Photoredox-catalyzed multicomponent 1,2-difunctionalization of activated alkenes with silyl enol ethers and oxime esters

Abstract

The photoredox-catalyzed multicomponent 1,2-cyanoalkylacylmethylation of electron-deficient alkenes using silyl enol ethers and cycloketone oxime esters has been described. This protocol provides access to alkyl cyanide group-anchored 1,4-dicarbonyl compounds with generally excellent yields. Furthermore, the 1,2-difunctionalization of acrylates can be achieved by adjusting the types of inorganic bases and replacing cycloketone oxime esters with indanone oxime esters in this catalytic system. Mechanistic studies demonstrated that the transformation proceeds via a radical pathway involving deconstructive carbon–carbon cleavage of oxime esters, radical addition, single-electron oxidation, and a desilylation cascade.

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Introduction

Modern synthetic chemistry strongly promotes the pursuit of developing strategies that are not only efficient and cost-effective but also environmentally sustainable, aimed at achieving molecular assembly.¹ In this context, multicomponent reactions (MCRs) have become a powerful and straightforward approach for constructing highly functionalized target molecules with excellent atom efficiency.² In contrast to the traditional sequential synthetic approach, MCRs enable the assembly of complex molecules by combining three or more starting materials in a single operation. Numerous elegant examples of employing these reactions in both ionic and radical processes have been documented in the literature.³ On the other hand, given the environmental friendliness, renewability, and ample availability of light sources, visible-light-promoted photoredox catalysis has attracted considerable interest from a wide range of chemical researchers.⁴ Famous organic chemists, such as MacMillan,⁵ Ritter,⁶ Rovis,⁷ and others,⁸ have made outstanding contributions and achieved extraordinary achievements in this field. Considering the great significance of MCRs and photoredox catalysis, there is a high demand for the exploration of mild and potent methodologies that combine their advantages.

In recent years, direct difunctionalization of alkenes has emerged as a potent strategy for synthesizing highly functionalized molecular frameworks in modern organic methodology research, enabling the simultaneous introduction of two distinct functional groups across the carbon–carbon double bond and significantly enhancing molecular complexity. Consequently, the pursuit of novel functionalization strategies remains a focal point of interest among synthetic organic chemists.⁹ Among these elegant transformations, alkene difunctionalization reactions involving radical processes have made significant progress. Generally, radical difunctionalization processes are initiated by the addition of radical species to the C=C bond of the alkenes, delivering a carbon-centered radical intermediate Int-I that combines with an external radical source to produce vicinal difunctionalized products (Scheme 1A, path a).¹⁰ Alternatively, the radical intermediate Int-I undergoes further oxidation to generate a carbocation, which is subsequently trapped by an extrinsic nucleophile, thereby accomplishing difunctionalization (Scheme 1A, path b).¹¹ In contrast, three-component radical-mediated difunctionalization reactions involving two different alkenes remain underexplored. In this transformation, the radical intermediate Int-I undergoes a second radical addition to the carbon–carbon double bond of another alkene analogue to form radical intermediate Int-II, which then further reacts to realize the difunctionalization of alkenes via an oxidation–deprotection pathway (Scheme 1B, path c). O-Silyl enol ethers are important synthons in organic chemistry, and they have been widely applied in C–C bond formation for the synthesis of diverse carbonyl compounds through metal-catalyzed or radical pathways.¹² For example, Liao and co-workers reported a multicomponent 1,2-alkylacylmethylation of electron-deficient olefins with NHPI esters and silyl enol ethers through light-activated electron donor–acceptor (EDA) complexes, which resulted in a series of structurally diverse 1,4-dicarbonyl compounds (Scheme 1C).¹³ To the best of our knowledge, 1,4-dicarbonyl compounds exist widely in many pharmaceutical molecules and serve as key building blocks for synthesizing heterocyclic compounds such as pyridazines, pyrroles, furans, thiophenes and so on.¹⁴ Given the significant application of 1,4-dicarbonyl compounds and encouraged by previous works, we wonder whether we can develop a class of more challenging radical 1,2-difunctionalization reactions involving two different alkenes by using oxime esters as radical precursors via photocatalytic multicomponent reactions and furnish a class of architecturally elongated 1,4-dicarbonyl compounds. As a continuation and deepening of our systematic research on radical chemistry¹⁵ and photoredox catalysis,¹⁶ we herein present a photoredox-catalyzed multicomponent 1,2-cyanoalkylacylmethylation of activated alkenes with cycloketone oxime esters and silyl enol ethers (Scheme 1D, path a). Furthermore, by adjusting the types of inorganic bases and utilizing indanone oxime esters as radical donors, the reaction proceeds smoothly, enabling a similar deconstructive process to synthesize polycarbonyl compounds (Scheme 1D, path b).

Scheme 1 Profiles for the radical difunctionalization of alkenes.

Results and discussion

The optimal reaction conditions were determined by means of successive screening, as shown in Table 1. At the outset, to evaluate the feasibility of this approach, silyl enol ether 1a, methyl acrylate 2a, and cycloketone oxime esters 3a were chosen as the model substrates to explore the reaction conditions. Gratifyingly, under 10 W blue LED irradiation, 52% yield of the anticipated product 1,4-dicarbonyl compound 4a was obtained after 12 hours of reaction utilizing Na₂CO₃ as the base, fac-Ir(ppy)₃ as the photocatalyst and anhydrous acetonitrile (CH₃CN) as the solvent (Table 1, entry 1). Compared to CH₃CN, EtOH proved to be a suitable alternative, but its yield is relatively lower (entry 2). Meanwhile, other solvents including tetrahydrofuran (THF), N,N-dimethylformamide (DMF) and 1,4-dioxane yielded product 4a in significantly lower amounts under the same reaction conditions (entries 3–5). We tested other photoredox catalysts. Among them, [Ir(dF(CF₃)ppy)₂(dtbbpy)]PF₆ (Ir-1), 4CzIPN, Eosin Y and Mes-Acr⁺ClO₄⁻ all gave relatively lower yields of product 5a (entries 6–9). Subsequently, a comprehensive evaluation of alternative inorganic bases in the CH₃CN solvent system demonstrated that K₃PO₄ represented the optimal selection (entries 10–12). In contrast, organic bases like Et₃N, pyridine, and DABCO significantly decreased the yield, while DIPEA and DBU, which were ineffective, resulted in no detection of product 4a (entries 13–17).

Table 1 Optimization of reaction conditions^a

Entry	PC	Solvent	Base	Yield^b (%)
a Reaction conditions: 1a (0.3 mmol, 1.5 equiv.), 2a (0.3 mmol, 1.5 equiv.), 3a (0.2 mmol, 1.0 equiv.), base (0.3 mmol, 1.5 equiv.), photocatalyst (2 mol%) and solvent (a2.0 mL), 10 W blue LED, at room temperature for 12 hours under a N2 atmosphere. b Isolated yield based on 3a. ND = not detected.
1	fac-Ir(ppy)3	CH3CN	Na2CO3	56
2	fac-Ir(ppy)3	EtOH	Na2CO3	53
3	fac-Ir(ppy)3	THF	Na2CO3	36
4	fac-Ir(ppy)3	1,4-Dioxane	Na2CO3	43
5	fac-Ir(ppy)3	DMF	Na2CO3	Trace
6	Ir-1	CH3CN	Na2CO3	38
7	4CzIPN	CH3CN	Na2CO3	33
8	EosinY	CH3CN	Na2CO3	41
9	Mes-Acr^+ClO4^−	CH3CN	Na2CO3	35
10	fac-Ir(ppy)3	CH3CN	K2CO3	71
11	fac-Ir(ppy)3	CH3CN	Cs2CO3	49
12	fac-Ir(ppy)3	CH3CN	K3PO4	74
13	fac-Ir(ppy)3	CH3CN	Et3N	41
14	fac-Ir(ppy)3	CH3CN	Py	49
15	fac-Ir(ppy)3	CH3CN	DABCO	34
16	fac-Ir(ppy)3	CH3CN	DIPEA	ND
17	fac-Ir(ppy)3	CH3CN	DBU	ND

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With the optimal conditions established, we proceeded to evaluate the generality of multicomponent 1,2-difunctionalization reactions by employing a wide range of silyl enol ethers, electron-deficient alkenes and cycloketone oxime esters (Scheme 2). The reactions of silyl enol ethers 1 bearing groups with electron-donating (–Me, –ⁿPent, –^tBu and –OMe) or electron-withdrawing (–Cl and –Br) properties at different positions on the phenyl ring with 2a and 3a all performed well under the optimal conditions and afforded the desired 1,4-dicarbonyl products 4b–4j in 39–61% yields. The reaction could also proceed successfully for naphthylcyclic silyl enol ethers, giving a 61% yield of product 4k. Then, we examined the scope of electron-deficient alkenes 2 for this radical 1,2-difunctionalization. Besides methyl acrylate (2a), similar α,β-unsaturated compounds such as benzyl acrylate (2b), 1-adamantylacrylate (2c), acrylic esters derived from L-menthol (2d), Tulipalin A (2e), and acrylonitrile (2f) all underwent this multicomponent reaction with 1a and 3a. The reaction successfully yielded the targeted products 4l–4p in 36–80% yield. We continued our assessment of the scope of cycloketone oxime esters 3 under the standard reaction conditions. Cycloketone oxime esters with an ethoxycarbonyl group (3b) and a benzyloxy group (3c) at the 3-position both reacted effectively, yielding the desired products 4q–4t in 36–55% yields. Moreover, 3,3-diphenyl-substituted analogue 3d was also a good substrate for the formation of 4u in 53% yield. In particular, the cycloketone oxime ester derived from 1-Boc-3-azetidinone (3e) reacted smoothly to generate product 4v in 56% yield. Unfortunately, phenyl vinyl sulfone (2g) and electron deficient aromatic alkenes (2h) were ineffective and did not yield the desired products.

Scheme 2 Substrate scope for accessing products 4.

After successfully achieving multicomponent 1,2-difunctionalization of α,β-unsaturated compounds using cycloketone oxime esters, we focused on evaluating the feasibility of difunctionalization by using indanone oxime esters 5 as C-radical precursors, which underwent selective cleavage of the C–C bond, enabling the assembly of functionalized diketones. When 1a was treated with 2a and 1-indanone-derived oxime ester 5a under the aforementioned conditions (as shown in Table 1, entry 12), the anticipated product 6a was successfully obtained with a yield of 45%. Furthermore, upon adjusting the inorganic base to potassium carbonate, the yield of product 6a increased to 64% (Scheme 3). In general, the silyl enol ethers with diverse functional groups at the para-position of the benzene ring and common acrylic derivatives did not significantly decrease the reaction efficiency, and the desired products 6a–6f could be isolated with yields ranging from 54% to 69%. Indanone-derived oxime esters 5 with both electron-donating groups (–Me and –OMe) and electron-withdrawing groups (–Cl and –Br) on the C5 or C6 position of the benzene ring showed good compatibility with our reaction conditions, and the corresponding products 6g–6k were obtained in 48–70% yields (Scheme 4a). Finally, through the utilization of acyl oxime ester 7 as the acyl radical precursor, the 1,2-acylalkylation of activated alkenes was successfully realized within an intermolecular, three-component cascade reaction (Scheme 4b).

Scheme 3 Synthesis of functionalized diketones 6a.

Scheme 4 Substrate scope for accessing products 6 and 8.

1,4-Dicarbonyl compounds that are produced through this transformation have the potential to be readily transformed into other significant derivatives. For example, the ester group in product 4a could be easily hydrolyzed to carboxylic acid 9 in 65% yield (Scheme 5a). In addition, several control experiments were performed to elucidate this multicomponent reaction mechanism. We carried out an investigation into the feasibility of a radical-involved pathway through the addition of an abundant quantity of the radical scavenger 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) under standard conditions. TEMPO completely inhibited the reaction, with no product 4a formed. Meanwhile, the cyanoalkyl radical adducts with TEMPO were captured by HR-MS, which shows that a radical-triggered difunctionalization process may play a crucial role in this reaction (Scheme 5b). Light on/off experiments clearly confirmed that light is essential for the progression of the photoredox-catalyzed multicomponent 1,2-difunctionalization protocol (Scheme 5c). Stern–Volmer quenching experiments between the photocatalyst fac-Ir(ppy)₃ and reactants revealed that oxime esters 3 and 5 are capable of quenching the excited state of fac-Ir(ppy)₃, with the quenching intensity showing a linear relationship with the concentration of the quencher (Scheme 5d, for details see the SI).

Scheme 5 Derivatization and control experiments.

Based on our experimental results and known literature reports,^13,17 we proposed a plausible reaction mechanism for multicomponent 1,2-difunctionalization in Scheme 6. Upon irradiation, the photocatalyst in its excited state (Ir^III*) initiates the reaction by engaging in an oxidative SET with cycloketone oxime esters 3a, which leads to the generation of iminyl radicals A and the concomitant release of the corresponding carboxylic anion (ArCO₂⁻). After that, iminyl radicals A undergo β-scission to generate cyanoalkyl radicals B, which are trapped by methyl acrylate 2a to form alkyl radical intermediate C. This intermediate is then further captured by silyl enol ether 1a to deliver the corresponding radical intermediate D. The oxidation of intermediate D by Ir^IV species to cationic intermediate E results in the regeneration of the photocatalyst (Ir^III). Finally, in the presence of a base, desilylation leads to the formation of product 4a.

Scheme 6 Proposed reaction mechanism.

Conclusions

In conclusion, we have established a new photoredox-catalyzed multicomponent 1,2-difunctionalization of activated alkenes with silyl enol ethers and oxime esters for the synthesis of a wide range of functionalized diketones with acceptable yields. The method facilitates direct deconstructive C–C bond cleavage of cyclobutanone and indanone oxime esters, which serve as radical precursors for forging multiple chemical bonds under mild conditions via a sequential radical-induced 1,2-addition/single-electron oxidation/desilylation process. Further studies on radical-mediated 1,2-difunctionalization between two different alkenes are underway in our laboratory.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data underlying this study are available in the article and its supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5qo01314c.

Acknowledgements

We would like to acknowledge the Jiangsu Higher Education Institution of China's Natural Science Foundation (24KJB150001).

References

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