Highly selective electrochemical hydrogenation of alkynes: Rapid construction of mechanochromic materials.

Electrochemical hydrogenation has emerged as an environmentally benign and operationally simple alternative to traditional catalytic reduction of organic compounds. Here, we have disclosed for the first time the electrochemical hydrogenation of alkynes to a library of synthetically important Z-alkenes under mild conditions with great selectivity and efficiency. The deuterium and control experiments of electrochemical hydrogenation suggest that the hydrogen source comes from the solvent, supporting electrolyte, and base. The scanning electron microscopy and x-ray diffraction experiments demonstrate that palladium nanoparticles generated in the electrochemical reaction act as a chemisorbed hydrogen carrier. Moreover, complete reduction of alkynes to saturated alkanes can be achieved through slightly modified conditions. Furthermore, a series of novel mechanofluorochromic materials have been efficiently constructed with this protocol that showed blue-shifted mechanochromism. This discovery represents the first example of cis-olefins-based organic mechanochromic materials.

INTRODUCTION

Cis-alkenes are important scaffolds in various natural products, pharmaceuticals, and organic functional materials. They are also key building blocks for developing molecular complexity from their stereospecific transformations (–). Thus, considerable efforts have been devoted to the development of straightforward and efficient approaches to access these thermodynamically less stable alkenes (–). The transition metal–catalyzed selective semihydrogenation of alkynes is among the most common methods for the synthesis of cis-olefins and is an extremely important process in both industry and academia (–). However, this process is often complicated with the use of stoichiometric amounts of reducing reagents (Fig. 1A) (–). Moreover, poor chemo- and stereoselectivity and over-reduction of alkenes to alkanes are observed in most cases (, , –). As a result, the development of efficient and environmentally friendly hydrogenation methods is an important subject in organic synthesis (–).

Fig. 1

Hydrogenation of alkynes to Z-alkenes and construction of mechanochromic materials. (A) The reported prevailing methods for Z-olefins. (B) Electrochemical selective hydrogenation of alkynes to Z-alkenes.

Electrochemical synthesis has emerged as a powerful synthetic tool and has gained great interest from both scientific and engineering viewpoints because of its innate advantages, including environmental benignity, operational simplicity, ease of scalability, reaction tenability, and economic efficiency from the use of cheap and/or recyclable electrodes (–). Therefore, the electrochemical hydrogenation of alkynes to Z-alkenes with great selectivity and efficiency is doubtless one of the most ideal strategies (Fig. 1B) (–, –). In this process, the hydrogen source comes from the solvent or supporting electrolyte rather than the hydrogen gas (–), which avoids a series of problems associated with hydrogen gas transportation and storage. As an ideal alternative to traditional catalytic hydrogenation, electrochemical hydrogenation can serve as an environmentally friendly and sustainable synthetic method in organic chemistry (–, –). Electrochemical reduction of carbon dioxide, ketones, esters, amides, and alkyl halides has been well established (). In comparison, reports on reduction of alkynes to Z-olefins are rare. In 1943, Campbell and Young disclosed this process using spongy nickel as the cathode and sulfuric acid as an anolyte under a constant current of 2 A (). Since then, several groups have attempted this transformation (, ). Unfortunately, these reactions are often associated with various disadvantages such as the use of corrosive sulfuric acid, poor chemo- and stereoselectivity, limited substrate scope, and over-reduction to alkanes (, ). Thus, the development of an innovative electrochemical method to overcome these drawbacks is urgently needed. In this study, we report the first example of electrochemical hydrogenation reactions of alkynes to a library of synthetically important Z-alkenes in great selectivity and efficiency (Fig. 1B).

RESULTS

We began our investigation by using 1,2-diphenylethyne (1a) as a model substrate to evaluate the feasibility of the electrochemical hydrogenation strategy (table S1). To our delight, 1a was hydrogenated efficiently to afford (Z)-1,2-diphenylethene (2a) in 81% yield with great selectivity (E/Z, 1:99) under a constant current of 0.1 A in a user-friendly undivided three-necked round-bottomed flask at 60°C (table S1, entry 1). Encouraged by these gratifying results, we further optimized the reaction conditions. Among the electrolytes investigated (e.g., Bu4NI, Bu4NPF6, and Bu4NBF4), Bu4NI was the most effective one (table S1, entries 1 to 3). After an extensive solvent screening, methanol proved to be the most effective (table S1, entries 4 to 7). It was also found that the selectivity and yield of 2a were reduced when other bases were used or in the absence of Me2NH (table S1, entries 8 to 10). Furthermore, replacement of PdCl2 with Pd(OAC)2 led to a slightly decreased reaction yield (table S1, entry 11). Moreover, a substantially lower reaction yield was obtained by decreasing the operating current or temperature (table S1, entries 12 and 13), while no desired product could be obtained in the absence of electric current (table S1, entry 14). It was also found that a very low yield was observed when the same electrode (either Pt/Pt or C/C) was used (table S1, entries 15 and 16). In addition, 2a was obtained in 18% yield when unwashed and reused Pt electrode was used as the cathode (table S1, entry 17). Delightfully, the desired product was obtained in 76% yield with high selectivity using the recycled palladium nanoparticles (table S1, entry 18), indicating that this protocol has the potential for industrial production. It is worth noting that the potential over-reduced alkane byproduct was not detected in the reaction, making this process unique and extremely valuable ().

With the optimized conditions in hand, a library of alkynes was subjected to electrochemical hydrogenation, and the corresponding Z-alkenes were obtained in high yields with excellent chemo- and stereoselectivity (Table 1), revealing the general applicability of this method. It is noteworthy that (Z)-1,2-diphenylethene (2a) could also be obtained on a gram scale in 78% yield with high Z/E selectivity. In addition, di(hetero)arylethynes with either an electron-donating or electron-withdrawing substituent resulted in good yields of the desired Z-alkenes (2a to 2j). Notably, heteroarylethynes were exclusively reduced to the corresponding Z-alkenes without affecting the heteroaromatic rings (2e and 2f). Furthermore, hydrogenation of unactivated dialkyl acetylenes also proceeded smoothly to provide the corresponding Z-olefins in high yields with excellent selectivity (2l to 2n and 2q to 2x). Moreover, terminal alkynes can be easily hydrogenated (2o and 2p). As shown in Table 1, a variety of valuable functionalities such as amino, chloro, cyano, ether, fluoro, methoxyl, methyl, silicon, trifluoromethyl, and heterocycle were all well tolerated. It is also worth noting that benzyl and naphthalene were all compatible under the present conditions (2q to 2t), making it a unique process because these two functional groups can be easily hydrogenated by most conventional hydrogenation reactions.

Table 1

Electrochemical selective hydrogenation of various alkynes to Z-alkenes.

*Reaction conditions: C anode, Pt cathode, constant current = 0.1 A, 1 (0.80 mmol), PdCl2 (0.5 mol %), Me2NH (0.5 equiv), Bu4NI (1.0 equiv), MeOH (8.0 ml), 60°C, 2.5 hours. Isolated yields and Z/E ratios are shown. Selectivity was determined by gas chromatography (GC) or nuclear magnetic resonance (NMR) analysis. †3.5 hours. Constant current = 0.2 A, PdCl2 (2.0 mol %), 5 hours. ‡PdCl2 (1.0 mol %), 3.5 hours.

To our delight, complete reduction of alkynes to saturated alkanes was also achieved under slightly modified reaction conditions (Table 2). Compared with the previous studies of electrochemical hydrogenation of alkynes to alkanes, the metal electrode is not sacrificed under the present conditions (). Moreover, deuterated 1,2-diphenylethane was obtained with 100% of deuterium incorporation in 83% yield with CD3CN as the solvent under the standard reaction conditions (scheme S2). Alkenes were also reduced cleanly to alkanes with this protocol. This process also showed a good catalytic activity toward mono-, di-, tri-, and tetra-substituted alkenes (Table 3). It should be mentioned that traditional catalytic hydrogenation of tri- or tetra-substituted alkenes often requires harsh reaction conditions such as high temperatures and high H2 pressures ().

Table 2

Electrochemical selective hydrogenation of alkynes to alkanes.

*Reaction conditions: C anode, Pt cathode, constant current = 0.3 A, 1 (0.80 mmol), PdCl2 (0.5 mol %), Me2NH (1.0 equiv), Bu4NI (2.0 equiv), MeCN (8.0 ml), 60°C, 2.5 hours. †PdCl2 (1.0 mol %), Me2NH (2.0 equiv), Bu4NI (3.0 equiv), 8 hours.

Table 3

Electrochemical selective hydrogenation of alkenes to alkanes.

*Reaction conditions: C anode, Pt cathode, constant current = 0.3 A, 2 (0.80 mmol), PdCl2 (0.5 mol %), Me2NH (1.0 equiv), Bu4NI (2.0 equiv), MeCN (8.0 ml), 60°C, 2.5 hours. †PdCl2 (1.0 mol %). ‡PdCl2 (2.0 mol %), 10 hours.

In recent years, the development of mechanochromic materials has attracted considerable attention in view of their great potential for various applications such as chemosensors, security systems, optical displays, and rewritable optical media (–). However, the design strategy for organic small-molecule mechanochromic materials still remains obscure (, –). Mechanochromism of organic materials mainly depends on efficient molecular packing from intermolecular interactions such as hydrogen bond and π-π and dipole-dipole interactions (, –). The weak intermolecular interactions and relatively loose packing usually exist in highly twisted molecules. The highly twisted conformation is susceptible to external perturbation to endow a chromic response. The triphenylamine (TPA) group has been well recognized for its efficient hole-transporting capability, good electron-donating property, and propeller-like nonplanar characteristic (–). As a result, we designed and synthesized a series of novel TPA-bearing (z)-2-(4-styrylphenyl)oxazole scaffolds (4a to 4d) (Fig. 1 and Table 4). It was envisaged that the propeller-like geometry of TPA and the π-conjugated nonplanar geometry of (Z)-1,2-diphenylethenes may endow them with a strong emission and highly twisted conformation.

Table 4

Synthesis of TPA-containing (z)-2-(4-styrylphenyl)oxazoles.

*For detailed reaction conditions, see the Supplementary Materials. †Emission maximum and quantum yields in toluene (2 × 10−5 M). ‡Emission maximum in pristine powder. §Emission maximum in ground powder.

Given that the highly selective electrochemical hydrogenation protocol developed here can tolerate the reactive chloride, (z)-1-chloro-4-styrylbenzene and (z)-1,2-bis(4-chlorophenyl)ethane were used as starting materials to avoid tedious multiple-step synthesis and thus greatly streamline synthetic routes. The palladium-catalyzed C–H/C–Cl cross-coupling of TPA-bearing oxazoles (5) with 2g or 2j was performed to obtain the corresponding TPA-bearing (z)-2-(4-styrylphenyl)oxazole scaffolds 4a to 4d (Table 4).

The photophysical properties of these scaffolds (4a to 4d) were measured with respect to the emission maximum, along with quantum yields in toluene solution (2.0 × 10−5 M) and emission maxima before and after grinding (Table 4 and Fig. 2). As shown in Fig. 2A and fig. S1B, their emission wavelengths in toluene are located in the blue region (452 to 490 nm), and high fluorescence quantum yields in toluene (53 to 62%) have been determined. Importantly, 4b displayed deep-blue emission with International Commission on Illumination 1931 (CIE1931) of (0.15, 0.08), which is very close to European Broadcasting Union (EBU) coordinates of (0.15, 0.06) (fig. S2). The absorption and emission spectra of 4a to 4d in toluene are shown in fig. S1.

Fig. 2

Fluorescence images.

(A) Fluorescence images of 4a to 4d in toluene (2.0 × 10−5 M) under ultraviolet (UV) light (365 nm). (B) Fluorescence images of 4a to 4d before and after grinding.

Molecules 4a to 4d displayed blue-shifted mechanochromic luminescence properties (Fig. 2B). Grinding of pristine powders 4a to 4d induced a blue shift with emission color change from yellow (λem = 503 to 568 nm) to blue-green (λem = 483 to 502 nm), approximately 31, 34, 20, and 66 nm, respectively (Table 4, Fig. 2, and fig. S3). The fluorescence blue shift, after grinding powders 4a to 4d, could be ascribed to weakened intermolecular π-π interactions. To the best of our knowledge, this is the first example of cis-olefin–based organic mechanochromic materials (–).

To explore the mechanochromic mechanism, 4b was further investigated as a representative sample, and its powder phase characteristics were studied by differential scanning calorimetry (DSC) and powder x-ray diffraction (PXRD) analysis. The DSC experiment of unground 4b did not present endothermic or exothermic peaks. In contrast, ground 4b exhibited an obvious endothermic peak, indicating a transition from the metastable state to the stable state (fig. S4). The PXRD patterns of the pristine solid of 4b exhibited sharp and intense reflections, whereas the sharp peaks disappeared after grinding (fig. S5). These observations demonstrated a morphological transition from the crystalline to amorphous phase (–). Furthermore, its thermal stability was also evaluated by thermal gravimetric analysis (fig. S6). The thermal decomposition temperature (Td) of 4b is 343°C, which indicates that it is thermally stable.

DISCUSSION

To get some insights into the mechanism of the selective electrochemical hydrogenation reaction, a series of deuterium-labeling experiments were performed to verify the hydrogen source of Z-alkenes (scheme S1 and section S9). With CD3OD as the solvent, (Z)-1,2-diphenylethene ([D]-2a) was obtained with 83% of deuterium incorporation in the absence of a base (scheme S1A), indicating that the hydrogen comes from both CD3OD and the electrolyte Bu4NI.Furthermore, a slightly smaller proportion of deuterated product was observed with the addition of diisopropylamine instead of diisopropylamine-d (scheme S1, B and C). This observation implied that diisopropylamine may also provide hydrogen in the reaction. In addition, with CH3OD as the solvent, the corresponding [D]-2a (D incorporation: ca. 82%) was observed (scheme S1D). This result suggests that hydrogen was not provided by the methyl group in methanol in the reaction. As a result, the hydrogen sources are believed to be the hydroxyl group in methanol, electrolyte Bu4NI, and dimethylamine under the standard reaction conditions (Fig. 3). Next, a series of control experiments were conducted to probe the reaction pathway. The reaction either failed or proceeded with low efficiency when the solvent methanol was changed to formaldehyde or formic acid (see scheme S1, F and I). These results indicate that a potential pathway involving the combination of carboxylic acid and zerovalent palladium catalyst could be excluded. In addition, when the reaction was performed with PdCl2 or Pd(PPh3)4 as a catalyst under 1 atm H2, only 2 to 6% of (Z)-1,2-diphenylethene was observed (see scheme S1, J to L), which excludes the possibility of the mechanism of hydrogenation with molecular hydrogen (H2). Furthermore, cyclic voltammetry (CV) of 1,2-diphenylethyne (1a) in CH3OH was measured (fig. S7). 1a showed a single irreversible reduction peak at −0.67 V (versus Ag/Ag+), indicating that 1a can be reduced in CH3OH. Moreover, the morphologic analyses of all palladium particles including those from the cathode surface and the solution were carried out with scanning electron microscopy (SEM) (Fig. 4). SEM micrographs revealed irregular palladium deposits of nanometric dimensions. X-ray diffraction (XRD) of these palladium particles showed that they are completely the Pd0 (Fig. 4D). On the basis of the above observed results and previous reports, a plausible mechanism for the selective electrochemical hydrogenation reaction of alkynes to Z-alkenes was proposed (Fig. 3) (–, –, , ). Initially, methanol is used as a hydrogen source to generate chemisorbed hydrogen at the cathode. Palladium(0) nanoparticles are generated on the cathode and adsorb hydrogen. Next, a hydrogen transfer process occurs with alkynes to give intermediate C. Subsequently, intermediate C adsorbs another hydrogen atom to generate intermediate D. Finally, Z-alkene products are generated and desorped, and adsorption sites are regenerated. In the reaction, the tetrabutylammonium ion is reduced by the cathode to generate tributylamine, which then loses an electron on the anode to form amine radical cation E (, ). This radical cation species could also transfer a hydrogen atom to intermediate C to afford the product 2 ().

Fig. 3

A plausible mechanism of electrochemical selective hydrogenation of alkynes.

Fig. 4

SEM micrographs and XRD diffractograms.

(A to C) SEM micrographs of palladium nanoparticles formed on the cathode surface. (D) XRD of the palladium nanoparticles. (E to G) SEM micrographs of the Pd nanoparticles from the solution. a.u., arbitrary units.

CONCLUSION

In summary, we have developed the first example of electrochemical hydrogenation reactions of alkynes to a library of Z-alkenes in high yields with excellent chemo- and stereoselectivity. Detailed mechanistic studies of electrochemical hydrogenation were also conducted. In the electrochemical selective hydrogenation process, deuterium-labeling experiments showed that hydrogen comes from the solvent, supporting electrolyte, and base. As demonstrated by SEM and XRD, palladium nanoparticles generated in the electrochemical reaction act as a chemisorbed hydrogen carrier in the process. A library of alkynes readily underwent electrochemical hydrogenation to provide the corresponding Z-alkenes in high yields with excellent chemo- and stereoselectivity. As a green synthetic method, this approach provides a great opportunity for synthetic application. As shown in this study, this newly developed method provided a straightforward access to TPA-bearing (z)-2-(4-styrylphenyl)oxazoles, which opens up a new avenue for a rapid buildup of new mechanofluorochromic materials. Furthermore, complete reduction of alkynes to saturated alkanes was also achieved by slightly tuning the conditions.

MATERIALS AND METHODS

General procedure

The electrochemical hydrogenation was carried out in a three-necked round-bottomed flask (10 ml), with a graphite rod anode and a platinum disc cathode. 1 (0.80 mmol), PdCl2 [0.5 mole percent (mol %), 0.7 mg], Me2NH (0.5 equiv, 0.2 ml, 2.0 M in the methanol), Bu4NI (1.0 equiv, 295.5 mg), and MeOH (8.0 ml) were placed in a three-necked round-bottomed flask at 60°C, with a constant current of 0.1 A maintained for 2.5 to 5 hours. The mixture was cooled to room temperature and diluted with 20 ml of EtOAc. The organic mixture was then washed with brine, dried over anhydrous Na2SO4, and evaporated under vacuum. The residue was purified by flash column chromatography (n-hexane) on silica gel to provide the desired product 2.