1,2-Alkylarylation of activated alkenes with dual C–H bonds of arenes and alkyl halides toward polyhalo-substituted oxindoles

Abstract

We describe here a new visible light facilitated radical strategy for 1,2-alkylarylation of activated alkenes with a C(sp²)–H bond of arenes and a C(sp³)–H bond of alkyl halides. This method achieves selective scission of the C(sp³)–H bond adjacent to halide atoms leading to a halo-substituted alkyl radical, and provides a new synthetic utilization of aryl halides toward polyhalo-substituted oxindoles in good to excellent yields. Moreover, the concise transformation of the products, polyhalo-substituted oxindoles, into vinyl halides and alkynyl halides was also illustrated.

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Polyhalogenated hydrocarbons, a class of organic compounds with multiple substitutions of halogens, are of particular importance in organic synthesis and chemical industries (particular pharmaceuticals and materials), albeit arousing controversy for the effects of these compounds on the environment and on human and animal health.¹ For example, the polychloromethyl unit is a structural feature presented in numerous bioactive natural products and pharmaceutical drugs wherein it usually provides the key component for the potent activity role in these compounds (Fig. 1).^1–3 As a result, the introduction of polychloromethyl groups into known bioactive molecules remains an active area.^2,3 Generally, the synthesis of polyhalogenated hydrocarbons from organohalides (often alkyl halides) is performed by thermo- or photo-initiated scission of the carbon–halogen bonds in organohalides leading to carbon-centered radicals, in which radical initiators, such as AIBN (azobis(isobutyronitrile)) and organotins (often Bu₃SnH), are usually used to accomplish these transformations.⁴ However, methods for carbon-centered radical formation from organohalides by selectively splitting the carbon–hydrogen bond, not the carbon–halogen bond, are quite rare.⁵ The reason is that in organohalides the reactivity of the carbon–halogen bond is far higher than that of the carbon–hydrogen bond under thermo- or photo-initiation.

Fig. 1 Important compounds with polychloro groups.

Recently, a new radical strategy for the oxidative cyclization of N-arylmethacrylamides with alkyl C(sp³)–H bonds to access functionalized oxindoles has been developed (Schemes 1a and 1b).^6,7 We have first reported an Fe-catalyzed oxidative 1,2-alkylarylation of activated alkenes with an aryl C(sp²)–H bond and a C(sp³)–H bond adjacent to a heteroatom for building oxindoles using TBHP oxidant, which proceeds via a radical process (Scheme 1a).^6a Subsequently, the Guo/Duan group,^7a–d Liang group,^7e Liu group^7f and our group^6b have independently developed other new radical cyclizations of N-arylmethacrylamides with alkyl C(sp³)–H bonds, including the C(sp³)–H bonds adjacent to an aryl,^6b,7b,f a hydroxyl^7a,e or dicarbonyl groups^7c,d and the C(sp³)–H bonds in simple alkanes^7f (Schemes 1a and 1b). However, these methods required unfriendly radical initiators including peroxides and K₂S₂O₈ under harsh conditions (often at over 100 °C), thereby greatly restricting their applications in synthesis and industry.^5b,6–8 Thus, a new mild radical strategy avoiding the use of unfriendly radical initiators for the cyclization of N-arylmethacrylamides with alkyl C(sp³)–H bonds is highly desirable. Very recently, we described a visible light catalysis strategy for the cyclization of N-arylmethacrylamides with alkyl C(sp³)–H bonds adjacent to a carbonyl group, which proceeds under mild conditions.^6c Inspired by these results, we reasoned that the visible light catalysis strategy might be a better alternative for the cyclization of N-arylmethacrylamides with various alkyl C(sp³)–H bonds.^9,10

Scheme 1 1,2-Alkylarylation of activated alkenes.

We began our investigation with the reaction between N-methyl-N-phenylmethacrylamide (1a) and dichloromethane (DCM, 2a) in the presence of visible light catalysts and 4-methoxybenzenediazonium tetrafluoroborate (Table 1). In the presence of 4-MeOC₆H₄N₂BF₄ and Na₂CO₃, the desired oxindole 3aa was obtained in 55% yield from substrate 1a with DCM 2a (entry 1). It has been reported that the visible light catalysis strategy could promoted the 4-MeOC₆H₄N₂BF₄-based reaction.^5a,6c,10,11 As expected, treatment of substrate 1a with DCM 2a, Ru(bpy)₃Cl₂, 4-methoxybenzenediazonium tetrafluoroborate and 36 W compact fluorescent light increased the yield of oxindole 3aa from 55% to 96% yield (entry 2). The results showed that bases played an important role in the reaction, and Na₂CO₃ was the most effective (entries 2–5). While Na₂CO₃ as the base gave 96% yield of oxindole 3aa (entry 2), Et₃N as the base decreased the yield to only 31% (entry 3), Cs₂CO₃ as the base to 37% (entry 4) and the absence of base to trace (entry 5). Among the reaction temperatures examined, it turned out that 50 °C was preferred for the reaction (entries 2 and 6–7). Gratifyingly, good yield was still achieved even at 2 mol% Ru(bpy)₃Cl₂ (entry 8). It is noteworthy that the reaction proceeds in MeCO₂ⁿBu smoothly, albeit with slightly lowering the yield (entry 9). Two other visible light photoredox catalysts, Ir(ppy)₃ and Eosin Y, were found to effect the reaction (entries 1 vs. 10–11). These results also suggest that the role of both Ru and visible light is mainly to improve the reaction. Extensive screening revealed that the amount of 4-methoxyphenyldiazonium tetrafluoroborate has a fundamental influence on the reaction: the yield decreased from 96% (entry 2) to 59% at 1 equiv. diazonium salt (entry 12), and the absence of diazonium salt resulted in no detectable product 3aa (entry 13).

Table 1 Screening of optimal conditions^a

Entry	[M] [mol%]	Base	T [°C]	Isolated yield [%]
a Reaction conditions: 1a (0.3 mmol), 2a (15 mmol), [M], 4-MeOC6H4N2BF4 (2 equiv.) and base (2 equiv.) with 36 W compact fluorescent light for 20 h under argon atmosphere. b Without additional light. c 2a (6 mmol) and MeCO2^nBu (anhydrous, 0.5 mL). d 4-MeOC6H4N2BF4 (1 equiv.). e Without 4-MeOC6H4N2BF4.
1^b	—	Na2CO3	50	55
2	Ru(bpy)3Cl2 (5)	Na2CO3	50	96
3	Ru(bpy)3Cl2 (5)	Et3N	50	31
4	Ru(bpy)3Cl2 (5)	Cs2CO3	50	37
5	Ru(bpy)3Cl2 (5)	—	50	Trace
6	Ru(bpy)3Cl2 (5)	Na2CO3	25	8
7	Ru(bpy)3Cl2 (5)	Na2CO3	80	83
8	Ru(bpy)3Cl2 (2)	Na2CO3	50	80
9^c	Ru(bpy)3Cl2 (5)	Na2CO3	50	75
10	Ir(ppy)3 (5)	Na2CO3	50	95
11	Eosin Y (5)	Na2CO3	50	68
12^d	Ru(bpy)3Cl2 (5)	Na2CO3	50	59
13^e	Ru(bpy)3Cl2 (5)	Na2CO3	50	0

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Encouraged by these results, we next set out to examine the effect of diazonium salts (Scheme 2).¹¹ The results demonstrated that three other aryldiazonium salts showed high reactivity, but they were less effective than 4-methoxybenzenediazonium tetrafluoroborate (Scheme 2vs. entry 2 in Table 1).

Scheme 2 Screening of effect of diazonium salts.

With the optimal conditions in hand, we first employed N-arylacrylamides 1 to explore the scope of the above radical cyclization protocol in the presence of DCM 2a (Table 2). While analogous amides with N-Bn, N-(2-iodobenzyl) or N-Ph were found to be viable substrates for the reaction (products 3ba–3da), changing to N-Ac resulted in a lower reactivity (product 3ea). Gratifyingly, we found that a number of substituents, including alkyl, MeO, Cl, F, CF₃, I and SMe groups, on the aromatic ring of the N-aryl moiety were tolerated well (products 3fa–ra), and the reactive order is as follow: para- > meta- > ortho-substituents. For example, para-Me-substituted substrate 1f afforded oxindole 3fa in 88% yield under the optimized conditions. Good yields were still achieved from substrates 1g–1i with other electron-rich groups, such as Et, n-Bu and MeO groups (products 3ga–ia). The reaction was not constrained by an electron-withdrawing CF₃ group, giving product 3la in 80% yield. However, the yields of oxindoles 3ma–pa from the corresponding ortho-substituted substrates 1m–1p decreased to moderate. It was noted that meta-substituted substrates 1q or 1r gave a mixture of two regioselective oxindoles (products 3qa and 3ra). Most importantly, halo groups, I, Cl and F groups, could be tolerated, thereby facilitating additional modifications at the halogenated position (products 3ca, 3ja, 3ka, 3na, and 3oa).¹² Screening disclosed that substrates 1s–1u bearing substituents, Ph, CH₂OH or CH₂OAc, at the 2 position of the acrylamide moiety were compatible with the optimal conditions (products 3sa–ua). Free CH₂OH-substituted substrate 1t, for instance, was converted into product 3ta in 50% yield. However, substrate 1v with a hydrogen atom at the 2 position had no reactivity for the reaction (product 3va). Interestingly, N-methyl-N-phenylcinnamamide (1w), an internal alkene, could also be cyclized to oxindole 3wa in 53% yield.

Table 2 Screening scope of N-arylacrylamides (1)^a

a Reaction conditions: 1 (0.3 mmol), 2a (15 mmol), Ru(bpy)₃Cl₂ (5 mol%), 4-MeOC₆H₄N₂BF₄ (2 equiv.) and Na₂CO₃ (2 equiv.) with 36 W compact fluorescent light at 50 °C for 20 h under argon atmosphere. b A side de-I product 3aa was isolated in 36% yield.

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As shown in Table 3, a variety alkyl halides 2 was next investigated in the presence of N-methyl-N-phenylmethacrylamide (1a), Ru(bpy)₃Cl₂, 4-methoxybenzenediazonium tetrafluoroborate, Na₂CO₃ and 36 W compact fluorescent light. Using 1,1-dichloroethane (2b), the C(sp³)–H bond adjacent to two chloride atoms was selectively cleaved, giving product 3ab in 93% yield. The results indicate that halo groups are necessary for selective scission of the C(sp³)–H bond. Indeed, both 1,1,2-trichloroethane (2c) and 1,1,2,2-tetrachloroethane (2d) were suitable substrates; moreover, the C(sp³)–H bond adjacent to dichloride atoms, not the C(sp³)–H bond adjacent to monochloride atoms, was selected to react with substrate 1a (products 3ac and 3ad). However, no reaction was observed using 1-chlorobutane (2e), a monochloro-substituted substrate (product 3ae). Gratifyingly, using 1,2,3-trichloropropane (2f) and 1,2-dichloroethane (2g) to build the corresponding products 3af and 3ag was successful in moderate yields. For chloroform (2h), good yield of the desired oxindole 3ah was also obtained under the optimal conditions.

Table 3 Screening scope of alkyl halides (2)^a

a Reaction conditions: 1a (0.3 mmol), 2 (15 mmol), Ru(bpy)₃Cl₂ (5 mol%), 4-MeOC₆H₄N₂BF₄ (2 equiv.) and Na₂CO₃ (2 equiv.) with 36 W compact fluorescent light at 50 °C for 20 h under argon atmosphere.

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However, this reaction was dependent on the leaving nature of the halo group: both C–H bond and C–Br bond cleavage took place using alkyl bromides (products 3ai/3ai′ and 3ak/3ak′). For example, treatment of CH₂Br₂ (2i) with substrate 1a afforded a mixture of C–H bond cleavage and C–Br bond cleavage oxindoles 3ai and 3ai′ in 80% total yield with 1 : 1 ratio. For bromoform (2j), however, the ratio of C–H/C–Br cleavage is 2 : 1. Using bromodichloromethane (2k), however, the reactivity of the C–Br bond is higher than the C–H bond in view of the ratio of products 3ak and 3aa.

To understand the current radical reaction, the mechanisms outlined in Scheme 3 are proposed on the basis of the present results¹³ and the literature reports.^5–11 Initially, ˙CHCl₂ radical may be generated via two pathways: (1) single-electron oxidation of DCM 2a by Ru(bpy)₃³⁺˙ gives a carbocation A, followed by deprotonation leading to the ˙CHCl₂ radical (pathway I), and/or (2) hydrogen abstraction of DCM 2a by a phenyl radical which was generated from the diazonium salt forms the ˙CHCl₂ radical (pathway II).^6c,10,11 Subsequently, addition of ˙CHCl₂ radical to alkene produces radical intermediate B, followed by intramolecular cyclization of radical intermediate B with an arene which gives rise to radical intermediate C. Hydrogen atom abstraction of radical intermediate C by Ru(bpy)₃²⁺˙ takes place to yield product 3aa and Ru(bpy)₃³⁺. Notably, this photoinduced mechanism is supported by the quantum yield (Φ_x = 0.056).^13,14

Scheme 3 Possible mechanisms.

On the basis of the results in Table 1, the ˙CHCl₂ radical can also be directly formed from the reaction between CH₂Cl₂ (2a) and 4-methoxyphenyl radical in the presence of a base under heating conditions. Thus, there are two key roles for the base in the present reaction: initiation of the radical reaction and neutralization of the in situ formed BF₄⁻.

Vinyl halides or alkynyl halides are important intermediates in organic synthesis. Having established a hydrogen abstraction–cyclization tandem method for the preparation of polyhalo-substituted oxindoles 3, we finally explored their concise transformation into vinyl halides or alkynyl halides (Scheme 4).¹⁵ After a brief screening of bases, ^tBuOK was employed for the dehalogenation of dichloro-substituted oxindoles 3aa, 3ha, 3ia, 3ka and 3la, exclusively providing the corresponding (Z)-vinyl chlorides 4 in moderate yields (eqn (1)). Interestingly, dehalogenation of trichloro-substituted oxindole 3ah resulted in alkynyl chloride 4ah in good yield (eqn (2)). Using a mixture of bromo-substituted oxindoles 3aj and 3aj′, (Z)-vinyl bromide 4aj was formed alone in 61% yield (eqn (3)).

Scheme 4 Utilizations of products 3.

In summary, selective scission of the C–H bond adjacent to halide atoms in alkyl halides under visible light photoredox catalysts-facilitated conditions has been illustrated for the synthesis of polyhalo-substituted oxindoles. Importantly, the polyhalo-substituted oxindoles could be further converted into (Z)-vinyl halides or alkynyl halides simply by treatment with ^tBuOK.

This research was supported by the Hunan Provincial Natural Science Foundation of China (no. 13JJ2018), Natural Science Foundation of China (no. 21172060), and Specialized Research Fund for the Doctoral Program of Higher Education (no. 20120161110041).

Notes and references

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13. The detailed data, including kinetic isotope effect experiments (Scheme S1†) and the quantum yield (Fig. S1†), are summarized in the ESI†.
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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4qo00251b

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