α-Ketocarbenium Ions Derived from Orthoquinone-Containing

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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α‑Ketocarbenium Ions Derived from Orthoquinone-Containing Polycyclic Aromatic Compounds Kazuki Urakawa,† Yuta Kawabata,† Masaki Matsuda,‡ Michinori Sumimoto,*,§ and Hayato Ishikawa*,† †

Division of Organic Chemistry, Department of Chemistry, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan ‡ Division of Physical Chemistry, Department of Chemistry, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan § Division of Computational Chemistry, Graduate School of Science and Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan S Supporting Information *

ABSTRACT: α-Ketocarbenium ions derived from synthesized orthoquinone-containing polycyclic aromatic compounds were generated in the presence of Brønsted acids such as sulfuric acid, trifluoromethanesulfonic acid, and fluorosulfonic acid. The prepared α-ketocarbenium ions were stabilized by conjugation of the aromatic moiety. In addition, unique absorption properties of the α-ketocarbenium ions were observed and identified on the basis of the calculated absorption spectra. It was suggested that the zigzag-shaped architecture stabilizes the newly discovered α-ketocarbenium ions derived from orthoquinone-containing polycyclic aromatic compounds.

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in organic synthesis.9 We recently developed redox switching technology using originally synthesized orthoquinone-containing polycyclic aromatic compounds.10 In this context, we envisaged that in the presence of a suitable Brønsted acid, novel α-ketocarbenium ions could be generated from the carbonyl group of orthoquinone-containing polycyclic aromatic compounds (Figure 1). We anticipated that such carbocations would be stabilized by conjugation through the aromatic moiety.

arbenium ions are a historically important chemical species in chemistry, and the preparation of stable carbocations is one of the most attractive and challenging areas of organic research. For instance, the discovery of the triphenylmethyl (trityl) cation, which is the archetypal stable carbenium ion stabilized by conjugation over three phenyl groups, contributed to the progress of organic and physical chemistry.1 In addition, diphenylhydroxycarbenium ions prepared from benzophenones and stabilized by conjugation of aromatics have been reported.2 By contrast, α-ketocarbenium ions, in which the carbenium ion is adjacent to an electronwithdrawing carbonyl group, are intrinsically unstable, and the preparation of such species is not easy. Thus, NMR studies at cryogenic temperature 3 and theoretical studies 4 of αketocarbenium ions have been reported. Furthermore, the existence of this rare species as a fragmentation ion in mass spectrometry5 and as a nonisolatable intermediate in a chemical reaction6 has been suggested. The generation of a stable αketocarbenium ion would lead to a better understanding of the chemical and physical properties of these elusive species and may facilitate their application in chemical synthesis. Furthermore, stable α-ketocarbenium ions at room temperature may be applied to materials such as organic electrolytes, surfactants, or alternative ionic liquid materials. On the other hand, the chemical properties of orthoquinones have attracted the attention of chemists. For instance, this functional group possesses redox ability7 and coordinating properties.8 Furthermore, orthoquinones are good electrophiles © XXXX American Chemical Society

Figure 1. Proposed α-ketocarbenium ion prepared from orthoquinone-containing polyaromatic compounds.

Herein we describe the preparation of the α-ketocarbenium ion from the originally designed orthoquinone-containing polycyclic aromatic compounds in sulfuric acid, trifluoromethanesulfonic acid, or fluorosulfonic acid. The structure of the α-ketocarbenium ion was supported by density functional Received: February 27, 2018

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DOI: 10.1021/acs.orglett.8b00682 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters theory (DFT) calculations, and a unique red shift of the absorption maximum of the prepared α-ketocarbenium ions was observed. Picene-13,14-dione (1), which was prepared according to our previously reported protocol,10 was selected as the substrate to generate the α-ketocarbenium ion. Several Brønsted acids were employed to protonate the orthoquinone moiety of 1. When 1 was treated with AcOH (pKa = 4.76 in H2O), HCO2H (pKa = 3.77 in H2O), or TFA (pKa = −0.25 in H2O), the absorption spectrum of the solution was the same as that of a chloroform solution of 1 (yellow color). On the other hand, when 1 was dissolved in concentrated H2SO4 (pKa = −3.0 in H2O), the color of the solution changed dramatically to bright violet (Figure 2). Importantly, the color of the violet solution

Figure 3. Optimized geometry of 2 obtained by DFT calculations.

be 3.998%, which is larger than that of benzene calculated under the same conditions (2.998%). This result indicates that the electrons of 2 are delocalized. The atoms with a large positive charge were C6 (+0.473), C9 (+0.484), and H37 (+0.515). Therefore, the cation charge is probably delocalized around the C6 and C9 atoms. The NICS(0) and NICS(1) values at the center ring of α-ketocarbenium ion 2 were calculated to be +15.6 and +8.9 ppm, respectively. These large positive values reflect an antiaromatic character. The good agreement between the calculated and experimental absorption spectra of 2, depicted in Figure 2B, supports the conclusion that the proposed cation is generated in situ. Furthermore, treatment of 1 with CF3SO3H (pKa = −14.0 in H2O), which is a stronger acid than H2SO4, provided the similar αketocarbenium ion 3. The absorption spectrum of 3 completely matched that of 2 (see Figure S1). Having established a method for the preparation of αketocarbenium ions, pentaphen-6,7-dione (4), which was previously prepared by us,10 was employed as a second substrate (Figure 4). Surprisingly, the absorption spectrum of 4

Figure 2. Generation of α-ketocarbenium ion 2 (eq 1). (A) Change in color upon formation of 2. (B) Absorption spectra of 1 in CHCl3 and 2 in H2SO4 and the calculated absorption spectrum of 2.

returned to yellow upon addition of water, and the starting material 1 was completely recovered by extraction with organic solvent (see details in the Supporting Information). Thus, no significant decomposition occurred upon treatment with sulfuric acid, and the reaction was established as a reversible system. At this stage, we assumed that the desired αketocarbenium ion 2 was generated in situ. The absorption spectra of 2 in H2SO4 and 1 in CHCl3 are shown in Figure 2B. The maximum absorption of 2 was at 593.5 nm; thus, a large red shift occurred upon conversion of 1 into 2. The 1H NMR spectra of 2 in D2SO4 recorded at a wide range of temperatures showed broad signals (see Figure S2). Thus, full assignment of the signals could not be accomplished, although the presence of aromatic protons was confirmed. To establish the structure of α-ketocarbenium ion 2, DFT calculations11 were carried out. The optimized structure of 2 is depicted in Figure 3. The twist strain of the O35−C6−C9−O36 moiety in 2 was 0.9°, and there was no large distortion. The C6−C9 distance of 1.520 Å indicated that it was essentially a single bond. To investigate the electronic structure and the site of the cation charge, natural bond orbital (NBO) analysis of 2 was performed. The percentage of total non-Lewis structure in 2 was calculated to

Figure 4. Generation of α-ketocarbenium ion 5 (eq 2). (A) Change in color upon formation of 5. (B) Absorption spectra of 4 in CHCl3 and 5 in FSO3H and the calculated absorption spectrum of 5.

recorded after treatment with H2SO4 was very similar to that of 4 in CHCl3 (see Figure S3). By contrast, when FSO3H (pKa = −15.1 in H2O) was employed as Brønsted acid, the solution became violet. These results indicate that the generation of αketocarbenium ion 5 from 4 required a stronger acid than that for 1. It may be that the zigzag architecture of phenacene-type molecules such as 2 could stabilize the α-ketocarbenium ion. B

DOI: 10.1021/acs.orglett.8b00682 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters The absorption spectrum of 5 was characterized by absorption maxima at 557.5 and 928.5 nm. We also performed DFT calculations on 5. The twist strain of the O35−C6−C9−O36 moiety in 5 was calculated to be 0.06°; thus, the structure of 5 is closer to a plane than that of 2. The percentage of total nonLewis structure in 5 was 3.828%, which indicates that the electrons of 5 are also delocalized. In addition, the percentages in 2 (3.998%) and 5 (3.828%) are both sufficiently large, but the difference of 0.17% indicates that zigzag-shaped 2 is more stable. The NICS(0) and NICS(1) values at the center ring of 5 were calculated to be +9.9 and +4.1 ppm, respectively. These are also large positive values that reflect an antiaromatic character. The experimental UV−vis and calculated spectra showed good agreement (Figure 4B). Although it is relatively small, we consider that the absorption band in the near-IR is a significant discovery for such a small molecule. Our interest then moved to π-extended α-ketocarbenium ion 7, prepared from orthoquinone-containing [7]phenacene derivative 6 (Scheme 1 and Figure 5). A large red shift of the Scheme 1. Synthesis of 2,13-Di-tertbutyldibenzo[c,m]picene-7,8-dione (6)

Figure 5. Structure of α-ketocarbenium ion 7 (top). (A) Change in color upon formation of 7. (B) Absorption spectra of 6 in CHCl3 and 7 in H2SO4.

presence of BPO. After the solvent was removed under reduced pressure, the obtained crude product was directly treated with aqueous potassium carbonate solution to provide the corresponding benzyl alcohol. Subsequent oxidation of the obtained benzyl alcohol provided 10 in 78% yield (three steps). Demethylation of the methoxy group of 10 with boron tribromide proceeded in 92% yield, and triflation of the phenolic alcohol with Tf2O in the presence of DMAP afforded 11 in 89% yield. The latter was treated with catalytic [1,1′bis(diphenylphosphino)ferrocene]palladium(II) dichloride and bis(pinacolato)diboron (0.5 equiv) to provide the homocoupling product 12 in quantitative yield. The key benzoin condensation reaction10 of 12 with 3-ethyl-5-(2-hydroxyethyl)4-methylthiazol-3-ium bromide, followed by oxidation in air gave the desired orthoquinone 6 in 61% yield. Upon completion of the total synthesis of orthoquinonecontaining [7]phenacene derivative 6, it was treated with concentrated H2SO4 to generate α-ketocarbenium ion 7 (Figure 5). As a result, the solution became bright blue with a maximum absorption at 710.5 nm. Thus, a clear red shift was observed in going from picene-derived carbenium ion 2 to [7]phenacene-derived carbenium ion 7 (593.5 to 710.5 nm). In addition, we noted that the acidity of H2SO4 was sufficient to generate the desired α-ketocarbenium ion. These results also support that the zigzag architecture stabilizes α-ketocarbenium ions derived from orthoquinone-containing polycyclic aromatic compounds. In conclusion, α-ketocarbenium ions derived from the carbonyl group of orthoquinone-containing polycyclic aromatic compounds were generated in the presence of Brønsted acids. In this study, normally unstable α-ketocarbenium ions were observed at room temperature because of stabilization afforded through conjugation of the aromatic framework. A unique red shift of the prepared α-ketocarbenium ions compared with the corresponding orthoquinones was observed. In addition, we recognized that the zigzag architecture offers some stabilization

absorption of 7 was anticipated because of the highly conjugated π system and the stabilization effect of the αketocarbenium ion from a longer zigzag-shaped architecture. The synthesis of 6 started with the reduction of commercially available 3-methoxy-2-methylbenzoic acid (8) with LiAlH4, followed by oxidation of the obtained benzyl alcohol to give the benzaldehyde derivative. The obtained aldehyde was treated with p-(tert-butyl)benzylphosphonium bromide and nBuLi to afford alkene 9 (E/Z ratio = 5:4; 99% yield over three steps). The E/Z mixture of 9 was then irradiated with a high-pressure mercury lamp in the presence of iodine to induce isomerization and six-π-electron cyclization. The benzyl position of the obtained cyclized product was brominated with NBS in the C

DOI: 10.1021/acs.orglett.8b00682 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters of the α-ketocarbenium ion. As a result, new functionality on the orthoquinone moiety within polycyclic aromatic compounds was established. Further applications of the novel αketocarbenium ions to materials such as organic electrolytes and the development of new functionality of orthoquinones are underway.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b00682. Figures S1−S5, experimental details, and characterization data for all new compounds (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hayato Ishikawa: 0000-0002-3884-2583 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the JSPS through a Grant-in-Aid for Challenging Exploratory Research (15K13647 to H.I.) and from the TOBE MAKI Scholarship Foundation (to H.I.).

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DOI: 10.1021/acs.orglett.8b00682 Org. Lett. XXXX, XXX, XXX−XXX