Letter pubs.acs.org/acscatalysis
Selective Hydrogenation of Functionalized Alkynes to (E)‑Alkenes, Using Ordered Alloys as Catalysts Shinya Furukawa* and Takayuki Komatsu* Department of Chemistry, Graduate School of Science and Enginnering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8550, Japan S Supporting Information *
ABSTRACT: Intermetallic Pd3Pb acts as a highly selective alkyne semihydrogenation catalyst that is greatly superior to the conventional Lindlar catalyst. Density functional theory (DFT) calculations demonstrate an ideal adsorption property of Pd3Pb, where the surface holds alkynes while releasing alkenes. A tandem catalytic system that is comprised of Pd3Pb/SiO2 for alkyne semihydrogenation and RhSb/SiO2 for alkene isomerization allows one-pot (E)-alkene synthesis from a functionalized alkyne, which is the first success using heterogeneous catalysts. A variety of functionalized alkynes with aldehyde, ketone, carboxylic acid, and ester moieties are hydrogenated into the corresponding (E)-alkene in good to excellent yields under 1 atm H2 at room temperature. KEYWORDS: alkyne hydrogenation, alkene isomerization, (E)-alkene, ordered alloy, intermetallic compound, Pd3Pb, RhSb
T
hydrogenation of CC bonds is favored kinetically and thermodynamically compared with heteroatom-containing unsaturated bonds such as CO, CN, CN, and NO. This hydrogenation is also expected to be compatible with acidlabile substrates, such as amines. Moreover, the use of hydrogen would allow for a one-pot protocol for semihydrogenation and isomerization under a single reaction condition. However, hydrogen-mediated alkene isomerization is always associated with irreversible overhydrogenation to alkane, which significantly lowers the yield of the desired (E)-alkene at a high conversion. This drawback has been a significant hindrance for the successful development of (E)-alkene synthesis reactions. Recently, we reported on the discovery of an unprecedented catalysis that overcomes this problem, where a RhSb-ordered alloy (intermetallic compound) selectively catalyzes the isomerization of (Z)-stilbene to (E)-stilbene under 1 atm H2 with a negligible byproduction of the overhydrogenated diphenylethane.8 A detailed study combined with several characterizations and theoretical calculations revealed that the surface of RhSb is dominated by one-dimensionally aligned Rh rows separated by Sb atoms (1D-planes), which allows surface hydrogen diffusion in one direction (Figure 1). The geometric constraints of the 1D-planes and steric hindrance from one alkyl group of (Z)-alkene limit hydrogen access toward the alkenyl carbon to one direction, enabling one-atom hydrogenation for isomerization but inhibiting twoatom hydrogenation for overhydrogenation (Figure 1).8 In the present study, the unprecedented surface stereochemistry
he semihydrogenation of alkynes to alkenes is a very important chemical transformation in many industrial and synthetic applications.1 To date, several catalytic systems applicable to alkyne semihydrogenation, represented by Lindlar reduction, have been reported.2 The catalytic hydrogenation of inner alkynes, being either homogeneous or heterogeneous, shows an intrinsic stereoselectivity to (Z)-alkenes, because of their syn addition style. In contrast, the catalytic hydrogenation of alkynes to (E)-alkenes, in principle, barely occurs. Although a Birch-type reduction can be applied to (E)-alkene synthesis, the reaction is stoichiometric and the strongly reductive condition with alkali metals in liquid ammonia is essentially incompatible to functionalized alkynes with base-labile or reducible moieties.3 In this context, the development of an efficient catalytic system for (E)-alkene synthesis that is both mild and functional-grouptolerant is highly attractive and challenging. A simple and promising synthetic root involves a two-step conversion via catalytic hydrogenation to (Z)-alkene, followed by isomerization to the thermodynamically stable (E) counterpart. Alkene isomerization generally requires the rotation of an alkenyl CC bond. The conventional protocols to rotate the CC bond employ photoexcitation to a triplet state,4 where a π-system is twisted, or acid-catalyzed formation of carbocation intermediates.5 However, these protocols are not very compatible with functionalized alkynes, because the functional groups typically undergo undesired photodecomposition (e.g., Norrish reactions of carbonyl6) or acid-catalyzed side reactions (e.g., aldol condensation or hydrolysis of esters). A promising candidate may be hydrogen-mediated alkene isomerization. In this process, the half-hydrogenation of a (Z)-alkene to an alkyl intermediate allows for C−C rotation, and the subsequent β−H elimination can produce the corresponding (E)-alkene.7 The © XXXX American Chemical Society
Received: December 26, 2015 Revised: February 15, 2016
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in Table 1). Similar results were obtained when PdCu and PdZn were used as catalysts (entries 2 and 3, respectively, in Table 1). These catalysts produced relatively high (E)-alkene contents, which was likely due to the occurrence of isomerization, as well as overhydrogenation. The catalysts Pd20Sn13, PdIn, PdFe, and Pd2Ga exhibited low or moderate conversions (5%−58%), even after 90 min. Both PdAg (an industrial catalyst for acetylene semihydrogenation) and Pd− Pb/CaCO3 resulted in relatively good selectivities (entries 8 and 9 in Table 1). However, they were below 90%, because of overhydrogenation. The selectivity was improved in the presence of quinoline modifier (entry 10 in Table 1), as typically applied in organic synthesis.2a,b Conversely, Pd3Bi and Pd3Pb exhibited excellent selectivities (entries 11 and 12 in Table 1), even at the 99% conversion of alkyne. The highest selectivity was obtained using Pd3Pb, resulting in an almost stoichiometric conversion into the corresponding alkene. Figure 2 shows the time course of the product yield in the hydrogenation of 4-phenyl-3-butyn-2-one with the Pd−Pb/ CaCO3 and Pd3Pb/SiO2 catalysts.
Figure 1. Equilibrium crystal shape of RhSb and atomic arrangement on the surface (left). Schematic illustration of limited hydrogen access to the alkenyl carbon on the 1D-plane ((020) plane, right).8
governed by RhSb/SiO2 was employed as an effective alkene isomerization methodology for (E)-alkene synthesis. Herein, we report the first example of a heterogeneous catalytic system for (E)-alkene synthesis from various functionalized alkynes. First, we optimized the alkyne semihydrogenation catalysts using Pd-based bimetallic materials, including a commercially available Lindlar catalyst (Pd−Pb/CaCO3, Pd 5 wt %, TCI). A series of Pd-based solid solution alloys and intermetallic compounds supported on silica (PdxMy/SiO2, where M = Ag, Bi, Cu, Fe, Ga, In, Pb, Sn, and Zn) were prepared by conventional impregnation, followed by H2 reduction at 400 °C−800 °C (see the Supporting Information for the experimental details). The formation of the desired alloy or intermetallic phase was confirmed by X-ray diffraction (XRD) (see Figure S1 in the Supporting Information). The catalytic performances of these palladium-based catalysts were compared in the hydrogenation of 4-phenyl-3-butyn-2-one under 1 atm H2 at 25 °C (see Table 1). For all catalysts, an overhydrogenated alkane (4-phenylbutan-2-one) was formed as the main byproduct. As commonly known, monometallic Pd/SiO2 shows low alkene selectivity, because of the significant overhydrogenation to alkane (entry 1 Table 1. Semihydrogenation of 4-Phenyl-3-butyn-2-one Using Various Palladium-Based Catalystsa
entry
catalyst
1 2 3 4 5 6 7 8 9c 10c,d 11 12 13
Pd/SiO2 PdCu/SiO2 PdZn/SiO2 Pd20Sn13/SiO2 PdIn/SiO2 PdFe/SiO2 Pd2Ga/SiO2 PdAg/SiO2 Pd−Pb/CaCO3 Pd3Bi/SiO2 Pd3Pb/SiO2 RhSb/SiO2
time (min)
conversion (%)
alkene selectivity (%)b
16 20 25 90 90 90 90 30 16 16 60 95 90
98 98 98 5 7 23 58 99 99 99 99 99 4
65 31 78 95 96 95 91 84 88 97 94 98 86
Figure 2. Time course of the product yield in the hydrogenation of 4phenyl-3-butyn-2-one over Pd−Pb/CaCO3 (left) and Pd3Pb/SiO2 (right).
Z:E 73:27 74:26 87:13 88:12 90:10 92:8 93:7 85:15 91:9 94:6 92:8 86:14 78:22
For Pd−Pb/CaCO3, overhydrogenation to alkane proceeded significantly after the complete hydrogenation of alkyne, resulting in a lower alkene yield. Thus, the commercial Lindlar catalyst could not prevent the undesired overhydrogenation and was not suitable for (E)-alkene synthesis. Conversely, when Pd3Pb/SiO2 was used, the overhydrogenation was successfully inhibited even after the complete conversion of alkyne. Note that the alkene selectivity was maintained at 93% at twice the reaction time for the complete alkyne conversion, which was higher than those for Pd−Pb/CaCO3 (54%) and the quinolinemodified Pd−Pb/CaCO3 (86%, Figure S2 in the Supporting Information). According to a report in the literature, the Pb species of the Pd−Pb/CaCO3 catalyst is deposited on the surface of monometallic palladium,9 which is different from the Pd3Pb/SiO2 catalyst, which comprised the intermetallic phase. Based on these results, we concluded that Pd3Pb/SiO2 was the catalyst most suitable for the (E)-alkene synthesis. We also checked the catalytic performance of RhSb/SiO2 for the alkyne hydrogenation, which showed a very low conversion (Table 1, entry 13). This may be explained by the geometric effect of RhSb 1D-plane, as shown in Figure 1: for alkyne, hydrogen
a
Reaction conditions: alkyne, 0.5 mmol; catalyst, 50 mg (Pd or Rh: 3 wt %); solvent, 5 mL (THF); atmosphere, 1 atm H2; temperature, 25 °C. bSelectivity to (Z)- and (E)-alkenes. cPd 5 wt %, 20 mg. d Quinoline (2 mg) was added in the reaction mixture. 2122
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The monometallic Pd(111) surface showed largely (−107 kJ mol−1) and moderately (−49 kJ mol−1) negative Ead values for acetylene and ethylene adsorption, respectively. The latter indicates that alkene desorption from palladium is relatively slow, which causes overhydrogenation to alkane. For other bimetallic surfaces, the Ead values for ethylene were higher than those of palladium in the following order:
access to the alkynyl carbons is completely blocked by the connecting functional groups. The observed difference in alkene selectivity among the tested catalysts has often been explained by the adsorption energy of alkene.10 The adsorption energy of alkene is typically reduced in the presence of the secondary metal atoms adjacent to Pd atoms, which accelerates alkene desorption, thereby, inhibiting overhydrogenation. However, alkyne adsorption is also weakened, which lowers the hydrogenation rate of alkyne. Based on this concept, we performed density functional theory (DFT) calculations to compare the adsorption energies of alkene and alkyne on the surface of the palladium-based materials used in this study (Pd, PdAg, Pd−Pb, Pd3Bi, Pd3Pb, and PdIn). As models of adsorbed alkene and alkyne, ethylene and acetylene with di-σ adsorption to Pd atoms were considered, respectively. Figure 3 shows the optimized
Pd < PdAg < Pd−Pb < Pd3Bi < Pd3Pb ≪ PdIn
This order is consistent with those of alkene selectivity at the complete conversion of alkyne (Pd < PdAg < Pd−Pb < Pd3Bi < Pd3Pb), demonstrating the aforementioned rationale for alkene selectivity. Although PdIn showed the highest Ead value for ethylene, that for acetylene was also quite positive. This agrees with the very low catalytic activity of PdIn (Table 1, entry 5). Thus, appropriate modification in Ead is needed to develop an efficient catalyst for alkene semihydrogenation. In this context, Pd3Pb provides an ideal adsorption property, i.e., slightly positive Ead for ethylene and moderately negative Ead for acetylene. Note that this property cannot be obtained with the Pb-deposited Pd surface (Pd−Pb). The modifier second metal atoms should be incorporated into the lattice, as observed for intermetallic Pd3Pb. We subsequently performed a one-pot synthesis of (E)alkene in the presence of Pd3Pb/SiO2 and RhSb/SiO2. Figure 5 shows the time course of the product yield in the hydrogenation of 3-phenyl-2-propyn-1-one using these catalysts.
Figure 3. Optimized structures of di-σ bonded ethylene on Pbdeposited-Pd(111) (left) and Pd3Pb(111) (right) surfaces. For the former, Pb atoms were deposited on fcc hollow sites of Pd(111) with a (3 × 3) structure.
structures of ethylene adsorbed on Pb-deposited-Pd(111) and Pd3Pb(111) surfaces, the former of which represents a simplified model surface of the Lindlar catalyst (structures for other surfaces and acetylene are shown in Figure S3 in the Supporting Information). These models clearly distinguish whether the Pb atoms are incorporated into the surface lattice or not, which is the crucial difference between the Lindlar catalyst and intermetallic Pd3Pb. The calculated adsorption energy (Ead) of acetylene and ethylene on the surfaces of various Pd-based materials are summarized in Figure 4.
Figure 5. Time course of the product yield in the hydrogenation of 3phenyl-2-propyn-1-one in the presence of Pd3Pb/SiO2 and RhSb/ SiO2. Reaction conditions: alkyne, 0.5 mmol; catalyst, Pd3Pb/SiO2 (Pd: 3 wt %) 100 mg, RhSb/SiO2 (Rh: 3 wt %) 100 mg; solvent (THF), 5 mL; atmosphere, 1 atm H2; temperature, 25 °C.
At the early stage of the reaction, the semihydrogenation of the alkyne to the (Z)-alkene mainly proceeded. In the latter half, the isomerization of the (Z)-alkene to the (E) counterpart took place almost exclusively. Thus, the tandem catalytic reaction system allowed for the successive reactions of semihydrogenation and isomerization. During the reaction, overhydrogenation to the alkane was successfully inhibited,
Figure 4. Calculated adsorption energies of di-σ bonded acetylene and ethylene on the surfaces of various Pd-based surfaces: Pd(111), PdAg(110), Pd(111)-(3 × 3)-2Pb (Pd−Pb), Pd3Bi(100), Pd3Pb(111), and PdIn(110). For each material, the most densely packed surface was employed as a most stable surface. 2123
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convenient handling of the synthesis. This catalytic system worked well also in a gram-scale condition, which afforded a good isolated yield of the desired (E)-alkene (76%, entry 5). Recycling tests revealed that the catalysts could be used at least 4 times without reducing the selectivity (see Table S1 in the Supporting Information). Although the conversion rate decreased in the reuse, high product yields comparable to that for the initial run could be obtained at longer reaction times. In this catalytic system, inhibiting the overhydrogenation to alkane is one of the critical points to obtain a high (E)-alkene yield. The conventional Lindlar catalyst, in this context, is not suitable for this reaction, because the unavoidable overhydrogenation during the second isomerization step would significantly decrease the alkene yield. The other critical point is that the RhSb/SiO2 catalyst and its hydrogen-mediated isomerization methodology are highly compatible to various functional groups. However, our attempt to hydrogenation of nitro-containing alkyne (1-nitro-4-phenylethynyl-benzene) to the corresponding (E)-alkene resulted in hydrogenation of both alkyne and nitro moieties (data not shown). A control experiment using RhSb/SiO2 alone revealed that RhSb/SiO2 is active for nitro-hydrogenation. In conclusion, we have discovered that a Pd3Pb ordered alloy, as a highly selective alkyne semihydrogenation catalyst, is greatly superior to the conventional Lindlar catalyst. DFT calculations have demonstrated an ideal adsorption property of Pd3Pb, where the surface holds alkynes while releasing alkenes. The combination of the two original catalysts, i.e., Pd3Pb/SiO2 for alkyne semihydrogenation and RhSb/SiO2 for alkene isomerization, allowed for one-pot (E)-alkene synthesis from a functionalized alkyne, which is the first success using heterogeneous catalysts. A variety of functionalized alkynes with aldehyde, ketone, carboxylic acid, and ester moieties were hydrogenated into the corresponding (E)-alkene in good to excellent yields under 1 atm H2 at room temperature.
resulting in a 94% yield of the desired (E)-alkene and the production of only a small amount of the overhydrogenated alkane. Alcoholic species originating from carbonyl hydrogenation were not detected. In a similar fashion, the hydrogenation of various functionalized alkynes was performed using the tandem catalytic system. Table 2 shows the resulting alkyne conversion, alkene selectivity and E:Z ratios, which reflects the (Z)-alkene conversion in the second step. Table 2. Hydrogenation of Various Functionalized Alkynes to the Corresponding (E)-Alkenes Using Pd3Pb/SiO2 and RhSb/SiO2 Catalystsa
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02953. Experimental and computational details, XRD patterns, recycling test, and optimized structures (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (S. Furukawa). *E-mail:
[email protected] (T. Komatsu).
a
Reaction condition is identical to that described in Figure 5. b Selectivity to (E) and (Z) alkenes. cAlkyne, 6.0 mmol; Pd3Pb/SiO2, 200 mg, RhSb/SiO2, 200 mg. dIsolated yield. eAlkyne, 0.1 mmol; Pd3Pb/SiO2, 25 mg, RhSb/SiO2, 75 mg.
Notes
The authors declare no competing financial interest.
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REFERENCES
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