Recyclable and reusable PdCl 2 (PPh 3 ) 2 /PEG-400/H 2 O system for the hydrophenylation of alkynes with sodium tetraphenylborate

Abstract

A stable and efficient PdCl₂(PPh₃)₂/PEG-400/H₂O catalytic system for the hydrophenylation reaction of alkynes has been developed. In the presence of 3 mol% PdCl₂(PPh₃)₂ and 2 equiv. of HOAc, the hydrophenylation of both terminal and internal alkynes with sodium tetraphenylborate proceeded smoothly in a mixture of PEG-400 and water at room temperature or 50 °C to afford a variety of phenyl-substituted alkenes in moderate to high yields. The isolation of the products was easily performed by extraction with petroleum ether, and the PdCl₂(PPh₃)₂/PEG-400/H₂O system could be readily recycled and reused six times without apparent loss of catalytic activity.

Introduction

Styrenes are important structural motifs extensively presented in pharmaceuticals, natural or synthetic products, and functional materials.^1–5 Traditionally, styrenes are prepared by the Wittig ⁶ or Peterson olefinations ⁷ and insertions of alkynes into organometallic reagents,^8–10 which are waste-intensive and require prefunctionalized starting materials. Catalytic methods such as the olefin metathesis^11,12 and Heck coupling reactions ¹³ are more atom-economic but also employ prefunctionalized arenes as substrates. In addition, C–H vinylations of the Fujiwara–Moritani type have provided an efficient route to styrenes, but stoichiometric oxidants are required.^14–16

In recent years, catalytic hydroarylation of alkynes has attracted considerable interest because it allows for the atom-economical construction of functionalized alkenes from relatively simple arenes and alkynes.^17–19 The hydroarylations of alkynes catalyzed by transition metal complex or Lewis acid through C–H bond activation of arenes have been widely investigated, but most research has mainly focused on the electron-rich arenes, and the regiocontrol of the reaction is still difficult, thereby providing a mixture of alkene derivatives.^20–23 Recently, the employment of arylboron compounds in hydroarylation reactions drew considerable attention because of their ready availability, stability, and high functional group compatibility.^24–28 Hayashi et al. ²⁹ reported hydroarylation of alkynes with arylboronic acids, and the reaction has been expanded to the arylative cyclization of aldehydes and alkyne-tethered alkenes.^30,31 Sodium tetraphenylborate is a stable and inexpensive phenylating reagent and has been widely used in organic synthesis. Palladium-catalyzed hydrophenylation of alkenes ³² and alkynes ³³ with sodium tetraphenylborate has been reported to proceed smoothly under mild conditions. However, the hydrophenylation reaction generally proceeds in the presence of a homogeneous palladium catalyst such as Pd(OAc)₂ or PdCl₂(PPh₃)₂, which makes the recovery of the expensive palladium catalyst tedious if not impossible and might cause unacceptable palladium contamination of the product. Therefore, from the standpoint of green and sustainable chemistry, the development of recyclable and reusable palladium catalytic systems for these important organic transformations is highly desirable.

As green organic synthesis is attracting more and more attention, the use of eco-friendly, green solvents is highly desirable. To address the recyclability of metal catalysts and environmental concerns, a convenient and efficient way is to anchor the catalyst in a liquid phase by dissolving it into a nonvolatile and nonmixing liquid, such as poly(ethylene glycols) (PEGs)^34–36 and ionic liquids. ³⁷ Generally, ionic liquids require a complicated preparative procedure, and their environmental safety is still being debated because the toxicity and environmental burden data are unknown for most of the ionic liquids. By contrast, PEGs are easily available and cheap, thermally stable, biodegradable, recoverable, and nontoxic liquid polymers which can be used as efficient media for eco-friendly and safe chemical reactions. Recently, PEGs have been widely utilized as green solvents for the palladium-catalyzed carbon–carbon bond-forming reactions such as the Heck coupling, ³⁸ the Suzuki coupling,^39–41 the homo-coupling and cross-coupling of aryl halides, ⁴² the direct arylation of 1,2,3-triazoles with aryl bromides, ⁴³ the Hiyama coupling, ⁴⁴ carbonylative Suzuki coupling, ⁴⁵ carbonylative Sonogashira coupling, ⁴⁶ and the homo-coupling of arylboronic acids ⁴⁷ with easy recyclability of the solvents and Pd catalysts. However, to the best of our knowledge, no palladium-catalyzed hydrophenylation reaction of alkenes or alkynes with sodium tetraphenylborate in PEGs has been described until now. Herein, we report the application of PdCl₂(PPh₃)₂/PEG-400/H₂O system as a highly efficient and reusable catalytic medium for the hydrophenylation of alkynes with sodium tetraphenylborate. This affords a variety of phenyl-substituted alkenes in moderate to high yields. The developed methodology shows important practical advantages deserving special note.

Results and discussion

In our initial screening experiments, the hydrophenylation of 1,2-diphenylacetylene (1a) with NaBPh₄ was selected as model reaction to optimize the reaction conditions. The effects of acids, solvents, and palladium catalysts on the reaction were studied, and the results are given in Table 1. At first, the acid effect was examined using PdCl₂(PPh₃)₂ (1 mol%) as catalyst with PEG-400 as solvent at room temperature (entries 1–3). It is evident that the reaction produced only a trace of target product 2a in the absence of any acid additive (Table 1, entry 1), and a low yield of 2a was obtained when 1 equiv. of HCl was used as the additive (Table 1, entry 2). However, when 1 equiv. of HOAc was used as the additive, the reaction was improved and afforded the desired 2a in 38% yield (Table 1, entry 3). Increasing the amount of HOAc or palladium catalyst loading was found to further promote the reaction, and moderate yields were achieved (Table 1, entries 4–6). Our next studies focused on the influence of solvent on the model reaction. It was found that a mixture of PEG-400 and water was more effective than PEG-400 alone as solvent (Table 1, entries 7–10). The reaction performed in PEG-400/H₂O (V/V, 1:1) gave the desired 2a in 94% yield (Table 1, entry 9). In addition, the efficiency of various chain length PEGs on the model reaction was investigated (Table 1, entries 11 and 12). PEG-400 was found to be superior to PEG-600 and PEG-1000. Other palladium catalysts such as Pd(OAc)₂, PdCl₂, and Pd(PPh₃)₄ were also tested, but none of them exhibited good catalytic activity (Table 1, entries 13–15). Reducing palladium loading from 3 to 1 mol% resulted in a significant decrease in the yield of 2a (Table 1, entry 16). Thus, the optimal catalytic system involved the use of PdCl₂(PPh₃)₂ (3 mol%), HOAc (2.0 equiv.) in PEG-400/H₂O (V/V, 1:1) at room temperature under Ar for 6 h (Table 1, entry 9).

Table 1.

Reaction of 1,2-diphenylacetylene (1a) with NaBPh₄. ^a

Entry	Pd catalyst (mol%)	Solvent (V/V)	Acid (equiv. to 1a)	Yield (%) b
1	PdCl2(PPh3)2 (1)	PEG-400	None	Trace
2	PdCl2(PPh3)2 (1)	PEG-400	HCl (1.0)	9
3	PdCl2(PPh3)2 (1)	PEG-400	HOAc (1.0)	38
4	PdCl2(PPh3)2 (1)	PEG-400	HOAc (2.0)	48
5	PdCl2(PPh3)2 (2)	PEG-400	HOAc (2.0)	58
6	PdCl2(PPh3)2 (3)	PEG-400	HOAc (2.0)	67
7	PdCl2(PPh3)2 (3)	PEG-400/H2O (3:1)	HOAc (2.0)	78
8	PdCl2(PPh3)2 (3)	PEG-400/H2O (2:1)	HOAc (2.0)	86
9	PdCl2(PPh3)2 (3)	PEG-400/H2O (1:1)	HOAc (2.0)	94
10	PdCl2(PPh3)2 (3)	PEG-400/H2O (1:2)	HOAc (2.0)	91
11	PdCl2(PPh3)2 (3)	PEG-600/H2O (1:1)	HOAc (2.0)	87
12	PdCl2(PPh3)2 (3)	PEG-1000/H2O (1:1)	HOAc (2.0)	74
13	Pd(OAc)2 (3)	PEG-400/H2O (1:1)	HOAc (2.0)	11
14	PdCl2 (3)	PEG-400/H2O (1:1)	HOAc (2.0)	15
15	Pd(PPh3)4 (3)	PEG-400/H2O (1:1)	HOAc (2.0)	29
16	PdCl2(PPh3)2 (1)	PEG-400/H2O (1:1)	HOAc (2.0)	48

a

Reactions were carried out with 1a (1.0 mmol), NaBPh₄ (1.0 mmol), palladium catalyst, and acid in solvent (2 mL) in a sealed tube at room temperature under Ar for 6 h.

b

Isolated yield.

Having achieved satisfactory results in the hydrophenylation of 1,2-diphenylacetylene with NaBPh₄, then, various internal and terminal alkynes were examined to explore the scope of substrates under the optimized reaction conditions and the results are listed in Table 2. As expected, symmetrical internal alkynes 1b–d displayed a similar reactivity as 1a and the hydrophenylation reactions proceeded smoothly to give the corresponding trisubstituted phenyl alkenes 2b–d in 70%–84% yields. Furthermore, the hydrophenylation of electron-poor internal alkyne 1e provided the adduct 2e in 80% yield. As for unsymmetrical internal alkynes, the reactions also took place effectively, and the regioselectivity depends on the nature of substituents. For example, the use of methyl but-2-ynoate 1f or ethyl but-2-ynoate 1g led to the formation of methyl 3-phenyl-2-butenoate 2f or ethyl 3-phenyl-2-butenoate 2g, respectively, in excellent yields and regioselectivity, indicating that the phenyl group was almost exclusively added to the β-carbon of the ynoate due to the presence of the strongly polarized C–C triple bond. The reaction of ethyl oct-2-ynoate 1h also proceeded smoothly with high yield, but a slightly lower regioselectivity of 93% was observed. Notably, ethyl 3-phenylpropiolate 1i showed a similar reactivity as 1f and 1g to furnish the expected product 2i in 92% yield and excellent regioselectivity. But, in the case of 1-phenylpropyne 1j, the two regioisomers 2j and 3j resulted from the cis-addition of NaBPh₄ to 1j were obtained in 43% and 32% yields, respectively. To our delight, when terminal alkynes were used as substrates, the hydrophenylation reactions also worked well with high regioselectivity in favor of the Markovnikov adducts. For instance, the hydrophenylation reactions of 1-hexyne 1k, 1-octyne 1l, and 5-phenyl-1-pentyne 1n afforded the corresponding phenyl-substituted alkenes 2k, 2l, and 2n, respectively, in 78%–89% yields. However, in the cases of phenylacetylene 1m, 5-chloro-1-pentyne 1o, and 5-cyano-1-pentyne 1p, the corresponding adducts 2m, 2o, and 2p were obtained in relatively lower yields of 48%–60%.

Table 2.

PdCl₂(PPh₃)₂-catalyzed hydrophenylation reaction of alkynes with NaBPh₄ in PEG-400/H₂O.^a,b

a

Reactions were carried out with alkyne (1.0 mmol), NaBPh₄ (1.0 mmol), HOAc (2.0 mmol), and PdCl₂(PPh₃)₂ (0.03 mmol) in a mixture of PEG-400 (1 mL) and H₂O (1 mL) in a sealed tube at room temperature under Ar for 6 h.

b

Isolated yield.

c

Determined by GC.

Sodium tetraarylborates are usually prepared by the reaction of arylmagnesium bromides with trimethyl borate, followed by hydrolysis. To explore the scope of the borates, we prepared sodium tetrakis(4-methylphenyl)borate via a complicated procedure starting from 4-bromotoluene, magnesium turnings, and trimethyl borate. The reaction of ethyl but-2-ynoate 1g with sodium tetrakis(4-methylphenyl)borate also proceeded smoothly under the conditions presented in Table 2 to afford the desired (E)-ethyl 3-p-tolylbut-2-enoate 4b in 88% yield with regioselectivity of 99%.

To explore the possibility for further efficient transfer of phenyl groups in NaBPh₄, we examined the reaction of 1a with 0.33 equiv. of NaBPh₄ in PEG-400/H₂O (1:1) under different conditions. It was found that when the reaction was carried out at 50 °C for 12 h, the desired product 2a was produced in 91% yield, indicating that at most three phenyl groups in NaBPh₄ could be efficiently utilized. We next performed the reaction of 0.33 equiv. of NaBPh₄ with several other alkynes at 50 °C in PEG-400/H₂O (1:1), and the results are given in Table 3. As shown in Table 3, both symmetrical and unsymmetrical internal alkynes underwent the hydrophenylation reaction smoothly to afford the corresponding adducts in good to high yields. But, the reactions of terminal alkynes 1l–n with 0.33 equiv. of NaBPh₄ gave the desired products 2l–n in only 39%–54% yields.

Table 3.

Palladium-catalyzed hydrophenylation of alkynes with NaBPh₄ (0.33 equiv.) in PEG-400/H₂O.^a,b

a

Reactions were carried out with alkyne (1.0 mmol), NaBPh₄ (0.33 mmol), HOAc (2.0 mmol), and PdCl₂(PPh₃)₂ (0.03 mmol) in a mixture of PEG-400 (1 mL) and H₂O (1 mL) in a sealed tube at 50 °C under Ar for 12 h.

b

Isolated yield.

c

Determined by GC.

It was reported that the reaction of NaBPh₄ with HOAc could produce Ph₃B, benzene, and NaOAc, and Ph₃B could react with H₂O further to form di- and monophenylboronic acid. ³² Thus, we also studied the reactivity of arylboronic acids with alkynes at room temperature (Scheme 1). As shown in Scheme 1, the palladium-catalyzed hydroarylation reaction of internal alkynes with various arylboronic acids also took place smoothly at room temperature in PEG-400/H₂O (1:1) to give the corresponding trisubstituted aryl alkenes in moderate yields. We next carried out the reaction of 1,2-diphenylacetylene 1a with PhB(OH)₂ under the conditions shown in Scheme 1 for direct comparison to the sodium tetraphenylborate, but the desired 2a was isolated in only 55% yield.

Scheme 1.

Palladium-catalyzed hydroarylation reaction of alkynes with arylboronic acids in PEG-400/H₂O (1:1).

A possible mechanism for the palladium-catalyzed hydrophenylation of alkynes with NaBPh₄ is illustrated in Scheme 2. First, oxidative addition of the C–B bond of Ph₃B formed in situ from reaction of NaBPh₄ with HOAc to (Ph₃P)₂Pd(0) produces intermediate A. ³² Subsequent selective insertion of the alkyne 1 into the Pd–C bond gives intermediate B, which undergoes a hydrolysis reaction to provide intermediate C. Finally, reductive elimination of intermediate C affords the desired phenyl-substituted alkene 2 and regenerates (Ph₃P)₂Pd(0) to complete the catalytic cycle. In addition to Ph₃B, Ph₂BOH and PhB(OH)₂ could also be possible intermediates in the palladium-catalyzed hydrophenylation reaction of alkynes with NaBPh₄.

Scheme 2.

Proposed catalytic cycle.

To evaluate the reusability of the solvent and the catalyst, the hydrophenylation reaction of 1,2-diphenylacetylene (1 mmol) with NaBPh₄ (1 mmol) was investigated in the presence of PdCl₂(PPh₃)₂ (3 mol%) and HOAc (2.0 equiv.) in PEG-400/H₂O (V/V = 1:1, 2 mL) at room temperature. As shown in Table 4, we were pleased to observe that the PdCl₂(PPh₃)₂/PEG-400/H₂O system could be recycled and reused up to six times without apparent loss of catalytic activity. After initial experimentation, the reaction mixture was extracted with petroleum ether (3 × 10 mL), and the PdCl₂(PPh₃)₂/PEG-400/H₂O system was then subjected to a second run of the reaction by charging with the same starting materials (1,2-diphenylacetylene, NaBPh₄, and HOAc) without addition of PdCl₂(PPh₃)₂. In addition, the leaching of palladium in the product was also determined, and inductively coupled plasma (ICP) analysis of the crude petroleum ether extract after the first cycle showed that palladium content was less than 0.80 ppm.

Table 4.

Recyclability of PdCl₂(PPh₃)₂/PEG-400/H₂O system. ^a

Cycle	Time (h)	Yield (%) b	Cycle	Time (h)	Yield (%) b
1	6	94	4	6	93
2	6	93	5	7	92
3	6	94	6	8	91

a

Reaction conditions: 1a (1 mmol), NaBPh₄ (1 mmol), PdCl₂(PPh₃)₂ (3 mol%), HOAc (2 mmol), and PEG-400/H₂O (1:1, 2.0 mL) at room temperature under Ar.

b

Isolated yield.

In conclusion, an efficient and recyclable catalytic system for the palladium-catalyzed hydrophenylation reaction of alkynes with sodium tetraphenylborate has been developed. In the presence of 3 mol% PdCl₂(PPh₃)₂ and HOAc, the hydrophenylation reaction of a variety of alkynes with NaBPh₄ proceeded smoothly at room temperature in PEG-400/H₂O (1:1) to afford the corresponding phenyl-substituted alkenes in moderate to high yields. Furthermore, the PdCl₂(PPh₃)₂/PEG-400/H₂O system could be recycled up to six times without apparent loss of catalytic activity. The present protocol will serve as an efficient and green way to prepare a variety of phenyl-substituted alkenes. Currently, further efforts to extend the application of the system in other palladium-catalyzed organic transformations are underway in our laboratory.

Experimental

All reagents were purchased from different commercial sources and used as received without further purification unless otherwise indicated. All reactions were carried out under Ar in a sealed reaction tube with magnetic stirring and the solvents needed to be de-gassed. ¹H NMR spectra were recorded on a Bruker Avance 400 (400 MHz) spectrometer with tetramethylsilane (TMS) as an internal standard using CDCl₃ as the solvent. ¹³C NMR spectra were recorded on a Bruker Avance 400 (100 MHz) spectrometer using CDCl₃ as the solvent. HRMS spectra were recorded on a quadrupole–time of flight Bruker MicroTOF-Q II mass spectrometer equipped with an electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) source. Palladium content was determined with inductively coupled plasma atom emission Atomscan16 (ICP-AES, TJA Corporation).

General procedure for palladium-catalyzed hydrophenylation of alkynes with NaBPh₄ in PEG-400/H₂O (1:1)

A mixture of alkyne 1 (1.0 mmol), NaBPh₄ (1.0 mmol), HOAc (2.0 mmol), PdCl₂(PPh₃)₂ (0.03 mmol), PEG-400 (1.0 mL), and H₂O (1.0 mL) was stirred under Ar in a sealed tube at room temperature for 6 h. After completion of the reaction, the reaction mixture was extracted three times with petroleum ether (3 × 10 mL). The combined ether phase was concentrated under reduced pressure, and the residue was purified by flash column chromatography on silica gel (eluting with petroleum ether) to afford the desired product 2.

The residue (the aqueous layer) of the extraction was heated to 60 °C in vacuum for 20 min to remove the residual petroleum ether, and then subjected to a second run of the reaction by charging with the same starting materials (alkyne, NaBPh₄, and HOAc) without addition of PdCl₂(PPh₃)₂ under identical conditions. After the first reaction cycle, the crude petroleum ether extract was subjected to ICP-AES analysis before any further purification.

1,1,2-Triphenylethene (2a): Pale yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.36-6.90 (m, 16H); ¹³C NMR (100 MHz, CDCl₃): δ = 143.5, 142.7, 140.4, 137.5, 130.5, 129.6, 128.7, 128.3, 128.2, 127.7, 127.5, 127.4, 126.8, 126.6. ³³

(E)-3-phenylhex-3-ene (2b): Pale yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.37-7.16 (m, 5H), 5.64 (t, J = 7.2 Hz, 1H), 2.51 (q, J = 7.4 Hz, 2H), 2.22-2.18 (m, 2H), 1.06 (t, J = 7.4 Hz, 3H), 0.99 (t, J = 7.6 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 143.2, 141.1, 130.1, 128.2, 126.5, 126.3, 22.8, 21.8, 14.4, 13.8. ³³

(E)-4-phenyloct-4-ene (2c): Pale yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.36-7.18 (m, 5H), 5.66 (t, J = 7.2 Hz, 1H), 2.48 (t, J = 7.6 Hz, 2H), 2.19-2.15 (m, 2H), 1.49-1.44 (m, 2H), 1.38-1.33 (m, 2H), 0.97 (t, J = 7.4 Hz, 3H), 0.88 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 143.7, 140.1, 129.3, 128.2, 126.4, 31.8, 30.8, 23.2, 21.9, 14.1. ³³

(E)-5-phenyldec-5-ene (2d): Pale yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.34-7.18 (m, 5H), 5.64 (t, J = 7.2 Hz, 1H), 2.49 (t, J = 6.8 Hz, 2H), 2.22-2.18 (m, 2H), 1.43-1.26 (m, 8H), 0.93 (t, J = 7.0 Hz, 3H), 0.87 (t, J = 6.6 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 143.6, 140.1, 129.1, 128.1, 126.3, 114.8, 32.1, 31.0, 29.5, 28.3, 22.7, 22.5, 14.1, 14.0. ³³

(Z)-dimethyl 2-phenylmaleate (2e): Yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.51-7.37 (m, 5H), 6.31 (s, 1H), 3.94 (s, 3H), 3.78 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 168.3, 165.4, 149.0, 133.2, 130.6, 129.0, 126.8, 117.1, 52.7, 52.0. ³³

(E)-methyl 3-phenylbut-2-enoate (2f): Pale yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.49-7.34 (m, 5H), 6.14 (q, J = 1.4 Hz, 1H), 3.76 (s, 3H), 2.59 (d, J = 1.4 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 167.4, 155.9, 142.2, 129.1, 128.6, 126.3, 116.8, 51.2, 18.1. ³³

(E)-ethyl 3-phenylbut-2-enoate (2g): Pale yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.48-7.26 (m, 5H), 6.13 (s, 1H), 4.22 (q, J = 7.2 Hz, 2H), 2.57 (s, 3H), 1.31 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 166.8, 155.4, 142.3, 128.9, 128.5, 126.3, 117.3, 59.8, 17.9, 14.3. HRMS calcd for C₁₂H₁₄O₂⁺ [M⁺]: 190.0994; found: 190.0988.

(E)-ethyl 3-phenyloct-2-enoate (2h): Pale yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.45-7.32 (m, 5H), 6.01 (s, 1H), 4.21 (q, J = 7.2 Hz, 2H), 3.09 (t, J = 7.6 Hz, 2H), 1.45-1.26 (m, 9H), 0.85 (t, J = 6.8 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 166.5, 160.8, 141.6, 128.7, 128.5, 126.7, 117.3, 59.8, 31.9, 31.0, 28.7, 22.4, 14.3, 14.0. ³³

Ethyl 3,3-diphenylacrylate (2i): Pale yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.42-7.21 (m, 10H), 6.39 (s, 1H), 4.08 (q, J = 7.2 Hz, 2H), 1.13 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 166.1, 156.5, 140.8, 129.4, 129.1, 128.5, 128.4, 128.3, 127.9, 117.5, 60.1, 14.0. ³³

(E)-1,2-diphenylpropene (2j): White solid; m.p. 81–82 °C. ¹H NMR (400 MHz, CDCl₃): δ = 7.53 (d, J = 7.2 Hz, 2H), 7.39-7.22 (m, 8H), 6.84 (s, 1H), 2.28 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 144.2, 138.5, 137.7, 129.3, 128.6, 128.3, 127.7, 127.3, 126.7, 126.2, 17.6. ³³

1,1-Diphenylpropene (3j): Colorless oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.39-7.16 (m, 10H), 6.17 (q, J = 7.2 Hz, 1H), 1.75 (d, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 143.1, 142.7, 140.2, 130.3, 128.3, 128.2, 127.4, 127.1, 126.8, 124.4, 15.8. ³³

2-Phenyl-1-hexene (2k): Pale yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.46-7.28 (m, 5H), 5.30 (s, 1H), 5.09 (d, J = 1.2 Hz, 1H), 2.54 (t, J = 7.4 Hz, 2H), 1.50-1.36 (m, 4H), 0.94 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 148.9, 141.6, 128.2, 127.2, 126.1, 112.0, 35.1, 30.5, 22.4, 13.9. ⁴⁸

2-Phenyl-1-octene (2l): Pale yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.40-7.23 (m, 5H), 5.25 (s, 1H), 5.04 (s, 1H), 2.49 (t, J = 7.2 Hz, 2H), 1.46-1.27 (m, 8H), 0.85 (t, J = 6.6 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 148.8, 141.6, 128.2, 127.2, 126.1, 112.0, 35.4, 31.7, 29.0, 28.3, 22.6, 14.1. ³³

1,1-Diphenylethene (2m): Brown oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.34-7.23 (m, 10H), 5.46 (s, 2H); ¹³C NMR (100 MHz, CDCl₃): δ = 150.2, 141.6, 128.3, 128.2, 127.7, 114.2. ³³

2,5-Diphenyl-1-pentene (2n): Yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.38-7.13 (m, 10H), 5.28 (s, 1H), 5.07 (s, 1H), 2.64 (t, J = 7.0 Hz, 2H), 2.54 (t, J = 7.2 Hz, 2H), 1.79-1.74 (m, 2H); ¹³C NMR (100 MHz, CDCl₃): δ = 148.4, 142.3, 141.4, 128.5, 128.3, 127.3, 126.2, 125.7, 112.4, 35.5, 34.9, 29.9. HRMS calcd for C₁₇H₁₈⁺ [M⁺]: 222.1409; found: 222.1414.

5-Chloro-2-phenyl-1-pentene (2o): Yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.43-7.25 (m, 5H), 5.33 (d, J = 1.4 Hz, 1H), 5.11 (d, J = 1.4 Hz, 1H), 3.54 (t, J = 6.4 Hz, 2H), 2.68 (t, J = 7.4 Hz, 2H), 1.96-1.87 (m, 2H); ¹³C NMR (100 MHz, CDCl₃): δ = 146.9, 140.7, 128.4, 127.5, 126.1, 113.3, 44.4, 32.4, 31.0. ⁴⁸

5-Cyano-2-phenyl-1-pentene (2p): Yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.42-7.28 (m, 5H), 5.38 (s, 1H), 5.16 (s, 1H), 2.72 (t, J = 7.2 Hz, 2H), 2.35 (t, J = 7.2 Hz, 2H), 1.86-1.79 (m, 2H); ¹³C NMR (100 MHz, CDCl₃): δ = 146.2, 140.2, 128.5, 127.8, 126.1, 119.4, 114.0, 34.0, 23.8, 16.4. HRMS calcd for C₁₂H₁₃N⁺ [M⁺]: 171.1048; found: 171.1046.

General procedure for palladium-catalyzed hydroarylation of alkynes with ArB(OH)₂ in PEG-400/H₂O (1:1)

A mixture of alkyne 1 (1.0 mmol), ArB(OH)₂ 3 (1.0 mmol), HOAc (2.0 mmol), PdCl₂(PPh₃)₂ (0.03 mmol), PEG-400 (1.0 mL), and H₂O (1.0 mL) was stirred under Ar in a sealed tube at room temperature for 6 h. After the reaction, the mixture was extracted three times with petroleum ether (3 × 10 mL). The combined ether phase was concentrated under reduced pressure, and the residue was purified by flash column chromatography on silica gel (eluting with petroleum ether) to afford the target product 4.

(Z)-dimethyl 2-p-tolylmaleate (4a): Yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.39 (d, J = 8.4 Hz, 2H), 7.22 (d, J = 8.0 Hz, 2H), 6.31 (s, 1H), 3.97 (s, 3H), 3.80 (s, 3H), 2.39 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 168.5, 165.6, 149.0, 141.2, 130.4, 129.8, 126.7, 115.9, 52.7, 52.0, 21.4. HRMS calcd for C₁₃H₁₄O₄⁺ [M⁺]: 234.0892; found: 234.0887.

(E)-ethyl 3-p-tolylbut-2-enoate (4b): Pale yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.38 (d, J = 8.0 Hz, 2H), 7.17 (d, J = 7.6 Hz, 2H), 6.13 (s, 1H), 4.20 (q, J = 7.2 Hz, 2H), 2.56 (s, 3H), 2.36 (s, 3H), 1.31 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 166.6, 153.9, 139.1, 137.7, 128.4, 126.4, 118.2, 51.9, 21.3, 18.2, 14.4. HRMS calcd for C₁₃H₁₆O₂⁺ [M⁺]: 204.1150; found: 204.1152.

(E)-ethyl 3-(4-methoxyphenyl)but-2-enoate (4c): Pale yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.45 (d, J = 8.8 Hz, 2H), 6.89 (d, J = 8.8 Hz, 2H), 6.11 (s, 1H), 4.20 (q, J = 6.8 Hz, 2H), 3.83 (s, 3H), 2.56 (s, 3H), 1.31 (t, J = 6.8 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 167.1, 160.4, 154.9, 134.4, 127.7, 115.4, 113.9, 59.7, 55.4, 17.7, 14.4. HRMS calcd for C₁₃H₁₆O₃⁺ [M⁺]: 220.1099; found: 220.1096.