Highly stereoselective heterogeneous palladium-catalyzed transfer semihydrogenation of internal alkynes to access cis -alkenes

Abstract

A highly efficient heterogeneous palladium-catalyzed transfer semihydrogenation of internal alkynes was achieved in dimethylformamide at 145 ºC by using an MCM-41-immobilized bidentate nitrogen palladium(II) complex [MCM-41-2N-Pd(OAc)₂] as catalyst and dimethylformamide/KOH as hydrogen source, yielding a variety of cis-alkenes in good to high yields with excellent stereoselectivity. This supported palladium catalyst can be easily recovered by filtration of the reaction solution and recycled up to seven times with almost consistent activity.

Introduction

Alkenes are highly useful building blocks in organic synthesis because of their versatile further functionalization. They are commonly used as substrates in a variety of organic reactions including Markovnikoff’s addition, epoxidation, ozonolysis, hydroboration, hydrogenation, hydroformylation reactions, and so on.^1–3 This diversity in application makes them important intermediates in the synthesis of drug molecules, natural products, and biologically active compounds.^4–6 In addition, well-defined (E)- or (Z)-alkenes constitute an integral part of many biologically active molecules. ⁴ As a result, many synthetic methods such as the Wittig reaction, ⁷ Horner–Emmons–Wadsworth reaction, ⁸ Peterson reaction, ⁹ Julia–Kocienski reaction, ¹⁰ Takai olefination, ¹¹ olefin metathesis, ¹² and cross-coupling reactions ¹³ have been developed to construct C–C double bond. Besides, the reduction of alkynes is an attractive alternative route to alkenes due to its high atom economy.^4,14

The selective semihydrogenation of internal alkynes to cis-alkenes is a highly useful reaction in organic synthesis, and there are mainly two types of catalytic processes to complete this transformation. One process involves hydrogenation using hydrogen gas (H₂) in the presence of Lindlar’s catalyst,^4,15 or other heterogeneous palladium catalysts.^16–18 The other is transfer hydrogenation with hydrogen donors. ¹⁹ The latter has obvious advantages such as safer operation and easy control of chemoselectivity of alkenes. Recently, transition metal-catalyzed transfer semihydrogenation of internal alkynes has been shown to be a highly efficient method for the synthesis of cis-alkenes and various transition metal catalysts such as palladium,^20–24 ruthenium,^25,26 gold, ²⁷ nickel, ²⁸ and copper ²⁹ based catalytic systems have been developed for this transformation. However, the use of expensive palladium, ruthenium, and gold as well as difficult recovery and non-recyclability of the metal catalysts make these methods of limited synthetic utility from economic and environmental points of view. Catalyst recycle is one of the most important features for many green synthetic methods. The heterogenization of the existing homogeneous metal catalysts appears to be a logical solution to this problem. There has been considerable interest in the development of heterogeneous transition metal catalytic systems that can be facilely recycled for several times with almost consistent activity. However, to the best of our knowledge, no examples of supported palladium complexes–catalyzed transfer semihydrogenation of internal alkynes to cis-alkenes have been described until now, despite the practical benefits of heterogeneous catalysis.

Mesoporous MCM-41 materials have recently emerged as powerful supports for immobilization of homogeneous catalysts owing to their outstanding properties.^30–32 Recently, we reported the synthesis of MCM-41-immobilized bidentate nitrogen palladium(II) complex [MCM-41-2N-Pd(OAc)₂] and found that it was a highly efficient and recyclable catalyst for the Suzuki–Miyaura reaction of aryl bromides, ³³ the cross-coupling of acyl chlorides with terminal alkynes, ³⁴ and the carbonylative cross-coupling of triarylbismuths with aryl iodides. ³⁵ In continuation of our effort to develop economical and eco-friendly synthetic pathways for organic transformations,^33–35 herein, we reported a heterogeneous palladium-catalyzed transfer semihydrogenation of internal alkynes by using the MCM-41-2N-Pd(OAc)₂ complex as catalyst with dimethylformamide (DMF)/KOH as hydrogen source to provide cis-alkenes in good to high yields with excellent stereoselectivity (Scheme 1).

Scheme 1.

Heterogeneous palladium-catalyzed transfer semihydrogenation of internal alkynes.

Results and discussion

The MCM-41-immobilized bidentate nitrogen palladium(II) complex [MCM-41-2N-Pd(OAc)₂] could be conveniently prepared via a simple two-step procedure as shown in Scheme 2. ³⁴ First, the mesoporous MCM-41 was condensed with commercially readily available and inexpensive 3-(2-aminoethylamino)propyltrimethoxysilane in toluene under reflux for 24 h, followed by the silylation with Me₃SiCl at room temperature for 24 h to produce the 3-(2-aminoethylamino)propyl-functionalized MCM-41 (MCM-41-2N). Then the latter was reacted with Pd(OAc)₂ in acetone under reflux for 72 h to afford the MCM-41-immobilized bidentate nitrogen palladium(II) complex [MCM-41-2N-Pd(OAc)₂] as a pale yellow powder. The palladium content of this heterogeneous palladium complex was determined to be 0.29 mmol g⁻¹ by inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis.

Scheme 2.

Preparation of MCM-41-2N-Pd(OAc)₂.

In continuation of our research to employ this heterogeneous palladium system for other organic transformations, we commenced our studies by investigating the transfer semihydrogenation of 1,2-diphenylacetylene 1a with DMF as both solvent and hydrogen source to optimize reaction conditions. We selected DMF as the hydrogen source since it was reported that DMF could be employed as a hydrogen source via its decomposition in the presence of palladium(II) complexes. ³⁶ Table 1 shows the results of heterogeneous palladium(II)-catalyzed semihydrogenation of 1a with DMF as both solvent and hydrogen source under various conditions. When a mixture of 1a and MCM-41-2N-Pd(OAc)₂ (2 mol%) in DMF was heated at 145 °C for 6 h, only trace amounts of stilbenes were observed and the recovery yield of 1a exceeded 95% (entry 1). We next examined the reaction by the addition of bases due to the fact that bases could promote the hydrolysis of DMF. ³⁷ It was found that the addition of K₂CO₃ or Cs₂CO₃ was ineffective (entries 2 and 3), but the addition of KOH (1.0 equiv.) could remarkably accelerate the semihydrogenation reaction of 1a to provide cis-stilbene 2a in 78% yield with excellent stereoselectivity (entry 4). To our delight, when 1.5 equiv. of KOH was used, the stilbenes were obtained in almost quantitative yield, with negligible amounts of 1,2-diphenylethane which arises from the over-reduction of stilbenes (entry 5). Besides, the stereoselectivity of 2a was up to 98%, and 2a could be isolated in 94% yield from the reaction mixture. Even if the reaction was carried out for 24 h, cis-stilbene 2a could not be isomerized into trans-stilbene 3a (entry 6). However, if the reaction was conducted at 100 ºC for 24 h, the decrease in the stereoselectivity of 2a was observed (entry 7). The reaction run in an air atmosphere resulted in a decrease in the total yield of stilbenes, which may be due to the fact that the oxidation state of the palladium catalyst was mainly Pd(II) other than Pd(0) at high temperature (145 ºC) under air (entry 8). When Pd(OAc)₂ (2 mol%) was used as the catalyst with KOH (1.5 equiv.) as the base at 145 ºC under Ar, the desired 2a was also isolated in excellent yield, which indicating that catalytic activity of MCM-41-2N-Pd(OAc)₂ was comparable to that of Pd(OAc)₂ (entry 9). Reducing the amount of MCM-41-2N-Pd(OAc)₂ to 1 mol% resulted in a decreased yield (entry 10). Therefore, the optimized conditions for this transformation are the MCM-41-2N-Pd(OAc)₂ (2 mol%), KOH (1.5 equiv.) as base in DMF as both solvent and hydrogen source at 145 °C under argon atmosphere for 6 h (Table 1, entry 5).

Table 1.

Heterogeneous Pd-catalyzed semireduction of 1,2-diphenylacetylene 1a. ^a

Entry	Pd Catalyst (mol%)	Additive (equiv.)	GC yield (%) b	2a:3a c
1	MCM-41-2N-Pd(OAc)2 (2.0)	–	<5
2	MCM-41-2N-Pd(OAc)2 (2.0)	K2CO3 (1.5)	<5
3	MCM-41-2N-Pd(OAc)2 (2.0)	Cs2CO3 (1.5)	<5
4	MCM-41-2N-Pd(OAc)2 (2.0)	KOH (1.0)	78	99:1
5	MCM-41-2N-Pd(OAc)2 (2.0)	KOH (1.5)	98 (94)	98:2
6 d	MCM-41-2N-Pd(OAc)2 (2.0)	KOH (1.5)	98	98:2
7 e	MCM-41-2N-Pd(OAc)2 (2.0)	KOH (1.5)	98	85:15
8 f	MCM-41-2N-Pd(OAc)2 (2.0)	KOH (1.5)	38	89:11
9	Pd(OAc)2 (2.0)	KOH (1.5)	99	97:3
10 g	MCM-41-2N-Pd(OAc)2 (1.0)	KOH (1.5)	56	99:1

GC: gas chromatography; DMF: dimethylformamide.

a

Reaction conditions: 1a (0.5 mmol), DMF (1.0 mL) in a sealed tube at 145 ºC for 6 h under Ar.

b

The number in parentheses is isolated yield.

c

Determined by GC analysis of reaction mixture.

d

The reaction was carried out for 24 h.

e

The reaction was performed at 100 ºC for 24 h.

f

The reaction was carried out under air.

g

The reaction was performed for 12 h.

The cis-stilbene skeleton is found in many natural products that exhibit a variety of biological activities, ³⁸ thus, development of a general, efficient, and practical method for construction of cis-stilbenes by heterogeneous palladium-catalyzed transfer semihydrogenation of diarylacetylenes with high stereoselectivity is highly desirable. With the optimized conditions in hand, we next investigated heterogeneous palladium-catalyzed semireduction of a variety of internal alkynes and the results are summarized in Table 2. In general, all the reactions proceeded smoothly under the standard reaction conditions to afford the cis-alkenes in good to high yields with excellent stereoselectivity. Diarylacetylenes bearing various substituents 1b–h, regardless of their electronic properties and substitution positions, reacted well in this transformation, thus giving the corresponding cis-stilbenes 2b–h in 85%–93% yields with stereoselectivity of 94%–98% (entries 2–8). The electron-rich diarylacetylenes 1b–e showed a slightly lower reactivity than the electron-deficient ones 1f–h and required a longer time (9 h) to complete the reaction. The reaction also worked well with 4-(phenylethynyl)biphenyl 1i to furnish the desired cis-stilbene 2i in 94% yield (entry 9). Bulky 1-naphthyl or 2-naphthyl-substituted internal alkynes 1j and k proved to be good substrates and gave the desired products 2j and k in high yields with stereoselectivity of 99% (entries 10 and 11). Notably, 2-phenylethynylthiophene 1l was also compatible with the employed conditions and could undergo the highly selective transfer semihydrogenation to afford the target product 2l in good yield (entry 12). In addition, alkyl-substituted internal alkynes 1m–r also underwent the transfer semihydrogenation smoothly to provide the corresponding cis-alkenes 2m–r in high yields with excellent stereoselectivity, and the associated double-bond shift isomerization was not observed (entries 13–18).

Table 2.

Heterogeneous Pd-catalyzed semireduction of internal alkynes. ^a

Entry	R	R1	Product	Yield (%) b	Selectivity (%) c
1	Ph	Ph	2a	94	98:2
2 d	4-MeC6H4	4-MeC6H4	2b	92	98:2
3 d	4-MeC6H4	4-MeOC6H4	2c	85	97:3
4 d	Ph	4-MeOC6H4	2d	88	94:6
5 d	Ph	2-MeOC6H4	2e	90	95:5
6	Ph	4-FC6H4	2f	93	96:4
7	Ph	4-ClC6H4	2g	87	97:3
8	Ph	2-CF3C6H4	2h	91	98:2
9	Ph	4-PhC6H4	2i	94	98:2
10	Ph	1-Naphthyl	2j	89	99:1
11	Ph	2-Naphthyl	2k	92	99:1
12	Ph	2-Thienyl	2l	75	93:7
13 d	Ph	PhCH2CH2CH2	2m	81	96:4
14	Ph	C2H5	2n	87	97:3
15	Ph	n-Bu	2o	89	95:5
16	Ph	n-Hexyl	2p	91	95:5
17	1-Naphthyl	Cyclopropyl	2q	83	98:2
18	n-Bu	n-Bu	2r	85	99:1

DMF: dimethylformamide; GC: gas chromatography.

a

Reaction conditions: 1 (1 mmol), KOH (1.5 mmol), MCM-41-2N-Pd(OAc)₂ (2 mol%), DMF (2 mL), in a sealed tube at 145 ºC for 6 h under Ar.

b

Isolated yield.

c

Determined by GC analysis of the reaction mixture.

d

For 9 h.

To verify whether the observed catalysis was due to the heterogeneous catalyst MCM-41-2N-Pd(OAc)₂ or to a leached palladium species in solution, we conducted the hot filtration test. ³⁹ We focused on the semihydrogenation of diphenylacetylene 1a. We removed the MCM-41-2N-Pd(OAc)₂ complex from the reaction mixture by filtration after 2 h of reaction time and allowed the filtrate to react further under the identical conditions. The catalyst filtration was performed at 145 °C to avoid possible recoordination or precipitation of soluble palladium upon cooling. It was found that, in this case, no significant increase in conversion of 1a was observed, indicating that leached palladium species from MCM-41-2N-Pd(OAc)₂ (if any) are not responsible for the observed conversion. In addition, ICP-AES analysis showed that no palladium species could be detected in the filtrate (below 0.1 ppm). These results exclude any contribution to the observed conversion from a homogeneous palladium species, demonstrating that the semihydrogenation reaction catalyzed by palladium was intrinsically heterogeneous.

A plausible mechanism for this heterogeneous palladium-catalyzed transfer semihydrogenation of internal alkynes is illustrated in Scheme 3. ²³ First, oxidative addition of HCOOH, generated in situ by the hydrolysis of DMF in the presence of KOH, to MCM-41-2N-Pd(0) provides an MCM-41-anchored HCOO-Pd(II)-H complex intermediate A. Subsequent selective insertion of internal alkyne 1 into the Pd–H bond of intermediate A gives intermediate B. Then intermediate B undergoes a decarboxylation to form an MCM-41-anchored vinyl-Pd(II)-H complex intermediate C. Finally, reductive elimination of intermediate C affords the desired cis-alkene 2 and regenerates the MCM-41-2N-Pd(0) complex to complete the catalytic cycle.

Scheme 3.

Proposed catalytic cycle.

For a supported precious metal catalyst, it is important to evaluate its ease of separation, recoverability, and reusability. We next investigated the recyclability of MCM-41-2N-Pd(OAc)₂ by using the transfer semihydrogenation reaction of diphenylacetylene 1a. After completion of the reaction, the catalyst was separated from the reaction mixture by simple filtration and washed with distilled water and acetone. After being air-dried, it can be reused directly without further purification. The recovered palladium catalyst was used in the next run, and almost the same yield of 2a was observed for eight consecutive cycles (94%, 93%, 93%, 92%, 91%, 92%, 90%, and 89%, respectively). The excellent recyclability of the catalyst should be attributed to the chelating action of bidentate nitrogen ligand on palladium and the mesoporous structure of the MCM-41 support.

In conclusion, we have developed an efficient and practical heterogeneous palladium-catalyzed transfer semihydrogenation of internal alkynes leading to cis-alkenes by using an MCM-41-supported bidentate nitrogen palladium(II) complex [MCM-41-2N-Pd(OAc)₂] as catalyst and DMF/KOH as the hydrogen source. The reactions generated a wide variety of cis-alkenes in good to excellent yields with excellent stereoselectivity and were applicable to a range of diarylacetylenes and alkyl-substituted internal alkynes. Importantly, this heterogeneous palladium catalyst can be easily prepared via a simple procedure from commercially readily available reagents, and recovered by filtration of the reaction solution and recycled up to seven times with almost consistent activity, thus making this approach economically and environmentally more acceptable.

Experimental

All reagents were analytical grade and used as received without further purification. The MCM-41-immobilized bidentate nitrogen palladium(II) complex [MCM-41-2N-Pd(OAc)₂] was prepared according to our previous method, ³⁴ the palladium content was determined to be 0.29 mmol g⁻¹ by ICP-AES analysis. DMF was chemical pure and used as received without drying. All reactions were carried out under Ar in oven-dried glassware with magnetic stirring. The products were purified by flash chromatography on silica gel using cyclohexane as eluent. All products were characterized by comparison of their spectra and physical data with authentic samples. The gas chromatography (GC) analysis of the reaction mixture was performed using Perkin Elmer Clarus 400 GC equipped with flame ionization detector with a capillary column and n-octadecane was used as an internal standard. 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. Palladium content was determined with inductively coupled plasma atom emission Atomscan16 (ICP-AES, TJA Corporation).

General procedure for the heterogeneous palladium-catalyzed transfer semihydrogenation of internal alkynes

Under an argon atmosphere, a thick-walled reaction tube was charged with KOH (84 mg, 1.5 mmol) and DMF (1.5 mL). The mixture was stirred at 145 °C under Ar for 0.5 h. After being cooled to room temperature, MCM-41-2N-Pd(OAc)₂ (69 mg, 0.02 mmol) and internal alkyne (1.0 mmol) were added to the tube and the resulting mixture was stirred at 145 °C under Ar for 6–9 h. After completion of the reaction, the reaction mixture was cooled to room temperature, diluted with ethyl acetate (15 mL), and filtered. The MCM-41-2N-Pd(OAc)₂ catalyst was washed with distilled water (2 × 5 mL) and acetone (2 × 5 mL), and reused in the next run. The filtrate was concentrated under reduced pressure and the residue was purified by flash column chromatography on silica gel (eluent: cyclohexane) to provide the desired product 2.

Cautionary note. the base-induced thermal decomposition of DMF with base generates CO₂ and Me₂NH, both gases at the temperatures involved. It is likely that significant pressure build up would occur at larger scales of reaction.

cis-Stilbene (2a). ²³ Oil. ¹H NMR (400 MHz, CDCl₃): δ 7.27–7.14 (m, 10H), 6.60 (s, 2H). ¹³C NMR (100 MHz, CDCl₃): δ 137.3, 130.3, 128.9, 128.3, 127.2.

cis-4,4′-Dimethylstilbene (2b). ²³ Oil. ¹H NMR (400 MHz, CDCl₃): δ 7.06 (d, J = 8.0 Hz, 4H), 6.92 (d, J = 8.0 Hz, 4H), 6.40 (s, 2H), 2.20 (s, 6H). ¹³C NMR (100 MHz, CDCl₃): δ 136.8, 134.7, 129.7, 129.0, 128.9, 21.4.

cis-4-Methyl-4′-methoxystilbene (2c). ⁴⁰ Oil. ¹H NMR (400 MHz, CDCl₃): δ 7.19 (d, J = 8.4 Hz, 2H), 7.16 (d, J = 8.0 Hz, 2H), 7.03 (d, J = 7.6 Hz, 2H), 6.75 (d, J = 8.4 Hz, 2H), 6.47 (s, 2H), 3.77 (s, 3H), 2.31 (s, 3H). ¹³C NMR (100 MHz, CDCl₃): δ 158.6, 136.6, 134.7, 130.1, 129.9, 129.1, 128.9, 128.8, 128.7, 113.6, 55.2, 21.2.

cis-1-(4-Methoxyphenyl)-2-phenylethene (2d). ²³ Oil. ¹H NMR (400 MHz, CDCl₃): δ 7.29–7.21 (m, 5H), 7.18 (d, J = 8.4 Hz, 2H), 6.75 (d, J = 8.8 Hz, 2H), 6.51 (s, 2H), 3.77 (s, 3H). ¹³C NMR (100 MHz, CDCl₃): δ 158.7, 137.6, 130.2, 129.8, 129.7, 128.8, 128.7, 128.2, 126.9, 113.6, 55.2.

cis-1-(2-Methoxyphenyl)-2-phenylethene (2e). ²³ Oil. ¹H NMR (400 MHz, CDCl₃): δ 7.25–7.12 (m, 7H), 6.88 (d, J = 8.0 Hz, 1H), 6.75 (t, J = 7.4 Hz, 1H), 6.69 (d, J = 12.4 Hz, 1H), 6.62 (d, J = 12.4 Hz, 1H), 3.81 (s, 3H). ¹³C NMR (100 MHz, CDCl₃): δ 157.2, 137.4, 130.3, 130.1, 128.9, 128.6, 128.0, 126.9, 126.3, 125.8, 120.2, 110.7, 55.5.

cis-1-(4-Fluorophenyl)-2-phenylethene (2f). ⁴¹ Oil. ¹H NMR (400 MHz, CDCl₃): δ 7.24–7.15 (m, 7H), 6.89 (t, J = 8.4 Hz, 2H), 6.58 (d, J = 12.4 Hz, 1H), 6.53 (d, J = 12.4 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 161.9 (d, ¹J_C–F = 245.2 Hz), 137.1, 133.2 (d, ⁴J_C–F = 3.4 Hz), 130.5 (d, ³J_C–F = 7.8 Hz), 130.3, 129.1, 128.8, 128.3, 127.2, 115.2 (d, ²J_C–F = 21.3 Hz).

cis-1-(4-Chlorophenyl)-2-phenylethene (2g). ⁴² Oil. ¹H NMR (400 MHz, CDCl₃): δ 7.26–7.14 (m, 9H), 6.62 (d, J = 12.0 Hz, 1H), 6.52 (d, J = 12.4 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 136.9, 135.7, 132.7, 131.0, 130.2, 128.9, 128.8, 128.4, 128.3, 127.3.

cis-1-Phenyl-2-(2-trifluoromethylphenyl)ethene (2h). ⁴³ Oil. ¹H NMR (400 MHz, CDCl₃): δ 7.68 (d, J = 6.4 Hz, 1H), 7.31–7.18 (m, 3H), 7.16–7.02 (m, 5H), 6.82 (d, J = 12.4 Hz, 1H), 6.71 (d, J = 12.4 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 136.8, 136.2, 132.2, 131.5, 131.3, 129.2, 128.7 (q, ²J_C–F = 29.6 Hz), 128.2, 127.3, 127.1, 126.9, 125.9 (q, ³J_C–F = 5.3 Hz), 124.4 (q, ¹J_C–F = 272.1 Hz).

4-(cis-Styryl)biphenyl (2i). ²³ Oil. ¹H NMR (400 MHz, CDCl₃): δ 7.57 (d, J = 7.6 Hz, 2H), 7.49–7.37 (m, 4H), 7.34–7.18 (m, 8H), 6.62 (s, 2H). ¹³C NMR (100 MHz, CDCl₃): δ 140.7, 139.8, 137.4, 136.3, 130.5, 129.8, 129.4, 128.9, 128.8, 128.3, 127.3, 127.2, 126.9, 126.8.

cis-1-(1-Naphthyl)-2-phenylethene (2j). ⁴⁴ Oil. ¹H NMR (400 MHz, CDCl₃): δ 8.07 (d, J = 8.8 Hz, 1H), 7.85 (d, J = 6.8 Hz, 1H), 7.75 (d, J = 8.0 Hz, 1H), 7.51–7.41 (m, 2H), 7.37–7.30 (m, 2H), 7.20–7.01 (m, 6H), 6.82 (d, J = 12.0 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 136.8, 135.3, 133.8, 132.1, 131.6, 129.1, 128.5, 128.1, 127.6, 127.1, 126.5, 126.1, 126.0, 125.7, 125.0.

cis-1-(2-Naphthyl)-2-phenylethene (2k). ²³ Oil. ¹H NMR (400 MHz, CDCl₃): δ 7.76–7.66 (m, 3H), 7.62 (d, J = 8.8 Hz, 1H), 7.43–7.38 (m, 2H), 7.35–7.32 (m, 1H), 7.28–7.25 (m, 2H), 7.22–7.16 (m, 3H), 6.74 (d, J = 12.4 Hz, 1H), 6.66 (d, J = 12.0 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 137.3, 135.0, 133.5, 132.6, 130.7, 130.2, 129.0, 128.3, 128.1, 128.0, 127.7, 127.5, 127.3, 127.0, 126.0, 125.9.

cis-2-Styrylthiophene (2l). ²³ Oil. ¹H NMR (400 MHz, CDCl₃): δ 7.37–7.25 (m, 5H), 7.07 (d, J = 5.2 Hz, 1H), 6.95 (d, J = 3.2 Hz, 1H), 6.89–6.84 (m, 1H), 6.69 (d, J = 12.0 Hz, 1H), 6.57 (d, J = 12.0 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 139.8, 137.4, 129.0, 128.9, 128.6, 128.2, 127.6, 126.5, 125.5, 123.4.

cis-Pent-1-ene-1,5-diyldibenzene (2m). ⁴⁵ Oil. ¹H NMR (400 MHz, CDCl₃): δ 7.34–7.12 (m, 10H), 6.43 (d, J = 11.6 Hz, 1H), 5.68 (dt, J = 12.0, 7.2 Hz, 1H), 2.63 (t, J = 7.8 Hz, 2H), 2.41–2.34 (m, 2H), 1.81–1.73 (m, 2H). ¹³C NMR (100 MHz, CDCl₃): δ 142.3, 137.7, 132.6, 129.3, 128.8, 128.5, 128.3, 128.2, 126.5, 125.8, 35.5, 31.7, 28.2.

cis-1-Ethyl-2-phenylethene (2n). ⁴⁶ Oil. ¹H NMR (400 MHz, CDCl₃): δ 7.36–7.16 (m, 5H), 6.38 (d, J = 11.6 Hz, 1H), 5.64 (dt, J = 11.6, 7.4 Hz, 1H), 2.39–2.30 (m, 2H), 1.06 (t, J = 7.4 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃): δ 137.8, 134.8, 128.8, 128.3, 128.1, 126.5, 22.0, 14.5.

cis-1-Phenyl-1-hexene (2o). ²³ Oil. ¹H NMR (400 MHz, CDCl₃): δ 7.27–7.10 (m, 5H), 6.33 (d, J = 11.6 Hz, 1H), 5.59 (dt, J = 11.6, 7.2 Hz, 1H), 2.29–2.20 (m, 2H), 1.41–1.20 (m, 4H), 0.82 (t, J = 7.2 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃): δ 137.9, 133.3, 128.8, 128.7, 128.1, 126.4, 32.2, 28.4, 22.5, 14.0.

cis-Oct-1-en-1-ylbenzene (2p). ⁴⁷ Oil. ¹H NMR (400 MHz, CDCl₃): δ 7.35–7.26 (m, 4H), 7.24–7.19 (m, 1H), 6.38 (d, J = 11.6 Hz, 1H), 5.66 (dt, J = 11.6, 7.2 Hz, 1H), 2.36–2.28 (m, 2H), 1.49–1.38 (m, 2H), 1.37–1.23 (m, 6H), 0.87 (t, J = 6.8 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃): δ 137.9, 133.3, 128.8, 128.7, 128.1, 126.4, 31.7, 30.0, 29.0, 28.7, 22.6, 14.0.

cis-1-(2-Cyclopropylvinyl)naphthalene (2q). Oil. ¹H NMR (400 MHz, CDCl₃): δ 8.09–8.05 (m, 1H), 7.85–7.82 (m, 1H), 7.74 (d, J = 8.0 Hz, 1H), 7.57 (d, J = 6.8 Hz, 1H), 7.51–7.42 (m, 3H), 6.79 (d, J = 11.6 Hz, 1H), 5.27 (t, J = 10.8 Hz, 1H), 1.71–1.58 (m, 1H), 0.75–0.70 (m, 2H), 0.51–0.44 (m, 2H). ¹³C NMR (100 MHz, CDCl₃): δ 138.5, 135.1, 133.7, 132.0, 128.4, 127.1, 126.6, 125.8, 125.7, 125.3, 125.2, 124.8, 11.4, 7.7. Anal. calcd for C₁₅H₁₄: C, 92.74; H, 7.26. found: C, 92.48; H, 7.04.

cis-Decene (2r). ²³ Oil. ¹H NMR (400 MHz, CDCl₃): δ 5.37–5.32 (m, 2H), 2.11–1.94 (m, 4H), 1.39–1.20 (m, 8H), 0.88 (t, J = 7.0 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃): δ 130.1, 32.1, 27.2, 22.5, 14.1.