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. 2014 Apr 2;136(13):4817-20.
doi: 10.1021/ja412715m. Epub 2014 Mar 18.

An efficient approach to mechanically planar chiral rotaxanes

Robert J Bordoli  1 , Stephen M Goldup
Affiliations

An efficient approach to mechanically planar chiral rotaxanes

Robert J Bordoli et al. J Am Chem Soc. .

Abstract

We describe the first method for production of mechanically planar chiral rotaxanes in excellent enantiopurity without the use of chiral separation techniques and, for the first time, unambiguously assign the absolute stereochemistry of the products. This proof-of-concept study, which employs a chiral pool sugar as the source of asymmetry and a high-yielding active template reaction for mechanical bond formation, finally opens the door to detailed investigation of these challenging targets.

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Figures

Figure 1
Figure 1
Schematic of our proposed approach to mechanically planar chiral rotaxanes. Stereochemical labels are arbitrary.
Scheme 1
Scheme 1. AT-CuAAC Synthesis of Mechanically Diastereoisomeric Rotaxanes 4
1 equiv 2, 1.5 equiv each 1 and 3, 0.96 equiv [Cu(MeCN)4]·PF6, and 10 equiv NEtiPr2. Isolated yield after flash chromatography.
Figure 2
Figure 2
X-ray structures of (a) (d,Rmp)-4 and (b) (d,Smp)-4. Selected distances (Å): (i) 2.9, (ii) 2.5, (iii) 2.5, (iv) 2.3, and (v) 2.6.
Figure 3
Figure 3
Partial 1H NMR (600 MHz, CDCl3, 300 K), with selected signals assigned, of (i) macrocycle 2, (ii) (d,Rmp)-4, (iii) (d,Smp)-4, and (iv) their corresponding non-interlocked thread. See the Supporting Information for full assignment. Labeling is as shown in Scheme 1. Residual solvent signals are shown in gray.
Scheme 2
Scheme 2. Substitution of the Chiral Glucose Unit To Give Mechanically Planar Chiral Rotaxanes 6
3.8 equiv 5, 2.5 equiv AlMe3, PhMe, 80 °C, 16 h. Isolated yield after column chromatography.
Figure 4
Figure 4
(a) CSP-HPLC and (b) CD analysis of 6 and [6·Cu]·PF6.
Figure 5
Figure 5
Partial 1H NMR (600 MHz, CDCl3, 300 K), with selected signals assigned, of (i) (Rmp)-6, (ii) their corresponding non-interlocked thread, and (iii) (Smp)-6. See the Supporting Information for the complete assignment. Labeling is as shown in Scheme 2. Residual solvent signals are shown in gray.
See this image and copyright information in PMC

Comment in

References

    1. Eliel E.; Wilen S.; Mander L.. Stereochemistry of Organic Compounds; John Wiley and Sons: New York, 1994.
    1. Schill G.Catenanes, Rotaxanes and Knots; Academic Press: New York, 1971.

    1. Catenanes and knots also display molecular chirality in the absence of covalent chirality although in their case molecular asymmetry can be the result of molecular topology, whereas rotaxanes are topologically trivial:

    2. Wasserman E.; Frisch H. L. J. Am. Chem. Soc. 1961, 83, 3789.
    3. Walba D. M. Tetrahedron 1985, 41, 3261.

    1. For recent reviews on the synthesis of mechanically interlocked molecules, including aspects of their mechanical stereochemistry, see:

    2. Beves J. E.; Blight B. A.; Campbell C. J.; Leigh D. A.; McBurney R. T. Angew. Chem., Int Ed. 2011, 50, 9260. - PubMed
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    4. Neal E. A.; Goldup S. M. Chem. Commun. 2014, 10.1039/C3CC47842D. - DOI - PubMed

    1. Although Schill proposed the term “cyclochiral” to describe mechanical asymmetry of the form exhibited by rotaxanes V, we feel this has the potential to lead to confusion, as the term properly belongs to the study of rotationally unsymmetrical cyclic molecules displaying multiple instances of asymmetry around the ring. Here we adopt the planar chiral interpretation, as proposed by Takata et al.

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