Iron/zinc-catalyzed benzannulation reactions of 2-(2-oxo-alkyl)benzketones leading to naphthalene and isoquinoline derivatives

Changyuan Zhang, Lianfen Chen, Kai Chen, Huanfeng Jiang and Shifa Zhu*
Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, P. R. China. E-mail:

Received 24th December 2017 , Accepted 9th January 2018

First published on 9th January 2018

An efficient method for the synthesis of poly-substituted naphthalene derivatives via Fe(III)-catalyzed [4 + 2] cycloaddition of 2-(2-oxo-alkyl)benzketones with alkynes or enols (generated in situ from alkyl aldehydes or alkyl ketones) is described. A variety of poly-substituted naphthalene derivatives could be synthesized in up to 90% yield under mild reaction conditions. Poly-substituted 1,2-dihydronaphthalene derivatives could be obtained instead through Zn(II)-catalyzed cyclization of 2-(2-oxo-alkyl)benzketones with alkenes. Furthermore, the Fe(III)-catalyzed intramolecular version could lead to isoquinoline derivatives in high yields.

Aromatic hydrocarbons and heteroaromatic rings are one of the most important organic compounds in nature.1 They are widely used in coal chemical industry, pharmaceutical chemistry and materials science.2,3 For example, naphthalene derivatives have attracted much attention due to their applications in the design of chiral catalysts.4 Among different heteroaromatic rings, isoquinoline derivatives display a wide range of pharmacological activity.5 Therefore, the development of efficient methods to synthesize naphthalene and isoquinoline derivatives, especially polysubstituted ones, would be highly valuable in organic synthesis.

The transition metal-catalyzed benzannulation reaction of o-alkynyl(oxo)benzenes with unsaturated compounds (alkenes, alkynes or enols) or amine sources is one of the most powerful tools for the preparation of various naphthalene derivatives6–11 or isoquinoline derivatives.12 As Liu et al. have reported that the selective hydrolysis of alkyne of o-alkynyl(oxo)benzene could be expected under the catalysis of Lewis acid via a 6-endo-dig cyclization intermediate, 2-(2-oxoethyl)-benzaldehydes A could be regarded as a surrogate of o-alkynylbenzaldehydes to some extent.9e–f While the biggest advantage of 2-(2-oxo-alkyl)benzaldehydes (or 2-(2-oxo-alkyl)benzketones) relative to o-alkynyl(oxo)benzenes is that another substitute R4 can be introduced, then various tri-/tetra-substituted naphthalenes or even heteroaromatic rings can be constructed easily in one-pot synthesis (Scheme 1). Recently, we reported that the reaction of 2-(2-oxo-alkyl)benzaldehydes A with alkynes under the catalysis of FeCl3 could afford various naphthalene derivatives as well.13 Considering the limitation of our previous studies (only mono- and di-substituted naphthalene derivatives can be produced), we would like to report our recent achievement in this system that not only different polysubstituted naphthalenes (especially tri- and tetra-substituted ones) can be obtained easily from readily available materials 2-(2-oxo-alkyl)benzketones with unsaturated compounds (alkenes, alkynes or enols), but also polysubstituted isoquinoline derivatives can be afforded directly from an intramolecular version (Scheme 1).

image file: c7qo01152k-s1.tif
Scheme 1 Benzannulation reactions of 2-alkylbenzaldehydes or 2-(2-oxo-alkyl)benzketones.

In our previous work, mono- or di-substituted naphthalene derivatives were obtained via FeCl3-catalyzed benzannulation of 2-(2-oxoethyl)-benzaldehydes and alkynes.13 To extend the generality of this methodology, 2-(2-oxoethyl)-benzophenone (or simply named diketone) 1a and phenylacetylene 2a were used as model substrates. When diketone 1a reacted with phenylacetylene 2a (1.1 equiv.) in the presence of FeCl3 (5 mol%) in DCE at room temperature for 12 h (Table 1, entry 1), only a trace amount of the 1,3-diphenyl naphthalene product 3a was detected. Raising the temperature to 80 °C, the yield was increased to 56% (entry 2). The yield of 3a was further improved to 88% by increasing the amount of catalyst FeCl3 and substrate 2a (entry 4). ZnCl2 was also highly efficient for this transformation (entry 5). Control reactions showed that the metal salts were crucial for this transformation. No reaction occurred in the absence of the catalyst (entry 6). And the substrate remained unchanged in the presence of TfOH or TFA, which suggested that the reaction was not a proton-catalyzed process (entries 7 and 8).

Table 1 Optimization of the reaction conditionsa

image file: c7qo01152k-u1.tif

Entry Cat. 1a[thin space (1/6-em)]:[thin space (1/6-em)]2a T/°C Yieldb
a Unless otherwise noted, the reaction was performed with 1a (0.25 mmol) in DCE (2.5 mL), 12 h, under N2.b Determined by 1H NMR using CH3NO2 as an internal standard.c Isolated yield.
1 FeCl3 (5 mol%) 1[thin space (1/6-em)]:[thin space (1/6-em)]1.1 r.t. Trace
2 FeCl3 (5 mol%) 1[thin space (1/6-em)]:[thin space (1/6-em)]1.1 80 56%
3 FeCl3 (10 mol%) 1[thin space (1/6-em)]:[thin space (1/6-em)]1.1 80 80%c
4 FeCl3 (10 mol%) 1[thin space (1/6-em)]:[thin space (1/6-em)]2.0 80 88%c
5 ZnCl2 (10 mol%) 1[thin space (1/6-em)]:[thin space (1/6-em)]2.0 80 87%c
6 None 1[thin space (1/6-em)]:[thin space (1/6-em)]2.0 80 NR
7 TfOH (20 mol%) 1[thin space (1/6-em)]:[thin space (1/6-em)]2.0 80–100 Trace
8 TFA (20 mol%) 1[thin space (1/6-em)]:[thin space (1/6-em)]2.0 80–100 Trace

With the optimized conditions in hand (entry 4 or 5, Table 1), the substrate scope was then explored. As shown in Scheme 2, a wide range of diketones 1 and alkynes 2 were tolerated for this catalytic process. For example, under the FeCl3-catalyzed conditions, various terminal alkyne derivatives reacted with 1a efficiently, giving the desired 1,3-disubstituted naphthalenes (3a–c and 3e–j) in 30–90% yields. When 4-methoxylphenyl acetylene 2d was utilized as the substrate, however, the FeCl3-catalyzed reaction was very messy and the desired naphthalene product 3d was not detected, which might be due to the high sensitivity of electron-rich 4-methoxylphenyl acetylene to oxidative FeCl3. To our delight, the desired product 3d could be furnished in 83% yield when using non-oxidative ZnCl2 as the catalyst instead. The yield dropped dramatically to 30% when an extremely electron-deficient alkyne, 3,5-bis(trifluoromethyl) phenylacetylene was used as a substrate (3h). Terminal alkyl alkynes reacted with 1a as well, yielding the products 3i and 3j in yields of 85% and 56%, respectively. When internal alkyne, 1-phenylpropyne, reacted with 1a under the catalysis of FeCl3, 1,2,3-trisubstituted naphthalene 3k was obtained in 72% yield. In addition to the scope of alkyne, different diketone analogues 1 were also tested (3l–p, 40–74%). When R1 was alkyl groups in diketones 1, the yields of products 3l and 3m were reduced to 53% and 40%, respectively. The inferior results may be attributed to the aldol-type self-condensation of the starting material diketone 1.14 When diketone 1 with R4 being the methyl group was used as a substrate, 1,3,4-trisubstituted naphthalene 3o was generated in 62% yield. It is noteworthy that 1,2,3,4-tetrasubstituted naphthalene can be prepared from the reaction of diketone 1 (R1 = Ph, R4 = methyl) with internal alkyne 1-phenylpropyne (3p, 70%). Interestingly, when 1,3,5-triethynylbenzene was used to react with 1a, a C3-symmetric polycyclic aromatic hydrocarbon (PAH) 3q could be obtained in 73% yield. Such PAHs were possible to be applied in electro-optic materials,4 thermochromics,15 and molecular spin batteries.16

image file: c7qo01152k-s2.tif
Scheme 2 FeCl3-catalyzed benzannulation of 2-(alkanoyl phenyl) ethyl ketones with alkynes to generate polysubstituted naphthalenes. Reaction conditions: 10 mol% FeCl3, 80 °C, under N2; [1] = 0.25 M, isolated yield. a 10 mol% ZnCl2, 100 °C under N2; b 10 mol% FeCl3, 100 °C, under N2.

After the protocol for the synthesis of polysubstituted naphthalenes was set up, we explored the methodology towards the formation of polysubstituted dihydronaphalenes (Scheme 3). For this purpose, styrene derivatives were chosen as the substrates to replace the alkynes. Preliminary condition screening showed that FeCl3 was not a suitable catalyst for this reaction. It seems that FeCl3 might induce the polymerization or oxidation of the alkene substrates.17 To our delight, the non-oxidative catalyst ZnCl2 was effective, and the substrate scope is as shown in Scheme 3. A range of styrene derivatives could be applied as the substrates, affording the desired 1,2-dihydronaphthalene derivatives 4a–i in 63–94% yields. Internal aromatic alkene was also converted into the desired product (4j) successfully.

image file: c7qo01152k-s3.tif
Scheme 3 ZnCl2-catalyzed benzannulation of 2-(benzoylphenyl) phenylethanone with alkenes. Reaction conditions: 10 mol% ZnCl2, 100 °C, under N2, [1] = 0.25 M, isolated yield. a120 °C.

After the intermolecular reaction of 2-(2-oxo-alkyl) benzketones 1 with alkynes or alkenes, we were very interested in the possibility of an intramolecular version. As shown in Scheme 4, the substrate 1r, tethered with an allylic group, was subjected to the catalysis of FeCl3 (10 mol%), which was supposed to give the cyclopropane-fused dihydronaphthalene product 5. To our disappointment, the desired product 5 was not detected, but an isochromenol derivative 1r′ was isolated in 30% yield (Scheme 4, eqn (1)). It was found that 1r′ transferred easily and reversibly to the starting material 1r at room temperature in the presence of FeCl3. Furthermore, naphthalene 3r could also be obtained when isochromenol 1r′ reacted with phenylacetylene under the catalytic conditions. It indicated that the reaction probably proceeded through isochromenol 1r′, which could then be converted easily into benzo-isopyrylium intermediate I (Scheme 4, eqn (2)). To prove this hypothesis, N,N-dimethylaniline was used to trap the proposed benzo-isopyrylium intermediate I. As expected, the desired product isochromene 6 could be successfully isolated in 63% yield (Scheme 4, eqn (3)). Although the diketone with an allylic group was not a suitable substrate for this intramolecular reaction (Scheme 4, eqn (1)), those tethered with the nitrile group could serve as effective substrates (Scheme 5, eqn (1)), giving rise to the corresponding isoquinolines 7a–b in 88% and 96% yields. However, the corresponding isoquinolines were not obtained by the intermolecular reaction of 2-(2-oxo-alkyl) benzketone 1a with 1.2 equiv. of acetonitrile or cyanobenzene (Scheme 5, eqn (2)), even when these reagents were used as solvent (Scheme 5, eqn (3)).

image file: c7qo01152k-s4.tif
Scheme 4 Control experiments.

image file: c7qo01152k-s5.tif
Scheme 5 FeCl3-catalyzed synthesis of polysubstituted isoquinolines.

On the basis of these experimental observations and our previous work,13,18 the reaction mechanism was proposed as shown in Scheme 6. The reaction might start from the Lewis acid-catalyzed enolization of diketone 1 to form enol II, which then underwent intramolecular cyclization to give the hemiketal intermediate III. A dehydroxylation process was then followed to afford the benzoisopyrylium intermediate IV. In the presence of alkyne, a sequential intermolecular [4 + 2] reaction and a decarboxylation process occurred to produce the final naphthalene product 3.

image file: c7qo01152k-s6.tif
Scheme 6 Proposed reaction mechanism.

In order to gain more insights into the above reaction mechanism, we also investigated the benzannulation of 2-(2-oxo-alkyl)benzketones 1 with the enolates, which could be generated in situ from aldehydes or ketones.6c As shown in Scheme 7, several aldehydes and ketones could be used as effective substrates for this reaction as well. For examples, paraacetaldehyde, phenylacetaldehyde, and butyraldehyde were all successfully converted into the desired mono- or di-substitituted naphthalene derivatives 3s–v in 55–82% yields. It is noteworthy that acetophenone, which is cheap and easily available, can be used as an alternative to phenylacetylene, giving the naphthalene 3a in 62% yield. Besides, the addition of TFA was necessary, which might assist the enolization process of the aldehydes or ketones.19 These results further support the mechanism proposed in Scheme 6.

image file: c7qo01152k-s7.tif
Scheme 7 FeCl3-catalyzed benzannulation of 2-(alkanoyl phenyl) ethyl ketones with enols. Reaction conditions: [1a] = 0.25 M; isolated yield.

In summary, we investigated the iron/zinc-catalyzed benzannulation reactions of 2-(2-oxo-alkyl)benzketones 1 with alkynes, establishing an efficient method for the synthesis of polysubstituted naphthalene derivatives. In addition to alkynes, alkenes, alkyl aldehydes or ketone can also be used as substrates. Mono-/di-/tri-/tetra-substituted naphthalenes and dihydronaphthalenes could be achieved in high selectivity and efficiency. Furthermore, isoquinoline derivatives could also be obtained in high yields by the Fe(III)-catalyzed intramolecular reaction of diketones 1 tethered with the nitrile group. In these reaction courses, the hemiketal III and the benzoisopyrylium IV were proposed as the key intermediates. This system holds the advantages of wide substrate scope and mild reaction conditions. The obvious merits of catalysts FeCl3 and ZnCl2 (cheap and abundant) employed in this system make it highly appealing for the construction of different types of naphthalene or isoquinoline derivatives.

Conflicts of interest

There are no conflicts to declare.


We are grateful to Ministry of Science and Technology of the People's Republic of China (2016YFA0602900), the NSFC (21372086, 21422204, and 21672071), Guangdong NSF (2016A030310433), the Science and Technology Program of Guangzhou (201707010316), and the Fundamental Research Funds for the Central Universities, SCUT.

Notes and references

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Electronic supplementary information (ESI) available: Spectroscopy details have been deposited. See DOI: 10.1039/c7qo01152k

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