Copper-catalyzed acyloxylation of the C(sp³)–H bond adjacent to an oxygen by a cross-dehydrogenative coupling approach

Abstract

The acyloxylation of the C(sp³)–H bond adjacent to oxygen by adopting a copper catalyzed dehydrogenative cross-coupling reaction between simple ethers and benzyl alcohols has been disclosed. The advantages of the dehydrogenative cross-coupling reaction are the adoption of unactivated ethers as substrates and good tolerance of many functional groups.

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The construction of carbon–carbon (C–C) and carbon–heteroatom (C–O & C–N) bonds using transition-metal catalyzed C–H bond functionalization has emerged as an attractive and powerful tool in modern organic chemistry.^1,2 In particular, the functionalization of C(sp³)–H bonds under oxidative conditions has fascinated much attention among synthetic chemists, since it eliminates the prefunctionalization step and thus make synthetic schemes considerably shorter and straight forward. In recent years, tremendous efforts have been placed on the formation of these bonds through oxidative C–H bond functionalization adjacent to heteroatoms by the cross dehydrogenative coupling approach (CDC).^3,4,6 Moreover, the majority of the reported methods have focused on the functionalization of the C(sp³)–H bond (via reactive iminium ion intermediates) adjacent to a nitrogen atom under oxidative conditions with various pro-nucleophiles.⁴ However, the functionalization of the C(sp³)–H bond (via reactive oxonium ion intermediates) adjacent to an oxygen atom is relatively inert due to its higher oxidation potential and the required strong hydrogen acceptor. In this regard, most of the reported methods were devoted to allylic and benzylic C–H bonds in the α-position to the oxygen with different carbon based pro-nucleophiles.⁵

In contrast, to our knowledge, there have been rare reports existing on α-C–H bond functionalization on simple alkyl ethers for the construction of C–O, C–N and C–S bonds using oxygen, nitrogen and sulphur based pro-nucleophiles via the CDC reaction.⁶ For example, Wan and co-workers^6c first described a Bu₄NI-catalyzed C–O bond formation by adopting a CDC reaction between carboxylic acids and unactivated simple ethers using tert-butyl hydrogen peroxide as an oxidant (Scheme 1a). Recently, Duan's^6e group also described the decarboxylative acyloxylation of the C(sp³)–H bond adjacent to an oxygen atom with α-oxocarboxylic acids in the presence of TBAI and TBHP as an oxidant (Scheme 1b). Most recently, Patel et al.,⁶ⁱ developed the copper catalyzed synthesis of α-acyloxy ethers from alkylbenzenes and simple ethers (Scheme 1c).

Scheme 1 CDC approach for C–O bond forming reaction via C–H bond functionalization.

Traditionally, benzyl alcohols are known to be acylation⁷ and acyloxylation reagents.⁸ Because benzyl alcohols are naturally abundant, stable, easy to handle and readily oxidized into aldehydes and acids, they can thus be likely used as ideal acyl and acyloxylating sources. α-acyloxy ethers appear as structural motifs in biologically active, pharmaceutical and natural products compounds.⁹ Herein, we report a copper-catalyzed oxidative CDC reaction between benzyl alcohols and simple cyclic and acyclic ethers in the presence of tert-butyl hydrogen peroxide (TBHP) as an oxidant (Scheme 1d), affording the acyloxylated products in moderate to good yields.

At the outset of our study, copper catalyzed oxidative C(sp³)–H bond acyloxylation was investigated with 4-methylbenzyl alcohol (1a) and 1,4-dioxane (2a) as model substrates (Table 1). The initial reaction was carried out with 10 mol% Cu(OAc)₂ in presence of 70% aqueous solution of TBHP (2 equiv.) at 80 °C for 8 h and we obtained the desired product 3a in 15% yield (Table 1, entry 1). Next we examined different Cu sources, such as Cu(acac)₂, CuCl₂, CuBr₂, CuCl, CuI and Cu(OAc)₂·H₂O; however, the expected product was not observed in all the cases (Table 1, entries 2–6) except for the Cu(OAc)₂·H₂O catalyst, which gave only 12% yield (Table 1, entry 7). Then, it was necessary to check the quantity of TBHP and the role of the temperature. Accordingly, the aq. TBHP amount was raised to 3 equiv. and the temperature from 80 °C to 100 °C (Table 1, entry 8), then the product yield increased from 15% to 35%. We evaluated the role of the oxidant by screening different oxidants such as O₂, H₂O₂, and di-tert-butyl peroxide (DTBP). However, we were not able to observe the required product (Table 1, entries 9–11). Moving from the aqueous TBHP to a commercially available TBHP in decane solution, the product yield was increased from 35% to 57% (Table 1, entry 12).¹⁰ By decreasing the Cu(OAc)₂ quantity from 10 mol% to 5 mol% (Table 1, entry 13), the yield was further enhanced from 57% to 68%. Further optimization studies revealed that the product yield was increased to 82% (Table 1, entry 14) when the reaction was carried out with 5 mol% Cu(OAc)₂ and 4 equiv. of TBHP. No significant improvement in the product yield was observed with a loading of 2 mol% of Cu(OAc)₂ and prolonging the reaction time (Table 1, entry 15). The lack of product formation in the control experiments in the absence of the metal or oxidant clearly showed the significance of both the metal catalyst and oxidant for this transformation (entry 16 and 17).

Table 1 Optimization of reaction conditions^a

Entry	Catalyst (mol%)	Oxidant (equiv.)	T [°C]	Yields^c 3a [%]
a Reaction conditions: unless specified, the reaction was carried out with: 1a (1 mmol), 2a (2 mL), Cu catalyst (5 mol%), oxidant (4 equiv.), T [°C], 8 h.b Reaction time was 18 h.c Yield of isolated product.d Product not observed by TLC.e H2O2 = hydrogen peroxide.f DTBP = di-tert-butyl peroxide.g 2a (1 mmol).
1	Cu(OAc)2(10)	TBHP in H2O(2)	80	15
2	Cu(acac)2(10)	TBHP in H2O(2)	80	—^d
3	CuCl2(10)	TBHP in H2O(2)	80	—^d
4	CuBr2(10)	TBHP in H2O(2)	80	—^d
5	CuCl(10)	TBHP in H2O(2)	80	—^d
6	CuI(10)	TBHP in H2O(2)	80	—^d
7	Cu(OAc)2·H2O	TBHP in H2O(2)	80	12
8	Cu(OAc)2(10)	TBHP in H2O(3)	100	35
9	Cu(OAc)2(10)	O2	100	—^d
10	Cu(OAc)2(10)	H2O2^e(3)	100	—^d
11	Cu(OAc)2(10)	DTBP^f	100	—^d
12	Cu(OAc)2(10)	TBHP in decane(3)	100	57
13	Cu(OAc)2(5)	TBHP in decane(3)	100	68
14	Cu(OAc)2(5)	TBHP in decane(4)	100	82
15	Cu(OAc)2(2)	TBHP in decane(4)	100	80^b
16	—	TBHP in decane(4)	100	—^d
17	Cu(OAc)2(5)	—	100	—^d
18	Cu(OAc)2(5)	TBHP in decane(4)	100	12^g

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Having the optimized reaction conditions with us (Table 1, entry 14), we screened different benzyl alcohol substrates 1 for the C(sp³)–H bond acyloxylation with different unactivated ethers 2. For example, substrates bearing electron-rich groups, such as methyl, isopropyl, methoxy, benzyloxy on aromatic ring, showed good activation with 1,4-dioxane to offer the desired products (3a–e) in good yields. Ortho-substituted benzyl alcohol did not affect the reaction and underwent efficient coupling with 1,4-dioxane to give the desired product 3f in 73% yield. Benzyl alcohols containing electron-poor groups such as CN, NO₂ and F worked well under the optimized reaction conditions (3g–3i). The present protocol was tolerated for the halogen bearing benzyl alcohols (Cl & Br) without any difficulties (3j & 3k). The catalytic system showed good activity in the case of a fused ring (naphthalene) containing benzyl alcohol and the product (3l) yield was obtained in 68%. Our efforts to activate hetero aromatic benzyl alcohols were not successful; instead of the acyloxylation product (3m) we observed the alcohol attack at the carbon atom (adjacent to the oxygen) (3n, 3o) (see ESI†). In addition, the acyloxylation of 1,4-dioxane 2a was also unsuccessful with allyl alcohols and aliphatic alcohols (3p, 3q, 3r) (Scheme 2) under the optimized reaction conditions.

Scheme 2 Cu(OAc)₂-catalyzed coupling of 1,4-dioxane with a variety of benzyl alcohols. Reaction conditions: benzyl alcohol (1 mmol), 1,4-dioxane (2 mL), Cu(OAc)₂ (5 mol%), TBHP in decane (4 equiv.), 100 °C, 8 h.

To evaluate the role of the ethers, we carried out the reactions with cyclic (2b–2e) and acyclic (2f, 2g) (Table 2) ethers under standard reaction conditions. Cyclic ethers like THP (2b) and THF (2c) reacted smoothly with the benzyl substituted alcohol to provide the corresponding products (4b, 4c) in good yields of 68% and 63%, but 2-methyl THP (2d) and 1,3-dioxolane (2e) were both inactive for the coupling reaction with benzyl alcohols. To compare the reactivity between the internal methylene and terminal methyl carbons, we carried out the reaction with 1,2-dimethoxyethane (2f) under standard conditions and we observed the six products (4fa–4fc′) with their respective combined good yields. In all cases, the acyloxylation products at the internal methylene carbon were slightly higher. Next, we carried out the reaction of 1a with diethylether (b.p 35 °C) (2g) but we could not scrutinize the acyloxy ether (4g) under the standard reaction conditions.

Table 2 Reaction of benzyl alcohols with ethers^a

Entry	Reactant	Ether	Product	Yield^b
a Reaction conditions: benzyl alcohol 1a (1 mmol), 2b–2g (2 mL), Cu(OAc)2 (5 mol%), TBHP in decane (4 equiv.), 100 °C, 8 h.b Yield of isolated product.
1				68%
2				63%
3				—
4				—
5				46%
				31%
6				43%
				30%
7				36%
				30%
8				—

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To explore the mechanism of the reaction, several control experiments were performed. In this process, we observed that the coupling product 3a was obtained in <5% yield^7a,c when 4-methylbenzoic acid was used instead of benzyl alcohol under the optimum reaction conditions (Scheme 3, eqn (1)); moreover from this process, it is apparent that the transformation did not proceed via an acid intermediate.¹¹ Surprisingly, no product was observed when the reaction was carried out between 2-(4-methylbenzyloxy)-1,4-dioxane (i.e., protected alcohol) and 2a (Scheme 3, eqn (2)) in our catalytic system. The acyloxylated product 3a was obtained in 80% yield when the reaction was performed with 4-methyl benzaldehyde (Scheme 3, eqn (3)). In addition, when 2,2,6,6-tetramethylpiperidinyloxy (TEMPO), a radical scavenger, was added to the reaction mixture of 1a and 1,4-dioxane (2a) under the optimized reaction conditions, no coupling product 3a was observed (Scheme 3, eqn (4)). On the basis of these observations and consistent with earlier reports,^6b,12 it is suggested that the mechanism of the dehydrogenative cross-coupling reaction may undergo a radical pathway in the presence of Cu(OAc)₂ and TBHP in decane as an oxidant, which is depicted in Scheme 3.

Scheme 3 Investigation of the reaction mechanism.

The mechanism involves five steps: initially, tert-butoxyl and tert-butyl peroxyl radicals are formed through interaction of TBHP with the Cu(II)–Cu(I) redox couple (Scheme 4, step a).¹³ Next the activation of the C–H bonds adjacent to the oxygen atom of the benzyl alcohol is activated to produce an aldehyde. Subsequently, this aldehyde undergoes a similar hydrogen abstraction to form an acyl radical, which is coupled with the tert-butyl peroxy radical and provides the per ester intermediate (Scheme 4, step b).¹⁴ The intermediacy of tert-butyl perbenzoate in this transformation (step b) is supported by a control experiment, in which the treatment of a commercially available tert-butyl perbenzoate with 1,4-dioxane 2a under the optimized reaction conditions afforded the coupling product 3e in moderate yields (Scheme 3, eqn (5)).⁶ⁱ From this, we can predict the plausible mechanism of the reaction, which is described in Scheme 4. The per ester intermediate undergoes homolytic cleavage from the acyloxy radical (Scheme 4, step c), which acts as a pro-nucleophile. The tert-butoxyl radical generated by the dissociation of t-BuOOH may have extracted the α-hydrogen of 1, 4-dioxane to form a radical and undergo a single electron transfer,¹⁵ which led to the formation of oxonium species (Scheme 4, step d). Then, finally the acyloxylated product 3a was formed by the coupling of the acyloxy radical with the oxonium species (Scheme 4, step e).

Scheme 4 Plausible pathway for the reaction.

Conclusions

In summary, we disclosed the Cu-catalyzed oxidative C(sp³)–H bond acyloxylation of unactivated cyclic and acyclic ethers with different substrates of benzyl alcohols by adopting TBHP as an oxidant. Ortho-substituted, halogen bearing and fused ring aromatic substrates are tolerated under the optimized reaction conditions. This novel strategy provides a simple, efficient, and direct access to acyloxylated products. Further investigation towards the scope, mechanism, and synthetic applications of this reaction are expected in due course.

Acknowledgements

We thank the Director, IICT, for the generous support. CSIR, New Delhi, for funding through the programme ORIGIN XII FYP (CSC0108). K. B. Raju thanks Council of Scientific and Industrial Research for CSIR-SRF fellowship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06233g

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