Functionalized α,β-ynones: efficient ligand for Cu catalyzed Sonogashira-type cross-coupling reaction

Abstract

Under the classic reaction conditions, a large excess of copper catalyst and N, O donor ligands were mandatory for the catalytic cross-coupling of Csp²–Csp bonds. Herein, we wish to report α,β-ynones as σ-, π-electron donating ligands for copper catalyzed Sonogashira-type reaction. As low as 0.25–2.5 mol% of L₁₁ (3-(4-bromophenyl)-1-(4-methoxyphenyl)prop-2-yn-1-one) significantly accelerated the 0.1–1.0 mol% of CuI catalyzed cross-coupling of aryl iodides with terminal alkynes and alkynylcarboxylic acids, respectively. This low-mol% catalyst system showed satisfactory activity and tolerance with 36 examples of substituted alkynes.

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The Sonogashira reaction is a powerful tool for preparing alkyne compounds, which has been widely applied in the synthesis of natural products, pharmaceuticals, and organic materials.¹ Recently, much attention has been attracted to the use of copper catalysts,² because copper is abundant, economic and less toxic.³ To enhance the catalytic efficiency of copper catalysts, many hard donor ligands were employed in Cu-catalyzed Sonogashira reactions.⁴ Generally, a large excess of multi-dentated ligands such as N,N-dimethylglycine (30 mol%),⁵ L-proline (10 mol%),⁶ 8-hydroxyquinoline (20 mol%),⁷ rac-BINOL (10 mol%),⁸ choline chloride (20 mol%),⁹ 6-methylpicolinic acid (10 mol%),¹⁰ diamines (10 mol%),¹¹ 1,10-phenanthroline (20 mol%),¹² DABCO (20 mol%),¹³ N,N-dibenzyl BINAM (20 mol%),¹⁴ diketone (20 mol%),¹⁵ salicylic acid (20 mol%),¹⁶ bis(pyrazolyl)methane (10 mol%)²⁵ is required to facilitate the Cu-catalyzed cross-coupling reaction of aryl iodides and terminal alkynes. More recently, C. Bolm et al., observed the ligand accelerating effect of 20–30 mol% DMEDA (N,N′-dimethyl-ethane-diamine), by which as low as 0.5 mol% CuI catalyzed cross-coupling reactions with decent yields.¹⁷ Until now, the addition of excess ligands (10–30 mol%) is mandatory to maintain the coupling efficiency for most homogeneous systems.¹⁸ Despite of the ligand-accelerating effect of these classic ligands, to explore new ligand system is a promising way to enhance the efficiency of copper catalysts.

Recently, few aromatic ligands, such as xantphos,¹⁹ pyrene,²⁰ BIMs (bisindoles)²¹ and TTCs (triainze trizole)²² were established as efficient ligands for Sonogashira cross-coupling reaction with low-mol% copper loading (Scheme 1). 2.5 mol% aromatic bidentated phosphine ligands successfully facilitated low-mol% Cu-catalyzed Sonogashira cross-coupling reaction, in which xantphos and Dppf showed excellent activity for 1.0 mol% Cu₂O. The polycyclic aromatic hydrocarbon ligands pyrene enhanced the activity of Cu(OTf)₂, affording 52% to 90% yields of alkyne products. BIMs bearing π-electron-rich aromatic moiety of 4-chloro-phenyl group was modulated as a powerful ligand for the Cu-catalyzed Sonogashira reaction with the yield up to 99%. TTCs with multiply hard donor sites stabilized catalytic active copper by the diverse coordination modes and afforded 61% to 96% yields. In light of these multi-functionalized ligands, particularly the beneficial accelerating effect of aromatic functional groups for Cu catalysis, we herein report α,β-ynones as σ-, π-electron donating ligands for copper catalyzed Sonogashira-type cross-coupling reaction. Ligand screening experiments of 20 substituted ligands found that L₁₁ containing methoxy- and Br- was the most efficient ligand. In presence of 2.5 equivalent L₁₁, 1.0 mol% CuI afforded 96% yield, whilst as low as 0.5 mol% of CuI gave 71% yield. An efficient copper catalyst system of CuI (1.0 mol%)/L₁₁ (2.5 mol%) showed satisfactory catalytic activity and compatibility in 36 examples of Sonogashira coupling and decarboxylative coupling, respectively (Scheme 2).

Scheme 1 Efficient ligands for Cu-catalyzed Sonogashira reactions.

Scheme 2 Ligands screening for Cu-catalyzed Sonogashira reactions. Reaction conditions: CuI (1.0 mol%), α,β-ynones (4.0 mol%), 4-iodo-anisole (0.5 mmol), phenylacetylene (0.6 mmol), DMF (3.0 ml), K₂CO₃ (0.5 mmol), 130 °C, N₂, 24 h. ¹H-NMR yields.

In presence of 4.0 mol% ligands (L₁–L₂₀), 1.0 mol% of CuI catalyzed cross-coupling reaction of 4-iodoanisole and phenyl-acetylene were used as a model reaction to evaluate the catalytic efficiency of α,β-ynones.^26a The simple aromatic α,β-ynone L₁ afforded moderate 50% yield, and L₂, L₃ and L₄ bearing methyl groups gave 40% to 36% yields. It was reported that chloro-benzene significantly accelerated transition metal catalyzed C–C cross-coupling reaction.²³ Herein, α,β-ynones ligands bearing halobenzene group were evaluated systematically. Six chlorinated α,β-ynones, L₅ to L₁₀ showed the different activity, in which L₈ bearing 4-OMe on benzoyl group accelerated the reaction with 75% yield. Interestingly, L₁₁ bearing 4-OMe on benzoyl group and 4-Br on phenylethenyl group gave the highest yield of 90%. Other ligands bearing methoxy groups (L₁₂–L₁₈) showed the decent activity and afforded up to 80% yield. Extra S-donor failed to further enhance the activity of α,β-ynone ligands compared with Ph(L₁). L₁₉ and L₂₀ bearing thiophenyl group gave 76% and 34% yield, respectively. These experiments clearly demonstrated that functionalized α,β-ynones ligands^26b,c system accelerated Cu catalyzed cross-coupling of Csp²–Csp bonds.

The efficiency of α,β-ynone ligands were further investigated for various Cu loadings. In the control experiment without ligand, 1.0 mol% CuI gave 40% yield (Table 1, entry 1). The accelerating effect were observed by adding 1.0–5.0 mol% of L₁₁, which significantly enhanced the catalytic activity of Cu catalyst and afforded 65–96% yields (Table 1, entries 2–7). Notably, 2.5 mol% L₁₁ gave the highest yield of 96% (Table 1, entries 4), 4.0 mol% and 5.0 mol% ligands slightly inhibited the activity of Cu catalyst, affording 92% and 89% yield (Table 1, entries 6 and 7), respectively. Notably, the ligand accelerating effect was also observed in the sub-mol% Cu catalyzed reaction. 1.25 mo% L₁₁ and 0.5 mol% CuI afforded 71% yield (Table 1, entry 8). As low as 0.25 mol% of L₁₁ and 0.1 mol% of CuI gave 50% yield (Table 1, entry 9).

Table 1 L₁₁ accelerated cross-coupling of 4-iodoanisole and phenylacetylene^a

Entry	CuI	L11	Yield^b (%)
a Reaction conditions: 4-iodo-anisole (0.5 mmol), phenylacetylene (0.6 mmol), DMF (3.0 ml), 130 °C, N2, 24 h, K2CO3 (0.5 mmol).b ^1H-NMR yields (average on three parallel reactions).
1	1	0	40
2	1	1	88
3	1	2	94
4	1	2.5	96
5	1	3	92
6	1	4	89
7	1	5	65
8	0.5	1.25	71
9	0.1	0.25	50

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The scope and limitation of α,β-ynones/CuI catalyst system were extensively explored with functionalized aryl iodides and terminal alkynes (Table 2). The cross-coupling reaction displayed remarkable tolerance towards the substitution pattern and steric effect.^26d,e Both electron-rich, electron-neutral or electron-deficient substituents and sterically encumbering substituents in the aryl ring gave good results. The aryl iodides containing electron-donating groups (4-OMe, 4-CH₃, 4-C₂H₅, 2-CH₃, 2-OMe, 3-OMe or 3-CH₃) gave isolated yields of 76% to 96% (Table 2, entries 1, 2, 4, 12, 13, 17, 18). When aryl iodides with electron-withdrawn groups (4-OCF₃, 4-CF₃, 4-NO₂, 4-COCH₃, 4-COOC₂H₅, 4-Br, 4-Cl, 4-CF₃, 2-CF₃, 2-Cl, 3-I or 3-CF₃) were used, the products were isolated with 56% to 90% yields (Table 2, entries 3, 5–10, 14–16, 19). When 1-octyne, 1-heptyne and 1-hexyne were employed instead of phenylacetylene, the lower yields (63%, 61%, 55%) were obtained (Table 2, entries 25–27). When 4-OMe, 4-CH₃, 4-C₂H₅, 4-C₃H₇, 3-CH₃, 3-OMe and 3-NH₂ phenylacetylene were involved in the coupling reaction, the good yields were obtained 69%, 63%, 56%, 80%, 74% and 58%, respectively (Table 2, entries 21–24, 28 and 29). Polycyclic compounds also reacted with phenylacetylene in excellent yields, 1-iodo-naphthalene was isolated in 63% yield (Table 2, entry 30).

Table 2 Cu-catalyzed Sonogashira cross-coupling reaction of aryl iodides with terminal alkynes^a

Entry	R1	R2	Yield^b (%)
a Reaction conditions: CuI (1.0 mol%), L11 (2.5 mol%), K2CO3 (0.5 mmol), DMF (3.0 ml), 130 °C, 24 h, aryl iodide (0.5 mmol), alkyne (0.6 mmol).b Isolated yields.
1	4-CH3O	C6H5	96
2	4-CH3	C6H5	89
3	4-NO2	C6H5	85
4	4-C2H5	C6H5	83
5	4-COCH3	C6H5	86
6	4-COOC2H5	C6H5	90
7	4-CF3	C6H5	76
8	4-OCF3	C6H5	74
9	4-Br	C6H5	78
10	4-Cl	C6H5	72
11	H	C6H5	89
12	2-CH3O	C6H5	75
13	2-CH3	C6H5	73
14	2-CF3	C6H5	65
15	2-Cl	C6H5	58
16	3-I	C6H5	56
17	3-CH3O	C6H5	81
18	3-CH3	C6H5	76
19	3-CF3	C6H5	70
20	H	CH3(CH2)5CCH	53
21	4-CH3O	4-CH3C6H4	69
22	4-CH3O	4-C2H5C6H4	63
23	4-CH3O	4-C4H9C6H4	56
24	4-CH3O	4-CH3OC6H4	80
25	4-CH3O	CH3(CH2)3CCH	63
26	4-CH3O	CH3(CH2)4CCH	61
27	4-CH3O	CH3(CH2)5CCH	55
28	4-CH3O	3-NH2C6H4	74
29	4-CH3O	3-CH3C6H4	58
30	1-Naphthalene	C6H5	63

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In the past several years, decarboxylative couplings of alkynylcarboxylic acids have been considered as a sub-category of Sonogashira coupling reactions since they are readily available and inexpensive.²⁴ The CuI (1.0 mol%)/α,β-ynones (2.5 mol%) system was also suitable for the decarboxylative coupling. We successfully applied the CuI/L₁₁ catalyst system in the decarboxylative cross-coupling reaction. The results were summarized in Table 3. The decarboxylative coupling of alkynylcarboxylic acid with aryl iodides containing electron-donating groups (–OMe or –CH₃) gave good yields. The moderate yields were obtained when aryl iodides bearing an electron-withdrawing groups (–NO₂, –Br) were used.

Table 3 CuI-catalyzed decarboxylative coupling reaction of aryl iodides with alkynylcarboxylic acid^a

Entry	R1	Yield^b (%)
a Conditions: CuI (1.0 mol%, 0.0009 g), L11 (2.5 mol%, 0.004 g), K2CO3 (0.5 mmol, 0.0689 g), aryl iodide (0.5 mmol), alkynylcarboxylic acid (0.5 mmol, 0.073 g), DMF (3.0 ml), 130 °C, 24 h.b Isolated yields.
1	4-CH3O	83
2	4-CH3	80
3	H	61
4	3-CH3O	42
5	4-NO2	58
6	4-Br	40

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Conclusions

In conclusion, we have disclosed α,β-ynones as efficient ligand system for copper catalyzed Sonogashira and decarboxylative cross-coupling reaction, respectively. The functionalized ligand L₁₁ bearing bromo- and methoxy-group is the most efficient ligand, by which 1.0 mol% copper catalyzed Sonogashira-type reactions. The catalyst system of CuI/L₁₁ were applied for the cross-coupling of both terminal alkynes and phenylpropiolic acids with various aryl iodides, respectively. The coupling reactions were compatible with a broad scope of functional groups and affords products in good to excellent yields. The activation mechanism of these α,β-ynone ligands is under investigation.

Experimental section

A mixture of aryl iodide (0.5 mmol), aryl alkyne (0.6 mmol), CuI (1.0 mol%), L₁₁ (2.5 mol%), K₂CO₃ (0.5 mmol), and dry DMF (3.0 ml) in a Schlenk tube was stirred under a nitrogen atmosphere at 130 °C for 24 h. After that, the mixture was poured into separating funnel (150 ml) and washed with water (50 ml). And then, it extracted with ethyl acetate (3 × 15 ml). The organic layer was dried by anhydrous MgSO₄, then filtered and evaporated under vacuum. The residue was separated by flash column chromatography.

Acknowledgements

This work was supported by the 111 Project (B14041), the grant from National Natural Science Foundation of China (21271124, 21272186, 21371112), the Fundamental Funds Research for the Central Universities (GK201501005, GK201302015, GK201503029), Natural Science Basic Research Plan in Shaanxi Province of China, Shaanxi Innovative Team of Key Science and Technology (2013KCT-17), the Program for Changjiang Scholars and Innovative Research Team in University (IRT-14R3).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23742h

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