Cascade cyclization reactions of alkylidenecyclopropanes for the construction of polycyclic lactams and lactones by visible light photoredox catalysis

Abstract

A visible light photocatalytic cascade cyclization reaction of alkylidenecyclopropanes for the rapid construction of seven- and eight-membered ring-containing polycyclic lactams and lactones has been developed. The process is proposed to proceed through a radical pathway, and the suggested radical intermediate was captured by TEMPO successfully. An intermolecular version of the reaction was also achieved, affording a variety of methyl dialinoacetate products.

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The past ten years have witnessed a flourish in visible light photocatalysis,¹ and the photocatalytic variants of many important reactions have been achieved, such as cross-coupling reactions, cycloadditions, fluorinations, etc.² Using light as the driving force in the reaction, the photocatalytic reaction can often proceed under mild reaction conditions. Meanwhile, photocatalysts can serve as both electron donors and acceptors in catalytic reaction cycles^1b,3 and many stoichiometrically used oxidants and reductants in traditional reactions can be replaced by air, oxygen and amines, leading to a more economical and sustainable chemical synthesis.^1a,4

Alkylidenecyclopropanes are important building blocks due to their high activities and diverse reactivities, and they have been widely used in organic synthesis.⁵ To further explore their potential usefulness in organic chemistry, persistent efforts have been made by our group and many other groups, and significant progress and interesting results have been achieved, including some light driven reactions about a decade ago.⁶ Considering the immense potential of visible light photocatalysis in organic synthesis, we attempt to combine it with alkylidenecyclopropane chemistry, in order to achieve new and efficient transformations of alkylidenecyclopropanes into useful structural motifs.

Inspired by the previous work,⁷ we hypothesized that substrate 1a having the alkylidenecyclopropane moiety under visible light photocatalysis could produce a radical intermediate by the cleavage of the C–Br bond at the α-position of the carbonyl group, and a cascade cyclization reaction would probably take place to afford polycyclic compounds. Thus, 1a was prepared, and we subsequently examined its reactivity under visible light photocatalysis (Scheme 1). To our delight, when 1a (0.2 mmol) together with Ir(ppy)₃ (3 mol%) and K₂CO₃ (3.0 equiv.) was exposed to blue LED light for 12 h, polycyclic product 2a was obtained successfully in a yield of 55% (Scheme 1 and Table 1, entry 6). The structure of 2a was confirmed by X-ray crystal diffraction. Its ORTEP drawing is shown in Fig. 1 and the CIF data are presented in the ESI.†

Scheme 1 Our design and first attempt of the photo-catalyzed reaction.

Fig. 1 The X-ray crystal structure of 2a.

Table 1 Optimization of reaction conditions

Entry^a	Catalyst	Base	Solvent	Yield^b/%
a Reaction conditions: 1a (0.2 mmol), photocatalyst (3.0 mol%) and base (3.0 equiv.) were placed in a reaction tube and Ar was charged. Then 2.0 mL solvent was added and the mixture was stirred exposing to blue LED light (8 W) at room temperature for 12 h. NR = no reaction. b Isolated yield. All the starting materials had been consumed and instead of forming 2a, the rest of them might have decomposed into unknown complexes. c White light (40 W) and green LED light (8 W) were also employed and no desired product 2a was obtained. d 1.5 mL DCE. e 2.5 mL DCE.
1^c	Eosin Y	K2CO3	DCE	NR
2^c	Fluorescein	K2CO3	DCE	NR
3^c	Ru(bpy)3·Cl26H2O	K2CO3	DCE	NR
4	Ir(ppy)2(dtbpy)PF6	K2CO3	DCE	10
5	Ir[dF(CF3)ppy]2(dtbpy)PF6	K2CO3	DCE	5
6	Ir(ppy) 3	K 2 CO 3	DCE	55
7	Ir(ppy)3	Cs2CO3	DCE	52
8	Ir(ppy)3	Pyridine	DCE	10
9	Ir(ppy)3	Et3N	DCE	15
10	Ir(ppy)3	K2CO3	Toluene	47
11	Ir(ppy)3	K2CO3	CH3CN	33
12	Ir(ppy)3	K2CO3	DCM	50
13^d	Ir(ppy)3	K2CO3	DCE	54
14^e	Ir(ppy)3	K2CO3	DCE	53

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Encouraged by the success of the first attempt of this reaction, we tried to optimize the reaction conditions (Table 1). Firstly, we screened several commonly used photocatalysts. Photocatalysts such as eosin Y, fluorescein and Ru(bpy)₃Cl₂·6H₂O could not catalyze this reaction, and the reaction still did not work when we employed white and green light sources instead of blue LED light (Table 1, entries 1–3). However, Ir-relevant photocatalysts smoothly catalyzed this reaction to afford the desired product 2a (Table 1, entries 4 and 5) and Ir(ppy)₃ used in our first attempt was still the best (Table 1, entry 6). This maybe implied that the redox potentials of Ir-relevant photocatalysts were the most suitable for this reaction. Then, different bases were screened. The use of the inorganic base Cs₂CO₃ afforded 2a in a yield of 52% (Table 1, entry 7), which is similar to that obtained using K₂CO₃. However, employing organic bases including pyridine and NEt₃ decreased the yield of 2a to 10% and 15%, respectively (Table 1, entries 8 and 9), which implied that organic bases might hamper the redox cycle of the photocatalyst in the reaction. Different solvents were also screened, and the use of toluene, CH₃CN and DCM as solvents afforded product 2a in yields ranging from 30% to 50% (Table 1, entries 10–12). In comparison, DCE was the most appropriate choice. The adjustment of the volume of solvent did not improve the yield of 2a (Table 1, entries 13 and 14). The reaction conditions used in the first attempt were still the best reaction conditions.

With the optimal conditions in hand, we then examined the substrate scope (Scheme 2). Generally speaking, the products were obtained in moderate yields; although all the starting materials were consumed, some of them were decomposed to unknown complexes. Substrates 1b–1d bearing the F/Cl substituent afforded the desired products 2b–2d in a similar yield of about 50%; substrates 1e and 1f bearing the –Br substituent afforded the corresponding products 2e and 2f in relatively lower yields of 45% and 49%, respectively. Substrates 1g and 1h bearing a methoxyl group also afforded the corresponding products 2g and 2h in yields of 53% and 51%, respectively. It seemed that the electronic effect of the substituents on the substrates did not have remarkable influence on the yields of the products. Alkyl group substituted substrates 1i–1l underwent the reaction smoothly, affording products 2i–2l in yields ranging from 35% to 54%. The phenyl substituted substrate 1m afforded product 2m in a yield of 46%. Substrate 1n having the CF₃ substituent underwent the reaction smoothly, affording product 2n in a yield of 42%. A polycyclic lactone product 2o was also obtained in a yield of 48%.

Scheme 2 Substrate scope of 1. ^a Reaction conditions: 1 (0.2 mmol), Ir(ppy)₃ (3.0 mol%) and K₂CO₃ (3.0 equiv.) were placed in a reaction tube and Ar was charged. Then 2.0 mL solvent was added and the mixture was stirred upon exposure to blue LED light (8 W) at room temperature for 12 h. ^b Isolated yield.

After successfully obtaining a series of polycyclic lactams and lactones containing seven-membered rings, we further proceeded to synthesize polycyclic compounds containing eight-membered rings. With a slight adjustment of the conditions, we found that when DCE was reduced to 1.0 mL, the temperature was increased to 70 °C and reaction time was prolonged to 24 h (see the ESI†), and product 4a was also obtained smoothly, though in a relatively lower yield (Scheme 3).

Scheme 3 Attempt to construct polycyclic compound 4a using substrate 3a.

The substrate scope was also examined by employing a variety of alkylidenecyclopropanes 3 bearing –Cl/Me/ⁱPr/Ph groups. The desired polycyclic lactams and lactones 4b–4j containing eight-membered rings were obtained in yields ranging from 10% to 30% (Scheme 4). In comparison, the yield of 4 was much lower than that of 2 presumably due to the large size of the eight-membered ring. The reactions of substrates 3d and 3i took place, affording products 4d and 4i as mixtures of regioisomers in 2.3 : 1 and 2 : 1 ratios, respectively.

Scheme 4 Substrate scope of 3. ^a Reaction conditions: 3 (0.2 mmol), Ir(ppy)₃ (3.0 mol%) and K₂CO₃ (3.0 equiv.) were placed in a reaction tube and Ar was charged. Then 2.0 mL solvent was added and the mixture was stirred upon exposure to blue LED light (8 W) at 70 °C for 12 h. ^b Isolated yield.

To gain some insights into the mechanism, TEMPO was used to capture radical intermediates involved in the reaction. Fortunately, compound 1a-TMP was successfully obtained in a yield of 60% (Scheme 5). This provided evidence for the proposed radical pathway.

Scheme 5 Capture of the radical intermediate with TEMPO.

Based on the previous reports^7b,c,8 and the above experimental results, we proposed a radical reaction mechanism for this photo-catalyzed reaction (Scheme 6). Firstly, substrate 1a produces an active radical intermediate I through a SET process, and then intermediate I undergoes cascade cyclization to afford intermediates II and III. Afterwards, through another SET process and deprotonation, the desired polycyclic product 2a can be generated. It should be noted here that K₂CO₃ as the auxiliary base is also critical for this reaction; without the base, the substrate will be quickly decomposed in the presence of HBr generated during the reaction process and the yield will be dramatically decreased.

Scheme 6 A plausible pathway for the reaction.

Furthermore, we developed an intermolecular version of the visible light photocatalyzed reaction on the basis of the above results. The reaction of 5a and methyl bromoacetate (MBA) occurred under the same conditions, and the desired product 6a was obtained in a yield of 33%, together with a radical chain product 7a in a yield of 24% (Scheme 7a). Based on the proposed mechanism (Scheme 6), we proposed that the key radical intermediate M probably underwent either cyclization (via path a) to give product 6 or a chain reaction with MBA (via path b) to generate product 7. We hypothesized that if the concentration of MBA was kept low enough, intermediate M would be favorably transformed into 6a through path a while path b would be greatly suppressed (Scheme 7b). Thus, MBA was injected dropwise into the reaction system through a syringe pump within 4 h and the yield of 6a successfully increased to 54% while the yield of 7a decreased to 10% (Scheme 7c; for details, see the ESI†).

Scheme 7 Intermolecular reaction of 5a with MBA.

We also investigated the substrate scope of 5 for this reaction (Scheme 8). Substrates 5b–5h bearing –F/Cl/Me/MeO/Ph/OBn groups underwent this reaction to afford products 6b–6h in yields ranging from 35% to 47% while the radical chain products 7b–7h were obtained in dramatically suppressed yields. As for substrate 5e, it transformed regiospecifically into product 6e. For substrate 5i, the corresponding product 6i was only obtained in a yield of 15% while the yield of 7i increased to 30%, presumably due to the electronic effect (see the ESI†).

Scheme 8 Substrate scope for the intermolecular reaction. ^a Reaction conditions: 5 (0.2 mmol), Ir(ppy)₃ (3.0 mol%) and K₂CO₃ (3.0 equiv.) were placed in a reaction tube. Ar was charged and 0.5 mL DCE was added. Then upon exposure to blue LED light (8 W), MBA (0.2 mmol) dissolved in 4.0 mL DCE was injected dropwise through a syringe pump within 4 h. ^b Isolated yields of product 6. ^c Yields of 7 determined by ¹H NMR spectroscopy with 1,3,5-trimethoxybenzene as an internal standard.

Products 6 can be used as precursors of many valuable compounds. For example, 6d can be dehydrogenated to form 6′d (available at a high price on the market) and then naproxen (non-steroidal anti-inflammatory drug) after further methylation and hydrolysis (Scheme 9).⁹

Scheme 9 Synthetic transformation of 6d.

In summary, a facile method for the synthesis of polycyclic lactams and lactones containing seven-membered or eight-membered rings by visible light photocatalysis has been developed and a plausible radical pathway is proposed. An intermolecular version of the reaction was also achieved which could afford various substituted methyl dialinoacetate products used as valuable precursors for further transformations.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

We are grateful for the financial support from the National Basic Research Program of China (973)-2015CB856603, the Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDB20000000 and sioczz201808, the National Natural Science Foundation of China (21372241, 21572052, 21421091, 21372250, 21121062, 21302203, 21772037, 21772226 and 21861132014), and the Shenzhen Nobel Prize Scientists Laboratory Project.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures and characterization data of new compounds. CCDC 1842217. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9qo01360a

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