Communication Cite This: J. Am. Chem. Soc. 2018, 140, 7385−7389
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Nickel-Catalyzed Highly Regioselective Hydrocyanation of Terminal Alkynes with Zn(CN)2 Using Water as the Hydrogen Source Xingjie Zhang, Xin Xie, and Yuanhong Liu* 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, People’s Republic of China
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S Supporting Information *
were quite limited; (3) The regioselectivity was not good. For example, in Ni-catalyzed hydrocyanation of terminal alkynes with HCN or acetone cyanohydrin, a mixture of linear and branched vinyl nitriles were usually formed. For phenyl acetylene, the linear product was strongly favored,5a,b whereas for aliphatic alkynes with small and moderately sized substituents, branched nitriles were predominant.3a,5 Hydrocyanation reactions of alkynes without the use of HCN have been accomplished by using a stoicheiometric amount of [Co(CN)5]3− and H26 or substoicheiometric amount of [Ni(CN)4]2− (0.5 equiv) in the presence of excess KCN with excess NaBH4 or Zn as the reducing agents in ethylene glycol or water.7 However, in most cases,7a the hydrocyanation is accompanied by a hydrogenation reaction leading to saturated nitriles. Morandi et al. reported an elegant nickel-catalyzed transfer hydrocyanation reaction through reversible alkene− nitrile interconversion for the synthesis of alkyl nitriles, and the reaction could be extended to internal alkynes.8 However, the reactivity of terminal alkynes has not been reported by this method. Recently, a nice Rh-catalyzed hydrocyanation of terminal alkynes with acetone cyanohydrin was reported by Ritter et al., which afforded the alkenyl nitriles with antiMarkovnikov selectivity.9 However, the efficient Markovnikov hydrocyanation of alkynes have not been reported. During our recent work on nickel-catalyzed cyanation of aryl/heteroaryl chlorides using less toxic Zn(CN)2 as the cyanide source,10 we discovered Zn(CN)2 could also be used for hydrocyanation of alkynes. In this report, we disclosed the first general and efficient nickel-catalyzed hydrocyanation of terminal alkynes with less toxic Zn(CN)2 in a mixed solvent of CH3CN/H2O. Remarkably, the method provides the alkenyl nitriles with excellent Markovnikov selectivity for aromatic as well as aliphatic substrates under extremely mild reaction conditions (25 to 50 °C) while obviating the use of the volatile and hazardous reagent of HCN. In addition, the competitive alkyne cyclotrimerization pathway11 could be minimized to a large extent. Deuterium-labeling experiments confirmed the role of water as the hydrogen source in the hydrocyanation reaction. We initially investigated the nickel-catalyzed hydrocyanation of 1-ethynyl-4-methylbenzene 1a with Zn(CN)2. Upon optimization of a variety of reaction parameters such as nickel catalysts, ligands, reducing agents, additives etc., we were pleased to find the hydrocyanation of 1a proceeded smoothly
ABSTRACT: The first efficient and general nickelcatalyzed hydrocyanation of terminal alkynes with Zn(CN)2 in the presence of water has been developed. The reaction provides a regioselective protocol for the synthesis of functionalized vinyl nitriles with a range of structural diversity under mild reaction conditions while obviating use of the volatile and hazardous reagent of HCN. Deuterium-labeling experiments confirmed the role of water as the hydrogen source in this hydrocyanation reaction.
N
itriles are versatile intermediates in organic synthesis because they can be used in preparation of a large number of pharmaceuticals, pesticides and functional materials used in daily life.1 They also serve as precursors for amides, amines, carboxylic acids, esters, aldehydes, ketones and alcohols etc.2 Transition-metal-catalyzed addition of hydrogen cyanide to alkynes (hydrocyanation) offers a straightforward way for synthesis of vinyl nitriles3,4 These reactions generally proceed via oxidative addition of HCN followed by syn addition of both H and CN groups to the carbon−carbon triple bond (Scheme 1). Unfortunately, these methods suffer from the following drawbacks: (1) the highly toxic and volatile hydrogen cyanide (bp 26 °C) was employed. In addition, a careful control of the HCN concentration is usually required to avoid catalyst poisoning during the reaction process. Although acetone cyanohydrin has been employed as a source of HCN, it still poses a significant risk; (2) The scope of the possible substrates Scheme 1. Transition-Metal-Catalyzed Hydrocyanation Reactions of Alkynes
Received: March 7, 2018 Published: May 31, 2018 © 2018 American Chemical Society
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DOI: 10.1021/jacs.8b02542 J. Am. Chem. Soc. 2018, 140, 7385−7389
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Journal of the American Chemical Society to afford branched nitrile 2a exclusively in 85% yield at 25 °C in the presence of 5 mol % Ni(acac)2 and 20 mol % Mn in CH3CN/H2O without adding additional ligands (Table 1, entry
solvents such as DMF or reducing agents were employed (entries 14−16). Importantly, with 2 equiv water or without water there was no reaction, indicating a crucial role of water (entry 17). The results indicate water may also play a role in improving the solubility of Zn(CN)2. Control experiments indicated the nickel catalyst and Mn were essential for the reaction to occur (entry 18). Only 14% and 24% of 2a were detected after 6 and 12 h, respectively, indicating a longer induction period was involved in this reaction (entry 19). Encouraged by these results, we turned our attention to investigate the generality of this hydrocyanation reaction. The scope of aryl alkynes 1 was first studied under the standard reaction conditions (Table 2). It was found the method was
Table 1. Optimization of the Reaction Conditionsa
Yield (%)b Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Deviation from standard conditions none with 6 mol % L1 with 6 mol % L2 with 6 mol % L3 with 6 mol % L4 with 6 mol % L5 with 6 mol % L6 with 6 mol % L7 with 12 mol % PPh3 with 6 mol % IPr NiCl2·6H2O instead of Ni(acac)2 NiI2 instead of Ni(acac)2 NiBr2(diglyme) instead of Ni(acac)2 THF instead of CH3CN DMF instead of CH3CN Zn instead of Mn with 2 equiv H2O or without H2O without either Ni(acac)2 or Mn 1, 3, 6 or 12 h instead of 24 h
2a c
85 (83 ) 82 74 1 7 88 27 20 72 84 3 18 7 83 2 5 0 0 0, 1, 14, 24
3a 0 0 0 0 0 0 1 10 (6c) 0 0 0 0 0 2 0 0 0 0 0
Table 2. Scope of Ni-Catalyzed Hydrocyanation of Aryl Alkynesa
Isolated yields. b25 h. c50 °C, 5 mol % Ni(acac)2 and 50 mol % Mn were used. d5 mol % Ni(acac)2, 6 mol % neocuproine and 50 mol % Mn were used. e36 h. f22 h. a
a
1a (0.5 mmol), Zn(CN)2 (0.4 mmol), Ni(acac)2 (0.025 mmol), Mn (0.1 mmol), in CH3CN/H2O (5:1, total 3.0 mL). bDetermined by 1H NMR of the crude reaction mixture using 1,3,5-trimethoxybenzene as an internal standard. cIsolated yields.
applicable to a wide range of substituted terminal aryl alkynes, and in all cases, the reaction proceeded with excellent Markovnikov selectivity as no linear nitriles were observed. The electron-rich aryl alkynes bearing p-Me, p-MeO, free amino and amide groups were well tolerated, furnishing 2a−2d in 79−85% yields. The presence of an ortho-substituent on the aryl ring resulted in a less efficient reaction at 25 °C, leading to branched nitrile 2e in 50% yield after 24 h. However, high yield of 2e (77%) could be achieved by increasing the reaction temperature to 50 °C and using 50 mol % of Mn. Sterically hindered 1-naphthyl-substituted alkyne afforded nitrile 2f in 73% yield by further adding 6 mol % neocuproine as the ligand. These results also indicated the regioselectivity was not affected by the steric effects on the alkyne substituents. Fluoro, chloro and bromo substituents on the aryl rings were also well tolerated (2g−2i). Although the Ar−X bonds are susceptible to undergo oxidative addition by low-valent nickel species, we did not observe the corresponding products derived from dehalogenation or cyanation reactions. Moderate yields were observed for aryl alkynes bearing electron-withdrawing groups such as p-CF3 (2j) and p-CONH2 (2k). The results might be ascribed to the consumption of the nitrile products through
1).12 The linear product 3a was not observed according to 1H NMR analysis of the crude reaction mixture. These results indicated the reaction proceeds with high regioselectivity via Markovnikov addition. The regioselectivity of arylalkynes observed in our reaction is in contrast to those reported in previous hydrocyanation events.5a,b We have also examined the effects of the ligands on the reaction efficiency. Among these ligands, neocuproine gave the best results, and bipyridine or Nheterocyclic carbene (IPr) ligand showed comparative activity with that of neocuproine (entries 2, 6 and 10). The use of C4-, C4′-substituted bipyridine led to a slightly lower yield (entry 3). Other 1,10-phenanthroline-type ligands of L3, L4 and L6 resulted in trace or low yields of 2a (entries 4, 5, 7). The high efficiency of neocuproine L5 is likely due to the enhanced stability of the nickel-containing intermediates with this ligand. Interestingly, the reaction was not interfered severely by the addition of PPh3 as the ligand, leading to 2a in 72% yield (entry 9). Other nickel catalysts such as NiCl2·6H2O, NiI2 and NiBr2(diglyme) gave 2a in low yields under ligandless conditions (entries 11−13). The reaction could also proceed effectively in THF. Inferior results were found when other 7386
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Journal of the American Chemical Society oligomerization or polymerization catalyzed by low-valent nickel species.13 In fact, the nitrile such as 2j was consumed almost completely upon subjection to the standard reaction conditions. Notably, the vinyl or internal alkynyl groups remained intact in the cases of 2m and 2n, indicating the reaction is highly chemoselective. Lower yield of 2o (39%) was obtained when 3-ethynylpyridine was employed as the substrate. Internal alkynes such as 1,2-diphenylethyne was also well suited, furnishing 2p in 78% yield at 80 °C. However, 1,3-butadiynes, ynamides or allenes were not suitable under the current reaction conditions.12 Next, we examined the hydrocyanation reactions of terminal aliphatic alkynes with Zn(CN)2. During this process, we found the alkynes could not be consumed completely under the standard reaction conditions for aryl alkynes. We then made an effort to reoptimize the reaction conditions for alkyl-substituted alkynes. After some trials, it was found the desired reactions proceeded smoothly with also excellent Markovnikov selectivity catalyzed by 5 mol % Ni(acac)2 and 6 mol % neocuproine in the presence of 50 mol % Mn at 50 °C. A large variety of nonactivated alkyl-substituted alkynes were cyanated efficiently under these modified reaction conditions (Table 3). For
method. Propargyl amines were proved to be also suitable substrates. In the case of N-(prop-2-ynyl)benzenamine bearing an α-NHPh group, the reaction appeared to proceed well by doubling the amounts of Ni(acac)2 and neocuproine (5m). However, when propargyl amine bearing a protected amino group was employed, high yield of 83% could be achieved under the standard reaction conditions (5n). It was noted usually low regioselectivity was observed in nickel-catalyzed hydrocyanation of propargyl alcohols or amines with HCN.5c,d An indolyl moiety could be successfully incorporated into the product (5o). However, silyl alkynes such as ethynyltriisopropylsilane only led to a trace amount of the product (5p). To demonstrate the practicality of the process, the reaction of 1h with Zn(CN)2 at 10 mmol scale was performed. Gratifyingly, high yield of 2a (74%) was achieved without further modification of the optimized reaction conditions. Thus, the protocol represents a low-cost and convenient route to vinyl nitriles from alkynes. In addition, alkyne 6 derived from drug Buclizine and Ethisterone 8, were cyanated in good yields by this Ni-catalyzed hydrocyanation (Scheme 2). Thus, the present method can be used for the late-stage functionalization of medicinally relevant compounds.
Table 3. Scope of Ni-Catalyzed Hydrocyanation of Alkyl Alkynesa
Scheme 2. Gram Scale and Modification of Drug Molecules
To understand the reaction mechanism, a deuterium labeling experiment using deuterium oxide (D2O) instead of water was performed. High deuterium incorporation of D1 (91%) cis to the cyano group was observed (Scheme 3, eq 1). Complete deuteration is not observed possibly due to alkynylic H/D exchange by D2O14 or the cis to trans double bond isomerization of vinyl nickel species during the process.15 The results verified water acts as the hydrogen source for hydrocyanation7a,16 and the reaction proceeds predominantly in a syn-addition manner. Employing TMSCN/MeOH as an in situ source of HCN17 resulted in no formation of 2a (Scheme 3, eq 2).18 Importantly, It was found Ni(COD)2 could catalyze the reaction leading to 2a in 38% yield, whereas Ni(I) complex Ni(acac)IPr15 failed to give a clean reaction (Scheme 3, eqs 3 and 4). The results suggest the reaction proceeds possibly via a Ni(0) species. Preliminary results implied semihydrogenation of 1a could occur in the absence of Zn(CN)2 with an excess amount of reductant, indicating a Ni−H species might be involved (Scheme 3, eq 5).19,20 Water is typically employed as a proton source, but it could also act as a hydride source through the reaction with the low-valent metals. Although quite rare, the oxidative addition of water by late transition-metal complexes21 leading to the formation of metal hydrido(hydroxo) complexes
a
Isolated yields. bNMR yield. c2-Methyl-4-phenylbutanenitrile was also isolated in 6% yield. d10 mol % Ni(acac)2 and 12 mol % neocuproine were used.
example, the common alkynes such as 1-dodecyne or 1-octyne transformed to vinyl nitriles 5a and 5b in 62% and 87% yields, respectively. Alkyl side chains bearing phenyl, ester, cyano, Cl or unprotected OH functional groups were also suitable for this reaction, furnishing 5c−5h in 60−85% yields. In the case of 5d, a saturated nitrile of 2-methyl-4-phenylbutanenitrile was also isolated in 6% yield, indicating further hydrogenation of 5d occurred during the process. In particular, good products yields were observed for propargyl alcohols, and not only secondary propargyl alcohols but also sterically hindered tertiary propargyl alcohols worked well to afford the corresponding 5i−5l in good yields. Especially, in the case of 5l, the vinyl moiety remained intact, highlighting again the excellent chemoselectivity of this 7387
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The reaction provides a novel and efficient protocol for the synthesis of functionalized vinyl nitriles with a wide range of structural diversity under extremely mild reaction conditions. Further mechanistic studies and the extension to internal alkynes and other π-systems are currently ongoing in our laboratory.
Scheme 3. Control Experiments
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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/jacs.8b02542. Experimental procedures, X-ray crystallography of compound 9 and spectral data (PDF) Data for C22H29NO2 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
has been proposed in a variety of catalytic processes such as the water gas shift reaction,22 alkyne−aldehyde coupling,23 semihydrogenation,19 etc. Our results suggested a process involving oxidative addition of water might take place in our reaction. To make clear whether the heterogeneous Ni catalysts act as real catalysts, the mercury poisoning experiments were carried out.12 It was found addition of mercury did not inhibit the reaction. Thus, the reaction may not proceed via heterogeneous system. Although the detailed mechanistic discussions should await further studies, we tentatively propose a reaction mechanism for hydrocyanation of terminal alkynes as depicted in Scheme 4.
Yuanhong Liu: 0000-0003-1153-5695 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Key R&D Program of China (2016YFA0202900), the National Natural Science Foundation of China (21772217), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000), Science and Technology Commission of Shanghai Municipality (18XD1405000) and Shanghai Institute of Organic Chemistry (sioczz201807) for financial support.
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Scheme 4. Possible Reaction Mechanism
REFERENCES
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The first step involves the formation of Ni(0) species by reduction of Ni(acac)2 with Mn. Oxidative addition of water to Ni(0) generates the Ni(II) intermediate 11.21f Alkyne insertion to 11 via cis-addition of Ni−H bond (hydronickelation) provides an alkenyl nickel complex 12. Possibly, the stability of the alkenyl nickel species influences the regioselectivity of this step. Transmetalation of 12 with Zn(CN)2 followed by reductive elimination delivers the nitriles. Alternatively, reaction of nickel π-alkyne complex 14 with H2O may also afford the same intermediate 12 (path b).20a The detailed process for this transformation is not clear yet, it may proceed through oxidative addition of water with 1419a or cleavage of nickelacyclopropene intermediate 15 by water. However, the direct attack of the cyanide nucleophile to nickel-coordinated alkyne followed by protonation could not be excluded.24 In summary, we have developed the first Ni-catalyzed regioselective hydrocyanation of terminal alkynes using Zn(CN)2 as the cyanide source and water as a hydrogen source. 7388
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