Rh(III)-Catalyzed Oxidative Spirocyclization of Isoquinolones with α

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

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Rh(III)-Catalyzed Oxidative Spirocyclization of Isoquinolones with α‑Diazo-1,3-indandiones Shenghai Guo,* Lincong Sun, Yangfan Liu, Nana Ma,* Xinying Zhang, and Xuesen Fan* Henan Key Laboratory of Organic Functional Molecule and Drug Innovation, Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, China

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S Supporting Information *

ABSTRACT: Rh(III)-catalyzed site-selective oxidative spirocyclization reactions of isoquinolones with α-diazo-1,3-indandiones leading to three kinds of spiro compounds by employing isoquinolones with different structural features or tuning the reaction parameters are presented. In addition, a plausible catalytic cycle is proposed on the basis of control experimental results and density functional theory calculations. Finally, the diverse transformations of the spiro products into other complex skeletons demonstrate the synthetic utility of this protocol.

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oxidative annulations (Scheme 1, eq 1).8 However, to the best of our knowledge, Rh(III)-catalyzed oxidative spirocyclizations

etal-catalyzed directing group-assisted C−H bond activation has attracted considerable attention in the past several decades and has been widely utilized for the transformation of simple organic molecules to complex products such as pharmaceuticals and natural products in an eco-friendly and cost-effective fashion.1 In this regard, directing groups usually play a vital role in the selective activation and functionalization of an inert C−H bond among many others. However, a pendant directing group is often undesirable in the final product and hardly modified or removed. One of the most commonly used strategies for addressing this problem is to install a functionalizable directing group, which not only can direct a specific C−H bond activation but also can undergo a late-stage transformation.2 For instance, the isoquinolone moiety has been employed as a good functionalizable directing group for the synthesis of various isoquinolone-containing polycyclic compounds.3 In particular, Ir- or Ru-catalyzed oxidative spirocyclization reaction of isoquinolones with 1,4benzoquinone could be used for the direct construction of hard-to-prepare spiroisoquinolone skeletons.4 Therefore, the development of novel and practical routes to a variety of spiro compounds with potential bioactivities via transition metalcatalyzed oxidative spirocyclization of isoquinolones with appropriate coupling partners is in high demand. Diazo compounds proved to be one of the most frequently used carbene precursors for performing a large number of transition metal-catalyzed carbene transformations.5 Recently, α-diazo carbonyl compounds have been widely used as C1 or C2 synthons for the construction of polycyclic molecules via a Rh(III)-catalyzed C−H activation/carbenoid insertion/intramolecular annulation.6,7 In particular, with cyclic α-diazo-1,3diketones as C2 synthons, more complex fused polycyclic compounds could also be prepared via Rh(III)-catalyzed © XXXX American Chemical Society

Scheme 1. Rh(III)-Catalyzed Oxidative Annulations of Cyclic α-Diazo-1,3-diketones

using cyclic α-diazo-1,3-diketones as C1 synthons to construct spiro skeletons have not been reported. As a continuation of our recent studies on the Rh(III)-catalyzed synthesis of polycyclic compounds with α-diazo carbonyl compounds as coupling partners,9 herein we report an unprecedented Rh(III)-catalyzed oxidative [4+1] spirocyclization of isoquinolones with α-diazo-1,3-indandione as a C1 synthon to afford three types of spiroisoquinolones by simply changing the substituents attached at position 4 of isoquinolones or the reaction parameters (Scheme 1, eq 2). Initially, the reaction parameters, including the catalyst, oxidant, solvent, and temperature, were screened by using 3,4diphenylisoquinolin-1(2H)-one 1a (0.2 mmol) and α-diazoReceived: April 10, 2019

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DOI: 10.1021/acs.orglett.9b01263 Org. Lett. XXXX, XXX, XXX−XXX

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Rh(III)-catalyzed reaction also proceeded to deliver 3k in 87% yield. Next, the effect of the different R2 substituents on this reaction was studied. The results revealed that either electron-donating or electron-withdrawing R2 groups attached at the para or meta position of the 3-phenyl ring of 1 were tolerated with the standard conditions to provide 3l−3n in 54−85% yields whereas o-chloro-substituted isoquinolone (1o) did not take part in this reaction. With 4-methyl (R3)substituted isoquinolone (1p), this reaction failed to give the expected product (3p), whereas the cycloalkyl-substituted form (1q) reacted with 2a well to yield 3q in 65% yield. Finally, the scope of 2 was explored with 1a as the reaction partner. It turned out that both methyl- and bromo-substituted diazo substrates 2b and 2c reacted with 1a well to afford 3r and 3s in 72% and 64% yields, respectively. Moreover, the reactions of naphtho-fused and dimethyl-substituted fivemembered diazo compounds 2d and 2e with 1a also proceeded smoothly to generate 3t and 3u in 78% and 40% yields, respectively. Subsequently, we also found that when sixmembered diazo compounds 2f and 2g were employed instead of five-membered diazo compounds 2a−2e, the oxidative spirocyclization could also proceed smoothly to give the expected products 3v and 3w, albeit in lower yields. Finally, a gram-scale reaction was run and afforded 3a in 78% yield. During our study of the scope of isoquinolones (1) for the construction of N-spiro skeletons (3), we were surprised to find that when 4-unsubstituted isoquinolone was treated with 2a, the expected N-spirocyclization leading to 3x was not observed. However, an unexpected C-spirocyclization reaction proceeded to give another new spiro product (4a), spiro[indene-2,11′-indeno[1,2-c]isoquinoline]-1,3,5′(6′H)-trione, in 94% yield (Scheme 3). Thus, we undertook an extensive

1,3-indandione 2a (0.3 mmol) as model substrates (Table S1). We observed that when a mixture of 1a and 2a in 1,4-dioxane was treated with [Cp*RhCl2]2 and AgOAc at 140 °C, the oxidative spirocyclization proceeded smoothly to afford Nspiro product 3a, 12′-phenyl-5′H-spiro[indene-2,7′-isoindolo[2,1-b]isoquinoline]-1,3,5′-trione, with a highest yield of 78%. The structure of 3a was confirmed by X-ray crystal diffraction (see the Supporting Information). Next, we tested the scope and limitation of this Rh(III)-catalyzed oxidative spirocyclization reaction leading to N-spiro compounds (3). As shown in Scheme 2, the reactions of different isoquinolones 1 (0.4 Scheme 2. Synthesis of N-Spiro Compounds (3)a,b

Scheme 3. Rh(III)-Catalyzed Oxidative Spirocyclization of 4-Unsubstituted Isoquinolone

study of the generality of this oxidative spirocyclization leading to C-spiro compounds (4), and the results are listed in Scheme 4. First, the effect of R1 substituents of 4-unsubstituted isoquinolones (1) on this reaction was investigated, and it turned out that both electron-donating methyl and electronwithdrawing trifluoromethyl and chloro attached at position 6 or 7 of isoquinolones were well tolerated to provide 4b−4d in 73−92% yields. With thieno-fused pyridone, this reaction could also proceed to afford 4e in 59% yield. Second, 4unsubstituted isoquinolones (1) having a methyl, methoxy, fluoro, chloro, or bromo group on the para, meta, or ortho position of the 3-phenyl unit also reacted with 2a well to afford 4f−4l in yields ranging from 58% to 83%. It is noted that the structure of 4f was confirmed by X-ray crystal diffraction (see the Supporting Information). In addition, 3-(thiophen-2yl)isoquinolone and 3-(pyridin-4-yl)isoquinolone also took part in this C-spirocyclization to provide 4m and 4n in 35% and 40% yields, respectively. Third, the substrate scope of diazo compounds (2) was then examined. Treatment of methyl- or bromo-substituted diazo compound 2b or 2c with 4-unsubstituted isoquinolone could afford the corresponding

a

Reaction conditions: 1 (0.4 mmol), 2 (0.6 mmol), [Cp*RhCl2]2 (0.01 mmol), AgOAc (0.8 mmol), 1,4-dioxane (6 mL). bIsolated yields. c1a (1.188 g) was used. dTwenty hours. e1o (85%) was recovered. fThe reaction became messy.

mmol) with 2a (0.6 mmol) were first investigated. We found that 1b−1e bearing different R substituents (such as Me, MeO, Cl, and NO2) at position 6 were all compatible with the standard conditions, giving 3b−3e, respectively, in 57−87% yields. In addition, 1f−1h with Me, Cl, and F groups at different positions of the isoquinolone’s phenyl ring also reacted with 2a to afford the desired products 3f−3h, respectively. It is noteworthy that isoquinolones (1) with electron-donating R1 groups (1b, 1c, 1f, and 1g) usually exhibit a reactivity lower than the reactivity of those with electron-withdrawing R1 groups. When naphtho- and thienofused pyridones 1i and 1j were employed, the corresponding spiro products 3i and 3j were obtained in 57% and 85% yields, respectively. With methyl-substituted pyridone (1k), this B

DOI: 10.1021/acs.orglett.9b01263 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 4. Synthesis of C-Spiro Compounds (4)a,b

oxidant and THF as the solvent, the reaction of 1p with 2a exclusively afforded another kind of spiro product (5a) with vicinal all-carbon quaternary centers, albeit in a lower yield of 32%. The structure of 5a was confirmed by X-ray crystal diffraction (see the Supporting Information). Moreover, we also examined the reactions of 4-ethyl- and 4-propylsubstituted isoquinolones with 2a under two different reaction conditions. It was observed that with CsOAc as an additive, the spirocyclization reactions could give rise to N-spiro products 3y and 3z in 22% and 27% yields, respectively, whereas C-spiro compounds 5b and 5c were obtained in 24% and 25% yields, respectively, by using Cu(OAc)2 as an oxidant. To explore the mechanism, we carried out several experiments as shown in Scheme 6. First, when N-methyl-substituted Scheme 6. Mechanistic Studies

a

Reaction conditions: 1 (0.4 mmol), 2 (0.6 mmol), [Cp*RhCl2]2 (0.01 mmol), AgOAc (0.8 mmol), 1,4-dioxane (6 mL). bIsolated yields. cIsoquinolone (1.105 g) was used. dTwenty hours.

C-spiro compound 4o or 4p in an excellent yield. With naphtho-fused and dimethyl-substituted diazo compounds 2d and 2e, the expected C-spirocyclization reactions proceeded smoothly to give 4q and 4r in 90% and 30% yields, respectively. Furthermore, we also carried out the Rh(III)catalyzed C-spirocyclization reactions of six-membered diazo compounds 2f and 2g, and the expected 4s and 4t were obtained in modest yields. Finally, a gram-scale reaction was run and afforded 4a in 88% yield. As mentioned above, the N-spirocyclization reaction of 4methyl-substituted isoquinolone (1p) with 2a failed to afford the N-spiro product (3p) under the conditions described above. To realize this transformation, a further optimization of the reaction parameters was carried out (Table S2). After several attempts, we found that when CsOAc was used as an additive, the target product (3p) could be obtained in 40% yield (Scheme 5). More interestingly, with Cu(OAc)2 as the

isoquinolone 6 or 7 was treated with 2a, no reaction was observed, suggesting that the free NH unit plays a crucial role in aromatic C−H bond activation (Scheme 6a). Second, an intermolecular competitive reaction between 1l and 1n was performed. As a result, 3l and 3n were obtained in a ratio of 1:4.29, indicating that the aryl C−H bond cleavage might go through a concerted metalation deprotonation (CMD) mechanism (Scheme 6b). Third, an H/D exchange of 4unsubstituted isoquinolone was carried out in the presence of CD3OD. From this reaction, the reactant was recovered in 81% yield and 1H NMR analysis revealed that only H/D exchange at the ortho position (75% D) of the 3-phenyl unit was observed (Scheme 6c). Finally, two parallel reactions of 1a and 1a-d10 with 2a were run, from which 3a and 3a-d9, respectively, were obtained in a ratio of 0.78:0.22 based on 1H NMR analysis, and a kH/kD value of 3.55 was calculated, indicating that C(sp2)−H bond cleavage is likely involved in the turnover-limiting step (Scheme 6d). On the basis of the results presented above and previous reports,9,10 a plausible catalytic cycle is proposed in Scheme 7. Initially, a Rh(III)-catalyzed N−H/C−H bond cleavage of 1 gives rise to a five-membered rhodacycle I, which then reacts with 2a to yield a metal carbene intermediate II. Subsequent 1,1-migratory insertion of II affords the key six-membered intermediate III. With 4-substituted isoquinolone as the reactant, intermediate III undergoes a C−N reductive

Scheme 5. Oxidative Spirocyclization of 4-Alkyl-Substituted Isoquinolones (1) with 2a

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DOI: 10.1021/acs.orglett.9b01263 Org. Lett. XXXX, XXX, XXX−XXX

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The Baeyer−Villiger oxidation of 3a with m-CPBA was also examined. To our surprise, it did not afford the desired ringexpanded lactone product but gave a ring-opened product (10) via a CC bond oxidative cleavage in 50% yield, whose structure was confirmed by X-ray crystal diffraction (see the Supporting Information). Literature searching revealed that the newly formed spiro skeleton (10) not only can be considered as an analogue of naturally occurring alkaloids11 but also could be ultilized as an efficient organocatalyst for asymmetric synthesis.12 On the other hand, the reaction of 4a with P2S5 gave thioamide (11) in 82% yield. Treating 4a with POCl3 or PBr3 afforded the chloro- or bromo-substituted isoquinoline (12 or 13, respectively). More intriguingly, heating 13 in morpholine provided an unexpected product (14) via a nucleophilic substitution and despirocyclization sequence.13 In summary, we have achieved an efficient procedure for the regioselective synthesis of three different types of novel spiro compounds through a Rh(III)-catalyzed controllable oxidative spirocyclization reaction. In addition, the obtained spiro compounds can be easily converted into other complex products. Further development of more metal-catalyzed oxidative spirocyclization reactions is in progress.

Scheme 7. Proposed Reaction Mechanism

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elimination to provide N-spiro product 3 and a Rh(I) complex, which is then reoxidized to the active Rh(III) catalyst by AgOAc. On the other hand, with 4-unsubstituted isoquinolone, it is proposed that with the promotion of HOAc, intermediate III undergoes a selective N−Rh bond cleavage and Nprotonation to afford IV. Then, a rollover C4−H activation affords a new six-membered rhodacycle V.10 V undergoes a C− C reductive elimination to provide C-spiro product 4 and a Rh(I) species, which is then oxidized by AgOAc to regenerate the active Rh(III) catalyst. It is noted that intermediate III (R = H) prefers to provide C-spiro product 4 rather than N-spiro product 3 mainly because the activation free energy for the formation of 4 is lower than that for the formation of 3 by 5.4 kcal/mol (for DFT calculations, see Figure S9). As for the formation of 5, a rollover C4 nucleophilic substitution of intermediate IV′ (R = alkyl) affords a new rhodacycle V′, which then undergoes a C−C reductive elimination to provide product 5. To showcase the synthetic application of the obtained spiro products, the transformations of 3a and 4a were studied (Scheme 8). Treatment of 3a with NaBH4 (0.6 equiv) or LiAlH4 (1.1 equiv) afforded alcohol 8 or cis-9 in 66% or 52% yield, respectively, with a highly diastereoselective manner. The relative configuration of 8 was identified as (1R,2S) or (1S,2R) by X-ray crystal diffraction (see the Supporting Information).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b01263. Experimental procedure, Table S1, Table S2, mechanistic studies, X-ray crystal structures of 3a, 4f, 5a, 8, and 10, DFT calculations, characterization, and spectral data (PDF) Accession Codes

CCDC 1884503, 1884512−1884514, and 1916218 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 Authors

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

Scheme 8. Diverse Transformations of 3a and 4a

ORCID

Shenghai Guo: 0000-0002-2797-4281 Nana Ma: 0000-0003-3225-9554 Xinying Zhang: 0000-0002-3416-4623 Xuesen Fan: 0000-0002-2040-6919 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the NSFC (Grants 21202040, 21603062, and 21572047), the China Postdoctoral Science Foundation (2014M552007 and 2015T80771), the Plan for Scientific Innovation Talents of Henan Province (184200510012), and the 111 Project (D17007) for financial support. D

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