Copper-catalyzed one-pot reactions of acetyl chloride, o-halobenzoic acids and Wittig reagents toward 3-methyl isocoumarin synthesis

Abstract

The one-pot reactions of acetyl chloride, o-halobenzoic acids and Wittig reagents providing 3-methyl isocoumarins have been furnished via tandem Wittig reaction, oxa-Michael addition and C–C cross coupling. Three new chemical bonds including one C–O, one C=C and one C–C bond are generated via the catalysis of a simple copper salt for the heterocycle construction.

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Introduction

The copper-catalyzed Ullmann-type C–C cross coupling reaction between aryl/vinyl halides and active carbon coupling partners is an important tool in the generation of new carbon–carbon bonds.¹ While significant contribution of such transformation has been furnished in the synthesis of divergent organic products via both direct coupling process or tandem reactions initiated by this coupling transformation, one of the major restrictions in the Ullmann C–C cross coupling reaction is that the scope of the carbon coupling partners is rather narrow. In most of the related known literature, active methylenes such as 1,3-dicarbonyl or analogous compounds are predominantly used as the reaction partners of aryl/vinyl halides,² and alternative reactants in such reactions are rarely known.³ Accordingly, the limited substrate scope has also hampered the application of this C–C cross coupling in the synthesis of more structurally diversified organic products. Therefore, in order to promote the application of the Ullmann C–C coupling as broad as those equivalent C-heteroatom coupling versions, discovering practical reaction partners constitutes the main present challenge. Our group has previously reported the copper-catalyzed C–C coupling initiated tandem reactions toward the synthesis of benzofurans⁴ and indoles⁵ wherein the in situ generated allenes are utilized as the carbon coupling partners of the Ar–halogen bond to enable the product construction. In despite of these successful examples, it should be noted that examples on the C–C coupling reactions employing allenes as coupling partners are still rather scarce.

Isocoumarins are a class of typical heterocycles with broad spectrum biological activity, and this backbone also constitutes the central fragment of many natural products.⁶ Based on known literature, isocoumarins can be synthesized with different strategies,⁷ such as the tandem annulations initiated by Ar–H bond addition to alkynes,⁸ diazo functionalized ketones⁹ or cyclic alkenyl carbonates,¹⁰ the intramolecular cross coupling of Ar–halogen and vinyl C–H bond,¹¹ the addition of Ar–halogen bond to alkynes,¹² ring expansion reactions,¹³ the oxidative coupling of benzoic acids and vinylarene,¹⁴ among others.¹⁵ Regardless the enriched availability on synthetic methods, it is notable that most of the reported methods on isocoumarin synthesis require the presence of noble metal reagent such as Pd, Ru, Rh, Ag or Au as catalyst with few exception. To our knowledge, a synthetic route via tandem reaction initiated by copper-catalyzed allene-based Ullmann-type C–C coupling has not yet been known. Considering the power of multicomponent reactions¹⁶ as well as related cascade reactions¹⁷ in providing efficient organic synthesis, and in continuous to our research efforts in developing copper-catalyzed cross coupling and their application in designing tandem reactions,¹⁸ we report herein a new tandem assembly consists of the in situ allene generation, the oxa-Michael addition and intramolecular C–C coupling for the synthesis of 3-methyl isocoumarins. The in situ generation and utilization of the unstable allene substrates as well as the low cost copper-catalyst of the present work feature as advantageous over similar known work employing noble metal Ag catalyst and two-step synthesis.¹¹

Results and discussion

The investigation started from the reaction of Wittig reagent 1a, acetyl chloride 2a and o-iodobenzoic acid 3a in the presence of CuI and Cs₂CO₃. By heating at 100 °C in DMSO, the target product 4a was acquired with 27% yield (entry 1, Table 1). Subsequently, entries employing different copper catalysts such as CuBr, CuO and Cu(OAc)₂ were examined, and Cu(OAc)₂ displayed much better catalytic efficiency than other candidates (entries 2–4, Table 1). The reactions conducted in the presence of different base additives, including K₂CO₃, K₃PO₄, NaHCO₃, EtONa and Et₃N, however, was not able to give better result (entries 5–9, Table 1). The employment of reaction medium of different polarity such as DMF, toluene and water etc. failed to improve the expect product generation, either (entries 10–13, Table 1). However, attempt in employing oxidative potassium iodate was found to be useful in enhancing the yield of 4a (entries 14–17, Table 2). Finally, the variation on the reaction temperature proved that 90 °C was proper for the reaction (entries 18–19, Table 1).

Table 1 Optimization on reaction conditions^a

Entry	Catalyst	Base	Additive	Solvent	Yield^b (%)
a The reactions were generally carried out with stepwise one-pot operation (see Experimental sections for details) 1a (0.6 mmol), 2a (0.9 mmol), 3a (o-iodobenzoic acid, 0.3 mmol), catalyst (0.03 mmol), base (0.75 mmol), additive (0.6 mmol, if applicable) and n-hexane (3 mL) in solvent (2 mL), stirred at 100 °C for 12 h (TLC); commercial Cu(OAc)2·H2O was used in all entries.b Yield of isolated product based on 1a.c The temperature was 90 °C.d The temperature was 110 °C.
1	CuI	Cs2CO3	—	DMSO	27
2	CuBr	Cs2CO3	—	DMSO	28
3	CuO	Cs2CO3	—	DMSO	21
4	Cu(OAc)2	Cs2CO3	—	DMSO	45
5	Cu(OAc)2	K2CO3	—	DMSO	27
6	Cu(OAc)2	K3PO4		DMSO	Trace
7	Cu(OAc)2	NaHCO3	—	DMSO	Trace
8	Cu(OAc)2	EtONa	—	DMSO	Trace
9	Cu(OAc)2	Et3N	—	DMSO	Trace
10	Cu(OAc)2	Cs2CO3	—	DMF	26
11	Cu(OAc)2	Cs2CO3	—	1,4-Dioxane	39
12	Cu(OAc)2	Cs2CO3	—	p-Xylene	NR
13	Cu(OAc)2	Cs2CO3	—	H2O	NR
14	Cu(OAc)2	Cs2CO3	KBr	DMSO	49
15	Cu(OAc)2	Cs2CO3	KI	DMSO	52
16	Cu(OAc)2	Cs2CO3	KIO3	DMSO	56
17	Cu(OAc)2	Cs2CO3	MnO2	DMSO	54
18^c	Cu(OAc)2	Cs2CO3	KIO3	DMSO	74
19^d	Cu(OAc)2	Cs2CO3	KIO3	DMSO	53

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Table 2 Scope in the three-component isocoumarin synthesis^a,b

a The reactions were generally carried out with one-pot stepwise operation (see Experimental sections for details): 1 (0.6 mmol), 2a (0.9 mmol), 3 (0.3 mmol), Cu(OAc)₂ (0.03 mmol), Cs₂CO₃ (0.75 mmol), KIO₃ (0.6 mmol) and n-hexane (3 mL)/DMSO (2 mL), stirred at 90 °C for 12 h.b Yield of isolated product.

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In subsequent work, the scope of this three-component tandem reaction on the synthesis of isocoumarins 4 was explored. As showing in Table 2, both o-iodo- and o-bromobenzoic acids could be used as building blocks in this kind of synthesis. The general tendency was that iodinated benzoic acids provided corresponding products with higher yield than those equivalent entries employing o-bromobenzoic acids, suggesting that the reactivity of the Ar–X bond evidently influenced the efficiency of the target product synthesis (comparing 4a, 4c, 4g with 4b, 4d, 4e, 4f, 4h, 4i and 4j in Table 2). On the other hand, the entries employing aryl ketone-based Wittig reagents gave much lower yield of corresponding isocoumarins than those ones constructed by alkyl ketone-based Wittig reagents (comparing 4k, 4l with 4m, 4n in Table 2), which indicated that the electron withdrawing effect resulting from the aryl ring was negative to expect reaction by reducing the nucleophilicity of the α-carbon in the in situ generated allene intermediate. In addition, the attempts in employing other linear acyl chlorides such as propionyl chloride and butyryl chloride for the reaction failed to give the expected isocoumarins. The consequence might be attributed to the additional steric effect resulting from the alkyl substitution with corresponding allene intermediate 5 which hampled the addition of the carboxylic acid ion of weak nucleophilicity.

To illustrate the possible process forming the isocoumarin products via the copper-catalyzed C–C coupling, a plausible mechanism for the present tandem reactions is proposed in Scheme 1. The reaction is supposed to start from the oxa-Michael addition of the carboxylic acid to the in situ generated allene intermediate 5 which provides vinyl ether 6. The tautomerization of 6 leads to the occurrence of the intermediate 7. Under the promotion of the Cs₂CO₃, the nucleophilic carbon site in intermediate 7 attacks Cu(OAc)₂ via a formal nucleophilic substitution to afford Cu(II) complex A. Subsequently, the oxidative addition of the copper site to the Ar–X bond takes place and generates the seven-membered Cu(IV) complex B. Finally, the reductive elimination on B allowed the production of target product 4 and the regeneration of the Cu(II) catalyst. The role of KIO₃ in the reaction is not yet clear, a possibility is that the KIO₃ can oxidise the halid (X⁻) produced during the product formation and promote the reaction to run toward the positive direction.

Scheme 1 The plausible reaction mechanism.

Conclusions

In summary, we have established a tandem reaction tactic wherein the Wittig reaction, oxa-Michael addition and a copper-catalyzed Ullmann-type C–C coupling have been involved for the facile synthesis of 3-methyl isocoumarins. This synthetic method starts from simple materials and require no noble metal catalyst, which can be a useful complementary approach in the synthesis of valuable isocoumarin scaffolds.

Experimental section

General procedure for the synthesis of isocoumarins 4

In a 25 mL round-bottom flask was charged with phosphorus ylide 1 (0.6 mmol), CH₂Cl₂ (1 mL), and Et₃N (0.66 mmol). A solution of acetyl chloride 2a (0.9 mmol) in CH₂Cl₂ (1 mL) was added dropwise with stirring. After an additional 4 h stirring, the CH₂Cl₂ was completely evaporated under reduced pressure. n-Hexane (3 mL), o-halobenzoic acid 3 (0.3 mmol), Cu(OAc)₂ (0.03 mmol), Cs₂CO₃ (0.75 mmol), KIO₃ (0.6 mmol) and DMSO (2 mL) were then consequently added. The resulting mixture was heated at 90 °C for 12 h (TLC). The reaction mixture was allowed to cool to rt, and H₂O (10 mL) was added. The resulting suspension was extracted with ethyl acetate (3 × 10 mL). The organic phases were combined and dried over Na₂SO₄. After filtration and removing the solvent at reduced pressure, the residue was subjected to silica gel column chromatography to give the pure product by using mixed petroleum ether/ethyl acetate (v/v = 20 : 1) as the eluent.

Ethyl 3-methyl-1-oxo-1H-isochromene-4-carboxylate (4a)¹¹. Yellow liquid; ¹H NMR (400 MHz, CDCl₃): δ 8.29 (d, J = 8.0 Hz, 1H), 7.78–7.72 (m, 2H), 7.52 (t, J = 7.2 Hz, 1H), 4.49–4.43 (m, 2H), 2.46 (s, 3H), 1.44 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): 165.8, 161.2, 157.7, 135.1, 134.6, 129.7, 128.2, 124.1, 119.5, 110.3, 61.7, 19.3, 14.3.

Ethyl 3,7-dimethyl-1-oxo-1H-isochromene-4-carboxylate (4b). White soild; mp 76–78 °C. ¹H NMR (400 MHz, CDCl₃): δ 8.07 (s, 1H), 7.67 (d, J = 8.4 Hz, 1H), 7.54 (d, J = 8.4 Hz, 1H), 4.48–4.42 (m, 2H), 2.45 (s, 6H), 1.43 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): 165.9, 161.4, 156.9, 138.4, 136.3, 132.1, 129.3, 124.1, 119.4, 110.1, 61.6, 21.2, 19.2, 14.3; ESI-HRMS calcd for C₁₄H₁₅O₄ [M + H]⁺ 247.0965, found 247.0966.

Ethyl 7-chloro-3-methyl-1-oxo-1H-isochromene-4-carboxylate (4d)¹¹. White soild; mp 97–99 °C; ¹H NMR (400 MHz, CDCl₃): δ 8.25 (d, J = 2.0 Hz, 1H), 7.79 (d, J = 8.8 Hz, 1H), 7.67 (dd, J₁ = 8.8 Hz, J₂ = 2.4 Hz, 1H), 4.48–4.42 (m, 2H), 2.48 (s, 3H), 1.43 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): 165.4, 160.1, 158.5, 135.4, 134.1, 133.1, 129.0, 126.0, 120.9, 109.6, 61.9, 19.5, 14.2.

Methyl 3-methyl-1-oxo-1H-isochromene-4-carboxylate (4g). Colorless liquid. ¹H NMR (400 MHz, CDCl₃): δ 8.28 (d, J = 7.6 Hz, 1H), 7.75–7.73 (m, 2H), 7.53–7.49 (m, 1H), 3.98 (s, 3H), 2.45 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): 166.2, 161.1, 135.1, 134.5, 129.6, 128.2, 124.2, 119.4, 110.0, 52.4, 19.4; ESI-HRMS calcd for C₁₂H₁₁O₄ [M + H]⁺ 219.0652, found 219.0655.

Methyl 6,7-dimethoxy-3-methyl-1-oxo-1H-isochromene-4-carboxylate (4i). White solid; mp 154–156 °C; ¹H NMR (400 MHz, CDCl₃): δ 7.64 (s, 1H), 7.33 (s, 1H), 3.99 (s, 3H), 3.98 (s, 3H), 3.97 (s, 3H), 2.48 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): 166.53, 166.52, 158.0, 155.3, 149.6, 130.3, 112.7, 109.5, 109.2, 105.5, 56.3, 56.2, 52.3, 19.8; ESI-HRMS calcd for C₁₄H₁₅O₆ [M + H]⁺ 279.0863, found 279.0862.

Acknowledgements

This work is financially supported by National Natural Science Foundation of China (21562024, 21202064).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: General experimental information, full characterization data and ¹H/¹³C NMR spectra. See DOI: 10.1039/c7ra06707k

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