Letter pubs.acs.org/OrgLett
Synthesis of π‑Extended Fluoranthenes via a KHMDS-Promoted Anionic-Radical Reaction Cascade Naoki Ogawa, Yousuke Yamaoka, Ken-ichi Yamada,† and Kiyosei Takasu* Graduate School of Pharmaceutical Sciences, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan S Supporting Information *
ABSTRACT: An unprecedented KHMDS-promoted domino reaction to furnish hydroxyfluoranthenes is described. Biaryl compounds bearing acyl and naphthylalkenyl moieties are transformed into 9-hydroxydibenzo[j,l]fluoranthenes in a single step through the formation of an aromatic and a pentagonal ring system. A variety of fluoranthenes including those with extended π-conjugation, a heteroaromatic ring, and unsymmetrical substituents could be synthesized. Mechanistic studies reveal a unique reaction cascade where KHMDS acts as both a base and a single-electron donor.
A
Scheme 1. KHMDS-Promoted (2 + 2)-Cycloaddition and Acid-Promoted Ring-Opening Reaction
diverse range of polycyclic aromatic hydrocarbons (PAHs) have been developed and are widely applied to organic materials. In recent years, nonalternant hydrocarbons, which possess nonhexagonal ring systems, such as pentagon and/or heptagon, have attracted considerable attention owing to their unique structural, optical, and electrochemical features.1 The fluoranthenes, whose skeleton features naphthalene and benzene rings are connected by a five-membered ring, display interesting properties such as large Stokes shifts and resistance to air-quenching.2 Therefore, their framework is widely used in organic materials such as luminescent twisted nanowires, OLEDs, and OFETs.3 The common strategies to synthesize fluoranthenes are based on Diels−Alder reactions4 or crosscoupling reactions.5 However, it is still desirable to develop facile and flexible methods for the preparation of unsymmetrically functionalized and/or π-extended fluoranthenes.6 We envisioned that such compounds are of interest because the properties of fluoranthenes can be modified by introducing substituents or π-extended systems.7 Herein, we report a novel synthetic method for unsymmetrical hydroxyfluoranthenes with extended π-conjugation in which an aromatic and pentagonal ring system is formed in one step from biaryl compounds. Mechanistic investigations suggest that potassium hexamethyldisilazide (KHMDS) acts as both a base and a single-electron donor in one reaction to enable this unique transformation. In addition, further transformation of the so-formed hydroxyfluoranthenes based on the recombination of the π-system are described. We previously reported a base-promoted (2 + 2)-cycloaddition of biaryls possessing acyl and vinyl groups at the 2 and 2′ positions, respectively, to form areno-fused cyclobutanols and acid-promoted rearrangement of these cyclobutanols to provide PAHs (Scheme 1).8 The polyaromatic framework and the substituents can be prepared by retrosynthetically designing biaryl substrates. During the course of our study, we found an interesting transformation reaction giving fluoranthenes. The reaction of © 2017 American Chemical Society
1a possessing a 1-(2-alkoxynaphthyl)vinyl moiety in the presence of KHMDS did not give the expected cyclobutanol 5a, but instead the heptacycles 2a and 3a were obtained in 74% and 11% yields, respectively (Scheme 2). Gratifyingly when 3a was heated to reflux in diglyme (bp 162 °C), a retro (2 + 2)cycloaddition occurred and 9-hydroxydibenzo[j,l]fluoranthene (4a) was obtained in 60% yield. The structures of 2a, 3a, and 4a were unambiguously determined by X-ray crystallographic studies. Compound 2a has a propellane structure connected with naphthofuran, phenanthrene, and cyclobutane rings. Compound 3a also has a propellane framework but fused with a 2-hydroxynaphthalene instead of naphthofuran. Compound 4a, obtained by removal of an isobutylene unit from 3a, possesses a highly planar skeleton. The reaction conditions giving 2a, 3a, and 4a from biaryl 1a were explored in detail (Table 1). When the reaction of 1a with KHMDS (5 equiv) was conducted at room temperature, the starting biaryl 1a was fully consumed within 0.5 h and only 2a was obtained in 79% yield (Table 1, entry 1). Compound 3a was produced when the reaction was conducted at reflux in diglyme. The yield of 3a increased with prolonged reaction time, suggesting that 3a was formed from 2a by recombination of the C−O bond as a C−C bond (Table 1, entries 2 and 3). Considering that fluoranthene 4a was obtained by heating 3a (Scheme 2), 4a could be obtained Received: May 20, 2017 Published: June 7, 2017 3327
DOI: 10.1021/acs.orglett.7b01538 Org. Lett. 2017, 19, 3327−3330
Letter
Organic Letters Scheme 3. Scope of the Reactiona
Scheme 2. Formation of 2a, 3a, and 4a and Their Crystallographic Structures
a
Reaction conditions: 1 (0.05 mmol), KHMDS (3 equiv) and cis-1,2cyclohexanediol (20 mol %) in diglyme (2 mL). Isolated yields in parentheses. bR = MOM. cR = Me. dFor 36 h.
Table 1. Optimization of Reaction Conditions for the Synthesis of 2a, 3a, and 4a from 1aa
and 4g in good yields. Notably, both the methyl and methoxymethyl (MOM) groups could be used as an ether substituent (R) on the ArC ring in the fluoranthene synthesis. This domino reaction could also be extended to double fluoranthene formation for constructing further extended πsystems (Scheme 4). The reaction of terphenyl 1h, bearing two sets of acyl and alkenyl moieties, proceeded smoothly to give the bisfluoranthene 4h in 72% yield.
b
yield (%) entry
base
temp
time (h)
2a
3a
4a
1 2 3 4 5 6 7e 8e 9e
KHMDS KHMDS KHMDS KHMDS KHMDSc KHMDSc,d LiHMDSc NaHMDSc KOtBuc
rt reflux reflux reflux reflux reflux reflux reflux reflux
0.5 0.17 1 24 24 24 24 24 24
79 74 34 0 0 0 94 75 23
0 11 46 0 0 0 0 0 0
0 0
