Nickel-Catalyzed Regio- and Stereoselective Hydrocarboxylation of

Mar 10, 2016 - Transition-metal-catalyzed transfer carbonylation with HCOOH or HCHO as non-gaseous C1 source. Jian Cao , Zhan-Jiang Zheng , Zheng Xu ...
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Nickel-Catalyzed Regio- and Stereoselective Hydrocarboxylation of Alkynes with Formic Acid through Catalytic CO Recycling Ming-Chen Fu, Rui Shang, Wan-Min Cheng, and Yao Fu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00276 • Publication Date (Web): 10 Mar 2016 Downloaded from http://pubs.acs.org on March 14, 2016

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Nickel-Catalyzed Regio- and Stereoselective Hydrocarboxylation of Alkynes with Formic Acid through Catalytic CO Recycling Ming-Chen Fu, Rui Shang,* † Wan-Min Cheng, and Yao Fu* iChEM, CAS Key Laboratory of Urban Pollutant Conversion, Anhui Province Key Laboratory of Biomass Clean Energy, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China. ABSTRACT: By the combination of a Ni(II) salt, a bisphosphine ligand, and a catalytic amount of carboxylic acid anhydride, atom-economic hydrocarboxylation of various alkynes with formic acid can be achieved with high selectivity and remarkable functional group compatibility, affording α,β-unsaturated carboxylic acids regio- and stereoselectively. Both terminal and internal alkynes are amenable substrates. A mechanism proceeding through carbon monoxide recycling in catalytic amount is demonstrated to be crucial for the success of this transformation. KEYWORD : Nickel, hydrocarboxylation, formic acid, alkynes, carbon monoxide

Nickel-catalyzed hydrocarboxylation1 of alkynes is a general strategy to access α,β-unsaturated carboxylic acids in a stereo- and regioselective manner. Except for several early reported carbonylation reactions using toxic CO gas,2 literature reports show these reactions were usually carried out by fixing CO2 with alkenyl nickel species generated through hydrometalation using sacrificing organometallic reagents,3,1e or through reductive CO2 fixation with Ni(0) followed by protonation.4,1f In these reports, a stoichiometric amount of organometallic reagent or metal reductant is necessary. We consider that formic acid, which can be produced from biomass5 and CO2 reduction,6 is potentially an ideal source for atomefficient alkyne hydrocarboxylation through formal addition of the C–H bond of formic acid to an alkyne. Although this process may proceed through an unachieved pathway involving nickel-catalyzed CO2 recycling via hydride elimination from formate anion followed by hydrometalation and CO2 fixation,7 we realized a possible pathway through catalytic CO recycling, as demonstrated in Figure 1. First, a Ni(0) species coordinates with alkyne (A), followed by hydrometalation to generate an alkenyl–Ni species (B).8 This alkenyl–Ni species can generate intermediate C through CO insertion.9 A reaction of formic acid decomposition to CO by acid exchange with a suitable acid anhydride can be used to generate CO.10 Although this decomposition of anhydride is well utilized in palladium catalysis to use formic acid as a CO surrogate,11,12 its utilization in nickel catalysis is rare, possibly because of the poisoning effect of CO in nickel catalysis.13 However, related literature reveals controlling the concentration of CO14 and utilizing a suitable supporting ligand to labilize the Ni–CO bond15 may solve the problem of CO poisoning in related catalytic reactions involving Ni–CO species. In our working hypothesis, the concentration of CO can be easily controlled by the amount of anhydride added. Furthermore, if the reaction can be conducted under a condition without addition of other nucleophiles, formate anion acts as the only nucleophile to participate in reductive elimination with acyl nickel species(C) to deliver formic acrylic anhydride (D) and regenerate the Ni(0) catalyst.

Figure 1. Working Hypothesis: Ni-Catalyzed Hydrocarboxylation using Formic Acid.

Formic acrylic anhydride can then easily decompose to deliver the hydrocarboxylation product and regenerate CO catalytically. Based on this hypothesis, we demonstrated in this work that through the combination of an inorganic Ni(II) salt, a bisphosphine ligand (1,2-bis(diphenylphosphino)benzene, dppbz) and a catalytic amount of acid anhydride, atomeconomical hydrocarboxylation of alkynes with formic acid can proceed with high chemo-, regio-, and stereoselectivity. The inexpensive nickel-catalytic system is capable of catalyzing a broad range of internal and terminal alkynes, and remarkable functional group compatibility was achieved. This work offers a new method to access various functionalized alkenyl carboxylic acids using inexpensive base-metal catalysis. It should be mentioned that a similar Pd-catalyzed version of this chemistry has been elegantly described by Zhou et al..11 However, the application of inexpensive, ubiquitous first-row transition-metal-catalyst to this process is still unexploited . A typical example of hydrocarboxylation of diphenylacetylene after scrupulous optimization is demonstrated in Scheme1. Simply heating a mixture of diphenylacetylene (0.5 mmol), formic acid (1.0 mmol), air stable Ni(acac)2 (5 mol %), dppbz (7 mol %), and pivalic acid anhydride (20 mol %) in

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a The reaction was performed on a 0.5 mmol scale, following the procedure described in the text. bThe yield was determined by isolation.

a The reaction was performed on a 0.5 mmol scale. bThe yield was determined by isolation. cUsing Ni(acac)2 (10 mol %), dppp (20 mol %), and Ac2O (20 mol %) instead. dppe = 1,2-bis(diphenylphosphino)ethane, dppen = cis-1,2-bis(diphenylphosphino)ethylene, dppp = 1,3-bis(diphenylphosphino)propane, dppm = bis(diphenylphosphino)methane, dppb = 1,4-bis(diphenylphosphino)butane, dppf = 1,1'-bis(diphenylphosphino)ferrocene, Xantphos = dimethylbisdiphenylphosphinoxanthene, BINAP = (+/-)-2,2'bis(diphenylphosphino)-1,1'-binaphthyl.

Scheme 1: Key Factors Controlling the Reaction.a,b toluene at 100 °C for 24 h afforded (E)-2,3-diphenylacrylic acid in 95% isolated yield. In contrast to other Ni-catalyzed hydrocarboxylations under basic conditions, this reaction proceeds under acidic conditions and affords the carboxylic acid without follow-up acidification. The amount of formic acid can be reduced to 1.5 equiv in a gram-scale reaction without decreasing the yield (94%, 1.06 g). The good chemo- and stereoselectivities of this transformation are notable. Although formic acid was previously reported to act as a reducing agent to transfer hydrogenate alkyne to alkene,16 under our optimized conditions, alkene and alkane by-products were not detected. The nickel catalyst selectively distinguishes the triple bond over the double bond because no further reduced aliphatic carboxylic acid or double hydrocarboxylated dicarboxylic acid was detected. The stereoselectivity of this reaction is excellent. No (Z)-2,3-diphenylacrylic acid was detected. Factors controlling this reaction were carefully studied (see SI), and some key factors affecting this reaction are discussed below (Scheme 1). Nickel precursors have a significant effect on the reaction outcome. As shown in Scheme 1, this reaction does not proceed at all in the absence of the nickel catalyst. Using nickel precursors containing a halide counter anion has a deleterious effect, probably because of the competitive coordination of halide with formate anion to the catalyst center. Ni(COD)2 also works as a good catalyst precursor. However, because of air sensitivity, air stable Ni(acac)2 is preferred. Ni(OTf)2 cannot catalyze this reaction, leading only to a side reaction of alkyne hydrogenation (see SI). The reaction strongly depends on the ligand used. As shown in Scheme 1, row 2, among all of the ligands tested, 1,2bis(diphenylphosphino)benzene (dppbz) gave optimal results. Dinitrogen ligand and monophosphine ligand are totally ineffective. It is also worth noting that the efficiency strongly depends on the bite angle and backbone structure of the bisphos-

Scheme 2: Effect of the Amount of Acid Anhydride.a,b phine ligand. The high ligand dependence may be ascribed to a suitable trans effect of the ligand to labilize the Ni–CO bond to facilitate CO insertion.17 Acid anhydride as additive appears to be another crucial factor. As show in Scheme 2, row 3, besides pivalic anhydride, acetic anhydride and benzoic anhydride also gave comparably good results, but trifluoroacetic anhydride and trifluoromethanesulfonic anhydride were ineffective. As we proposed in the working hypothesis, the concentration of CO in the catalytic recycle can be controlled by the amount of acid anhydride added. The results in Scheme 2 show that both the absence and using excess (>50%) amount of acid anhydride give no product. These parameter studies supported a suitable ligand and subtly controlling the concentration of CO are two keys to achieve this reaction, as mentioned in our working hypothesis. Table 1: Scope of Symmetric Alkynes.a,b

a

Alkyne (0.5 mmol), HCO2H (2.2 equiv), Ni(acac)2 (5 mol %), dppbz (7 mol %), Piv2O (20 mol %), toluene (1.0 mL), 100 °C, 24 h. bIsolated yields. cdppp (10 mol %), HCO2H (2.0 equiv). d HCO2H (1.5 equiv). eHCO2H (1.2 equiv).

With the understanding of this reaction, we explored the preparative scope. Table 1 shows the scope of diarylacetylenes. For various diarylacetylenes, syn-addition delivers (E)-2,3diarylacrylic acid as the sole product. No (Z)-isomer was detected. The (E)-isomer was also confirmed by an X-ray analysis of product 8. Both electron-rich (3) and electron-deficient (8) diarylacetylenes deliver the hydrocarboxylation product in good yield. Fluoro (6), chloro (4), and bromo (5) substituents

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on the aromatic ring are well tolerated. 1,2-Di(naphthalen-1yl)ethyne gives the product in moderate yield, probably because of steric hindrance (9). 3,3′-(Ethyne-1,2diyl)dibenzonitrile gives moderate conversion and yield, possibly because of the coordination effect of the cyano substituent (10). Besides diaryl alkyne, dialkyl alkyne also works well. The reaction of oct-4-yne gave (E)-2-propylhex-2-enoic acid in 95% yield (11). The amount of formic acid can be reduced to 1.5 equiv and 1.2 equiv, with slight decrease of the yield to 93% and 73%, respectively (11). Table 2: Scope and Selectivity of Unsymmetrical Internal Alkynes.a,b

alkyl substituent is secondary (19), but with decrease of the yield (57%, α/β = 6.1/1) due to the steric hindrance.19 When (3-(benzyloxy)but-1-yn-1-yl)benzene is used, good regioselectivity was achieved with moderate yield (20, 68%, α/β = 90/10). When 4,4-dimethyl-2-pentyne was applied, the carboxylation exclusively took place on the less sterically hindered position in excellent yield (21, 98%, α/β = 0/100). From these examples above, it is proved that the carboxylation selectively takes place on the α-position of the aryl substituent, the steric effect of the alkyl substituent acts as an additional factor to determine the regioselectivity. For unsymmetrical dialkyl acetylenes, steric effects act as the dominant factor. Table 3: Scope and Selectivity of Terminal Alkynes.a,b

a

Alkyne (0.5 mmol), HCO2H (2.2 equiv), Ni(acac)2 (5 mol %), dppen (7 mol %), Piv2O (20 mol %), toluene (1.0 mL), 100 °C, 24 h. bIsolated yields, the ratio of α and β was determined by 1H NMR analysis. cdppbz (7 mol %). dThe ratio of isomers was determined by isolation. eUsing dppbz (7 mol %), HCO2H (2.0 equiv).

For unsymmetrical alkynes, such as aryl alkyl alkynes, the hydrocarboxylation affords the α-arylated alkenyl carboxylic acids as the major product (Table 2). Dppen was chosen as the ligand because of its slightly better performance than dppbz (see SI). As shown by product 12, 1-phenylpropyne gave the hydrocarboxylation product in excellent yield (97%), in which the α-carboxylated product is the major isomer (α/β = 1.9/1). This result reveals that in the hydrometalation step, nickel prefers the α-position of the aryl substituent. The ratio of the two regioisomers (α/β) is also affected by the steric effect of the alkyl substituent of the alkyne. As shown by product 13, when 1-phenylbutyne was used, the reaction gave excellent yield with a higher ratio (93% yield, α/β = 4.6/1) of the two isomers compared with product 12. Using hex-1-yn-1ylbenzene gave the hydrocarboxylation product in a slightly lower yield with no further improvement in regioselectivity (14). Comparing products 15 and 16 shows that the electronic effect on the aryl substituents slightly affects the yield, but has no significant influence on the regioselectivity. Although several studies reported that a heteroatom in the alkyne significantly improves the regioselectivity of hydrometallation,18 our results (17 and 18) show putting a coordinative heteroatom on neither aromatic ring nor alkyl chain improves the regioselectivity. The α/β regioselectivity further increased when the

a

Alkyne (0.5 mmol), HCO2H (2.2 equiv), Ni(acac)2 (5 mol %), dppbz (7 mol %), Piv2O (20 mol %), toluene (1.0 mL), 100 °C, 24 h. bIsolated yields. cHCO2H (1.2 equiv). dHCO2H (1.1 equiv). e The ratio was determined by 1H NMR. fHCO2H (1.5 equiv). g1,4dioxane instead of toluene.

Terminal alkynes also act as suitable substrates for this reaction (Table 3). It should be noted for nickel-catalyzed hydrocarboxylation with CO2, terminal alkynes were not demonstrated to be amenable substrates.3e,4d Table 3 summarizes the results for terminal alkynes. For terminal alkyl alkynes, the branched selectivity can be achieved exclusively. Only when 3,3-dimethylbut-1-yne, a substrate with large steric hindrance on the α-position was used, 13% linear product was detected (24, 93%, α/β = 80/13). For terminal aryl alkyne, phenylacety-

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lene gave the branched product in moderate yield, the linear product was detected only in 3%. The excellent functional group compatibility is highlighted in Table 3. Alkyl chloride (26), cyano (27), aryl iodide (28), activated aryl bromide (29), aryl pinacol boronate (30), ester (31, 32, 40), aryl aldehyde (33), ketone with enolizable α-hydrogen (34), alkene (35), imide (36), and sulfide (37) are well tolerated. It is noteworthy that although Ar-Br bond is susceptible to low-valent nickel, species, we did not detect significant amount of hydrodebromination and hydrodeiodination under our reaction conditions. The tolerance of Ar-Br and Ar-I in the present reaction may be ascribed to the CO poison effect to low-valent nickel species retarding oxidative addition to aryl halide and also the acidic reaction condition making low-valent nickel species easily oxidized by formic acid (A to B in Figure 1). For some entries when moderate yields were observed, the starting materials were recovered (34, 37). Heteroaromatics, such as thiophene (17), indole (38), pyrrole (39), and furan (40) are well tolerated. An isolated cyclic alkene structure remained intact after the reaction, and no transesterification reaction was detected (41). It is notable that this reaction is highly chemoselective, no aliphatic carboxylic acid, alkene, or further hydrocarboxylation of the alkenyl carboxylic acid was detected. The amount of formic acid can be reduced to 1.1 equiv without significantly reducing the yield (22, 31, 32).

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.” Experimental details and characterization data for all materials and products (PDF) Crystallographic data for 8 (CIF) Crystallographic data for 33 (CIF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]

Present Addresses †R.S.: Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the 973 Program (2012CB215306), NSFC (21325208, 21172209, 21361140372), IPDFHCPST (2014FXCX006), CAS (KJCX2-EW-J02), FRFCU and PCSIRT.

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(2)

Alkyne (0.5 mmol), H13CO2H (1.1 equiv), Ni(acac)2 (5 mol %), dppbz (7 mol %), Piv2O (20 mol %), toluene (1.0 mL), 100 °C, 24 h. bIsolated yields.

a

Scheme 3: 13C-Labeling Experiment.a,b Atom economy becomes more important when utilizing expensive 13C-enriched reagent. The present reaction can be used to synthesize 13C-labeled carboxylic acids. Using 1.1 equiv of H13COOH, hydrocarboxylation affords 13C-labeled 2methylenehexanoic acid in high yield with excellent 13Cincorporation (Scheme 3). The reaction conducted under CO2 atmosphere also gave the desired product in high yield with excellent 13C incorporation (>99% 13C incorporation). In accordance with our working hypothesis, this result clearly supports that the carboxylation process with CO2 is not likely involved in the reaction mechanism. In summary, we have demonstrated that the combination of an inexpensive nickel precatalyst, a bisphosphine ligand, and catalytic amount of acid anhydride can catalyze atomeconomic hydrocarboxylation of a broad range of alkynes, including both internal and terminal alkynes with formic acid in a highly chemo-, regio-, and stereoselective manner. The operational simplicity, generality, and remarkable functional group compatibility make this reaction a user-friendly method to prepare functionalized α,β-unsaturated carboxylic acids. A mechanism through CO recycling in catalytic amount is critical to make the reaction successful. The methodology using formic acid through catalytic CO recycling may find future applications in related nickel-catalyzed carbonylation reactions.

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