
==== Front
Org Lett
Org Lett
ol
orlef7
Organic Letters
1523-7060
1523-7052
American Chemical Society

39255331
10.1021/acs.orglett.4c02712
Letter
Enantioselective Synthesis, Crystal Structures, and Stereoisomerism of Substituted o,m,o,p-Tetraphenylenes
Kawai Yuya
Oriki Tomohiro
Sato Yu
Nogami Juntaro
Kamiya Yoshinobu
Suzuki Shunsuke
https://orcid.org/0000-0003-0534-7559
Tanaka Ken *
Department of Chemical Science and Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan
* ktanaka@apc.titech.ac.jp
10 09 2024
20 09 2024
26 37 78697874
24 07 2024
06 09 2024
03 09 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by/4.0/ Permits the broadest form of re-use including for commercial purposes, provided that author attribution and integrity are maintained (https://creativecommons.org/licenses/by/4.0/).

We have achieved the enantioselective synthesis of highly strained, substituted o,m,o,p-tetraphenylenes (≤98% ee) via the cationic Rh(I)/(R)-H8-BINAP complex-catalyzed chemo-, regio-, and enantioselective intermolecular cross-[2+2+2] cycloaddition of teraryl diynes with dimethyl acetylenedicarboxylate. X-ray crystallographic analyses demonstrate the highly bent structures of the para-substituted benzene moieties, and density functional theory calculations reveal the large local strain of the paraphenylene unit. 1H nuclear magnetic resonance analyses and theoretical calculations elucidate the stereoisomerism, indicating that the nonrotatable ortho-disubstituted biphenyl structure results in cis and trans isomers.

Japan Society for the Promotion of Science 10.13039/501100001691 JP19H00893 Japan Society for the Promotion of Science 10.13039/501100001691 JP24H00005 Japan Society for the Promotion of Science 10.13039/501100001691 JP21K18949 Japan Society for the Promotion of Science 10.13039/501100001691 JP21J22287 document-id-old-9ol4c02712
document-id-new-14ol4c02712
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==== Body
pmco,o,o,o-Tetraphenylene1 is a cyclic aromatic hydrocarbon with a stable saddle-shaped structure, in which all four benzene rings are linked at the ortho position. As shown in the top of Figure 1a, introducing a substituent R at the ortho position of the benzene ring results in axially chiral tetraphenylene A due to the nonrotatable ortho-trisubstituted biphenyl structure.1c Molecules B (o,o,o,m-tetraphenylene) and C (o,o,o,p-tetraphenylene2), in which one of the benzene rings is linked to the meta or para positions, have never been synthesized, probably because of high strain and instability (Figure 1a, top). In contrast, the meta linkage of the two benzene rings stabilizes the molecule (o,m,o,m-tetraphenylene3) without distortion. In this molecule, the two meta-linked benzene rings cannot rotate around each other due to steric hindrance, and as shown in the bottom of Figure 1a, molecule D with substituent R introduced at the ortho position of the benzene ring should become axially and planarly chiral due to two rotational restrictions, resulting in enantiomers and diastereomers (cis/trans-isomers). Rajca et al. reported the enantioselective synthesis of o,o,o,o-tetraphenylenes by (−)-Sparteine-induced lithiation of 2,2′-dibromobiphenyl followed by the CuBr2-mediated enantioselective coupling.4 Subsequently, Shibata et al. reported the catalytic enantioselective synthesis of o,o,o,o-tetraphenylenes5 and o,m,o,m-tetraphenylenes6 by the Rh(I)-catalyzed intermolecular homo-[2+2+2] cycloaddition reactions7 of triynes.

Figure 1 Synthesis and stereoisomerism of tetraphenylenes.

On the other hand, linking the two benzene rings at the meta and para positions significantly increases the distortion. In 1988, Wong et al. first synthesized o,m,o,p-tetraphenylene,8 followed in 1993, they reported the synthesis of o,m,o,p-tetraphenylene E, in which two methyl groups were introduced at the ortho positions of the benzene ring (Figure 1a, bottom).9 In molecule E, the meta- and para-linked benzene rings cannot rotate due to steric repulsion and ring strain, resulting in axial chirality, which was expected to give rise to enantiomers. However, isolation of the enantiomers was unsuccessful.9

Here, we have succeeded in the synthesis of highly strained, enantioenriched o,m,o,p-tetraphenylenes 3 (up to 98% ee) via the Rh(I)-catalyzed chemo-, regio-, and enantioselective intermolecular cross-[2+2+2] cycloaddition of teraryl diynes with dimethyl acetylenedicarboxylate (2) (Figure 1b).10 Their structures were determined by X-ray crystallographic analyses, and their stereoisomerism was confirmed by 1H NMR analysis and theoretical calculations.

In 2003, we reported that a cationic Rh(I)/H8-BINAP complex catalyzes the chemo- and regioselective [2+2+2] cycloaddition of two molecules of terminal alkynes involving phenylacetylene with one molecule of dialkyl acetylenedicarboxylate at room temperature to produce the corresponding 3,6-disubstituted phthalate in high yield.11 Thus, we investigated the enantioselective synthesis of o,m,o,p-tetraphenylenes 3 using dimethyl acetylenedicarboxylate (2) and the cationic Rh(I)/(R)-H8-BINAP catalyst (10–40 mol % Rh), as shown in Figure 2, applying various substituted terphenyl diynes 1 instead of two phenylacetylenes to this reaction. The desired o,m,o,p-tetraphenylene 3 is highly strained, and thus we also assumed the formation of low-strain regioisomer 4. In addition, on the basis of the 1H NMR analysis of molecule E by Wong et al., cis-3 and trans-3 should exist if the rotation of the meta-linked benzene ring is inhibited by substituents.9

Figure 2 Rh-catalyzed [2+2+2] cycloaddition of terphenyl diynes 1 with dimethyl acetylenedicarboxylate (2). Yields are of the isolated products. [Rh(cod)2]BF4 (10–40 mol %), (R)-H8-BINAP (10–40 mol %), 1 (0.062–0.10 mmol), 2 (1.1 equiv), and CH2Cl2 (0.025 M for 1a and 1b and 0.0125 M for 1c–f) were used. aIsolated yield after repeated silica gel PTLC.

As expected, symmetrically substituted terphenyl diynes 1a and 1b reacted with 2 to give the desired o,m,o,p-tetraphenylenes 3a and 3b along with the regioisomeric o,m,o,m-tetraphenylenes 4a and 4b in 41% and 28% yields with 84:16 and 78:22 ratios of 3/4, respectively. Although cis-3b and trans-3b can exist due to rotational inhibition by the methyl groups, only cis-3b was produced in this reaction. From a mixture of cis-3b and 4b, pure cis-3b could be isolated after recrystallization. Unsymmetric 1-ethynylnaphthalene-derived diynes 1c and 1d reacted with 2 to yield chiral tetraphenylenes 3c and 3d with high regio- and enantioselectivity (97–98% ee). Unsymmetric 2-ethynylnaphthalene-derived diyne 1e reacted with 2 to give a mixture of cis-3e, trans-3e, and 4e in 48% yield due to rotational inhibition by the naphthyl group in 3e. In contrast to 3c, the ee value of cis-3e markedly dropped to 40%. Interestingly, the reaction of a hybrid diyne 1f bearing both 1-ethynyl- and 2-ethynylnaphthalene moieties with 2 also afforded a mixture of cis-3f, trans-3f, and 4f in 35% yield, but the ee of cis-3f improved to 93%. From this mixture, cis/trans-3f could be isolated by repeated silica gel preparative thin-layer chromatography (PTLC). 1H NMR of the methyl group of the ester moiety demonstrated the interconversion of cis-3 and trans-3. Broad peaks of the methyl groups were observed in 3a, 3c, and 3d; thus, the cis- and trans-forms interconvert at room temperature. In contrast, in 3b, 3e, and 3f, the peaks of the methyl groups corresponding to the isomers were observed sharply, and thus the cis-form and trans-form do not interconvert at room temperature. The thermal stability of the stereochemistry was evaluated for 3e. Heating a (CH2Cl)2 solution of 3e (cis/trans = 4:1, 40% ee) at 80 °C for 13 h resulted in slight epimerization but not racemization to give 3e (cis/trans = 3:1, 41% ee). Heating at 150 °C in dimethyl sulfoxide resulted in complete decomposition, and thus we performed DFT calculations to determine the racemization barrier of 3e. However, the barrier was significantly higher and did not converge.

Although we failed to obtain chiral single crystals of (+)-3c, the sign and value of experimental specific rotation of (+)-3c obtained from 1c and 2 with (R)-H8-BINAP as a ligand agrees well with those of the theoretical specific rotation of (R)-3c [B3LYP/6-311++G(2d,2p)] (Table S27), confirming the R absolute configuration of (+)-3c. This enantioselection can be rationalized by the preferential reaction of INT-A rather than INT-B from 1c, 2, and (R)-H8-BINAP due to avoiding steric repulsion between the naphthyl group (colored in blue) and the equatorial phenyl group (colored in red) on the ligand in INT-B (Figure 3a). In diyne 1c, the 1-ethynylnaphthalene moiety is more sterically hindered than the phenylacetylene moiety, and therefore, the phenylacetylene moiety in 1c selectively reacts with 2 and Rh to give INT-A or INT-B. However, the difference in steric hindrance between the 2-ethynylnaphthalene (colored in gray) and phenylacetylene moieties in diyne 1e is small. Accordingly, the reactions through INT-C and INT-D, giving opposite enantiomeric products, are not biased, which would reduce the ee value of the product (Figure 3b).

Figure 3 Plausible enantioselection mechanisms using (R)-H8-BINAP.

We performed single-crystal X-ray diffraction analyses of o,m,o,p-tetraphenylenes 3a (Figures S1–S3), (±)-cis-3b (Figure S4), (±)-3d (Figure S5), (±)-cis-3e (Figure S6), and (±)-cis-3f (Figure 4). cis-3a,d and trans-3a,d are interconvertible in solution at room temperature, crystals with a cis- to trans-isomer ration of 1:1 were obtained for 3a, whereas crystals of only the cis-isomer were obtained for 3d. For (±)-cis-3b, (±)-cis-3e, and (±)-cis-3f, their cis structures were confirmed unambiguously. As a representative example, X-ray crystal structures of (±)-cis-3f is shown in Figure 4. The top view shows that the two benzene rings partially overlap, and the side view shows that the para-linked benzene ring is curved significantly. Measurement of the distance between the two planes shows that the shortest interplane distance (2.72 Å) is shorter than that of o,p,o,p-tetraphenylene (2.80 Å).12 The benzene ring bonded to naphthalene at the meta position (highlighted in blue) is tilted 19.5° relative to the benzene ring bonded to naphthalene at the para position (highlighted in orange). The deformation of the benzene ring from planarity can be defined by the deviation angles (α/α′) of the para carbons from the base plane of the boat-shaped benzene ring (Table 1).13 The average deviation angles (17.9–19.2°) of cis-3 are larger than those of unsubstituted o,m,o,p-tetraphenylene (17.2°),9 and [5]cycloparaphenylene (CPP, 15.6°),14 indicating that the present substituted o,m,o,p-tetraphenylenes 3 are highly distorted cyclophenylene molecules.

Figure 4 X-ray crystal structures of (±)-cis-3f, showing thermal ellipsoids at the 50% probability level. The distance between the red plane and the edge carbon atom of the blue plane is indicated in black. The dihedral angle between the red plane and the blue plane is indicated in blue.

Table 1 Deviation Angles of 3 and Related Compounds

compound	α/α′	average	
cis-3a (trans-3a)	17.6/18.6, 18.5/19.0 (16.8/17.6, 16.1/16.8)	18.4 (16.8)	
cis-3b	17.8/17.9	17.9	
(±)-3d	18.8/19.5	19.2	
(±)-cis-3e	17.8/18.7	18.3	
(±)-cis-3f	18.1/19.0	18.6	
o,m,o,p-tetraphenylenea	16.7/17.7	17.2	
[5]CPPb	15.6	15.6	
a Data from ref (9).

b Data from ref (14).

We further evaluated the strain energies by DFT calculations. The strain energies of simplified cis-3a′ (Figure S7), cis-3c′ (Figure S8), cis-3e′ (Figure S9), and cis-3f′ (Figure 5) by removing substituents on the meta-linked benzene ring, based on the homodesmotic reaction method,15 are 18.3, 16.5, 17.9, and 16.2 kcal/mol, respectively, indicating that cis-3a is the most strained molecule (Table 2). For trans-3′, 3a′ and 3e′ are the most strained molecules. These values are markedly smaller than [2.2]paracyclophane (37 kcal/mol) and [6]CPP (91 kcal/mol).16 The StrainViz16 examination of the strain distribution for cis-3a′ (Figure S7), cis-3c′ (Figure S8), cis-3e′ (Figure S9), and cis-3f′ (Figure 5) shows that the paraphenylene unit of cis-3e′ is the largest with 3.98 kcal/mol, which is less than the 4.47 kcal/mol of [6]CPP16 but more than the 3.39 kcal/mol of [2.2]paracyclophane.16

Table 2 Strain Energies of 3a′, 3c′, 3e′, and 3f′ Determined by DFT Calculations at the B3LYP/6-31G(d) Level of Theory

compound	cis-isomer (kcal/mol)	trans-isomer (kcal/mol)	
3a′	18.3	18.5	
3c′	16.5	16.5	
3e′	17.9	18.5	
3f′	16.2	16.5	

Figure 5 StrainViz of cis-3f′ by DFT calculations at the B3LYP/6-31G(d) level of theory.

1H NMR spectra of 3a, 3c, 3e, and 3f show that the cis/trans isomers of 3a and 3c interconvert at room temperature, but the cis/trans isomers of 3e and 3f do not. The meta-linked biphenyl (highlighted in red, Figure 6a, left) in 3a and 3c is ortho-monosubstituted and would easily rotate at room temperature. On the other hand, the meta-linked biphenyl (highlighted in blue, Figure 6a, right) in 3e and 3f is ortho-disubstituted and would normally rotate readily at room temperature. However, the proximity of Ha and Hb due to ring strain may inhibit the rotation of 3e and 3f. Thus, we performed DFT calculations to determine the rotational barriers of cis/trans isomerization for simplified compounds 3a′, 3c′, 3e′, and 3f′. The DFT calculations showed that the activation energy for rotation of the biphenyl unit in 3a′ and 3c′ are ΔG‡ = 15.1 and 16.1 kcal/mol, respectively, indicating that cis/trans isomerization proceeds rapidly even at room temperature (Figure 6b, top). In contrast, the activation energy for rotation of the biphenyl unit in 3e′ and 3f′ are ΔG‡ = 25.4 kcal/mol, indicating that cis/trans isomerization does not proceed at room temperature (Figure 6b, bottom). Accordingly, for o,m,o,p-tetraphenylenes, the ortho-disubstituted biphenyl cannot rotate at room temperature.

Figure 6 Stereoisomerism of 3 and 3′.

In conclusion, we have achieved the enantioselective synthesis of highly strained, substituted o,m,o,p-tetraphenylenes 3 with up to 98% ee via the cationic Rh(I)/(R)-H8-BINAP complex-catalyzed chemo-, regio-, and enantioselective intermolecular cross-[2+2+2] cycloaddition of teraryl diynes 1 with dimethyl acetylenedicarboxylate (2). In diyne 1c, the 1-ethynylnaphthalene moiety is more sterically hindered than the phenylacetylene moiety, and thus the phenylacetylene moiety of 1c may react selectively with 2 and Rh to afford 3c with high enantioselectivity. X-ray crystallographic analyses of 3 revealed the highly bent structures of the para-substituted benzene moieties, and their deviation angles are larger than those of unsubstituted o,m,o,p-tetraphenylene and [5]CPP. DFT calculations of 3 revealed that the total strain energies are smaller than those of [2.2]paracyclophane and [6]CPP. Nevertheless, the local strain of the most distorted paraphenylene unit of cis-3a′ is smaller than [6]CPP but larger than [2.2]paracyclophane. 1H NMR analyses and theoretical calculations elucidated the stereoisomerism. The ortho-disubstituted biphenyl structure results in cis and trans isomers because this moiety cannot rotate at room temperature. Thus, we have demonstrated the enantioselective synthesis of substituted o,m,o,p-tetraphenylenes 3 and their structures and stereoisomerism.

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.orglett.4c02712.Detailed experimental procedures, characterization data, crystal data, theoretical calculations, chiral HPLC charts, and NMR spectra of the compounds (PDF)

Supplementary Material

ol4c02712_si_001.pdf

Author Contributions

Y.Kawai and T.O. contributed equally to this work. Y.Kawai, T.O., and Y.Kamiya conducted experimental work and analyzed the experimental data. Y.S. and J.N. conducted X-ray crystallographic analyses. J.N. carried out computational studies. S.S. provided experimental guidance to T.O. T.O. and K.T. designed the project. K.T. directed the project and wrote the manuscript. All authors edited the manuscript.

The authors declare no competing financial interest.

Acknowledgments

This work was supported by JSPS KAKENHI Grants JP24H00005, JP21K18949, and JP19H00893 to K.T. and Grant JP21J22287 to J.N. from JSPS. The authors thank Takasago International Corp. for the gift of H8-BINAP. A generous allotment of computational resources from TSUBAME (Tokyo Institute of Technology) is gratefully acknowledged.
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