the ligands into close proximity, favoring the engagement ring closure towards knotted structures rather than the unknoted one. Final treatment with Et4NF in DMSO-d6 smoothly afforded the organic 81-atom-loop trefoil knot 255 in quantitative deliver. According to the past strategy, the same research group demonstrated the power of chiral tridentate ligand to copy chiral information from asymmetric stereocenters to topological stereochemistry in the activity of enantiopure molecular trefoil knots.

Treatment of (R, R)-256 (99% ee) with Ln(CF3SO3)3 (Ln = European or Lu) gave the corresponding lanthanide complexes (R, R)-257a, b in 83% and 89% brings, respectively. The circular trimeric helicate processes were after that closed by simply RCM to offer trefoil knots of sole handedness Оº-(R6)-258a, b in 55% and 62% yields, respectively, together with the metal-free unknotted macrocycles. Final treatment with fluoride ions afforded the organic topological structure Оº-(R6)-259 in 74% yield. Because an extension with this strategy, Leigh et al. reported the stereoselective synthesis of a left-handed trefoil knot starting from a single ligand strand (R6)-260 folding around the lanthanide ion. The enantiopure complexes Оº-(R6)-261a, n underwent macrocyclization by RCM, enabling the organization of sole handedness trefoil knots Оº-(R6)-262a, b in high deliver (>90%).

Recently, Leigh et al. (J. Am. Chem. Soc. 2018, 140, 4982-4985) described the two-step synthesis of a molecular trefoil knot trough the self-assembly of a rare 12-component trimeric round helicate, received by development of an imine bond between 263 and 264 in the presence of Zn(II). The six au cours de alkenyl organizations were covalently captured by RCM employing HG2ND catalyst into topological trefoil knot 265.

3. several. 2 Synthesis of double (51) and triple (819) helicates by passive material template RCM approach

Leigh et ing. efficiently applied the spherical metal helicate methodology to get the planning of molecular pentafoil knot, a 51 knot in Alexander Briggs notation. The authors demonstrated how, simply by changing the experimental conditions of the self-assembly process, the tris-(2, 2′-bipyridine)-strand 90 was able to interwine about five Fe(II) cations resulting in the five crossing points onto the low rotate Fe(II) sophisticated 266. Submitting 266 to RCM reaction using HG2ND catalyst, a related legend of David catenane 267 was acquired in 98% yield, offering chloride anion whithin the central cavity.

Further, the same analysis group prolonged the circular helicate strategy from twice to triple helicate (819) structures utilizing the Fe(II) ion as template. In particular, the tetrameric intricate 269 (60%) was formed by the reaction of way more versatile tris (bypyridine) ligand follicle 268 with FeCl2. Covalent capture among strands by olefin metathesis reaction in the presence of HG2ND catalyst, after quenching with ethyl vinyl ether and treatment with aqueous KPF6, provided the Fe(II) complex 270 in 62% yield. Further more demetalation produced the desired rounded triple helicate 271.

Conclusions

During the last twenty years, olefin metathesis reaction is firmly established being a powerful isodesmic process pertaining to the activity of macromolecules and by artificial means interlocked elements alike. Their popularity with researchers arises from the inbuilt high useful group specificity to the gentle reaction conditions in which the response can be carried out. Moreover, the olefin metathesis compatibility towards the weak interactions mostly involved in template assembly techniques makes it a magic post-assembly modification effect for the preparation of very difficult non-covalently connected molecules and polymeric devices. Covalent record by a number of metathesis changes thus gives a growing tools for the construction of MIMs and a new generation of molecular equipment with unpredicted technological applications in the real-world.

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