Design and synthesis of the first triply twisted Möbius annulene

Journal name:
Nature Chemistry
Volume:
6,
Pages:
608–613
Year published:
DOI:
doi:10.1038/nchem.1955
Received
Accepted
Published online

Abstract

As long as 50 years ago theoretical calculations predicted that Möbius annulenes with only one π surface and one edge would exhibit peculiar electronic properties and violate the Hückel rules. Numerous synthetic attempts notwithstanding, the first singly twisted Möbius annulene was not prepared until 2003. Here we present a general, rational strategy to synthesize triply or even more highly twisted cyclic π systems. We apply this strategy to the preparation of a triply twisted [24]dehydroannulene, the structure of which was confirmed by X-ray analysis. Our strategy is based on the topological transformation of ‘twist’ into ‘writhe’. The advantage is twofold: the product exhibits a lower degree of strain and precursors can be designed that inherently include the writhe, which, after cyclization, ends up in the Möbius product. With our strategy, triply twisted systems are easier to prepare than their singly twisted counterparts.

At a glance

Figures

  1. Strategies to introduce multiple twists in a cycle.
    Figure 1: Strategies to introduce multiple twists in a cycle.

    a, Naive strategies may include the cyclotrimerization of three components each containing a single twist, or cyclization of a band with three twists. Both strategies are hampered because linear π systems cannot be twisted easily, and if constrained to do so would probably not cyclize easily. b, The transformation of twist (Tw) into writhe (Wr) in a closed band by winding around itself can release strain and suggests an alternative strategy. Tw is a measure of how often the upper edge of the band crosses the lower edge in 2D projections (local crossings, top), and Wr is a measure of the number of crossings of the ribbon axis with itself. The middle illustration shows three local self-crossings in the 2D projection of the view along the z axis (orthogonal to the paper plane). To calculate the writhe either the average of self-crossings in all projections from all vantage points has to be calculated or the double Gaussian integral has to be solved. Local crossings and self-crossings can be either positive or negative. To assign a sign to a crossing the band has to be given a direction and the crossings are analysed as shown in the bottom illustration (right handed, +; left handed, −). Tw and Wr can take any positive or negative real number, Lk is always an integer.

  2. Combining building blocks with writhe to produce a triply twisted system and identification of a target structure.
    Figure 2: Combining building blocks with writhe to produce a triply twisted system and identification of a target structure.

    ad, Different combinations of writhe units (ac) and formation of a Möbius ring with Lk = −3 (d). Structures with Lk = 3, Tw = 0 and Wr = 3 are difficult to draw. e,f, To derive clearly recognizable chemical structures that can be drawn in 2D, we use the homeomorphic Lk = 3, Tw = −3 and Wr = 6 projection (e). The bridging units that connect the loops are not twisted in the 3D structure (d). The twists arise from the topological transformation from d to e and projection into 2D. Target structure 1 is presented in this projection (f). However, for the discussion of the topology, and particularly of twist and writhe, or properties that rely on the 3D geometry (such as orbital overlap), please refer to the 3D structure as represented in d.

  3. Synthesis of the Möbius dehydroannulene.
    Figure 3: Synthesis of the Möbius dehydroannulene.

    a, Reagents: (i) (1) n-BuLi, THF, (2) DMF, yield of 3 = 63%; (ii) Bestmann–Ohira reagent, K2CO3, MeOH, yields of 5 = 74% and 4 = 23%; (iii) NBS, AgNO3, acetone, yield of 7 = 96%; (iv) (1) LiN(TMS)2, THF, (2) DMTS-Cl, yields of 8 = 65%, 5 = 25% and 6 = 10%; (v) TBAF, THF, yield of 8 = 98%; (vi) Pd2(dba)3, LiI, CuI, PMP, DMSO, yield of 9 = 32%; (vii) TBAF, THF, yield of 10 = 100%; (viii) Cu(OAc)2, pyridine, MeOH, yield of 1 = 11%. All references for the synthesis are given in the Supplementary Chapter 3. b, Owing to the axial chirality of the writhe units and the three possible ways to combine them, three diastereomers (enantiomeric pairs) are formed on the reaction of 7 with 8. In principle, the final ring closure of 10af can furnish two cyclic trimers with Möbius topology, besides oligomers and polymers. The Möbius structure 1c/d with Lk = 1 is not formed because ring closure is energetically unfavourable. The red stereo descriptors indicate those stereoisomers that have the correct stereochemistry to form the target (triply twisted) Möbius product (1a/b). The compounds that are not highlighted with red stereo descriptors form either oligomers or a Möbius product with Lk = 1. TMS, trimethylsilyl; dba, dibenzylideneacetone; DMTS, dimethyl-(1,1,2-trimethylpropyl)silyl; TBAF, tetra-n-butylammonium fluoride; NBS, N-bromosuccinimide; PMP, 1,2,2,6,6-pentamethylpiperidine; DMSO, dimethyl sulfoxide.

  4. Structure of the Möbius dehydroannulene 1a/b with Lk = 3 as determined by X-ray crystallography.
    Figure 4: Structure of the Möbius dehydroannulene 1a/b with Lk = 3 as determined by X-ray crystallography.

    a,b, Ball and stick model (a) and space-filling model (b). α, dihedral angle formed by two adjacent naphthyl units. Chloroform molecules are omitted for clarity. c, CD spectra of both enantiomers (1a/b, red and blue lines, respectively) of the Möbius dehydroannulene after separation on a chiral column.

  5. Topological procedure to determine the linking number Lk of the π system of 1.
    Figure 5: Topological procedure to determine the linking number Lk of the π system of 1.

    A band orthogonal to the π system is constructed and cut down the middle. The fact that a trefoil knot is obtained confirms the linking number, Lk = 3, of the original band (see also Supplementary Fig. 5).

Compounds

10 compounds View all compounds
  1. 1,6,11-(2,2')-Tri-1,1'binaphthyla-cycopentadecaphane-2,4,7,9,12,14-hexayne
    Compound 1 1,6,11-(2,2')-Tri-1,1'binaphthyla-cycopentadecaphane-2,4,7,9,12,14-hexayne
  2. 2,2'-Dibromo-1,1'-binaphthalene
    Compound 2 2,2'-Dibromo-1,1'-binaphthalene
  3. (1,1'-Binaphthalene)-2,2'-dicarbaldehyde
    Compound 3 (1,1'-Binaphthalene)-2,2'-dicarbaldehyde
  4. 2'-Ethynyl-(1,1'-binaphthalene)-2-carbaldehyde
    Compound 4 2'-Ethynyl-(1,1'-binaphthalene)-2-carbaldehyde
  5. 2,2'-Diethynyl-1,1'-binaphthalene
    Compound 5 2,2'-Diethynyl-1,1'-binaphthalene
  6. 2,2'-Bis(((2,3-dimethylbutan-2-yl)dimethylsilyl)ethynyl)-1,1'-binaphthalene
    Compound 6 2,2'-Bis(((2,3-dimethylbutan-2-yl)dimethylsilyl)ethynyl)-1,1'-binaphthalene
  7. 2,2'-Bis(bromoethynyl)-1,1'-binaphthalene
    Compound 7 2,2'-Bis(bromoethynyl)-1,1'-binaphthalene
  8. ((2'-(Bromoethynyl)-(1,1'-binaphthalen)-2-yl)ethynyl)(2,3-dimethylbutan-2-yl)dimethylsilane
    Compound 8 ((2'-(Bromoethynyl)-(1,1'-binaphthalen)-2-yl)ethynyl)(2,3-dimethylbutan-2-yl)dimethylsilane
  9. 2,2'-Bis((2'-(((2,3-dimethylbutan-2-yl)dimethylsilyl)ethynyl)-(1,1'-binaphthalen)-2-yl)buta-1,3-diyn-1-yl)-1,1'-binaphthalene
    Compound 9 2,2'-Bis((2'-(((2,3-dimethylbutan-2-yl)dimethylsilyl)ethynyl)-(1,1'-binaphthalen)-2-yl)buta-1,3-diyn-1-yl)-1,1'-binaphthalene
  10. 2,2'-Bis((2'-ethynyl-(1,1'-binaphthalen)-2-yl)buta-1,3-diyn-1-yl)-1,1'-binaphthalene
    Compound 10 2,2'-Bis((2'-ethynyl-(1,1'-binaphthalen)-2-yl)buta-1,3-diyn-1-yl)-1,1'-binaphthalene

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Author information

Affiliations

  1. Institute for Organic Chemistry, University of Kiel, Otto-Hahn Platz 4, 24098 Kiel, Germany

    • Gaston R. Schaller &
    • Rainer Herges
  2. Department of Chemistry, Nanoscience Center, University of Jyväskylä, PO Box 35, 40014 Jyväskylä, Finland

    • Filip Topić &
    • Kari Rissanen
  3. Nagoya University, 1E Miyahigashi-cho, Showa-ku, Nagoya 466-0804, Japan

    • Yoshio Okamoto
  4. College of Materials Science and Chemical Engineering, Harbin Engineering University, 145 Nantong Street, Harbin 150001, China

    • Jun Shen

Contributions

G.R.S. worked out the topological construction strategy, developed the syntheses, carried out the experiments and characterized the compounds. R.H. developed the topological strategy and directed the study. F.T. prepared the single crystals, collected the data and solved and refined the structure together with K.R. K.R. supervised the X-ray diffraction part of the work. Y.O. and J.S. performed the separation and CD measurement of the enantiomers. G.R.S., R.H., F.T. and K.R. wrote the manuscript.

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    Crystallographic data for racemic compound 1

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