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. 2023 Mar 10;14(15):3963-3972.
doi: 10.1039/d3sc00571b. eCollection 2023 Apr 12.

Stereoselective synthesis of [2.2]triphenylenophanes via intramolecular double [2 + 2 + 2] cycloadditions

Yuya Kawai  1 , Juntaro Nogami  1 , Yuki Nagashima  1 , Ken Tanaka  1
Affiliations

Stereoselective synthesis of [2.2]triphenylenophanes via intramolecular double [2 + 2 + 2] cycloadditions

Yuya Kawai et al. Chem Sci. .

Abstract

Planar chiral [2.2]cyclophanes with two aromatic rings in close proximity have attracted much attention for their applications as chiral materials and catalysts because of their stable chirality and transannular interactions. Although numerous [2.2]cyclophanes have been synthesized to date, only a few polycyclic aromatic hydrocarbon (PAH)-based ones have been reported, and the simultaneous control of two planar chiralities of the two aromatic rings facing each other has not been achieved. Here we report the enantio- and/or diastereoselective synthesis of planar chiral PAH-based [2.2]cyclophanes ([2.2]triphenylenophanes) via the high-yielding base-mediated intermolecular macrocyclization and Rh- or Ni-catalyzed intramolecular double [2 + 2 + 2] cycloadditions. DFT calculations have revealed that the second [2 + 2 + 2] cycloaddition kinetically determines the diastereoselectivity. Single crystal X-ray diffraction analyses have confirmed that the facing triphenylene or [5]helicene skeletons strongly repel each other, resulting in curved structures with bulged centers.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Fig. 1
Fig. 1. Research backgrounds. (A) Synthesis of racemic planar chiral π-extended [2.2]cyclophanes. (B) Enantioselective synthesis of [n.n]cyclophanes with one planar chirality. (C) Optical resolution of racemic [2.2]cyclophane with two planar chiralities (Morisaki, 2014). (D) Enantio- and diastereoselective synthesis of π-extended [2.2]cyclophanes (this work).
Fig. 2
Fig. 2. Synthesis of hexaynes 2. MOM = methoxylmethyl. Ts = p-toluenesulfonyl. Ns = 2-nitrobenzenesulfonyl. TBAF = tetrabutylammonium fluoride.
Fig. 3
Fig. 3. Analyses for steps determining enantio- and diastereoselectivities to produce (Rp,Rp)-3b from 2b. The changes of the Gibbs free energies (kcal mol−1) are calculated at the M06/6-31G(d) (C, H, O, P) and LANL2DZ (Rh) level. (A) Reaction pathways of Rh-catalyzed [2 + 2 + 2] cycloaddition of 2b leading to 3b. (B) Rotation isomerization of axial chirality. (C) The second [2 + 2 + 2] cycloaddition to produce (Rp,Rp)-3b.
Fig. 4
Fig. 4. X-ray crystal structures (a and b) and strain analyses (c and d). Hydrogen atoms are omitted for clarity. Atoms and the Ts group are colored green (carbon), red (oxygen), purple (nitrogen), and white (Ts group). (A) Unimolecular structures of [2.2]triphenylenophane (±)-3a. (B) Unimolecular structures of [2.2][5]helicenophane (±)-3e. (C) Strain energies of [2.2]triphenylenophane 3b and [2.2][5]helicenophane 3e. (D) StrainViz of 3b and [2.2]paracyclophane.
Fig. 5
Fig. 5. Absorption (solid line) and emission (dashed line) spectra. (A) 3a–3c. (B) 3e
Fig. 6
Fig. 6. Electronic structures calculated by TD-DFT at the B3LYP/6-31G(d) level. (A) Pictorial representations of six frontier MOs of 3b and 3e. (B) Energy diagrams of 3b and 3e.
Fig. 7
Fig. 7. ECD spectra. Dashed blue lines show ECD spectra calculated by TD-DFT at the B3LYP/6-31G(d) level. (A) (–)-3b, (+)-3b, and calculated (Rp,Rp)-3b. (B) (–)-3e, (+)-3e, and calculated (P,P,Rp,Rp)-3e.
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