Cyclopentadienones with

Letter Cite This: Org. Lett. 2018, 20, 966−970

pubs.acs.org/OrgLett

Diels−Alder Reaction of Isobenzofurans/Cyclopentadienones with Tetrathiafulvalene: Preparation of Naphthalene, Fluoranthene, and Fluorenone Derivatives Jayachandran Karunakaran and Arasambattu K. Mohanakrishnan* Department of Organic Chemistry, School of Chemical Sciences, University of Madras, Guindy Campus, Chennai 600 025, Tamil Nadu, India S Supporting Information *

ABSTRACT: Diels−Alder reaction of 1,3-diarylbenzo[c]furan/cyclopentadienone with TTF followed by triflic acid mediated cleavage of the resulting adducts led to the formation of the respective 1,4-diaryl substituted naphthalenes, fluoranthenes, and fluorenones. The photophysical properties of representative diaryl-substituted hydrocarbons are also reported. ince the discovery of the first metallic charge-transfer (CT) tetrathiafulvalene (TTF) complex,1 the search for new TTF-based donor molecules suitable for molecular organic metals has continued to gain the attention of the scientific community.2 Even though TTF 2 is stable for many synthetic transformations,2 functionalization of the same has been constrained due to its stability issues under strong acidic conditions and also with strong oxidizing agents. The recent synthetic strategies outlined for aryl-substituted TTFs involve the Pd-mediated C−H activation.3 By employing a similar concept, the synthesis of (2-azulenyl)TTFs has been achieved.4 Easy access to mono-, di-, tri-, and tetrasubstituted TTF derivatives, involving a selective lithiation using Mg− and Zn− TMP bases, has also been reported.5 The synthesis of TTF analogues through a combination of metalation followed by Pdmediated coupling reaction was also realized.6 An annulation of the aromatic ring to the TTF backbone, e.g., dibenzotetrathiafulvalene and dinaphthotetrathiafulvalene, has displayed high-performance OFETs.7 The synthesis of TTF-conjugated bistetracene was achieved involving Diels− Alder reaction of 1,3-diphenylisobenzofuran with a naphthoquinone−DTF (dithiafulvalene) framework followed by reductive aromatization and triethyl phosphite mediated dimerization.8 It is obvious that the oxidation potential of TTF can be easily tuned by an annulation of the aromatic ring system to its framework. Hence, in a further continuation of our studies on synthetic utility of IBF (isobenzofurans) derivatives,9 we report herein our preliminary findings on Diels−Alder reaction of TTF with 1,3-diarylisobenzofuran as well as cyclopentadienone derivatives. To date, there is no straightforward synthetic method available for the annulation of TTF. However, it should be noted that the half-TTF, 1,3-dithiol-2-thione,10 as well as TTF11 are not well-known for their dienophilic character. Our recent report on inverse-electron-demand Diels−Alder reaction of benzo[c]furans with acenaphthylene12 prompted us to

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© 2018 American Chemical Society

explore the reaction of the same with an electron-rich dienophile, TTF. As expected, the reaction of 1,3diphenylbenzo[c]furan 1a with TTF 2 in dry xylenes at reflux for 6 h furnished Diels−Alder adduct 3a as a brown solid in an excellent yield (Scheme 1). Having prepared the Diels−Alder Scheme 1. Diels−Alder Reaction of Isobenzofuran 1a with TTF 2

adduct 3a, the subsequent aromatization of the same using ptoluenesulfonic acid (PTSA) in xylenes at reflux for 6 h did not afford annulated TTF; instead, an insoluble black polymeric material was obtained. However, when the same reaction was performed with PTSA at a slightly lower temperature (90 °C), an unexpected 1,4diphenylnaphthalene 4a could be isolated as a yellow solid in a moderate yield. A plausible mechanism for the formation of 1,4-diarylnaphthalene 4a is depicted in Scheme 2. Obviously, the rupture of the furan unit was facilitated by the anti-orientation of dihydro TTF unit to produce intermediate I. The subsequent intramolecular cyclization of hydroxyl group may lead to a tetracyclic intermediate III, which upon aromatization through elimination of carbon disulfide and 1,3-dithiol-2-one 5 may give 1,4-diarylnaphthalene 4a (Scheme 2). Received: December 13, 2017 Published: February 8, 2018 966

DOI: 10.1021/acs.orglett.7b03686 Org. Lett. 2018, 20, 966−970

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Organic Letters

82% yield for 4b could be obtained using 0.5 equiv of triflic acid (TfOH) in DCM at 0 °C for 10 min.

Scheme 2. Plausible Mechanism for the Formation of 1,4Diphenylnaphthalene 4

Table 1. Optimization Reaction Conditions for Di(1,4-ptolyl)naphthalene 4b

The 1H NMR spectrum of the crude product indicated the appearance of a singlet at δ 6.8 ppm corresponding to the vinylic proton of 1,3-dithiol-2-one 5. Additionally, the mass spectral fragmentation of the adduct 3a also indicated a [MH]+ peak at m/z 119 corresponding to the 1,3-dithiol-2-one 5 (see the Supporting Information). The syn-orientation of the tetrahydrofuran bridge and hydrogen atoms is not conducive for the expected dehydration of 3a to produce the naphthannulated TTF. Obviously, the Diels−Alder reaction of 1,3diphenylbenzo[c]furan 1a with TTF 2 furnished a thermodynamically stable anti-endo adduct 3a as a sole product. Further, the proposed mechanism is also supported by the failure of cleavage with benzo[c]furan-1,3-dithiol-2-thione adduct 6 (see the SI). Next, the Diels−Alder reaction of 1,3-di-p-tolylbenzo[c]furan 1b with tetrathiafulvalene (TTF) 2 in dry xylenes at reflux followed by the aromatization of adduct 3b with PTSA in xylenes at 90 °C also furnished 1,4-di(p-tolyl)naphthalene 4b (Scheme 3). The structure of the 1,4-di(p-tolyl)naphthalene 4b was confirmed by a single-crystal X-ray diffraction analysis (see the SI).

entry

reagent (equiv)

solvent

temp (°C)

time

yielda (%)

1 2 3 4 5 6 7 8 9

PTSA (4.0) PTSA (4.0) AcOH (2.0) CF3CO2H (1.0) CH3SO3H (0.5) TMSOTf (0.5) TfOH (0.5) BBr3 (0.5) BF3·OEt2 (0.5)

xylenes xylenes xylenes xylenes DCM DCM DCM DCM DCM

145 90 95 95 0 0 0 0 0

4h 8h 9h 8h 40 min 10 min 10 min 30 min 30 min

dec 62 45 50 56 70 82 68 65

a

Isolated yield by column chromatography.

With the optimized conditions in hand, we proceeded to investigate the scope of the reaction with aryl/heteroaryltethered benzo[c]furan derivatives. Several symmetrical as well as unsymmetrical benzo[c]furans 1c−u were tested for their Diels−Alder reaction with TTF 2 followed by triflic acidmediated fragmentation process (Scheme 4). Irrespective of the nature of aryl substituents, all of the benzo[c]furans underwent Diels−Alder reaction followed by fragmentation to furnish the respective 1,4-diarylnaphthalenes in good yields. Relatively, the presence of sterically demanding 1-naphthyl units on benzo[c]furan (1e and 1f) reduced the yield of respective 1,4-diaryl naphthalenes 4e and 4f. As expected, xylene-, veratrole-, and bithienyl-tethered unsymmetrical isobenzofurans 1g−k also underwent a similar type of Diels−Alder reaction followed by aromatization to afford the corresponding unsymmetrical 1,4-diarylnaphthalenes 4g−k in 78−81% yields. Interestingly, p-chlorophenyl-linked benzo[c]furan 1l also produced the expected diarylnaphthalene 4l in 83% yield. Gratifyingly, diphenylmethane 1m and trimethoxybenzene 1n incorporated benzo[c]furans were also successfully transformed into the respective 1,4-diarylnaphthalenes 4m and 4n in good yields. Further, a facile one-pot Diels−Alder reaction of 3methylbenzo[b]thiophene-2-thienyl attached isobenzofuran 1o with TTF 2 followed by aromatization gave the disubstituted naphthalene 4o in 83% yield. The scope of the reaction could be further extended with the syntheses of pyrene as well as 9,9dihexylfluorene linked naphthalenes 4p and 4q. Finally, the syntheses of highly fluorescent dibenzoaryl/heteroaryl- and triphenylamine-based naphthalenes 4r−u have also been achieved. Next, the synthesis of annulated TTF through an inverse electron-demand Diels−Alder reaction with cyclopentadienones 7a−i13 was also initiated. As a representative case, the Diels−Alder reaction of tetraarylcyclopentadienone 7a/7b with TTF 2 in dry xylenes at reflux for 16 h did not afford the annulated tetrathiafulvalenes; instead, 1,2,3,4-tetraarylbenzenes 8a and 8b were obtained in 72 and 74% yields, respectively (Scheme 5). A plausible mechanism for the formation of 1,2,3,4tetraphenylbenzene 8a is depicted in Scheme 6. The thermally labile Diels−Alder adduct IV upon extrusion of carbon monoxide may lead to dihydro TTF V. Most likely, the dihydro TTF V upon subsequent fragmentation via elimination

Scheme 3. Diels−Alder Reaction of Isobenzofuran 1b with TTF 2

Even though the Diels−Alder reaction of 1,3-diaryl benzo[c]furan 1a/1b with TTF 2 was found to be facile, the unexpected problem encountered with subsequent aromatization of the adduct 3a/3b was found to be a setback for synthesis of annulated TTF analogues. On the other hand, the rupture of the dihydro-TTF unit as mentioned above is unknown. As a matter of fact, the TTF as a synthetic equivalent of an acetylene unit is indeed unique, and hence, it will be worthwhile to explore it further. To maximize the yield of di(1,4-p-tolyl)naphthalene 4b, the fragmentation reaction of the Diels−Alder adduct 3b was explored using different types of Lewis as well as Brønsted acids, and the results obtained are outlined in Table 1. Among the various Lewis/Brønsted acids employed, a maximum of 967


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of (1,3-dithiol-2-ylidene)methanethione 9 might furnish tetraphenylbenzene 8a. As expected, a similar type of Diels−Alder reaction of 7,9diphenyl-8H-cyclopenta[a]acenaphthylen-8-one 7c with TTF 2 was performed to furnish 7,10-diphenylfluoranthene 8c. Next, the reactions of p-anisyl- and veratrole-tethered acenaphthylene-fused cyclopentadienone 7d/7e with TTF 2 also gave the corresponding 7,10-diarylfluoranthenes 8d and 8e in 77% and 78% yields. Finally, a one-pot synthesis of 7,10-di(2-thienyl)fluoranthene 8f could also be achieved involving the Diels− Alder reaction of acenaphthylene-fused cyclopentadienone 7f with TTF 2. As expected, cyclopentadienone 7g/7h could be smoothly transformed into the corresponding 2,5-diphenylfluorenones 8g and 8h (Scheme 7). The structure of fluoranthenes 8d and 8f were confirmed by single-crystal Xray diffraction analyses (see the SI).

Scheme 4. Synthesis of 1,4-Diarylnaphthalene Analogues 4c−u

Scheme 7. Synthesis of Fluoranthenes and Fluorenones 8c− h

Surprisingly, the Diels−Alder reaction of 1,3-diphenyl-2Hcyclopenta[l]phenanthren-2-one 7i with TTF 2 did not give the expected 1,4-diphenyltriphenylene; instead, the intermediate Diels−Alder adduct 10 was isolated as a stable compound (Scheme 7). It has been observed previously that the Diels− Alder adduct obtained from cyclopenta[l]phenanthren-2-one 7i is highly stable and underwent extrusion of carbon monoxide only at an elevated temperature.14 The optical properties of representative 1,4-diarylnaphthalene 4a, 4c-e, 4p, and 4s−u, tetraarylbenzene 8a/8b, diarylfluoranthene 8c−f, and diarylfluorenone 8g/8h derivatives were investigated by UV−vis and fluorescence spectroscopy (Figures 1 and 2, Tables 2 and 3). The UV−vis spectra of diarylnaphthalene analogues 4a, 4c− e, 4p, and 4s−u displayed λmax absorption bands in the region 276−415 nm (Figure 1). Among the eight 1,4-diarylnaphthalenes, 1,4-dithienylnaphthalene 4d and ternaphthalene 4e showed second absorption λmax values greater than 400 nm. The attachment of a pyrene/N-hexylcarbazole unit on naphthalene 4p/4t induced the red shift due to extended conjugation as well as enhancement of π−π* electronic transition. The pyrene-linked naphthalene 4p exhibited higher fluorescence quantum efficiency (34%) than anthracene (29%). This clearly confirms that pyrene and naphthalene units are responsible for their enhancement of fluorescence. Similarly, thiophene and carbazole attached naphthalenes 4d/4t also displayed relatively better fluorescence quantum yields (Table 2). The large Stokes shift values of naphthalenes 4a, 4c−4e, 4s,

Scheme 5. Synthesis of 1,2,3,4-Tetraphenylbenzene 8a/8b

Scheme 6. Plausible Mechanism for Formation of Tetrarylbenzene 8a

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Table 3. Photophysical Studies of Tetraarylbenzenes 8a/8b, Diarylfluoranthenes 8c−f, and Diarylfluorenones 8g/8h entry product 1 2 3 4 5 6 7 8

8a 8b 8c 8d 8e 8f 8g 8h

absorptiona λmax(abs) (nm)

emissiona,b λmax(em) (nm)

Stokesc shift (cm−1)

quantum yield (φ)d

268 280 330, 366 288, 390 289, 390 294, 382 335, 406 263, 285

385, 469 412, 489 453 486 518 496 408, 512 394, 470

23321 15264 5247 14147 15298 13852 5341 16746

0.290 0.010 0.306 0.039 0.216 0.047 0.002 0.015

Recorded in DCM at 25 °C. bExcited at the longest wavelength of the absorption maximum. cStokes shift = λmax(abs) − λmax(emi) (cm−1). d Fluorescence quantum yield was calculated using anthracene as a standard (Φstd = 0.29 in ethanol, see the SI). a

Figure 1. Absorption and fluorescence spectra of 4a, 4c−e, 4p, and 4s−u.

The UV−visible spectra of benzene, fluoranthene and fluorenone derivatives 8a−h exhibited broad absorption bands in the region 263−406 nm (Figure 2, Table 3). Interestingly, 1,2,3,4-tetraphenylbenzene 8a showed higher Stokes shift value than the unsymmetrical diphenyldi-panisylbenzene 8b (Figure 2). The p-anisyl-substituted fluoranthene 8d and fluorenone 8h displayed greater Stokes shift values than the corresponding phenyl-substituted counterparts 8c and 8g. The lower Stokes shift values displayed by phenylsubstituted fluoranthene 8c and fluorenone 8g could be accounted for the reduced polarization. Out of eight compounds 8a−h, electron-deficient diarylfluorenones 8g and 8h have a lower fluorescence quantum efficiency than arylsubstituted benzenes 8a and 8b and fluoranthenes 8c−f. Most likely, the dianisylnaphthalene (8c) and tetraarylbenzenes (8a, 8b) displayed second emission due to the association induced emission (AIE) of benzene-cored fluorophores.15 As observed in the case of fluorenone,16 the second emission band appearing at 512 and 480 nm for diarylfluorenones 8g and 8h is attributed to the excimer fluorescence. For the first time, we demonstrated an inverse-electrondemand Diels−Alder reaction of 1,3-diarylbenzo[c]furans with TTF in xylenes at reflux. The benzo[c]furan−TTF adducts upon interaction with triflic acid led to the formation of an easily inaccessible unsymmetrical 1,4-diarylnaphthalene in very good yields. The protocol was also found to be applicable for the syntheses of 1,2,3,4-tetraarylbenzene and fluoranthene as well as fluorenone derivatives. The photophysical properties of representative 1,4-diarylnaphthalene, 1,2,3,4-tetraarylbenzene, diarylfluoranthene, and diarylfluorenone compounds have also been reported.

Figure 2. Absorption and fluorescence spectra of 8a−h.

Table 2. Photophysical Studies of Diarylnaphthalenes 4a, 4c−e, 4p, and 4s−u entry product 1 2 3 4 5 6 7 8

4a 4c 4d 4e 4p 4s 4t 4u

absorptiona λmax(abs) (nm)

emissiona,b λmax(em) (nm)

Stokesc shift (cm−1)

quantum yield (φ)d

279 277, 317 294, 415 287, 412 350 276 349 300, 340

387 419, 505 443 503 452 431 457 519

10003 12235 11440 16091 6448 13030 6772 14066

0.039 0.042 0.182 0.124 0.338 0.075 0.210 0.036

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ASSOCIATED CONTENT

S Supporting Information *

Recorded in DCM at 25 °C. Excited at the longest wavelength of the absorption maximum. cStokes shift = λmax(abs) − λmax(em) (cm−1). d Fluorescence quantum yield was calculated using anthracene as a standard (Φstd = 0.29 in ethanol; see the SI). a

b

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03686. Experimental procedures, analytical data, NMR and HRMS spectra, X-ray data, fluorescence quantum yield determination, and absorption/emission spectra for obtained compounds (PDF)

and 4u might be due to the presence of donor groups facilitating the charge separation. 969


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Karunakaran, J.; Dhivyapirabha, L.; Mohanakrishnan, A. K. ChemistrySelect 2017, 2, 7899. (10) Callahan, R.; Ramirez, O.; Rosmarion, K.; Rothchild, R.; Bynum, K. C. J. Heterocycl. Chem. 2005, 42, 889. (11) Khodorkovsky, V. Y.; Becker, J. Y.; Bernstein, J. Synth. Met. 1993, 56, 1931. (12) Karunakaran, J.; Mohanakrishnan, A. K. Eur. J. Org. Chem. 2017, 2017, 6747. (13) (a) Kato, S.-I.; Kijima, T.; Shiota, Y.; Yoshihara, T.; Tobita, S.; Yoshizawa, K.; Nakamura, Y. Tetrahedron Lett. 2016, 57, 4604. (b) Luo, J.; Xu, X.; Mao, R.; Miao, Q. J. Am. Chem. Soc. 2012, 134, 13796. (14) (a) Yoshitake, Y.; Misaka, J.; Abe, M.; Yamasaki, M.; Eto, M.; Harano, K. Org. Biomol. Chem. 2003, 1, 1240. (b) Callahan, R.; Ramirez, O.; Rosmarion, K.; Rothchild, R.; Bynum, K. C. J. Heterocycl. Chem. 2005, 42, 889. (c) Yamaguchi, K.; Eto, M.; Harano, K. Chem. Pharm. Bull. 2013, 61, 1065. (d) Wooi, G.; White, J. M. Org. Biomol. Chem. 2005, 3, 972. (15) Wang, H.; Liang, Y.; Xie, H.; Feng, L.; Lu, H.; Feng, S. J. Mater. Chem. C 2014, 2, 5601. (16) Heldt, J. R.; Heldt, J.; Jozefowicz, M.; Kaminski, J. J. Fluoresc. 2001, 11, 65.

CCDC 1562867, 1583971, and 1584282 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], mohan_67@hotmail. com. ORCID

Arasambattu K. Mohanakrishnan: 0000-0002-3758-4578 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial assistance from CSIR, New Delhi, is acknowledged. J.K. thanks CSIR, New Delhi. for an SRF fellowship. We thank DST-FIST for NMR and HRMS facilities. We also thank Prof. P. Ramamurthy, Director, National Centre for Ultrafast Processes, University of Madras, for discussions on optical studies.

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REFERENCES

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Cyclopentadienones with

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