Water as a Hydrogenating Agent: Stereodivergent Pd-Catalyzed

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Water as a Hydrogenating Agent: Stereodivergent Pd-Catalyzed Semihydrogenation of Alkynes Chuan-Qi Zhao, Yue-Gang Chen, Hui Qiu, Lei Wei, Ping Fang, and Tian-Sheng Mei* State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, China

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S Supporting Information *

ABSTRACT: Palladium-catalyzed transfer semihydrogenation of alkynes using H2O as the hydrogen source and Mn as the reducing reagent is developed, affording cis- and trans-alkenes selectively under mild conditions. In addition, this method provides an efficient way to access various cis-1,2-dideuterioalkenes and trans-1,2-dideuterioalkenes by using D2O instead of H2O.

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Scheme 1. Palladium-Catalyzed Transfer Hydrogenation from Water

onstruction of carbon−carbon double bond is of great importance in organic synthesis since alkenes are essential structural units in many natural products, pharmaceuticals, and agrochemicals.1 Transition-metal-catalyzed semihydrogenation of alkynes using hydrogen gas has been developed as a reliable tool for the stereoselective construction of alkenes in the laboratory and on an industrial scale.2,3 Alternatively, transfer hydrogenation has emerged as a promising tool to construct alkenes since it avoids the use of a flammable gas and elaborate experimental setups.4 In this context, various H donors such as formic acid,5 silanes,6 alcohols,7 and others have been used in catalytic semihydrogenation of alkynes.8 However, there are few examples of semihydrogenation reactions that use water as a hydrogenating agent despite the cost and safety advantages associated with its use.9,10 Further, due to the importance of deuterium-labeled molecules as tools for biological research and the elucidation of reaction mechanisms, the transfer deuteriation of alkynes using D2 O to obtain alkenes deuteriated at olefinic positions is highly important.11,12 In 2005, the Hayashi group developed catalytic semihydrogenation of diarylalkynes to provide trans-alkenes using H2O and hexamethyldisilane as the hydrogen source (Scheme 1a).9 In 2007, Oltra and co-workers reported catalytic semihydrogenation of alkynes to achieve cis-alkenes using H2O and a stoichiometric amount of titanocene(III) chloride with the aid of Lindlar catalyst (Scheme 1b).10a Transfer hydrogenation of alkenes to alkanes with D2O was also demonstrated, and incomplete deuterium incorporation was obtained. In 2016, Stokes and co-workers elegantly developed a diboron-mediated Pd-catalyzed transfer hydrogenation of alkenes and alkynes using H2O as a hydride source at room temperature (Scheme 1c).10b Nevertheless, catalytic stereodivergent semihydrogenation of alkynes using H2O to produce cis- and trans-alkenes with great selectivity and efficiency is still lacking.13,14 Herein, we report a Pd-catalyzed semihydrogenation of alkynes to access cis- and trans-alkenes using water as the hydrogenating agent (Scheme 1d). In © XXXX American Chemical Society

addition, we demonstrate that cis-1,2-dideuterioalkenes are selectively obtained from alkynes using D2O, the most inexpensive and convenient deuterium source. A preliminary mechanistic study indicates that trans-alkenes are derived from cis-alkenes through isomerization and palladium nanoparticles (PdNPs) with an average diameter of 1.3 nm are the active catalysts. Initially, we chose diphenylacetylene (1a) as a model substrate and H2O as a hydrogenating agent and probed Received: January 14, 2019

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DOI: 10.1021/acs.orglett.9b00148 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

Scheme 2. Cis-Selective Alkyne Semihydrogenationa,b

various reaction conditions for the envisioned semihydrogenation. After extensive optimization, we found that 88% isolated yield of cis-stilbene (2a) could be obtained in the presence of 1 mol % of Pd(OAc)2, 2 equiv of Mn, and 80 equiv of water in CH3CN at room temperature after 24 h (Table 1, Table 1. Reaction Optimization with Substrate 1a,b

entry

variation from standard conditions

1 2

none 20 equiv of H2O in lieu of 80 equiv of H2O 40 equiv of H2O in lieu of 80 equiv of H2O 160 equiv of H2O in lieu of 80 equiv of H2O DMF in lieu of CH3CN 5 mol % of Pd/C in lieu of 1 mol % of Pd(OAc)2 1 mol % of Pd(PPh3)4 in lieu of 1 mol % of Pd(OAc)2 Zn in lieu of Mn no Mn or no Pd(OAc)2 or no H2O

3 4 5 6 7 8 9

convb (%)

2a/3a

2a yieldb (%)

98 10

96/4 89/11

88c 8

55

96/4

52

82

95/5

75

32 14

95/5 99/1

31 14

0

0

0 0

0 0

a Reaction conditions: 1a (0.25 mmol), Pd(OAc)2 (1 mol %), Mn (2 equiv), H2O (80 equiv), CH3CN (2 mL), rt for 24 h. bThe yield and cis/trans ratio was determined by 1HNMR using dibromomethane as the internal standard. cIsolated yield.

a

Isolated yields are reported unless otherwise noted. bCis/trans ratio of alkenes in parentheses was determined by GC−FID. cThe yield was determined by 1H NMR with dibromomethane as the internal standard.

entry 1). Decreasing and increasing the amount of water resulted in lower yields (entries 2−4). Evaluating different solvents revealed that CH3CN was optimal (entry 5). The yield decreased significantly when Pd(OAc)2 was replaced with Pd/C or Pd(PPh3)4 (entries 6 and 7). Finally, control experiments indicated that Mn, Pd(OAc)2, and H2O are all essential for this reaction (entries 8 and 9). With the optimized reaction conditions in hand, the scope of alkynes was investigated to test the generality and limitations of this Pd-catalyzed semihydrogenation reaction. As shown in Scheme 2, both aromatic (2b−s) and aliphatic internal alkynes (2t−y) are readily hydrogenated to the desired cis-alkenes with good to excellent yield and cis-stereoselectivity. Alkynes substituted with a variety of functional groups such as ether, alkyl, chloro, acyl, ester, cyano, amide, formyl, aldehyde, hydroxyl, and amino groups (2b−x) were well tolerated under the standard conditions. It is worth noting that thiophene- and pyridine-substituted alkynes proved compatible (2h and 2i). To our delight, nonconjugated internal alkynes also gave excellent yield with high cis-stereoselectivity (2t, 2u, and 2w− y). Trans-selective semihydrogenation of alkynes is more challenging. We envisioned that trans-alkenes could be formed from cis-alkenes at elevated temperature through isomerization, since trans-alkenes are typically the thermodynamically favored products.15 To our delight, trans-stilbene (3a) could be obtained in moderate yield (42%) with 8% cis-stilbene (2a) in the presence of 5 mol % of Pd(OAc)2, 2 equiv of Mn, and 80 equiv of H2O in CH3CN at 80 °C (entry 1, Table S7; see the Supporting Information for details). The low yield is due to the formation of alkane 4a by over-reduction. When the

reaction was conducted in DMF, excellent trans-selectivity could be achieved with moderate yield (entry 2). Fortunately, lowering the amount water to 20 equiv could further improve the yield of the desired product (3a) to 80%, along with 20% over-reduction product (4a) (entry 3). Thus, controlling the amount of water is crucial to overcome the over-reduction side reaction. However, the trans-selectivity decreased with further lowering of the amount of water (entries 4 and 5). To our delight, the conversion increased rapidly when Mg(OAc)2 was added (entries 6−8). trans-Stilbene (3a) could be obtained in good yield (89%) with excellent trans-selectivity using 7 equiv of H2O and 3 equiv of Mg(OAc)2 (entry 8). The role of Mg(OAc)2 is not fully understood at the current stage, although it was proposed that Lewis acid may help to activate H2O.16 Other Lewis acids such as MgCl2, MgBr2, LiOAc, NaOAc, and Ba(OAc)2 are not as effective as Mg(OAc)2 (entries 9−13). Next, we explored the scope of trans-stereoselective semihydrogenation of alkynes. As shown in Scheme 3, aromatic internal alkynes are hydrogenated to the desired trans-alkenes with satisfactory yield and excellent stereoselectivity (3a−l). Importantly, substrates containing various functional groups, including alkyl, ether, amino, fluoro, ester, acyl, cyano, thiophene, amide, and hydroxyl, were tolerated. Unfortunately, 3-phenyl-2-propynenitrile and 3-phenyl-2-propyn-1-ol give low yields due to over-reduction and isomerization. B


Letter

Organic Letters Scheme 3. Trans-Selective Alkyne Semihydrogenationa,b

Scheme 4. Cis-Selective Alkyne Semireduction with D2Oa−c

a

Isolated yields are reported unless otherwise noted. bCis/trans ratio of alkenes in parentheses was determined by GC−FID. cMethod A: 1 (0.25 mmol), Pd(OAc)2 (5 mol %), Mn (2 equiv), Mg(OAc)2 (3 equiv), H2O (7−10 equiv), DMF (2 mL), 80 °C for 24 h. dMethod B: 1 (0.25 mmol), Pd(OAc)2 (5 mol %), Mn (2 equiv), H2O (20 equiv), DMF (2 mL), 80 °C for 24 h. eCis/trans ratio of alkene determined by 1H NMR.

a

Isolated yield unless otherwise noted. bCis/trans ratio of alkene in the parentheses was determined by GC-FID. cD/H ratio of alkene was determined by 1H NMR. dReaction conditions A: 1 (0.25 mmol, 1.0 equiv), Pd(OAc)2 (3−5 mol %), Mn (0.5 mmol, 2 equiv), D2O (20 mmol, 80 equiv), CH3CN (2 mL) at room temperature for 24 h. e Reaction conditions B: 1 (0.25 mmol), Pd(OAc)2 (5 mol %), Mn (2 equiv), D2O (25 equiv), DMA (2 mL), 80 °C for 24 h. fCis/trans ratio of alkene determined by 1H NMR.

Encouraged by the feasibility of catalytic stereodivergent semihydrogenation of alkynes using H2O as a hydride source, we moved on to examine the deuteration of alkynes using D2O. To our satisfaction, as shown in Scheme 4, cis-1,2dideuterioalkenes were obtained in good yields and excellent stereoselectivity. Various functional groups, including ether, halide, trifluoromethyl, ester, cyano, and amide, were tolerated under the reaction conditions (5a−m). Importantly, high deuterium incorporation was achieved under optimal reaction conditions. Meanwhile, high deuterium incorporation of trans1,2-dideuterioalkenes can also be prepared in this protocol (5n−p). To the best of our knowledge, this is the first example of Mn-mediated stereodivergent deuteriation of alkynes to achieve cis-alkenes and trans-alkenes using D2O. The scalability of this transfer semihydrogenation was evaluated using a reaction containing 10.7 g (60.0 mmol) of substrate 1a (Scheme S1). Specifically, the reaction between 1a and H2O furnished the desired product in 95% yield with excellent cis-stereoselectivity (96:4), which showcases the preparative utility of this semihydrogenation. The utility of this catalytic stereodivergent semihydrogenation was further demonstrated by synthesis of natural products of ciscombretastatin A-4 (7) and analogues (8−10) (Scheme 5).17 To gain insight into the reaction mechanism, the kinetic behavior of this reaction was investigated. The kinetic profile of this reaction clearly showed that the cis-alkene intermediate 2a was generated in the first 1.5 h and that 2a was further transformed to the trans-alkene 3a through an isomerization process (Figure S1; see the Supporting Information for details).15 Furthermore, the kinetic isotope effect (KIE) experiments were carried out (Scheme S2). Subjection of 1a to the reaction conditions in the presence of an equimolar

Scheme 5. Synthetic Applications

mixture of H2O and D2O revealed a primary kinetic isotope effect of 6.1. Parallel experiments show similar results(kH/kD = 5.0). These results indicate that the O−H cleavage of water is the rate-determining step.10b Additionally, a mercury drop experiment was performed and resulted in a significant decrease in the conversion of 1a, which indicates that the reaction may be involved in metal nanoparticles (Scheme S3).18 As direct evidence in support of the nanoparticulate nature of the catalyst, transmission electron microscopy (TEM) revealed the in situ generated, narrowly dispersed Pd C


Letter

Organic Letters nanoparticles with an average size of 1.3 nm (Figure S2). The nanoparticles were also analyzed using energy-dispersive X-ray spectroscopy (EDX) to confirm the composition of palladium nanoparticles rather than the Pd−Mn bimetallic particles (Figure S3). Thus, we establish for the first time that palladium nanoparticles could be prepared via such a convenient method using Mn as a reducing agent.19 Based on our data, we propose the mechanism depicted in Scheme 6. Initially, Pd(OAc)2 is reduced by Mn to generate

Tian-Sheng Mei: 0000-0002-4985-1071

Scheme 6. Proposed Reaction Mechanism

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000), “1000-Youth Talents Plan”, NSF of China (Grant Nos. 21772220 and 21821002), and S&TCSM of Shanghai (Grant Nos. 17JC1401200 and 18JC1415600).

Pd(0) catalyst, which may interact with water assisted by Mn to form palladium hydrides.19,20 Hydrogen could be generated by reductive elimination from the palladium surface. Indeed, gas-chromatographic headspace analysis confirmed the formation of molecular hydrogen (Figure S4). Next, the palladium hydrides could coordinate with an alkyne to generate complex 11, which undergoes migratory insertion to deliver complex 12. Upon reductive elimination, cis-alkene 2a is released and Pd(0) catalyst is regenerated, thereby completing the catalytic cycle. In addition, cis-alkene 2a could react with palladium hydride to generate intermediate 13 at elevated temperature, which could undergo β-hydride elimination to deliver trans-alkene 3a. Alternatively, complex 13 could undergo reductive elimination to give alkane 4a. In conclusion, we have demonstrated palladium-catalyzed stereodivergent transfer hydrogenations of alkynes to cis- and trans-alkenes using water and Mn. The current system operates under mild conditions and allows for the semihydrogenation of various alkynes with good yields and stereoselectivity. More work to better understand the mechanistic intricacies of this process are currently underway in our laboratory.

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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/acs.orglett.9b00148. Text, figures, and table giving experimental procedures and compound characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yue-Gang Chen: 0000-0001-6012-1849 Ping Fang: 0000-0002-3421-2613 D


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Water as a Hydrogenating Agent: Stereodivergent Pd-Catalyzed

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