Copper-catalyzed oxidative ipso-carboalkylation of activated alkynes with ethers leading to 3-etherified azaspiro[4.5]trienones

Abstract

Cu-catalyzed synthesis of 3-etherified azaspiro[4.5]trienones from N-arylpropiolamides and ethers is presented using TBHP oxidant. This is achieved through C(sp³)–H functionalization, ipso-carbocyclization and dearomatization, and this method represents a new example of alkyne oxidative 1,2-difunctionalization with an ipso-aromatic carbon and a C(sp³)–H bond by simultaneously forming two new carbon–carbon bonds.

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The spirocyclohexadienone ring system is a prevalent core structure in numerous natural products, pharmaceuticals and performance materials,¹ and it serves as a versatile synthon in organic synthesis.² The development of efficient and atom-economical methods for the synthesis of spirocyclohexadienones from readily accessible starting materials has therefore recently attracted considerable attention.^1–6 Traditionally, the vast majority of efforts focused on the dearomatization of phenols via either the radical ipso-carbocyclization of intermediate A, in situ generated from the corresponding iodides or diazonium salts (Scheme 1a),^3,4 or transition-metal-catalyzed intramolecular nucleophilic ipso-carbocyclization of 5-(4-hydroxyaryl)-1-alkenes or 5-(4-hydroxyaryl)-1-alkynes.⁵ However, these methods only modify the pre-existing frameworks, with no new functional groups being introduced into the system. Recently, the electrophilic ipso-carbocyclization of 4-aryl-1-alkynes with halogen electrophiles has emerged as a powerful and efficient method to construct spirocyclohexadienone rings (Scheme 1b):⁶ although these transformations could introduce new functional groups into the pre-existing systems, the functional groups were limited to halogen atoms. In light of these reported results, we hypothesized that the presence of an unsaturated bond could serve as a platform to trap radicals leading to a similar version of intermediate A, followed by ipso-carbocyclization and dearomatization affording the expected functionalized spirocyclohexadienone ring system. Herein, we report a new C(sp³)–H functionalization tandem strategy for selective synthesis of 3-etherified azaspiro[4.5]trienone architectures by copper-catalyzed oxidative ipso-carboalkylation of activated alkynes with aromatic ipso-carbon atoms and ethers; this tandem reaction is triggered by the CuCl/TBHP system and involves the use of 4-aryl-1-alkynes as radical acceptors to trap the ether radicals (Scheme 1c).

Scheme 1 Construction of spirocyclohexadienone rings.

Simple ethers, as important basic chemical feedstocks, are widely used solvents in organic synthesis and industry because of their chemical inertness under many reaction conditions. Nevertheless, the use of transition-metal catalysts affords the opportunity to use them as reactive functionalities. Although ethers, particularly cyclic ethers, frequently occur within the framework of many biologically active molecules,⁷ transition-metal-catalyzed methods employing simple ethers to construct higher-functionalized ethers through a C–C bond forming process have not been much investigated.^8,9 In this field, the methods proceed via a single electron transfer (SET) strategy, and focus on the reaction of the C(sp³)–H bonds adjacent to an oxygen atom (ethers) with other C(sp³)–H bonds,^8a–c C(sp²)–H bonds,^8d–e aryl organometallic reagents,^8f–h alkenes^8i–n or terminal alkynes.^8o However, methods for the difunctionalization of an alkyne with a C(sp³)–H bond adjacent to an oxygen atom (ethers) and an aromatic ipso-carbon atom to simultaneously form two new carbon–carbon bonds are lacking.

We began our investigation by examining the reaction between N-methyl-N,3-diphenylpropiolamide (1a) and tetrahydrofuran (THF, 2a) (Table 1). Initially, treatment of substrate 1a with THF 2a, FeCl₃ and TBHP (tert-butyl hydroperoxide, 5 M in decane) afforded the desired product 3aa in 37% yield (entry 1). Gratifyingly, the Cu catalysts, such as CuCl, CuBr, CuI, CuOAc and CuCl₂, were effective to improve the reaction, and CuCl was the most efficient (entries 2–6). In the presence of CuCl and TBHP, product 3aa was obtained in 83% yield (entry 2). It was noted that without Cu catalysts the reaction could take place in the presence of TBHP, albeit with a lower yield (entry 7). In addition, treatment of substrate 1a with THF 2a, TBHP and n-Bu₄NI afforded the desired product 3aa in only 26% yield (entry 8). However, the absence of TBHP resulted in no detectable product 3aa (entry 9). Screening the loading of TBHP revealed that the amount of TBHP affected the reaction, and the reaction at 1.2 equiv. TBHP was revealed as the most effective loading (entries 2, 10 and 11). Subsequently, a series of other oxidants, including aqueous TBHP, TAHP (tert-amyl hydroperoxide), CHP (cumene hydroperoxide), DTBP (di-tert-butyl peroxide), DCP (dicumyl peroxide) and BPO (benzoyl peroxide), were tested (entries 12–17). We were surprised to find that only hydroperoxides, aqueous TBHP, TAHP and CHP afforded any results (entries 12–14), and the other oxidants, DTBP, DCP and BPO, were inert (entries 15–17). After the effects of the amount of CuCl and the reaction temperature were examined, it turned out that the best results were obtained in the presence of 5 mol% CuCl at 120 °C (entry 2 and 18–21). Notably, 10 mmol of substrate 1a were successfully converted in good yield (entry 22).

Table 1 Screening optimal conditions^a

Entry	[M] [mol%]	[O] [equiv.]	T [°C]	Isolated yield [%]
a Reaction conditions: 1a (0.3 mmol), 2a (1.5 mmol), [Cu], [O], and n-BuOAc (2 mL) under argon atmosphere for 24 h. TBHP (5 M in decane). b n-Bu4NI (0.36 mmol) was added. c >90% of 1a was recovered. d TBHP (70% in water) was added. e 1a (10 mmol, 2.35 g).
1	FeCl3 (5)	TBHP (1.2)	120	37
2	CuCl (5)	TBHP (1.2)	120	83
3	CuBr (5)	TBHP (1.2)	120	50
4	CuI (5)	TBHP (1.2)	120	54
5	CuOAc (5)	TBHP (1.2)	120	59
6	CuCl2 (5)	TBHP (1.2)	120	48
7	—	TBHP (1.2)	120	41
8^b	—	TBHP (1.2)	120	26
9^c	CuCl (5)	—	120	0
10	CuCl (5)	TBHP (1.0)	120	61
11	CuCl (5)	TBHP (2.0)	120	83
12^d	CuCl (5)	TBHP (1.2)	120	82
13	CuCl (5)	TAHP (1.2)	120	62
14	CuCl (5)	CHP (1.2)	120	14
15^c	CuCl (5)	DTBP (1.2)	120	0
16^c	CuCl (5)	DCP (1.2)	120	0
17^c	CuCl (5)	BPO (1.2)	120	0
18	CuCl (2)	TBHP (1.2)	120	54
19	CuCl (10)	TBHP (1.2)	120	83
20	CuCl (5)	TBHP (1.2)	100	51
21	CuCl (5)	TBHP (1.2)	130	80
22^e	CuCl (5)	TBHP (1.2)	120	79

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With the optimal conditions in hand, the scope of this Cu-catalyzed tandem reaction of ethers 2 with respect to N-methyl-N,3-diphenylpropiolamide (1a) was firstly investigated (Table 2). The optimal conditions were found to be compatible with a wide range of ethers, such as 1,4-dioxane (2b), tetrahydro-2H-pyran (2c), oxepane (2d), 2,3-dihydrobenzofuran (2e), 2,3-dihydrobenzo[b][1,4]dioxine (2f), diethyl ether (2g) and 1,2-dimethoxyethane (2h). For example, 1,4-dioxane (2b) successfully underwent the reaction with propiolamide 1a, CuCl and TBHP, providing product 3ab in 77% yield. Gratifyingly, the reaction could be expanded to ethers 2c and 2d with different ring sizes (products 3ac and 3ad). Benzoheterocyclic rings are valuable compounds with important chemical, biological, and medicinal properties.¹⁰ To our delight, two benzoheterocyclic rings, 2,3-dihydrobenzofuran (2e) or 2,3-dihydrobenzo[b][1,4]dioxine (2f), could be readily introduced into the azaspiro[4.5]trienone structure (products 3ae and 3af). Simple ethers, diethyl ether (2g) and 1,2-dimethoxyethane (2h) were also viable, and their reactions proceeded regioselectively at the CH₂ position (products 3ag and 3ah).

Table 2 Scope of ethers (2)^a

a Reaction conditions: 1a (0.3 mmol), 2 (1.5 mmol), CuCl (5 mol%), TBHP (1.2 equiv., 5 M in decane), and n-BuOAc (2 mL) at 120 °C under argon atmosphere for 24 h.

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We next set out to examine the possibility of generating 3-etherified azaspiro[4.5]trienones by the Cu-catalyzed reaction of various N-arylpropiolamides 1 in the presence of ethers 2, CuCl and TBHP (Table 3). Changing the N-Me group to a N-Bn, N-(2-I-Bn) or N-allyl group furnished the corresponding products 3ba–da with slightly lower yields, but a N-H group resulted in no reaction (product 3ea). Subsequently, the impact of substituents on the aromatic ring of the N-aryl moiety was investigated (products 3fa–ja). N-Arylpropiolamides with an electron-donating group (o-Me, m-Me or o-MeO) or an electron-withdrawing group (I) on the aromatic ring of the N-aryl moiety were all well-tolerated (products 3fa–ia). Interestingly, the reaction of a naphthalene-derived substrate proceeded smoothly to afford a polycyclic spiro-compound 3ja in moderate yield, albeit with the requirement of a prolonged reaction time. In light of these results, a number of substituents at the terminal alkyne position, including electron-rich or electron-deficient aryl groups and aliphatic groups, were examined (products 3ka–qa). All aryl alkynes were consistent with the optimal conditions, and the order of the substituent reactivity is: electron-rich aryl groups (products 3ka–ma) > electron-deficient aryl groups (products 3na–oa). However, the aliphatic alkyne that was examined showed a lower activity (product 3pa). Gratifyingly, the heteroaryl alkyne that was investigated revealed itself as a suitable substrate (product 3qa). I- or MeO-substituted substrates 1i and 1l reacted with 1,4-dioxane (2b), giving the desired products 3ib and 3lb in moderate yields. It is noteworthy that 3-phenylpropiolate 1r is viable for the tandem reaction with THF 2a in moderate yield (product 3ra).

Table 3 Copper-catalyzed oxidative C–H functionalization/ipso-carbocyclization of N-arylpropiolamides (1) with ethers (2)^a

Entry	Substrate 1	Product/isolated yield (%)
a Reaction conditions: 1 (0.3 mmol), 2 (1.5 mmol), CuCl (5 mol%), TBHP (1.2 equiv., 5 M in decane) and nBuOAc (2 mL) at 120 °C under argon atmosphere for 24 h. b CuCl (10 mol%) and TBHP (2.4 equiv.) for 48 h.
1	R^3 = Bn (1b)	R^3 = Bn (3ba)/66%
2	R^3 = 2-I-Bn (1c)	R^3 = 2-I-Bn (3ca)/61%
3	R^3 = allyl (1d)	R^3 = allyl (3da)/67%
4	R^3 = H (1e)	R^3 = H (3ea)/trace
5
6
7	R^1 = OMe (1h)	R^1 = OMe (3ha)/74%
8	R^1 = I (1i)	R^1 = I (3ia)/63%
9
10	R^2 = 4-MeC6H4 (1k)	R^2 = 4-MeC6H4 (3ka)/67%
11	R^2 = 4-MeOC6H4 (1l)	R^2 = 4-MeOC6H4 (3la)/70%
12	R^2 = 2-MeOC6H4 (1m)	R^2 = 2-MeOC6H4 (3ma)/68%
13	R^2 = 4-IC6H4 (1n)	R^2 = 4-IC6H4 (3na)/66%
14	R^2 = 4-CNC6H4 (1o)	R^2 = 4-CNC6H4 (3oa)/62%
15	R^2 = n-C5H11 (1p)	R^2 = n-C5H11 (3pa)/trace
16
17
18
19

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To our surprise, p-MeO- or p-F-substituted N-arylpropiolamides 1s and 1t gave the corresponding spiro[4,5]trienone 3aa in 68% and 66% yields, respectively, by releasing the para-substituents (eqn (1); Scheme 2). However, p-Me-substituted substrate 1u afforded hydroxylated product 3ua in 36% yield (eqn (2)). Notably, no ¹⁸O atom was incorporated in the corresponding spiro[4,5]trienones 3 when the reaction of 1a, 1s, 1t or 1u with THF 2a was performed in the presence of 2 equiv. H₂¹⁸O, suggesting that the carbonyl and hydroxyl oxygen atom does not come from water.

Scheme 2 Control experiments.

The results in Table 1 were carefully reviewed, and we found that only hydroperoxides displayed catalytic activity for the current reaction (entries 2, 12–14 vs. entries 15–17). In light of this, we deduced that the oxygen atom of the newly formed carbonyl group comes from the hydroperoxide.¹¹ The results in eqn (3) show that a stoichiometric amount of radical inhibitor (1.2 equiv. TEMPO) resulted in no conversion of substrate 1a; moreover, THF 2a was converted into 2,2,6,6-tetramethyl-1-(tetrahydrofuran-2-yloxy)piperidine 4. These results imply that the tandem reaction includes a radical process. A large kinetic isotope effect was discovered using a mixture of THF 2a and THF-D8 [D₈]-2a (k_H/k_D = 3.0), indicating that the C(sp3)–H bond cleavage of THF constitutes a rate-limiting step (eqn (4)).⁸

Consequently, we proposed a working mechanism as outlined in Scheme 3 on the basis of the present results and the literature reports.^8,9 Initially, alkyl radical B is formed from THF (1a) by a single electron transfer from TBHP with the aid of the active Cu⁺ species.^8,9 Subsequently, alkylation of alkyne 2a with alkyl radical B affords vinyl radical intermediate C, followed by selective ipso-carbocyclization to give intermediate D.^4,5 Intermediate D reacts with Cu²⁺(OH) to yield intermediate E and regenerates the active Cu⁺ species.¹¹ Finally, oxidation of intermediate E by TBHP/[Cu] produces the desired spiro[4,5]trienone 3aa.¹²

Scheme 3 Possible mechanism.

In summary, we have developed a new Cu-catalyzed synthesis of 3-etherified azaspiro[4.5]trienones from N-arylpropiolamides and ethers through oxidative C(sp³)–H functionalization, ipso-carbocyclization and dearomatization cascade. Mechanistic studies suggested that a radical process is involved, and the oxygen atom of the newly formed carbonyl group comes from the hydroperoxide. Importantly, this tandem method makes the construction of higher-functionalized ethers from simple ethers easy by a C(sp³)–H oxidative functionalization strategy, and represents the first example of alkyne oxidative 1,2-difunctionalization to simultaneously form two carbon–carbon bonds and one carbon–oxygen double bond. Studies on applications of this oxidative difunctionalization method in organic synthesis are currently underway in our laboratory.

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

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4qo00006d

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