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. 2024 Dec 9;26(50):10702–10707. doi: 10.1021/acs.orglett.4c03519

Palladium-Catalyzed Carbonylative Sonogashira Coupling of Aliphatic Alkynes and Aryl Thianthrenium Salts to Alkynones and Furanones

Yan-Hua Zhao †, Xing-Wei Gu †, Xiao-Feng Wu †,#,*
PMCID: PMC11667721  PMID: 39651552

Abstract

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Herein, we developed a mild and efficient palladium-catalyzed carbonylative Sonogashira coupling of aryl thianthrenium salts with aliphatic alkynes and benzyl acetylene toward alkynones and furanones. Various desired products were prepared in good yields with broad functional group tolerance including the bromide group. In the case of using benzyl acetylene, the corresponding furanones can be obtained in good yields under the same conditions with two molecules of CO inserted.

Alkynones are an important class of compounds found in many bioactive molecules,1 natural products,2 and pharmaceuticals.3 They are the key intermediates in synthesizing heterocyclic compounds4 such as furans,5 pyrazoles,6 pyrroles,7 and flavonoids.8 Hence, various synthetic procedures were developed for their preparation. Traditionally, the reaction of alkynyl organometallic reagents9 or terminal alkynes10 with acyl chlorides plays an important role in the preparation of alkynones. However, besides storage issues, the requirement of an inert atmosphere protection and the use of dry solvents limited their applications. Moreover, the method often suffers from a narrow functional group tolerance and poor substrate stability. To solve these problems, alternative methods are highly desired, such as transition metal-catalyzed carbonylative Sonogashira reaction.11

Carbon monoxide (CO), as a cheap and easily available source of C1, has been recognized as an important synthon with the development of carbonylation chemistry.10d,12 In 1981, Kobayashi and Tanaka reported the preparation of acetylenic ketones by the palladium-catalyzed carbonylation Sonogashira coupling reaction of terminal alkynes with organic halides first.13 Afterward, the substrates were expanded aryl triflates,14 and aryl triazenes.15 In 2011, Lee’s group reported the synthesis of alkynyl ketones by the reaction of aryl iodides with alkynyl carboxylic acids in the presence of a Pd/Cu catalyst under a CO atmosphere.16 In 2017, Wu’s group reported a carbonylation cross-coupling method for aryl diazonium salts with terminal alkynes by using formic acid as the CO source.17 In the same year, El Ali’s group reported the carbonylation Sonogashira coupling reaction of aryl alkynes, alkyl alkynes and dialkynes with aryl iodides in the presence of (N-heterocyclic carbene) Pd(pyridine)Br2 complex.18 Recently, Lei’s group achieved a smooth synthesis of acetylenic ketones from arylhydrazines and terminal alkynes by electrochemical Pd-catalyzed oxidative Sonogashira carbonylation.19

Additionally, furanones are vital structural motifs20 in a variety of pharmaceutical molecules and natural products with a range of biological activities such as cardiotonic, anticancer, analgesic, antituberculosis, anti-inflammatory, antimicrobial, antimalarial, and antiviral activities.21 Presently, medicines incorporating a furanone substrate are even commercially available, e.g., narthogenin, ascorbic acid, butalactin, basidalin, and rofecoxib. Thus, far, there have been several reports on the synthesis of furanones, by transition metal-catalyzed cyclization of alkyne substrates.22

In recent years, thianthrenes have been brought to the attention of scientists inspired by the studies from groups including Shine, Ritter, and others.23 Thianthrenium salts have a higher activity and better chemoselectivity than aryl halides. In addition, thianthrenium salts are reactive electrophilic reagents that are readily available from inexpensive and readily available aromatic hydrocarbons. It was demonstrated in previous work by our group that the thianthrenium salt can undergo a carbonylation cross-coupling reaction with aryl alkynes in the presence of a palladium catalyst to give the corresponding alkynes in excellent yields.24 Unfortunately, aliphatic alkynes were failed in this catalytic system. In order to solve this challenge, we restudied this transformation with aliphatic alkynes. Herein, we report our new results on palladium-catalyzed carbonylative Sonogashira coupling of aliphatic alkynes with thianthrene salts. Notably, in the case of using benzyl acetylene, the corresponding furanones can be obtained in good yields under the same conditions with two molecules of CO inserted.

Initially, we started with readily available aryl thianthrenium salt 2a and heptane 1a as the model substrates to establish this reaction, and the target product 3a was obtained (Table 1). After a systematic investigation of the reaction parameters, we determined the optimal conditions: alkyl alkyne 1a (1.0 equiv), aryl thianthrenium salt 2a (1.5 equiv), PdI2 (2 mol %), PPh3 (6 mol %), DIPEA (2.0 equiv), DMF (0.1 mol L-1), under CO (10 bar) atmosphere, at 80 °C for 15 h, and the corresponding alkenone 3a was successfully obtained with a GC yield of 78% (72% isolated yield, Table 1, entry 1). In variation experiments, the reaction did not proceed in the absence of a ligand, catalyst, or base (Table 1, entries 2–4). Different bases were then examined, and only 56% of the product was detected when DABCO was used as the base (Table 1, entry 5). No product could be detected when inorganic bases (e.g., Cs2CO3 or K3PO4) were tested (Table 1, entries 6–7). Next, different catalyst precursors (e.g., Pd(OAc)2, PdBr2, (cinnamyl)2Pd2Cl2) were checked in the reaction, and the yield of the desired product decreased (Table 1, entries 8–10). Screening of other phosphine ligands showed that monodentate phosphine ligands (e.g., PCy3 or BuPAd2) exhibited good reactivity (Table 1, entries 11–12). In contrast, bidentate phosphine ligand BINAP was ineffective in promoting the reaction, and only a trace amount of the desired product was detected (Table 1, entry 13). Among the solvents tested, 61–62% of the target product was obtained using DCE and DMSO instead of DMF (Table 1, entries 14–15). The corresponding acetylenic ketone was formed in 40% yield when acetonitrile was used as the solvent (Table 1, entry 16). Moreover, modifying the reaction concentration resulted in a decreased yield (Table 1, entry 17). When the amount of arylthianthrene salt 2a was reduced, a 70% yield of the target product was achieved. In addition, the reaction was inhibited by the addition of CuI as an additive (Table 1, entry 19).

Table 1. Optimization of the Reaction Conditionsa.

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Entry Variation from the Standard conditions Yield (%)b
1 None 78 (72)c
2 No ligand n.o.
3 No catalyst n.o.
4 No base n.o.
5 DABCO instead DIPEA 56
6 Cs2CO3 instead of DIPEA n.o.
7 K3PO4 instead of DIPEA n.o.
8 Pd(OAc)2 instead of PdI2 65
9 PdBr2 instead of PdI2 65
10 (cinnamyl)2Pd2Cl2 instead of PdI2 61
11 PCy3 instead of PPh3 39
12 BuPAd2 instead of PPh3 63
13 BINAP instead of PPh3 trace
14 DCE as solvent 61
15 DMSO as solvent 62
16 CH3CN as solvent 40
17 0.2 mol L–1 DMF 66
18 1.0 equiv. 2a 70
19 CuI (10 mol %) as additive trace
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a

The reaction was conducted using 1a (0.2 mmol), 2a (x mmol), catalyst (2 mol %), ligand (6 mol %), base (2.0 equiv), CO (10 bar), 80 °C, 15 h.

b

Determined by GC with hexadecane as internal standard.

c

Isolated yield is shown in parentheses.

Under the optimal reaction conditions, we initially examined thianthrenium salts containing different substituents. Methyl, methoxy, and deuterated thianthrenium obtained the corresponding alkynes in high yields (Table 2, entries 1–3). Disubstituted salts can afford the target product in 71% yield (Table 2, entry 10). To investigate the chemoselectivity of the reaction, halogen-substituted thianthrenium salts were tested, and the corresponding products were obtained in moderate yields (Table 2, entries 4–6). Delightfully, the disubstituted substrates containing C(sp2)–X bonds (X = F, Cl, and Br) can also be successfully converted into the thianthrenium desired alkynones in good yields (Table 2, entries 7–9). However, the reaction become messy when iodide containing substrate was tested. In addition, halogen-modified trisubstituted aryl thianthrene salts were also compatible with this reaction system, yielding the targeted product in 60% yield (Table 2, entry 11). This indicates that the catalytic system has good chemoselectivity and provides the possibility of further structural modifications.

Table 2. Substrate Scope of Aryl Thianthrenium Saltsa.

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graphic file with name ol4c03519_0005.jpg

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a

Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), PdI2 (2 mol %), PPh3 (6 mol %), DIPEA (2.0 equiv), CO (10 bar), DMF (0.1 M), 80 °C, 15 h. yields of isolated products are shown. TT = thianthrene.

Subsequently, we examined several aliphatic alkynes (Table 3). Simple straight-chain alkynes showed good reactivity to the corresponding products in good yields (Table 3, entries 1–4, 7). Cyclohexylacetylene and cyclopropylacetylene were also used as substrates and transformed into the corresponding acetylenic ketones in high yields (Table 3, entries 5–6). Here, no compounds related with ring opening of the cyclopropane group could be detected from the reaction mixture. In addition, phenylacetylene was also amenable to these reaction conditions, and the target product was obtained in good yield (Table 3, entry 8). Notably, benzylacetylene was used as a substrate in this catalytic system as well, and the corresponding furanone was generated in good yield with two molecules installed (Table 4, entry 1). Under optimal conditions, the reaction of benzylacetylene with aryl thianthrenes containing different substituents all led to differently substituted furanones without problem (Table 4, entries 2–6).

Table 3. Substrate Scope of Acetylenea.

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graphic file with name ol4c03519_0007.jpg

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a

Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), PdI2 (2 mol %), PPh3 (6 mol %), DIPEA (2.0 equiv), CO (10 bar), DMF (0.1 M), 80 °C, 15 h. yields of isolated products are shown. TT = thianthrene.

Table 4. Substrate Scope of Acetylenea.

graphic file with name ol4c03519_0008.jpg

graphic file with name ol4c03519_0009.jpg

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a

Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), PdI2 (2 mol %), PPh3 (6 mol %), DIPEA (2.0 equiv), CO (10 bar), DMF (0.1 M), 80 °C, 15 h. yields of isolated products are shown. TT = thianthrene.

We performed several mechanistic experiments to illustrate the reaction mechanism. Adding TEMPO (2 equiv) or BHT (1–3 equiv) was added to the reaction under standard conditions, the reaction still progressed smoothly and provided the corresponding acetylenic ketones in 69–71% yields (Scheme 1a). In addition, 1,1-diphenylethylene (1,1-DPE) was added to the standard reaction mixture, which provided the desired acetylenic ketone 3a in 65% yield (Scheme 1b). Therefore, we can rule out the possibility of free radical intermediate involvement.

Scheme 1. Control Experiments.

Scheme 1

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A possible reaction mechanism was introduced based on the above results and related reports25 (Scheme 2). Initially, the active Pd(0) complex A was produced from the palladium precursor, which generated the palladium complex B by oxidative addition with an aryl thianthrenium salt. After that, the CO coordinates and inserts into complex B to produce intermediate C. Then terminal alkynes undergo a ligand exchange reaction with intermediate C to generate alkynyl palladium complex D. Finally, reductive elimination from complex D to give the target product and meanwhile regenerate complex A for the next catalytic cycle (cycle I). In the case of benzylacetylene, the produced alkynone will isomerize to the corresponding allene in the presence of a base. Then it is activated by palladium to give intermediate E which will give complex F after the coordination and insertion of the second molecule of CO. Finally, reductive elimination will release the isolated furanone product and regenerate palladium for the next catalytic cycle (cycle II).

Scheme 2. Proposed Mechanism.

Scheme 2

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In conclusion, we developed a new palladium-catalyzed carbonylative Sonogashira coupling reaction of aliphatic alkynes and benzylacetylenes with aryl thianthrenium salts. In general, moderate to good yields of the desired alkynones can be produced selectively. Additionally, good yields of furanones can be obtained using benzylacetylene as the feedstock. Among the obtained products, a range of halogen-containing acetylenic ketones and furanones were formed, which also provide opportunities for further transformations.

Acknowledgments

We thank the financial supports from National Key R&D Program of China (2023YFA1507500) and Chinese Scholarship Council. We also thank the analytical department of Leibniz-Institute of Catalysis for their excellent analytical service.

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.orglett.4c03519.

  • General comments, general procedure, optimization details, analytic data, and NMR spectra (PDF)

The authors declare no competing financial interest.

Supplementary Material

ol4c03519_si_001.pdf (5.7MB, pdf)

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Associated Data

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Supplementary Materials

ol4c03519_si_001.pdf (5.7MB, pdf)

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.

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