Base-Catalyzed Cyclization of 1,6-Diynyl Carboxylates Involving Propargyl-Allenyl Isomerization: Efficient Synthesis of Benzo[b]fluorene and Its Analogues

Article abstract of DOI:10.1002/adsc.201601074

An efficient protocol for the synthesis of benzo[b]fluorenes via base-catalyzed cyclization of 1,6-diynyl carbonates, esters or ethers has been developed. The reaction likely proceeds via base-induced propargyl-allenyl isomerization followed by Schmittel-type cyclization. Heterocycle-fused fluorenes such as thiophene- or pyridine-fused substrates could also be conveniently constructed by this method. The synthetic utility of this reaction was demonstrated by the preparation of up to seven-ring fused polycyclic aromatic hydrocarbons. (Figure presented.).

Full text of DOI:10.1002/adsc.201601074

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Base-Catalyzed Cyclization of 1,6-Diynyl Carboxylates Involving
Propargyl-Allenyl Isomerization: Efficient Synthesis of Benzo[b]
fluorene and Its Analogues
Ning Sun,a Xin Xie,a Gaonan Wang,a Haoyi Chen,a and Yuanhong Liua,*
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a
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences,
345 Lingling Lu, Shanghai 200032, People’s Republic of China
Fax: (+86) 021–64166128; e-mail: yhliu@sioc.ac.cn
Received: September 27, 2016; Revised: January 4, 2017; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201601074
metal-catalyzed reactions,[10] the synthetic methods for
Abstract: An efficient protocol for the synthesis of
benzo[b]fluorenes via base-catalyzed cyclization of
their polyfused derivatives such as benzo[b]fluorenes
are less explored.[11–15] Among the reported ap-
1,6-diynyl carbonates, esters or ethers has been
proaches, Schmittel-type cyclization of aryl-substituted
developed. The reaction likely proceeds via base-
enyne-allenes (C2-C6) is one of the most efficient
induced propargyl-allenyl isomerization followed
method for the synthesis of benzo[b]fluorenes.[12,13]
by Schmittel-type cyclization. Heterocycle-fused
However, due to the lack of diverse methods for the
fluorenes such as thiophene- or pyridine-fused
generation of the allenic substrates or intermediates,
only specially substituted benzo[b]fluorenes could be
substrates could also be conveniently constructed
by this method. The synthetic utility of this reaction
constructed. For example, most of the reported
was demonstrated by the preparation of up to
reactions afforded phosphine-substituted benzo[b]flu-
seven-ring fused polycyclic aromatic hydrocarbons.
orenes[12a–e] via [2,3]-sigmatropic rearrangement of the
in situ formed propargyl phosphinites (Scheme 1, eq
Keywords: base-catalysis; cyclization; propargyl-al-
1).[12a] The allenic intermediates could also be pro-
lenyl isomerization; Schmittel cyclization; benzo[b]
duced via gold-catalyzed 3,3-rearrangement.[13–14] For
fluorene
example, Shi et al. found that benzo[b]fluorenes could
be synthesized via gold-catalyzed propargyl vinyl
ether rearrangement followed by cyclization
[13a]
38 Fluorenes and their benzo-fused derivatives, classified (Scheme 1, eq 2).
We have also developed a gold-
39 as non-alternant polycyclic aromatic hydrocarbons catalyzed cyclization of 1,6-diynyl carbonates to benzo
40 (PAHs),[1] have attracted much attention in recent [b]fluorenes via gold-catalyzed 3,3-rearrangement of
41 years due to their unique electronic and photonic propargyl carbonate moiety followed by 6-endo-dig
42 properties, and their applications in organic functional cyclization and arylation/decarboxylative etherifica-
43 materials. For example, fluorenes have been widely tion. In this reaction, the aryl ring on the bottom
44 utilized in organic light-emitting diodes (OLEDs),[2] alkyne terminus undergoes the cyclization to form a
45 semiconductors,[3] and solar cells[4] etc. Benzo[b]fluo- benzo unit (Scheme 1, eq 3).[14] These reactions
46 rene unit has also been found in a series of functional require the use of high-cost metal-catalysts. Wang
47 materials which have potent applications in various prepared benzo[b]fluorenes via base-catalyzed prop-
48 fields of organic electronics.[5] The tetracyclic nucleus argyl-allenyl isomerization to enyne-allenes,[15] how-
49 of benzo[b]fluorenes are present in natural products ever, the reaction was restricted to the alkynes bearing
50 such as kinoscurinone, stealthin C and cysfluoretin,[6] a bulky tert-butyl substituent at the propargylic
51 structurally related to the kinamycins[7] which display position.
A
Pd-catalyzed tandem Sonogashira
52 a variety of important biological activities. Benzo[b] couplingÀisomerization reaction[16] of terminal prop-
53 fluorenes were also used as synthetic intermediates for argyl ethers with aryl iodides was utilized for the
54 the synthesis of bowl-shaped PAHs which have carbon formation of benzo[b]fluorenes.[17] However, since
55 frameworks represented on the surface of C60.[8] these reactions usually require an electron-withdraw-
56 Although fluorenes can be prepared through classical ing group on the aryl halides to trigger an efficient
57 acid-promoted ring closure of biaryl compounds[9] or alkyne-allene isomerization of the aryl-substituted
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Scheme 1. Synthesis of benzo[b]fluorenes from diynyl alcohol derivatives.
40 propargyl ether intermediate,[16] thus the reported fluorenes under base-catalyzed conditions, and the
41 method was limited to electron-deficient aryl io- products would be different from that obtained via
42 dides.[17] For example, the benzo[b]fluorene product gold catalysis (Scheme 1, eq 4). We expected that such
43 could not be formed using electron-neutral 1-iodoben- methodology would not only avoid the use of
44 zene as the substrate.[17] During our continuing inter- expensive metal-catalysts, but also provide the fused-
45 ests[18a] in the development of cascade reactions fluorenes with a wide range of structural diversity
46 involving based-induced propargyl-allenyl isomeriza- which may not be easily accessible through metal-
47 tion,[18] we noted that such isomerization could be catalyzed protocols. In this communication, we present
48 effected markedly by the substitution pattern of the a highly efficient synthetic route to benzo[b]fluorenes
49 substrates. Inspired by our recent work on gold- via DBU-catalyzed reaction of 1,6-diynyl carbonates,
50 catalyzed reaction to benzo[b]fluorenes[14] and base- esters or ethers. In addition, a heterocycle-fused
51 catalyzed cyano-Schmittel cyclization,[19] we envi- fluorenes such as thiophene- or pyridine-fused one
52 sioned that the presence of a heteroatom group such could also be conveniently constructed by this method.
53 as a carbonate group in 1,6-diynyl substrates might The synthetic utility of this reaction was also demon-
54 increase the acidity of the proton on the benzylic strated by the preparation of up to seven-ring-fused
55 position, thereby facilitating the expected base-cata- polycyclic aromatic hydrocarbons.
56 lyzed propargyl-allenyl isomerization. Thus it is possi-
At the beginning, the cyclization reactions of 1,6-
57 ble to cyclize 1,6-diynyl carbonates to benzo[b] diynyl carbonate 1a bearing phenyl substituents on
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Table 1. Optimization Studies for the Formation of 2a.
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[a] 0.3 mmol scale. [Substrate]=0.1 M. All yields are NMR yields using CH2Br2 as the internal standard. The NMR yields of
unreacted 1a are shown in parentheses.
[b] Isolated yield was 91%.
[c] In a sealed tube.
[d] Isolated yield.
46 both of the alkyne terminus in the presence of various room temperature, only 42% of 2a was formed after
47 bases were examined. We were pleased to find that stirring for 24 h, and the allene intermediate was also
48 treatment of 1a with 10 mol% of DBU in toluene at not observed (entry 3). The results indicated that C2-
49 808C for 13 h enabled complete consumption of 1a, C6 enyne-allene cyclization could occur even at room
50 leading to the desired benzo[b]fluorene 2a in 93% temperature, as also observed in other reports.[12e,13a]
51 yield (Table 1, entry 1). No allene intermediate was Other organic bases, such as DABCO, DMAP and Et3
52 observed during the reaction process upon monitoring N were not effective in this reaction, possibly due to
53 the reaction by 1H NMR in d8-toluene, most likely due that the allenic intermediate could not be formed in
54 to that the subsequent cyclization reaction occurs very an efficient way under these conditions (entries 4–6).
55 fast. Performing the reaction at 508C resulted in a Inorganic bases such as KOtBu and Cs2CO3, which
56 lower product yield of 78% with a longer reaction were proved to be efficient catalysts in our previous
57 time (entry 2). When the reaction was carried out at work involving cyano-Schmittel cyclization of o-
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Scheme 2. Synthesis of poly-fused aromatic hydrocarbons.
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13 (cyano)phenyl propargyl ethers, afforded only trace of ents on the aryl ring were well suited, providing the
14 the product 2a in toluene (entry 7 and 10). A slight corresponding benzo[b]fluorenes 2j–2q in good to
15 higher yield of 14% of 2a was observed by switching excellent yields. The functionalities such as p-MeO, p-
16 the solvent from toluene to THF and employing Me, p-Cl, p-F, p-CF3, p-CO2Et, o-Br, and 3,4,5-(MeO)3
17 1.0 equiv. KOtBu (entry 9). In this case, several by- were well tolerated. A sterically demanding 1-
18 products were observed, possibly due to the decom- naphthyl group was also compatible, furnishing 2r in
19 position of substrate 1a under this condition. Moder- 87% yield. Interestingly, the heteroaryl group such as
20 ate yield could be achieved at 808C in THF using 2-thienyl or 2-pyridyl groups could also be successfully
21 1.0 equiv. of Cs2CO3 as the base (entry 12). The incorporated into the fused products, highlighting the
22 reactions proceeded also well in THF or MeCN in the great diversity of the products. It should be noted that
23 presence of 10% of DBU, in which 2a was formed in pyridine ring rarely undergoes the [4+2] cycloaddi-
24 70% and 85% yields, respectively (entries 14–15). tion reaction due to its low reactivity.[21] In addition,
25 However, when DCE was used as the solvent, only the parent phenyl rings substituted with synthetically
26 38% of 2a was formed (entry 13). Decreasing the valuable ÀMeO, ÀCl, ÀF or methylenedioxy function-
27 catalyst loading to 5 mol% resulted in a lower product alities were also efficiently transformed into benzo[b]
28 yield of 85% with a longer reaction time (entry 16). In fluorenes 2u–2x, and high yields were achieved in
29 the absence of a catalyst, no reaction occurred at each case. The structures of benzo[b]fluorenes were
30 808C, and the starting material 1a was recovered in unambiguously confirmed by X-ray crystallographic
31 98% yield (entry 17).
Next, the substrate scope of this reaction was
analyses of 2a, 2o and 2t.[22]
32
The reaction can be readily applied to the synthesis
33 studied and the results are shown in Table 2. To our of polycyclic aromatic compounds from oligoalkynes.
34 delight, the reactions were proved to be quite general For example, when polyyne 3a or 3b bearing a 2,5-
35 with respect to substitution of R1 ~R3, showing a broad dialkynylated thienyl group as a linker were em-
36 scope and efficiency of this method. The effects of ployed, all the four alkyne moieties were fully
37 protecting groups on the alcohol moiety (R1) were converted into the seven-ring fused framework leading
38 first examined. Propargyl esters with -Ac or -Bz to polycycles 4a or 4b in 56–62% yields (Scheme 2
39 groups were well suited in this reaction, leading to and Figure 1). These products may find utilities in
40 2b–2c in 88–92% yields. Interestingly, Me or TMS materials science.
41 protected propargyl alcohols 1d and 1e also under-
To demonstrate the synthetic utility of benzo[b]
42 went this cascade reaction smoothly to afford 2d and fluorenes 2, deprotection and further oxidation of 2a
43 2e in 83% and 78% yields, respectively, however, with was performed, and the desired benzo[b]fluoren-11-
44 a longer reaction time of 48 h. Next, the effects of R3 one 6a was formed in high yield. We also developed a
45 group on the bottom alkyne terminus were inves- one pot protocol for the direct transformation of 2a to
46 tigated. The reactions accommodated aryl and hetero- 6a using CAN as the oxidant, in which polycycle 6a
47 aryl groups at this position, furnishing 2f–2g and 2i in was obtained in 78% yield (Scheme 3).
48 high yields of 81–89%. Especially, a TMS group was
The reactivity of 1,6-diynyl tosylcarbamate 7[23] was
49 also compatible to give 2h in 86% yield, which might also examined (Scheme 4). In this case, cyclization
50 be further functionalized through halogenation or occurred efficiently at room temperature catalyzed by
51 metal-catalyzed coupling reactions.[20] Unfortunately, 40 mol% DBU to afford the desired benzo[b]fluorene
52 no desired benzo[b]fluorene was observed when R3 8 in 41% yield, along with 53% ketone 6a. 6a is likely
53 was an alkyl group. The effects of R2 substituent on formed via formation of an imine intermediate
54 the upper alkyne terminus were examined next, which derived from NÀS bond cleavage[24] under basic
55 provided a fused benzo unit during the reaction. It conditions followed by hydrolysis.
56 was found that when R2 was an aryl group, both
A possible reaction mechanism for this cascade
57 electron-withdrawing and electron-donating substitu- transformation is shown in Scheme 5. First, a prop-
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Table 2. Synthesis of Benzo[b]fluorenes and its Analogues.
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[a] Isolated yields.
[b] 20 mol% of DBU was used.
53 argyl-allenyl isomerization of 1a via a fast 1,3-H shift stepwise (involving a diradical intermediate) or con-
54 occurs catalyzed by DBU to afford enyne-allene 10. certed reaction pathway[25] to give the intermediate 11.
55 This process may proceed via deprotonation and The subsequent aromatization via deprotonation/pro-
56 formation of a contact ion pair followed by reprotona- tonation furnishes the benzo[b]fluorene 2a. To under-
57 tion.[19] 10 undergoes Schmittel cyclization through stand the mechanism, DBU-catalyzed cyclization of
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Figure 1. X-ray crystal structure of 4b.
Scheme 3. Synthesis of benzo[b]fluoren-11-one 6a.
Scheme 4. Synthesis of benzo[b]fluorene from 1,6-diynyl tosylcarbamate.
48 1a in the presence of a radical scavenger such as constructed during the process. The reaction likely
49 TEMPO (2.0 equiv) was performed. The results in- proceeds via base-induced propargyl-allenyl isomer-
50 dicated that the reaction was not suppressed.[26] ization followed by Schmittel-type cyclization. Hetero-
51 However, the diradical pathway could not be excluded cycle-fused fluorenes such as thiophene- or pyridine-
52 based on this result due to the short lifetime of the fused one could also be conveniently constructed by
53 diradical intermediate.[12g]
this method. The synthetic utility of this reaction was
In summary, we have developed a highly efficient also demonstrated by the preparation of up to seven-
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55 protocol for the synthesis of benzo[b]fluorenes via ring fused polycyclic aromatic hydrocarbons. Further
56 base-catalyzed cyclization of 1,6-diynyl carbonates, studies to extend the scope of these base-catalyzed
57 esters or ethers. Three new cyclic frameworks were
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Scheme 5. Proposed reaction mechanism.
29 cascade reactions towards more complex polycycles 2a, 2o, 2t and 4b are given in the Supporting
30 are in progress in our laboratory.
Information file.
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Experimental Section
Acknowledgements
Typical procedure for the synthesis of methyl (5-phenyl-11H-
benzo[b]fluoren-11-yl) carbonate (2a). To a solution of 1a
(109.9 mg, 0.3 mmol) in toluene (3 mL) was added DBU
(4.5 mL, 0.03 mmol) under argon. After the reaction mixture
was stirred at 808C for 13 h, the solvent was evaporated
under the reduced pressure and the residue was purified by
column chromatography on silica gel (petroleum ether: ethyl
acetate=20:1) to afford 2a (100.1 mg, 91%) as a yellow
solid. M.p. 140–1428C. 1H NMR (400 MHz, CDCl3) d 8.12 (s,
1H), 7.87 (d, J=8.0 Hz, 1H), 7.62–7.54 (m, 4H), 7.49–7.33
(m, 5H), 7.22-7.18 (m, 1H), 7.05 (t, J=8.0 Hz, 1H), 6.82 (s,
1H), 6.37 (d, J=7.6 Hz, 1H), 3.91 (s, 3H); 13C NMR
(100 MHz, CDCl3) d 156.68, 141.76, 140.87, 138.83, 138.10,
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We thank the National Key Research and Development
Program (2016YFA0202900) and the National Natural Sci-
ence Foundation of China (Grant No. 21421091) for financial
support.
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Base-Catalyzed Cyclization of 1,6-Diynyl Carboxylates
Involving Propargyl-Allenyl Isomerization: Efficient
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