A copper-catalyzed insertion of sulfur dioxide via radical coupling

Abstract

A copper-catalyzed three-component reaction of O-acyl oximes, DABCO·(SO₂)₂, and 2H-azirines under mild conditions has been achieved. This protocol provides an efficient route for the construction of various tetrasubstituted β-sulfonyl N-unprotected enamines in moderate to good yields with excellent stereoselectivity and regioselectivity. Notably, this method represents a rare example of 2H-azirines as useful synthons for β-functionalized N-unprotected enamines. Preliminary mechanistic studies indicate that the reaction proceeds through coupling of a sulfonyl radical and α-carbon radical via copper-catalyzed ring-opening C–C bond cleavage of O-acyl oxime and C–N bond cleavage of 2H-azirine with the insertion of sulfur dioxide.

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Tetrasubstituted alkenes are found in many pharmaceuticals, natural products and material science, and can also serve as versatile building blocks in organic synthesis.¹ Owing to their inherent steric-hindrance and difficulties in controlling stereoselectivity, the preparation of tetrasubstituted alkenes remains a great challenge. On the other hand, N-unprotected enamines as ubiquitous structural elements are important to the organic synthetic community.² However, only a few examples toward the efficient synthesis of tetrasubstituted N-unprotected enamines have been reported so far, presumably due to the sensitivity for hydrolysis. General strategies for their construction involve the introduction of electron-withdrawing groups including sulfonyl and carbonyl at the β-position to confer stability.^3,4 For example, Lautens and co-workers reported the rhodium(I)-catalyzed addition of arylboronic acids to α-phenyl-substituted (arylsulfonyl)acetonitrile for the synthesis of tetrasubstituted β-sulfonyl N-unprotected enamines.^4e Jiang and co-workers described a route to sulfone derivatives through a copper-catalyzed oxidative coupling of oxime acetates with sodium sulfinates.^4b The metal-free intermolecular aminoarylation of internal alkynes to access tetrasubstituted enamine derivatives was developed by the group of Greaney.^3b Despite the progress in this area, practical and efficient approaches for the preparation of tetrasubstituted β-functionalized N-unprotected enamines are still in high demand.

2H-Azirines are highly valuable synthetic intermediates for the formation of heterocycles due to their high ring strain and easy ring opening.⁵ The high chemical reactivity of 2H-azirines leads to various transformations, such as nucleophilic addition to the C=N double bond,⁶ cycloaddition⁷ and ring-opening reaction.⁸ Recently, the radical ring-expansion of 2H-azirines via a single electron transfer (SET) process has attracted extensive attention.⁹ For instance, Guan and co-workers reported the synthesis of substituted pyrroles through an iron-catalyzed ring-opening radical cycloaddition of 2H-azirines with enamides via homolytic C–N bond cleavage.^9c However, to the best of our knowledge, the use of 2H-azirines as the N-unprotected enamines synthon to furnish the corresponding tetrasubstituted alkenes has not been achieved yet.

In recent years, driven by the importance of sulfonyl-containing compounds in pharmaceuticals and agrochemicals, we have focused on the methods development with the insertion of sulfur dioxide, using DABCO·(SO₂)₂ and inorganic sulfites as the sulfur dioxide surrogates.¹⁰ For instance, we demonstrated that 4-substituted Hantzsch esters could be used as radical reservoirs in the reaction between sulfur dioxide and vinyl azides under photoredox catalysis.^4d As a result, only trisubstituted β-sulfonyl N-unprotected enamines were obtained through the addition of alkylsulfonyl radicals to structurally limited vinyl azides. We also reported that the alkyl radicals generated from O-acyl oximes^11,12 through a SET reduction could react with sulfur dioxide leading to sulfonyl radicals for multicomponent sulfonylation process.^3d Encouraged by these results, we envisioned that 2H-azirines might undergo C–N bond cleavage to generate α-carbon radical in the presence of a copper catalyst, which would go through selective coupling with sulfonyl radical, thus providing various tetrasubstituted β-sulfonyl enamines (Scheme 1). Herein, we describe the copper-catalyzed radical coupling of O-acyl oximes and 2H-azirines with the insertion of sulfur dioxide, which represents the first example for the conversion of 2H-azirines into N-unprotected enamines.

Scheme 1 Strategy for the synthesis of N-unprotected β-sulfonyl enamines with the insertion of sulfur dioxide.

Initially, cyclobutanone O-(4-(trifluoromethyl)benzoyl) oxime 1a, DABCO·(SO₂)₂, and 2,3-diphenyl-2H-azirine 2a were chosen as the model substrates for condition optimization (Table S1, in ESI†). To our delight, the reaction occurred smoothly in acetonitrile in the presence of Cu(OAc)₂ and 1,10-phenanthroline at room temperature, giving rise to the desired product 3aa in 47% yield (Table S1, ESI†). Screening of other solvents including toluene, 1,4-dioxane and dichloromethane showed that acetonitrile was the best choice. The use of CuOAc in the presence of 2,3-diphenyl-2H-azirine 2a (1.5 equiv.) gave rise to a good result (71% yield). Control experiments revealed that no reaction occurred in the absence of copper catalyst, and the yield of 3aa was dramatically decreased to 26% without the use of 1,10-phen (see ESI†). Having established the optimized conditions, we started to explore the scope generality for the reaction of O-(4-(trifluoromethyl)benzoyl) oxime 1a, DABCO·(SO₂)₂, and 2H-azirines 2. As shown in Table 1, various substituents on the aromatic ring of 2H-azirines 2 did not affect the reaction efficiency, affording the desired products 3 in moderate to good yields. Both electron-donating and electron-withdrawing substituted 2H-azirines 2 were suitable in this transformation. Notably, 2H-azirines bearing thiophenyl and naphthyl substituents proceeded smoothly as well to produce the corresponding products 3af and 3aj in 54% and 32% yields, respectively. The structure of compound 3aj was confirmed by the X-ray diffraction analysis,¹³ which showed excellent stereoselectivity in the outcome.

Table 1 Scope exploration for the reaction of O-acyl oxime 1a, DABCO·(SO₂)₂, and 2H-azirines 2

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Subsequently, various O-acyl oximes 1 were evaluated in the reaction of DABCO·(SO₂)₂ and 2,3-diphenyl-2H-azirine 2a. The results are summarized in Table 2. As expected, a range of substituted O-acyl oximes 1 proceeded smoothly to provide the corresponding products 3 in morderate to good yields. For instance, functional groups including bromo, ester, and nitrogen-containing heterocycle were well compatible giving rise to the products 3da, 3fa and 3ka in 59–72% yields. Additionally, O-acyl oximes with allyl or benzyl functional groups at the 2-position could undergo selective C–C bond cleavage to give more stable secondary alkyl radical, thus leading to the products 3ia and 3ja in 55% and 70% yields, respectively. Moreover, the desired product 3ha was obtained in 29% yield when the oxygen-containing substrate 1h was employed in the reaction.

Table 2 Reaction of O-acyl oximes 1, DABCO·(SO₂)₂ and 2H-azirine 2a

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To gain more insight of the reaction mechanism, several control experiments were conducted (Scheme 2). Initially, 2.0 equiv. of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) was added as a radical scavenge to the reaction of O-acyl oxime 1n, DABCO·(SO₂)₂ and 2,3-diphenyl-2H-azirine 2a under the standard conditions (Scheme 2, eqn (a)). The β-sulfonyl enamine 3ea was obtained in 28% yield and the radical trapping products 4 and 5 were isolated in 18% and 27% yields, respectively. Additionally, the formation of 3ea was completely inhibited when the amount of TEMPO was increased to 3.0 equiv., revealing that a radical coupling process might be involved in the process. Subsequently, a reaction of O-acyl oxime 1b, DABCO·(SO₂)₂ and TEMPO was carried out in acetonitrile at room temperature, and the TEMPO-trapped product 6 was obtained in 20% yield in the presence of CuOAc and ligand. However, only a trace amount of product 6 was obtained without the addition of CuOAc (Scheme 2, eqn (b)). Similarly, the reaction of 2H-azirine 2a, DABCO·(SO₂)₂ and TEMPO under copper catalysis gave rise to the TEMPO-trapped product 5 in 39% yield (Scheme 2, eqn (c)). These results revealed that copper catalyst might play a vital role in the radical ring opening of O-acyl oximes and 2H-azirines. Furthermore, β-keto sulfone 7 was isolated in 29% yield when 10 equiv. of trimethyl((1-phenylvinyl)oxy)silane was subjected to the model reaction, whereas another possible product 8 was not detected. This outcome showed that the alkyl radical generated from O-acyl oxime 1a would prefer to combine with sulfur dioxide to form sulfonyl radical (Scheme 2, eqn (d)). These experiments confirmed that alkyl radical, sulfonyl radical and α-carbon radical were involved in this transformation.

Scheme 2 Mechanistic investigation.

On the basis of above results and previous reports,^9c–11 a plausible radical coupling mechanism is depicted in Scheme 3. Single-electron reduction of O-acyl oxime 1 by Cu(I) led to the formation of iminyl radical intermediate A, which would convert to the alkyl radical Bvia C–C bond cleavage. Then, the resulting radical B would react with DABCO·(SO₂)₂ leading to sulfonyl radical C. Meanwhile, the Cu(I)-promoted reductive radical ring opening of 2H-azirine 2 with sequential protonation would afford α-carbon radical E and Cu(II). Subsequently, selective cross-coupling of radical species C and E would give rise to the intermediate F, which would further experience tautomerization to provide the final product 3. On the other hand, Cu(II) generated in this system could be reduced to Cu(I) by DABCO·(SO₂)₂ at room temperature to finish the catalytic cycle. The (Z)-selectivity of tetrasubstituted β-sulfonyl enamines should be attributed to the intramolecular hydrogen bonding effect. For the role of 1,10-phenanthroline, we reasoned that it would stabilize the copper catalyst during the reaction process via coordination with the metal. Additionally, it could accelerate the reaction from the control experimental result (Table S1, entry 14, ESI†).

Scheme 3 Proposed mechanism.

In conclusion, we have developed a copper-catalyzed three-component reaction of O-acyl oximes, DABCO·(SO₂)₂ and 2H-azirines for the stereoselective synthesis of tetrasubstituted β-sulfonyl enamines. Notably, this is an example of 2H-azirines as novel and useful synthons for β-functionalized N-unprotected enamines. Radical trapping experiments show that alkyl radical, sulfonyl radical and α-carbon radical are involved in the transformation. Preliminary mechanistic studies indicate that the reaction proceeds through coupling of sulfonyl radical and α-carbon radical via copper-catalyzed ring-opening C–C bond cleavage of O-acyl oxime and C–N bond cleavage of 2H-azirine with the insertion of sulfur dioxide.

Financial support from National Natural Science Foundation of China (No. 21871053 and 21532001) is gratefully acknowledged. We thank Prof. Pornchai Rojsitthisak for English review.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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13. CCDC 1965442 contains the supplementary crystallographic data for this paper†.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and spectral data, copies of ¹H and ¹³C NMR spectra. CCDC 1965442. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0cc00375a

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