Visible-light-induced selective hydrolipocyclization and silylation of alkenes: access to ring-fused quinazolin-4(3H)-ones and their silicon-substituted derivatives

Abstract

A mild and novel visible-light-induced hydrolipocyclization or silylation of unactivated alkenes toward the synthesis of organosilanes and polycyclic quinazolinones was developed. Of note is that this is the first example using hydrosilanes as the silicon source and hydrogen source, respectively, under different light irradiation. These two transformations exhibit high atom economy, noble metal- and oxidant-free nature, high functional group tolerance, mild reaction conditions and direct synthesis of α-methyldeoxyvasicinones. Based on control experiments, two possible reaction mechanisms are proposed.

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Introduction

Organosilanes are of great interest in the fields of synthetic chemistry, medicinal chemistry, agrochemistry, and functional materials, given their unique chemical, physical, and biological properties.¹ Accordingly, the development of mild and efficient methods for the introduction of silicon groups into organic molecules has gained increased attention from synthetic chemists.² Recently, functionalization of alkenes with hydrosilanes, involving the interception of silyl radicals with alkenes, has emerged as a powerful tool to assemble silicon-containing molecules.³ Traditionally, the formation of silyl radicals relies on the combination of stoichiometric peroxide, such as DTBP, TBHP and DCP (Scheme 1a).⁴ However, the drawbacks of these reactions, including harsh reaction conditions, functional group incompatibility, and oxidized waste, have limited their further applications. Therefore, more efficient, green and sustainable synthetic methodologies are highly desirable. The utilization of energy from visible light in synthesis offers an appealing approach for the generation of silyl radicals.⁵ Particularly, the combination of a photocatalyst with a metal catalyst could realize new transformations that do not proceed with either catalyst alone.⁶ In the latter case, Wu and Lei have disclosed a series of photoredox-mediated hydrogen evolution cross-coupling reactions.⁷ In 2019, Xu's group developed a synergistic combination of photoredox, HAT (hydrogen-atom transfer) and cobalt catalysis to synthesize allylsilanes with H₂ evolution.⁸ Nevertheless, despite the tremendous achievements, it is still necessary and urgent to develop more diverse reaction patterns.

Scheme 1 Representative strategies for dehydrogenative silylation and hydrolipocyclization.

Polycyclic quinazolinone derivatives exhibit a broad spectrum of pharmacological and biological activities, including antibacterial, anti-inflammatory, antimitotic, and even potential anti-COVID-19 activities (Scheme 1c).⁹ Consequently, numerous synthetic methodologies have been developed for the construction of polycyclic quinazolinones.¹⁰ Using N-alkene-tethered quinazolinones as the substrates, various functionalized polycyclic quinazolinones were obtained via radical cyclization.¹¹ More recently, Deng and co-workers reported a visible-light-induced photocatalytic intramolecular hydrolipocyclization of unactivated alkenes using thiosulfonate compounds as mediators.¹² Pan's group developed a metal-free photoinduced hydrocyclization for the construction of ring-fused quinazoline-4(3H)-ones using THF as the hydrogen source (Scheme 1d).¹³ With our continuous interest in the development of green methods for the synthesis of silicon-containing compounds and polycyclic aromatic hydrocarbons (PAHs),¹⁴ herein, we report a novel visible-light-induced intermolecular hydrolipocyclization and silylation using hydrosilane as the hydrogen source and silicon source, respectively, which provided a selective and efficient pathway for the construction of organosilanes and polycyclic quinazolinones.

Results and discussion

We initiated the study by employing 3-(but-3-en-1-yl)quinazolin-4(3H)-one (1a) as the model substrate and 1,1,1,3,3,3-hexamethyl-2-(trimethylsilyl)trisilane (2a) as the Si–H partner under 12 W blue LED irradiation at room temperature (Table 1). First, lots of photoredox catalysts were screened, and the results showed that the organophotoredox catalyst 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN, 2.5 mol%) was the best choice for the construction of product 3a (Table 1, entries 1–5). Just as we expected, no product was detected in the absence of a photocatalyst (Table 1, entry 6). After that, different metal catalysts were screened, and Co(OAc)₂, Co(acac)₃ and NiBr₂ were considered to be less efficient for the reaction system than CoBr₂ (Table 1, entries 7–10). Notably, the yield of product 3a can be increased to 81% when using pyridine as the base to promote the reaction (Table 1, entry 11). Increasing the dosage of pyridine to 2 equivalents led to a lower yield of product 3a (Table 1, entry 12). Further screening of bases revealed that pyridine was superior to others such as KOH, K₂CO₃, and Cs₂CO₃ (Table 1, entries 13–15). Among the solvents examined, DMSO resulted in the best reactivity (Table 1, entries 11 and 16–19). The use of five equivalents of silane was necessary to achieve the desired product in good yield (Table 1, entries 11, 20 and 21). Finally, no silylation product was obtained in the absence of either a quinuclidinium catalyst or light, demonstrating that all these components were essential for the reaction (Table 1, entries 22 and 23).

Table 1 Optimization of the reaction conditions^a

Entry	Photocatalyst	Metal catalyst	Base (equiv.)	Solvent	Yield^b %
a Reaction conditions: 1a (0.1 mmol), 2a (0.5 mmol), metal catalyst (10 mol%), photocatalyst (2.5 mol%), quinuclidinium (40 mol%), base (0.1 mmol), and DMSO (2 mL) were stirred under irradiation (12 W 460–465 nm blue LEDs) at room temperature under open air for 12 h. b Isolated yields. c 3 equivalents of 2a instead of 5 equivalents. d 10 equivalents of 2a instead of 5 equivalents. e Without quinuclidinium. f Without light.
1	Ru(bpy)3Cl2·6H2O	—	—	DMSO	11
2	[Ir(dtbbpy)(ppy)2][PF6]	—	—	DMSO	29
3	Eosin Y	—	—	DMSO	0
4	fac-Ir(ppy)3	—	—	DMSO	0
5	4CzIPN	—	—	DMSO	46
6	None	—	—	DMSO	0
7	4CzIPN	CoBr2	—	DMSO	64
8	4CzIPN	Co(OAc)2	—	DMSO	49
9	4CzIPN	Co(acac)3	—	DMSO	20
10	4CzIPN	NiBr2	—	DMSO	Trace
11	4CzIPN	CoBr 2	Pyridine (1)	DMSO	81
12	4CzIPN	CoBr2	Pyridine (2)	DMSO	77
13	4CzIPN	CoBr2	KOH	DMSO	32
14	4CzIPN	CoBr2	K2CO3	DMSO	14
15	4CzIPN	CoBr2	Cs2CO3	DMSO	27
16	4CzIPN	CoBr2	Pyridine (1)	THF	8
17	4CzIPN	CoBr2	Pyridine (1)	DCE	13
18	4CzIPN	CoBr2	Pyridine (1)	CH3OH	0
19	4CzIPN	CoBr2	Pyridine (1)	Acetone	0
20^c	4CzIPN	CoBr2	Pyridine (1)	DMSO	47
21^d	4CzIPN	CoBr2	Pyridine (1)	DMSO	73
22^e	4CzIPN	CoBr2	Pyridine (1)	DMSO	0
23^f	4CzIPN	CoBr2	Pyridine (1)	DMSO	0

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Having the optimized conditions in hand, we then explored the reactivities of various N-alkylated quinazolin-4(3H)-ones, and the results are summarized in Scheme 2. By employing TTMSS as the silane reagent, substrates 1 with either electron-withdrawing or electron-donating groups on the benzene rings in quinazolinones were well tolerated, giving the corresponding silylation products in moderate to good yields (3b–3g). Substrates bearing strongly electron-withdrawing groups, such as CF₃ and COOMe, also gave the desired products 3h and 3i in 72% and 73% yields, respectively. The quinazolinone substrates with a bulky group also participated smoothly in this process to deliver product 3j in 71% yield. To confirm the practicability of this process, a gram-scale experiment was performed, and it afforded the corresponding product 3a in 60% yield.

Scheme 2 Substrate scope for N-alkylated quinazolin-4(3H)-ones. Reaction conditions: 1 (0.1 mmol), 2a (0.5 mmol), CoBr₂ (10 mol%), 4CzIPN (2.5 mol%), quinuclidine (40 mol%), pyridine (0.1 mmol), and DMSO (2 mL) were stirred under irradiation (12 W 460–465 nm blue LEDs) at room temperature under open air for 12 h. Isolated yields.

Next, we tested the silylation of N-alkylated quinazolinones with longer chains (Scheme 3). A variety of 3(pent-4-enyl)quinazolin-4(3H)-ones (1′) with electron-donating or electron-withdrawing groups were found to be competent substrates, giving the six-membered silicon-substituted piperidino-quinazolinones (4a–4i) in moderate yields. Unfortunately, 3-(hex-5-en-1-yl)quinazolin-4(3H)-one (1′j) and 3-(hept-6-en-1-yl)quinazolin-4(3H)-one (1′k) failed to produce the seven-membered and eight-membered ring-fused quinazolinones. When 1′j and 1′k were subjected to the reaction system, direct addition products were obtained (1′j-A and 1′k-A).

Scheme 3 Substrate scope of quinazolinones. Reaction conditions: 1′ (0.1 mmol), 2a (0.5 mmol), CoBr₂ (10 mol%), 4CzIPN (2.5 mol%), quinuclidine (40 mol%), pyridine (0.1 mmol), and DMSO (2 mL) were stirred under irradiation (12 W 460–465 nm blue LEDs) at room temperature under open air for 12 h. Isolated yields.

Finally, the procedure was extended successfully to other trialkylsilanes, such as triphenylsilane, tert-butyldimethylsilane, and triethylsilane, albeit with decreased yields (35–68%), which probably results from the higher Si–H BDE (bond dissociation energy) (Scheme 4).¹⁵

Scheme 4 Substrate scope for other trialkylsilanes. Reaction conditions: 1 or 1′ (0.1 mmol), 2 (0.5 mmol), CoBr₂ (10 mol%), 4CzIPN (2.5 mol%), quinuclidine (40 mol%), pyridine (0.1 mmol), and DMSO (2 mL) were stirred under irradiation (12 W 460–465 nm blue LEDs) at room temperature under open air for 12 h. Isolated yields.

To our surprise, a visible-light-induced intramolecular hydrolipocyclization of N-alkylated quinazolin-4(3H)-ones was developed when purple LEDs were used as the light source. As shown in Scheme 5, substrates bearing methyl or halogens on the benzene ring were compatible in this reaction system, affording the expected five-membered polycyclic quinazolinones 6a–6e in moderate yields. This discovery significantly expanded the application of this reaction. Based on control experiments, the initial hydrogen radical was generated from hydrosilanes. The majority of the hydrogen radicals were regenerated from deprotonation of N-alkene-tethered quinazolinones.

Scheme 5 Scope for the formation of α-methyldeoxyvasicinone derivatives. Reaction conditions: 1 (0.1 mmol), triphenylsilane (0.3 mmol), 4CzIPN (2.5 mol%), quinuclidine (40 mol%), and DMSO (2 mL) were stirred under irradiation (12 W 390–395 nm purple LEDs) at room temperature under open air for 20 h. Isolated yields.

To gain further insight into the reaction, we performed some control experiments (Scheme 6). The reaction was completely suppressed when TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxyl), BHT (butylated hydroxytoluene) or 1,1-diphenylethene was added. These results indicated that the reactions may undergo free radical mechanisms (Scheme 6a). Moreover, the light on–off experiment showed that the desired product 3a was not generated without light irradiation, which revealed that visible-light is essential for this transformation (Scheme 6b). In order to further illustrate the effect of light, we conducted a comparative experiment for the synthesis of disilane. The results disclosed that purple LEDs are favorable for the synthesis of disilane, which provided important evidence for the selective hydrolipocyclization and silylation process (Scheme 6c). On the other hand, a deuterium labeling study was performed to illustrate the source of the hydrogen (Scheme 6d). Very small amounts of deuterated 6a were detected, suggesting that only a small portion of hydrogen radicals were generated directly from hydrosilane. Last but not least, hydrogen was detected using a gas detector in these two processes, which provided important clues for the reaction mechanisms (Scheme 6e).

Scheme 6 Control experiments.

Based on the abovementioned investigations,¹⁶ a plausible mechanism for this visible-light-promoted dehydrogenative silylation is proposed in Scheme 7a. Initially, the photocatalyst 4CzIPN (PC) is excited to 4CzIPN* (PC*) under visible-light irradiation. Then, single electron transfer between the excited PC* and CoBr₂ generates Co^I species and PC⁺. The generated oxidizing PC⁺ species oxidizes quinuclidine to form the quinuclidine radical cation along with the completion of the photocatalytic cycle. After that, the resulting quinuclidine radical cation serves as the HAT catalyst to produce the silyl radical by abstracting the hydrogen atom from the hydridic Si–H bond of TTMSS. Subsequent addition of the silyl radical to the double bond of alkene gives alkyl radical species A. Next, intramolecular radical addition takes place to form the nitrogen-centered radical intermediate B, which undergoes a 1,2-hydrogen shift to afford a more stable carbon radical C. In the meantime, Co^I species can accept the radical to afford intermediate D. Finally, product 3a is obtained by Co–C bond cleavage and β-hydride elimination, and the hydrocobalt species E can react with PyH⁺ to release H₂. On the other hand, a plausible mechanism for this hydrolipocyclization reaction is proposed in Scheme 7b. The single electron transfer between PC* and quinuclidine generates the reactive quinuclidine radical cation which abstracts the hydrogen atom from TTMSS, forming the quinuclidine cation along with the completion of the HAT cycle. According to our control experiments, we infer that purple light could promote the homocoupling of the silyl radical. After further deprotonation, the key hydrogen cation was released from the quinuclidine cation. The resulting hydrogen cation was reduced by PC˙⁻ to form the hydrogen radical along with the completion of the photocatalytic cycle. The hydrogen radical was then preferentially added to the electron-rich alkene to generate the radical intermediate F, which underwent intramolecular cyclization, 1,2-hydrogen shift and deprotonation to give the final hydrolipocyclization product 6a.

Scheme 7 Proposed mechanisms.

Conclusions

In summary, we have established a novel and efficient visible-light-induced selective intramolecular hydrolipocyclization and intermolecular silylation of unactivated alkenes with hydrosilanes. A series of organosilanes and polycyclic quinazolinones were delivered in moderate to good yields. For the intermolecular silylation process, a tricatalytic process was proposed to synthesize organosilanes with H₂ evolution. For the intramolecular hydrolipocyclization process, a deuterium-labeling study confirmed that the initial hydrogen radical was generated directly from hydrosilanes.

Author contributions

C. L. and F. J. conceived the study; S. W. performed the experiments and prepared the ESI;† Z. C. participated in writing and reviewing the document; F. X., Y. Z., B. W., S. W., Y. X., X. Z., and G. J. synthesized some of the substrates.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (No. 22361044, 21961037 and 22201241), the Tianshan Talents Program for Leading Talents in Science and Technology Innovation (No. 2022TSYCLJ0016), the Key Program of Natural Science Foundation of Xinjiang Uygur Autonomous Region (No. 2022D01D06), the Excellent Doctoral Innovation Project (No. XJU2022BS041) and the Tianchi Talents Introduction Program (No. 5105240151a).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qo01119h

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