Bis -NHC–Ag/Pd(OAc) 2 Catalytic System Catalyzed Transfer Hydrogenation Reaction

: The bis -NHC–Ag/Pd(OAc) 2 catalytic system (NHC = N -heterocyclic carbene), a combina-tion of bis -NHC–Ag complex and Pd(OAc) 2 , was found to be a smart catalyst in the Pd-catalyzed transfer hydrogenation of various functionalized arenes and internal/terminal alkynes. The catalytic system demonstrated high efﬁciency for the reduction of a wide range of various functional groups such as carbonyls, alkynes, oleﬁns, and nitro groups in good to excellent yields and high chemoselectivity for the reduction of functional groups. In addition, the protocol was successfully exploited to stereoselectivity for the transformation of alkynes to alkenes in aqueous media under air. This methodology successfully provided an alternative useful protocol for reducing various functional groups and a simple operational protocol for transfer hydrogenation.

Due to the reductive elimination of the hydrido-palladium complexes of NHC, leading to catalyst deactivation [30][31][32][33][34], Pd-NHC-catalyzed transfer hydrogenation has less successful examples. In 2004, Cavell and co-workers demonstrated the first successful example of a stable tricarbene-Pd-hydrido complex [35], and Pd-NHC-catalyzed transfer hydrogenation has recently attracted much attention. Elsevier et al. and Cazin et al. also illustrated the Pd(NHC)-catalyzed reduction of alkynes employing triethylammonium formate (TEAF) as the hydrogen source. They also proposed a catalytic mechanism to illustrate the formation of Z-alkenes in the transfer hydrogenation of alkynes [36][37][38][39][40][41][42]. Bis-NHC binds to metal to form stable complexes compared to their monodentate NHC complexes. In addition, bis-NHC ligand diversity could be easily accessed by the modification of the linker and the wingtips. In 2013, Elsevier et al. established various bis-NHC-Pd complexes and applied them to the semihydrogenation of 1-phenyl-1-propyne [43]. Unfortunately, only 18% conversion was obtained by using formic acid (5.0 eq) as a hydrogen donor at 70 • C in acetonitrile. The authors mentioned that the transfer semihydrogenation gave Although an excellent Z/E ratio was displayed, the challenges are still functional group compatibility, chemoselectivity, stereoselectivity, and the control of over reduction. Therefore, it is desirable to develop an alternative method for efficient transfer hydrogenation. On the other hand, NHC ligands were recently found to play an important role in the synthesis of metallic nanoparticles (MNPs). Pd nanoparticles (Pd NPs) were formed by the decomposition of Pd-NHC bonds in a Pd/NHC catalytic system or by the reduction of the Pd precursor, especially in the presence of aliphatic amines such as triethylamine. The NHC-ligated Pd NPs present an efficient catalytic activity in the catalysis [44][45][46][47][48]. We recently reported an in situ-generated bis-NHC/Pd(OAc)2 catalytic system, which was derived from bis-benzimidazolium salt and Pd(OAc)2, as a catalyst for the Suzuki-Miyaura reaction, Mizoroki-Heck reaction, and Friedel-Crafts alkylation reaction of indole and nitrostyrene in good to excellent yields ( Figure 1a) [49,50]. Motivated by these results we continued our efforts to develop an efficient bis-NHC-Ag/Pd(OAc)2 catalytic system to catalyze chemoselective transfer hydrogenation with TEAF as a hydrogen donor ( Figure  1b).

Results
A testing protocol was examined by using trans-cinnamyl alcohol, formic acid, triethylamine, and a bis-NHC-Ag/Pd(OAc)2 catalytic system at 80 °C. To screen the optimized reaction conditions fast, various factors such as Pd loading, equivalents of TEAF,

Results
A testing protocol was examined by using trans-cinnamyl alcohol, formic acid, triethylamine, and a bis-NHC-Ag/Pd(OAc) 2 catalytic system at 80 • C. To screen the optimized reaction conditions fast, various factors such as Pd loading, equivalents of TEAF, and solvents were evaluated (Table 1). Preliminary results illustrated that N,N-dimethylformamide (DMF) was a suitable solvent for transfer hydrogenation (entry 2 vs. 4). trans-cinnamyl alcohol was hydrogenated completely to 3-phenylpropan-1-ol in the presence of 1.0 mol % catalyst loading and a 4-fold excess of HCO 2 H/NEt 3 in DMF (entry 5). A dramatic drop in conversion was obtained in the absence of bis-NHC-Ag complex (8% conversion, entry 6), which means the bis-NHC-Ag/Pd(OAc) 2 catalytic system raises the reactivity of the Pd metal center on the catalytic hydrogenation reaction (entry 5 vs. entry 6).  3 The reaction was carried out in the absence of bis-NHC-Ag complex.
After screening the optimal reaction conditions, the reduction of various functionalized substrates was studied. As illustrated in Table 2, quantitative conversions were observed in the reduction of olefins containing alcohol, acid, and ester functionalities (entries 1-3). Notably, the reduction of 1,2-diphenylacetylene 3a produced (Z)-4a as the only product, without the formation of the over-reduced product, 1,2-diphenylethane and (E)-4a (entry 4, 85%). The reduction of nitrobenzene catalyzed by the bis-NHC-Ag/Pd(OAc) 2 catalytic system gave aniline 6a in a 99% isolated yield (entry 5). With a substrate containing two active functional groups, 4-nitroacetophenone 6b, the catalytic system was found to selectively reduce the nitro group, while no reduction product of the carbonyl group was observed on benzene ring 6b (entry 6). This demonstrates that the reduction rate of the nitro group is faster than that of ketone. In addition, aldehyde-bearing strong electron-withdrawing group CF 3 , which could afford the corresponding alcohol 8a in a 92% yield (entry 7), indicates that our reaction works for both electron-donating and electron-withdrawing substituents.
Recently, numerous studies of Pd-catalyzed transfer hydrogenation using H 2 O as the hydrogen agent and the solvent have been reported [51][52][53][54][55][56][57][58][59]. We turned our attention to the possibility that the transfer hydrogenation of internal/terminal alkynes may also continue in the presence of the bis-NHC-Ag/Pd(OAc) 2 catalytic system in aqueous media under air. We began to develop a general method for the transfer hydrogenation of internal alkynes by using 1,2-diphenylacetylene 3a and formic acid (4 equiv) and triethylamine (4 equiv) as the model substrates. As described in Table 3, 3a was reduced to semihydrogenation product 4a with a Z/E ratio of 94/6 in DMF/H 2 O (9/1) mixed solvent (entry 1). The over-reduced product 9a was fully inhibited. When we continued to increase the amount of water, the conversion yields decreased because of the low solubility of 3a, but (Z)-4a was still the major product (entries [1][2][3][4]. Notably, the ratio of (Z)-4a and (E)-4a decreased slightly as the amount of water decreased (entries 1-4). cis-Stilbene 8a was afforded as a major product regardless of the proportion of water. This observation is contrary to recent reports, in which trans-alkenes were found to be a major product through in situ Z→E isomerization in the Pd-catalyzed semihydrogenation of internal alkynes in aqueous solution [52,53,[56][57][58]. In addition, K 2 CO 3 and K 3 PO 4 ·H 2 O were not the best choices as the base, indicating that the basicity of the base might affect the reaction rate (entries 2, 5, and 6) [60]. With Pd(II) sources, 3a was transformed to olefin efficiently, but Pd(OAc) 2 was the best choice (entries 2, 7-9). selectively reduce the nitro group, while no reduction product of the carbonyl group was observed on benzene ring 6b (entry 6). This demonstrates that the reduction rate of the nitro group is faster than that of ketone. In addition, aldehyde-bearing strong electronwithdrawing group CF3, which could afford the corresponding alcohol 8a in a 92% yield (entry 7), indicates that our reaction works for both electron-donating and electron-withdrawing substituents. selectively reduce the nitro group, while no reduction product of the carbonyl group was observed on benzene ring 6b (entry 6). This demonstrates that the reduction rate of the nitro group is faster than that of ketone. In addition, aldehyde-bearing strong electronwithdrawing group CF3, which could afford the corresponding alcohol 8a in a 92% yield (entry 7), indicates that our reaction works for both electron-donating and electron-withdrawing substituents. nitro group is faster than that of ketone. In addition, aldehyde-bearing strong electronwithdrawing group CF3, which could afford the corresponding alcohol 8a in a 92% yield (entry 7), indicates that our reaction works for both electron-donating and electron-withdrawing substituents. nitro group is faster than that of ketone. In addition, aldehyde-bearing strong electronwithdrawing group CF3, which could afford the corresponding alcohol 8a in a 92% yield (entry 7), indicates that our reaction works for both electron-donating and electron-withdrawing substituents. Recently, numerous studies of Pd-catalyzed transfer hydrogenation using H2O as the hydrogen agent and the solvent have been reported [51][52][53][54][55][56][57][58][59]. We turned our attention to the possibility that the transfer hydrogenation of internal/terminal alkynes may also continue in the presence of the bis-NHC-Ag/Pd(OAc)2 catalytic system in aqueous media under air. We began to develop a general method for the transfer hydrogenation of internal alkynes by using 1,2-diphenylacetylene 3a and formic acid (4 equiv) and triethylamine (4 equiv) as the model substrates. As described in Table 3, 3a was reduced to semihydrogenation product 4a with a Z/E ratio of 94/6 in DMF/H2O (9/1) mixed solvent (entry 1). The over-reduced product 9a was fully inhibited. When we continued to increase the amount of water, the conversion yields decreased because of the low solubility of 3a, but (Z)-4a was still the major product (entries [1][2][3][4]. Notably, the ratio of (Z)-4a and (E)-4a decreased slightly as the amount of water decreased (entries 1-4). cis-Stilbene 8a was afforded as a major product regardless of the proportion of water. This observation is contrary to recent reports, in which trans-alkenes were found to be a major product through in situ Z→ E isomerization in the Pd-catalyzed semihydrogenation of internal alkynes in aqueous solution [52,53,[56][57][58]. In addition, K2CO3 and K3PO4·H2O were not the best Recently, numerous studies of Pd-catalyzed transfer hydrogenation using H2O as the hydrogen agent and the solvent have been reported [51][52][53][54][55][56][57][58][59]. We turned our attention to the possibility that the transfer hydrogenation of internal/terminal alkynes may also continue in the presence of the bis-NHC-Ag/Pd(OAc)2 catalytic system in aqueous media under air. We began to develop a general method for the transfer hydrogenation of internal alkynes by using 1,2-diphenylacetylene 3a and formic acid (4 equiv) and triethylamine (4 equiv) as the model substrates. As described in Table 3, 3a was reduced to semihydrogenation product 4a with a Z/E ratio of 94/6 in DMF/H2O (9/1) mixed solvent (entry 1). The over-reduced product 9a was fully inhibited. When we continued to increase the amount of water, the conversion yields decreased because of the low solubility of 3a, but (Z)-4a was still the major product (entries [1][2][3][4]. Notably, the ratio of (Z)-4a and (E)-4a decreased slightly as the amount of water decreased (entries 1-4). cis-Stilbene 8a was afforded as a major product regardless of the proportion of water. This observation is contrary to recent reports, in which trans-alkenes were found to be a major product through in situ Z→ E isomerization in the Pd-catalyzed semihydrogenation of internal alkynes in aqueous solution [52,53,[56][57][58]. In addition, K2CO3 and K3PO4·H2O were not the best Recently, numerous studies of Pd-catalyzed transfer hydrogenation using H2O as the hydrogen agent and the solvent have been reported [51][52][53][54][55][56][57][58][59]. We turned our attention to the possibility that the transfer hydrogenation of internal/terminal alkynes may also continue in the presence of the bis-NHC-Ag/Pd(OAc)2 catalytic system in aqueous media under air. We began to develop a general method for the transfer hydrogenation of internal alkynes by using 1,2-diphenylacetylene 3a and formic acid (4 equiv) and triethylamine (4 equiv) as the model substrates. As described in Table 3, 3a was reduced to semihydrogenation product 4a with a Z/E ratio of 94/6 in DMF/H2O (9/1) mixed solvent (entry 1). The over-reduced product 9a was fully inhibited. When we continued to increase the amount of water, the conversion yields decreased because of the low solubility of 3a, but (Z)-4a was still the major product (entries 1-4). Notably, the ratio of (Z)-4a and (E)-4a decreased slightly as the amount of water decreased (entries 1-4). cis-Stilbene 8a was afforded as a major product regardless of the proportion of water. This observation is contrary to recent reports, in which trans-alkenes were found to be a major product through in situ Z→ E isomerization in the Pd-catalyzed semihydrogenation of internal alkynes in aqueous solution [52,53,[56][57][58]. In addition, K2CO3 and K3PO4·H2O were not the best choices as the base, indicating that the basicity of the base might affect the reaction rate (entries 2, 5, and 6) [60]. With Pd(II) sources, 3a was transformed to olefin efficiently, but Recently, numerous studies of Pd-catalyzed transfer hydrogenation using H2O as the hydrogen agent and the solvent have been reported [51][52][53][54][55][56][57][58][59]. We turned our attention to the possibility that the transfer hydrogenation of internal/terminal alkynes may also continue in the presence of the bis-NHC-Ag/Pd(OAc)2 catalytic system in aqueous media under air. We began to develop a general method for the transfer hydrogenation of internal alkynes by using 1,2-diphenylacetylene 3a and formic acid (4 equiv) and triethylamine (4 equiv) as the model substrates. As described in Table 3, 3a was reduced to semihydrogenation product 4a with a Z/E ratio of 94/6 in DMF/H2O (9/1) mixed solvent (entry 1). The over-reduced product 9a was fully inhibited. When we continued to increase the amount of water, the conversion yields decreased because of the low solubility of 3a, but (Z)-4a was still the major product (entries 1-4). Notably, the ratio of (Z)-4a and (E)-4a decreased slightly as the amount of water decreased (entries 1-4). cis-Stilbene 8a was afforded as a major product regardless of the proportion of water. This observation is contrary to recent reports, in which trans-alkenes were found to be a major product through in situ Z→ E isomerization in the Pd-catalyzed semihydrogenation of internal alkynes in aqueous solution [52,53,[56][57][58]. In addition, K2CO3 and K3PO4·H2O were not the best choices as the base, indicating that the basicity of the base might affect the reaction rate (entries 2, 5, and 6) [60]. With Pd(II) sources, 3a was transformed to olefin efficiently, but Recently, numerous studies of Pd-catalyzed transfer hydrogenation using H2O as the hydrogen agent and the solvent have been reported [51][52][53][54][55][56][57][58][59]. We turned our attention to the possibility that the transfer hydrogenation of internal/terminal alkynes may also continue in the presence of the bis-NHC-Ag/Pd(OAc)2 catalytic system in aqueous media under air. We began to develop a general method for the transfer hydrogenation of internal alkynes by using 1,2-diphenylacetylene 3a and formic acid (4 equiv) and triethylamine (4 equiv) as the model substrates. As described in Table 3, 3a was reduced to semihydrogenation product 4a with a Z/E ratio of 94/6 in DMF/H2O (9/1) mixed solvent (entry 1). The over-reduced product 9a was fully inhibited. When we continued to increase the amount of water, the conversion yields decreased because of the low solubility of 3a, but (Z)-4a was still the major product (entries 1-4). Notably, the ratio of (Z)-4a and (E)-4a decreased slightly as the amount of water decreased (entries 1-4). cis-Stilbene 8a was afforded as a major product regardless of the proportion of water. This observation is contrary to recent reports, in which trans-alkenes were found to be a major product through in situ Z→ E isomerization in the Pd-catalyzed semihydrogenation of internal alkynes in aqueous solution [52,53,[56][57][58]. In addition, K2CO3 and K3PO4·H2O were not the best choices as the base, indicating that the basicity of the base might affect the reaction rate (entries 2, 5, and 6) [60]. With Pd(II) sources, 3a was transformed to olefin efficiently, but Pd(OAc)2 was the best choice (entries 2, 7-9). Recently, numerous studies of Pd-catalyzed transfer hydrogenation using H2O as the hydrogen agent and the solvent have been reported [51][52][53][54][55][56][57][58][59]. We turned our attention to the possibility that the transfer hydrogenation of internal/terminal alkynes may also continue in the presence of the bis-NHC-Ag/Pd(OAc)2 catalytic system in aqueous media under air. We began to develop a general method for the transfer hydrogenation of internal alkynes by using 1,2-diphenylacetylene 3a and formic acid (4 equiv) and triethylamine (4 equiv) as the model substrates. As described in Table 3, 3a was reduced to semihydrogenation product 4a with a Z/E ratio of 94/6 in DMF/H2O (9/1) mixed solvent (entry 1). The over-reduced product 9a was fully inhibited. When we continued to increase the amount of water, the conversion yields decreased because of the low solubility of 3a, but (Z)-4a was still the major product (entries 1-4). Notably, the ratio of (Z)-4a and (E)-4a decreased slightly as the amount of water decreased (entries 1-4). cis-Stilbene 8a was afforded as a major product regardless of the proportion of water. This observation is contrary to recent reports, in which trans-alkenes were found to be a major product through in situ Z→ E isomerization in the Pd-catalyzed semihydrogenation of internal alkynes in aqueous solution [52,53,[56][57][58]. In addition, K2CO3 and K3PO4·H2O were not the best choices as the base, indicating that the basicity of the base might affect the reaction rate (entries 2, 5, and 6) [60]. With Pd(II) sources, 3a was transformed to olefin efficiently, but Pd(OAc)2 was the best choice (entries 2, 7-9). Recently, numerous studies of Pd-catalyzed transfer hydrogenation using H2O as the hydrogen agent and the solvent have been reported [51][52][53][54][55][56][57][58][59]. We turned our attention to the possibility that the transfer hydrogenation of internal/terminal alkynes may also continue in the presence of the bis-NHC-Ag/Pd(OAc)2 catalytic system in aqueous media under air. We began to develop a general method for the transfer hydrogenation of internal alkynes by using 1,2-diphenylacetylene 3a and formic acid (4 equiv) and triethylamine (4 equiv) as the model substrates. As described in Table 3, 3a was reduced to semihydrogenation product 4a with a Z/E ratio of 94/6 in DMF/H2O (9/1) mixed solvent (entry 1). The over-reduced product 9a was fully inhibited. When we continued to increase the amount of water, the conversion yields decreased because of the low solubility of 3a, but (Z)-4a was still the major product (entries 1-4). Notably, the ratio of (Z)-4a and (E)-4a decreased slightly as the amount of water decreased (entries 1-4). cis-Stilbene 8a was afforded as a major product regardless of the proportion of water. This observation is contrary to recent reports, in which trans-alkenes were found to be a major product through in situ Z→ E isomerization in the Pd-catalyzed semihydrogenation of internal alkynes in aqueous solution [52,53,[56][57][58]. In addition, K2CO3 and K3PO4·H2O were not the best choices as the base, indicating that the basicity of the base might affect the reaction rate (entries 2, 5, and 6) [60]. With Pd(II) sources, 3a was transformed to olefin efficiently, but Pd(OAc)2 was the best choice (entries 2, 7-9). Recently, numerous studies of Pd-catalyzed transfer hydrogenation using H2O as the hydrogen agent and the solvent have been reported [51][52][53][54][55][56][57][58][59]. We turned our attention to the possibility that the transfer hydrogenation of internal/terminal alkynes may also continue in the presence of the bis-NHC-Ag/Pd(OAc)2 catalytic system in aqueous media under air. We began to develop a general method for the transfer hydrogenation of internal alkynes by using 1,2-diphenylacetylene 3a and formic acid (4 equiv) and triethylamine (4 equiv) as the model substrates. As described in Table 3, 3a was reduced to semihydrogenation product 4a with a Z/E ratio of 94/6 in DMF/H2O (9/1) mixed solvent (entry 1). The over-reduced product 9a was fully inhibited. When we continued to increase the amount of water, the conversion yields decreased because of the low solubility of 3a, but (Z)-4a was still the major product (entries 1-4). Notably, the ratio of (Z)-4a and (E)-4a decreased slightly as the amount of water decreased (entries 1-4). cis-Stilbene 8a was afforded as a major product regardless of the proportion of water. This observation is contrary to recent reports, in which trans-alkenes were found to be a major product through in situ Z→ E isomerization in the Pd-catalyzed semihydrogenation of internal alkynes in aqueous solution [52,53,[56][57][58]. In addition, K2CO3 and K3PO4·H2O were not the best choices as the base, indicating that the basicity of the base might affect the reaction rate (entries 2, 5, and 6) [60]. With Pd(II) sources, 3a was transformed to olefin efficiently, but Pd(OAc)2 was the best choice (entries 2, 7-9). Recently, numerous studies of Pd-catalyzed transfer hydrogenation using H2O as the hydrogen agent and the solvent have been reported [51][52][53][54][55][56][57][58][59]. We turned our attention to the possibility that the transfer hydrogenation of internal/terminal alkynes may also continue in the presence of the bis-NHC-Ag/Pd(OAc)2 catalytic system in aqueous media under air. We began to develop a general method for the transfer hydrogenation of internal alkynes by using 1,2-diphenylacetylene 3a and formic acid (4 equiv) and triethylamine (4 equiv) as the model substrates. As described in Table 3, 3a was reduced to semihydrogenation product 4a with a Z/E ratio of 94/6 in DMF/H2O (9/1) mixed solvent (entry 1). The over-reduced product 9a was fully inhibited. When we continued to increase the amount of water, the conversion yields decreased because of the low solubility of 3a, but (Z)-4a was still the major product (entries 1-4). Notably, the ratio of (Z)-4a and (E)-4a decreased slightly as the amount of water decreased (entries 1-4). cis-Stilbene 8a was afforded as a major product regardless of the proportion of water. This observation is contrary to recent reports, in which trans-alkenes were found to be a major product through in situ Z→ E isomerization in the Pd-catalyzed semihydrogenation of internal alkynes in aqueous solution [52,53,[56][57][58]. In addition, K2CO3 and K3PO4·H2O were not the best choices as the base, indicating that the basicity of the base might affect the reaction rate (entries 2, 5, and 6) [60]. With Pd(II) sources, 3a was transformed to olefin efficiently, but Pd(OAc)2 was the best choice (entries 2, 7-9). Recently, numerous studies of Pd-catalyzed transfer hydrogenation using H2O as the hydrogen agent and the solvent have been reported [51][52][53][54][55][56][57][58][59]. We turned our attention to the possibility that the transfer hydrogenation of internal/terminal alkynes may also continue in the presence of the bis-NHC-Ag/Pd(OAc)2 catalytic system in aqueous media under air. We began to develop a general method for the transfer hydrogenation of internal alkynes by using 1,2-diphenylacetylene 3a and formic acid (4 equiv) and triethylamine (4 equiv) as the model substrates. As described in Table 3, 3a was reduced to semihydrogenation product 4a with a Z/E ratio of 94/6 in DMF/H2O (9/1) mixed solvent (entry 1). The over-reduced product 9a was fully inhibited. When we continued to increase the amount of water, the conversion yields decreased because of the low solubility of 3a, but (Z)-4a was still the major product (entries 1-4). Notably, the ratio of (Z)-4a and (E)-4a decreased slightly as the amount of water decreased (entries 1-4). cis-Stilbene 8a was afforded as a major product regardless of the proportion of water. This observation is contrary to recent reports, in which trans-alkenes were found to be a major product through in situ Z→ E isomerization in the Pd-catalyzed semihydrogenation of internal alkynes in aqueous solution [52,53,[56][57][58]. In addition, K2CO3 and K3PO4·H2O were not the best choices as the base, indicating that the basicity of the base might affect the reaction rate (entries 2, 5, and 6) [60]. With Pd(II) sources, 3a was transformed to olefin efficiently, but Pd(OAc)2 was the best choice (entries 2, 7-9). Based on this observation, transfer hydrogenation of internal alkynes 3 was subsequently investigated in aqueous media under air. The results summarized in Table 4 showed that various internal alkynes were readily reduced to the corresponding alkenes in moderate to excellent conversion yields. For 1,2-diphenylacetlene 3a as the substrate, the use of a DMF/H 2 O (5/5) solvent system at 80 • C (condition A) produced a 100% conversion yield confirmed by GC (Gas Chromatography) with a Z/E ratio of 93/7 (entry 1). On the other hand, the use of a DMF/H 2 O (9/1) solvent system at 80 • C (condition B) generated a 100% conversion GC yield and 94/6 Z/E ratio (entry 2). Surprisingly, the corresponding product 4b was formed in a 97% isolated yield with a Z/E ratio of 30/70 under condition B (entry 3) when using heteroaromatic alkynes 3b as a substrate. That might be the reason why the stereoselectivity of the product is affected by the pyridinyl group, the coordinative moiety [61,62]. It should be mentioned that 3c was successfully reduced to (Z)-4c with excellent stereoselectivity under condition B (entry 5). For the semihydrogenation of conjugated alkynes bearing ester 3d, good performance was demonstrated with a Z/E ratio of 95/5 under condition B (entry 6). In particular, only (Z)-alkenylamide, (Z)-4e, was prepared in an 83% isolated yield for 1 h at 60 • C (entry 8) and over-reduced amide 9e was the only product in a quantitative yield for 2 h at 80 • C (entry 7). This illustrated that the excellent chemoselectivity of the reduction of 3e can be controlled by prolonging the reaction time and conditions.   Table 4 showed that various internal alkynes were readily reduced to the corresponding alkenes in moderate to excellent conversion yields. For 1,2-diphenylacetlene 3a as the substrate, the use of a DMF/H2O (5/5) solvent system at 80 °C (condition A) produced a 100% conversion yield confirmed by GC (Gas Chromatography) with a Z/E ratio of 93/7 (entry 1). On the other hand, the use of a DMF/H2O (9/1) solvent system at 80 °C (condition B) generated a 100% conversion GC yield and 94/6 Z/E ratio (entry 2). Surprisingly, the corresponding product 4b was formed in a 97% isolated yield with a Z/E ratio of 30/70 under condition B (entry 3) when using heteroaromatic alkynes 3b as a substrate. That might be the reason why the stereoselectivity of the product is affected by the pyridinyl group, the coordinative moiety [61,62]. It should be mentioned that 3c was successfully reduced to (Z)-4c with excellent stereoselectivity under condition B (entry 5). For the semihydrogenation of conjugated alkynes bearing ester 3d, good performance was demonstrated with a Z/E ratio of 95/5 under condition B (entry 6). In particular, only (Z)-alkenylamide, (Z)-4e, was prepared in an 83% isolated yield for 1 h at 60 °C (entry 8) and over-reduced amide 9e was the only product in a quantitative yield for 2 h at 80 °C (entry 7). This illustrated that the excellent chemoselectivity of the reduction of 3e can be controlled by prolonging the reaction time and conditions.  Based on this observation, transfer hydrogenation of internal alkynes 3 was subsequently investigated in aqueous media under air. The results summarized in Table 4 showed that various internal alkynes were readily reduced to the corresponding alkenes in moderate to excellent conversion yields. For 1,2-diphenylacetlene 3a as the substrate, the use of a DMF/H2O (5/5) solvent system at 80 °C (condition A) produced a 100% conversion yield confirmed by GC (Gas Chromatography) with a Z/E ratio of 93/7 (entry 1). On the other hand, the use of a DMF/H2O (9/1) solvent system at 80 °C (condition B) generated a 100% conversion GC yield and 94/6 Z/E ratio (entry 2). Surprisingly, the corresponding product 4b was formed in a 97% isolated yield with a Z/E ratio of 30/70 under condition B (entry 3) when using heteroaromatic alkynes 3b as a substrate. That might be the reason why the stereoselectivity of the product is affected by the pyridinyl group, the coordinative moiety [61,62]. It should be mentioned that 3c was successfully reduced to (Z)-4c with excellent stereoselectivity under condition B (entry 5). For the semihydrogenation of conjugated alkynes bearing ester 3d, good performance was demonstrated with a Z/E ratio of 95/5 under condition B (entry 6). In particular, only (Z)-alkenylamide, (Z)-4e, was prepared in an 83% isolated yield for 1 h at 60 °C (entry 8) and over-reduced amide 9e was the only product in a quantitative yield for 2 h at 80 °C (entry 7). This illustrated that the excellent chemoselectivity of the reduction of 3e can be controlled by prolonging the reaction time and conditions.  4 GC yield is reported in parentheses and undecane was applied as an internal standard. 5 Isolated yield is reported in parentheses. , and HCO2H/TEAF (4 equiv) at 80 °C in DMF/H2O mixed solvent (5 mL) under air atmosphere. 2 Condition A: the reaction was carried out at 80 °C in DMF/H2O (5/5); condition B: the reaction was conducted at 80 °C in DMF/H2O (9/1); condition C: the reaction was carried out at 60 °C in DMF/H2O (9/1). 3 Determined by GC or 400 MHz NMR analysis (in Supplementary Materials). 4 GC yield is reported in parentheses and undecane was applied as an internal standard. 5 Isolated yield is reported in parentheses.
Similarly, the catalytic system was efficiently used to employ in the reduction of var- , and HCO2H/TEAF (4 equiv) at 80 °C in DMF/H2O mixed solvent (5 mL) under air atmosphere. 2 Condition A: the reaction was carried out at 80 °C in DMF/H2O (5/5); condition B: the reaction was conducted at 80 °C in DMF/H2O (9/1); condition C: the reaction was carried out at 60 °C in DMF/H2O (9/1). 3 Determined by GC or 400 MHz NMR analysis (in Supplementary Materials). 4 GC yield is reported in parentheses and undecane was applied as an internal standard. 5 Isolated yield is reported in parentheses.
Similarly, the catalytic system was efficiently used to employ in the reduction of various terminal alkynes 10 ( Table 5). The catalytic system was effective in the hydrogenation  %), bis-NHC-Ag (0.5 mol %), and HCO2H/TEAF (4 equiv) at 80 °C in DMF/H2O mixed solvent (5 mL) under air atmosphere. 2 Condition A: the reaction was carried out at 80 °C in DMF/H2O (5/5); condition B: the reaction was conducted at 80 °C in DMF/H2O (9/1); condition C: the reaction was carried out at 60 °C in DMF/H2O (9/1). 3 Determined by GC or 400 MHz NMR analysis (in Supplementary Materials). 4 GC yield is reported in parentheses and undecane was applied as an internal standard. 5 Isolated yield is reported in parentheses.
Similarly, the catalytic system was efficiently used to employ in the reduction of various terminal alkynes 10 ( Table 5). The catalytic system was effective in the hydrogenation of phenylacetylene under condition B and styrene was observed with an 89% GC yield  %), bis-NHC-Ag (0.5 mol %), and HCO2H/TEAF (4 equiv) at 80 °C in DMF/H2O mixed solvent (5 mL) under air atmosphere. 2 Condition A: the reaction was carried out at 80 °C in DMF/H2O (5/5); condition B: the reaction was conducted at 80 °C in DMF/H2O (9/1); condition C: the reaction was carried out at 60 °C in DMF/H2O (9/1). 3 Determined by GC or 400 MHz NMR analysis (in Supplementary Materials). 4 GC yield is reported in parentheses and undecane was applied as an internal standard. 5 Isolated yield is reported in parentheses.
Similarly, the catalytic system was efficiently used to employ in the reduction of various terminal alkynes 10 ( Table 5). The catalytic system was effective in the hydrogenation of phenylacetylene under condition B and styrene was observed with an 89% GC yield (entry 1). On the contrary, ethylbenzene 12a was obtained when the reaction time was Similarly, the catalytic system was efficiently used to employ in the reduction of various terminal alkynes 10 ( Table 5). The catalytic system was effective in the hydrogenation of phenylacetylene under condition B and styrene was observed with an 89% GC yield (entry 1). On the contrary, ethylbenzene 12a was obtained when the reaction time was spread to 5 h (entry 2). It was also noted that the terminal-alkyne-bearing electrondonating group on the benzene ring, i. e., 10b, 10c, and 10d, was easily reduced to the corresponding olefins within 30 min with conversion yields of 82-92% (entries 3 and 5). Completely reduced products, saturated alkanes 12, were achieved by extending reaction time to 3 h and moderate GC yields in the range 40-68% were obtained (entries 4 and 6). Notably, 4-nitrophenylacetylene, 10d, shows that the catalytic system displays excellent chemoselectivity between the alkyne and nitro groups (entry 7). Interestingly, the complete reduction product, 4-ethylaniline 12d, was generated in an 80% isolated yield when increasing reaction time to 2 h (entry 8). In addition, the catalytic system also tolerates well functionalities such as alcohol, methoxy, and nitro groups. Heteroaryl alkyne, 10e, was reduced to the corresponding olefin in a 72% GC yield (entry 9). In addition to aryl alkynes, the aliphatic alkynes, 10f and 10g, were also investigated in an aqueous medium under air. The semi-reduction products, 11f and 11g, were obtained in 99% and 78% isolated yields (entries 10 and 12), while the over-reduction products, 12f and 12g, were given in 97% and 72% isolated yields, respectively (entries 11 and 13). Table 5. Chemoselectivity studies for bis-NHC-Ag/Pd(OAc) 2 catalytic system catalyzed transfer hydrogenation of terminal alkynes in aqueous media 1 .

General Methods
Unless otherwise noted, commercially available materials, which were received from Aldrich and Acros, were used without further purification. Anhydrous solvent, DMF, was received from Aldrich. Acetonitrile was obtained by distillation over calcium hydride. Toluene and t-BuOH were distilled and used after treatment with sodium. Reactions were monitored with pre-coated silica gel 60 (F-254) plates. The purification of products was performed by column chromatography (silica gel, 0.040-0.063 μm) eluting with n-hexane/ethyl acetate. 1 H and 13 C NMR spectra, found in the Supplementary Materials, were analyzed with an Agilent Mercury 400 spectrometer. J-values are given in Hz . Chemical shifts (δ) were recorded from CDCl3 (δ = 7.26 ppm) in the 1 H NMR spectra and the central peak of CDCl3 (δ = 77.0 ppm) in the 13 C NMR spectra. GC-FID (Gas Chromatographyflame ionization detector) was analyzed on a Shimadzu GC-2014 equipped with a capillary column (SPB ® -5, 60 m × 0.25 mm × 0.25 μm). The conversion yields, GC-yield, and ratios were determined by using undecane as an internal standard. High-resolution mass spectra were obtained with a Finnigan/Thermo Quest MAT 95XL mass spectrometer using either the electron impact (EI) or the electrospray ionization (ESI) method. The synthesis of the bis-NHC-Ag complex was carried out according to our previous report [50].

General Procedures for Transfer Hydrogenation Reactions in Organic Solvent Under N2
A Schlenk tube was charged with bis-NHC-Ag complex (0.5 mol %), Pd(OAc)2 (1 mol %), NEt3 (4 mmol), HCO2H (4 mmol), substrate (1 mmol), and DMF (5 mL). After stirring at 80 °C for 24 h under N2, 5 mL of brine were added to the reaction mixture. The aqueous phase was extracted with EtOAc (5 mL × 3). The combined organic phases were dried (Na2SO4) and filtered. The filtrate was concentrated under reduced pressure and the residue was purified by column chromatography.  2 Determined by GC analysis. 3 GC yield is reported and undecane was applied as an internal standard. 4 Isolated yield is reported. 5 The product was 4-ethylaniline. 6 The reaction temperature was 60 • C.

General Methods
Unless otherwise noted, commercially available materials, which were received from Aldrich and Acros, were used without further purification. Anhydrous solvent, DMF, was received from Aldrich. Acetonitrile was obtained by distillation over calcium hydride. Toluene and t-BuOH were distilled and used after treatment with sodium. Reactions were monitored with pre-coated silica gel 60 (F-254) plates. The purification of products was performed by column chromatography (silica gel, 0.040-0.063 µm) eluting with n-hexane/ethyl acetate. 1 H and 13 C NMR spectra, found in the Supplementary Materials, were analyzed with an Agilent Mercury 400 spectrometer. J-values are given in Hz. Chemical shifts (δ) were recorded from CDCl 3 (δ = 7.26 ppm) in the 1 H NMR spectra and the central peak of CDCl 3 (δ = 77.0 ppm) in the 13 C NMR spectra. GC-FID (Gas Chromatography-flame ionization detector) was analyzed on a Shimadzu GC-2014 equipped with a capillary column (SPB ® -5, 60 m × 0.25 mm × 0.25 µm). The conversion yields, GC-yield, and ratios were determined by using undecane as an internal standard. High-resolution mass spectra were obtained with a Finnigan/Thermo Quest MAT 95XL mass spectrometer using either the electron impact (EI) or the electrospray ionization (ESI) method. The synthesis of the bis-NHC-Ag complex was carried out according to our previous report [50].

General Procedures for Transfer Hydrogenation Reactions in Organic Solvent under N 2
A Schlenk tube was charged with bis-NHC-Ag complex (0.5 mol %), Pd(OAc) 2 (1 mol %), NEt 3 (4 mmol), HCO 2 H (4 mmol), substrate (1 mmol), and DMF (5 mL). After stirring at 80 • C for 24 h under N 2 , 5 mL of brine were added to the reaction mixture. The aqueous phase was extracted with EtOAc (5 mL × 3). The combined organic phases were dried (Na 2 SO 4 ) and filtered. The filtrate was concentrated under reduced pressure and the residue was purified by column chromatography.

General Procedures for Transfer Hydrogenation Reactions in Aqueous Media under Air
A Schlenk tube was charged with bis-NHC-Ag complex (0.5 mol %), Pd(OAc) 2 (1 mol %), NEt 3 (4 mmol), HCO 2 H (4 mmol), substrate 3 (1 mmol), and DMF/H 2 O (5 mL). After stirring at 80 • C for 24 h under air, 5 mL of brine were added to the resulting solution. After extracting the aqueous layer with EtOAc (5 mL × 3), the combined organic layers were dried (Na 2 SO 4 ) and filtered. The filtrate was concentrated under reduced pressure and the residue was purified by column chromatography.