Nickel-Catalyzed Decarbonylation of Aromatic Aldehydes - The

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Nickel Catalyzed Decarbonylation of Aromatic Aldehydes Keying Ding, Shi Xu, Rajeh Alotaibi, Keshav Paudel, Eric W. Reinheimer, and Jessie Weatherly J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b00284 • Publication Date (Web): 11 Apr 2017 Downloaded from http://pubs.acs.org on April 11, 2017

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The Journal of Organic Chemistry

Nickel Catalyzed Decarbonylation of Aromatic Aldehydes Keying Ding*,†, Shi Xu†, Rajeh Alotaibi†, Keshav Paudel†, Eric W. Reinheimer‡, Jessie Weatherly† †

Department of Chemistry, Middle Tennessee State University, Murfreesboro, TN 37132, United States ‡

Rigaku Americas, The Woodlands, TX 77381, United States [email protected]

ABSTRACT: We report here the first systematic study of nickel catalyzed decarbonylation of aromatic aldehydes under relatively mild conditions. Aldehydes with electron donating groups at para and ortho positions are generally successful with our method. For aldehydes with electron withdrawing groups, significantly higher yields were achieved for ortho substituted substrates than para ones, probably due to the effects of steric hindrance or electron donors at the ortho position to suppress the Tishchenko reaction, an undesirable side reaction towards homo-coupled esters. Decarbonylation of aldehydes as an important organic transformation has recently received increasing research attention with the tremendous progress of biomass conversion and natural product synthesis in the chemical and pharmaceutical industries.1 Since the first discovery of this reaction,2 decarbonylation of aldehydes and related reactions have been accomplished by catalysts containing Rh,3 Ir,4 Ru5 and Pd.6 However, harsh reaction conditions such as high temperatures over 180 °C are normally required. Although significant improvements have appeared recently,7 these reactions exclusively

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rely on expensive precious transition metals. With the increasing concerns on depletion of natural resources, it becomes more appealing to replace precious metal catalysts with ones based on earth-abundant metals.8,9 Nevertheless, non-precious metal catalysis of decarbonylation of aldehydes is still very rare and lacking mechanistic understandings. The past decade has witnessed tremendous advances in Ni catalysis, however, the notorious strong bonding of Ni with CO is prone to deactivate the catalyst and imposes a challenge for decarbonylation reaction.10 Dong and co-workers studied a stoichiometric decarbonylation of 2-naphthaldehyde by Ni(COD)2/IPr (COD = 1,5-cyclooctadiene, IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene) with trifluoroacetophenone as an additive, 38% yield of naphthalene was obtained.11 The only known catalytic example used heterogeneous Ni on charcoal (Ni/C) catalyst in a flow reactor at 10 Pa, but the substrate scope is very limited.12 Hence, it is highly desirable to develop a general methodology of homogeneous Ni catalyzed decarbonylation of aldehydes under mild conditions using simple set-ups. Very recently, one of us reported a study on Ni catalyzed dehydrative decarbonylation of carboxylic acids to alkenes by continuous distillation process at 180 °C.13a Decarbonylation of ketones was just reported with Ni/Nheterocyclic carbene.13b Herein, we report results from the systematic study of Ni mediated decarbonylation of aromatic aldehydes with mechanistic insights explored. Our study was initiated by identifying the reaction conditions based on the readily available Ni(0) precursor Ni(COD)2. In order to optimize decarbonylation reaction condition, furfural 1a was chosen as the model substrate and tested by varying ligands, solvents, temperatures, reaction times and inorganic additives (see Supporting Information). After screening, gratifyingly we obtained an excellent yield of furan 2a


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(92%) when PCy3 was selected as the ligand in cyclohexane at 140 °C after 24 hours (Table 1, entry 6). Bidentate phosphine ligands have shown success in Ir4a and Rh3 mediated decarbonylation of aldehydes, however, significantly improved yields resulted when monodentate ligands are used instead with Ni (Table 1, entries 1-5). There are minor decarbonylated products when IPr or IMes was used (Table 1, entries 12-13). PnBu3 exhibited excellent yield of furan in 90% comparable to PCy3 (Table 1, entry 2). However, PCy3 outperforms PnBu3 with many other substrates, therefore PCy3 was chosen as the ligand for the rest of study.



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Table 1. Ligand and Solvent Optimization of Ni-catalyzed Decarbonylation of Furfurala

entry ligand 1 PPh3

solvent cyclohexane

yieldb 57

2

PnBu3

cyclohexane

90

3

DPPE

cyclohexane

36

4

DPPP

cyclohexane

41

5

DCPE

cyclohexane

54

6

PCy3

cyclohexane

92 (96c)

7

PCy3

toluene

67

8

PCy3

diglyme

18

9

PCy3

m-xylene

30

10

PCy3

dioxane

63

11

PCy3

diphenylethane 7

12

IPr

cyclohexnae

11

13

IMes

cyclohexane

4

14

N/A

cyclohexane

0

a

Reaction conditions: 1a (0.25 mmol), cyclohexane (0.7 mL), Ni(COD)2 (15 mol%), ligand (30 mol%), 140 °C. bNMR yield. cGC yield.


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It was observed that inorganic additives (K3PO4, KI, Cs2CO3, K2CO3) have little impact on the reaction (see Supporting Information). Molecular sieves were reported to greatly enhance Pd catalyzed decarbonylation of aldehydes due to its capability to remove trace amount of water,7c-e but in our reaction, addition of molecular sieves is not necessary. Temperature of 140 °C was found to be crucial for this reaction to succeed (see Supporting Information). With the optimized reaction conditions in hand (Table 1, entry 6), we then turned to explore the substrate scope of the reaction. Furfural derivatives 3-furaldehyde and 5methylfurfural were nicely converted to the corresponding decarbonylated products (Table 2, 2b and 2c) in good to excellent yields. Then, a series of para-substituted benzaldehydes were tested for decarbonylation with our Ni/PCy3 system. Interestingly, aldehydes bearing electron donating substituents perform significantly better than those with electron withdrawing ones (Table 2, 2d-i). The highest decarbonylation yield (83%) was achieved with 4-methoxybenzaldehyde (Table 2, 2d). Meanwhile, moderate yields were observed for 4-fluorobenzaldehyde and 4-trifluoromethylbenzaldehyde (Table 2, 2h-i). Besides, 1-naphthaldehyde was tested and a moderate yield of 31% was obtained (Table 2, 2j). No conversion was detected for 4-chlorobenzaldehyde since starting materials were returned after reaction. (Table 2, 2k). Decarbonylations of hydrocinnamaldehyde and cinnamaldehyde were unsuccessful (Table 2, 2l and 2m) with some unidentified by-products. Interestingly, when the reaction was performed at reflux condition under Ar with 4-methoxybenzaldehyde as the substrate,14 only 16% anisole product was formed (Table 2, 2de).



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Table 2. Ni-catalyzed Decarbonylation of Aromatic Aldehydesa

a

Reaction conditions: aldehyde 1b-m (0.25 mmol), cyclohexane (0.7 mL), Ni(COD)2 (15 mol%), PCy3 (30 mol%), 140 °C, 24 h. b GC yield. cNMR yield. disolated yield. e reflux at 140 °C for 24 h under Ar atmosphere. fisolated yield with 1 mmol scale.


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Upon further analysis, it was found that the challenging substrates bearing electron withdrawing groups suffer from undesirable aldehyde dimerization to homo-coupled ester (Table 3, 3) known as the Tishchenko reaction.15 For para-substituted benzaldehydes, it appears that the decarbonylation selectivity relies on the electronic effects of the substituents, i.e., the ester byproducts are observed more prominently for benzaldehydes with electron-withdrawing groups on the para position (Table 3, entries 14). Benzaldehyde also suffers from severe Tishchenko dimerization problem (Table 3, entry 5). We speculate that strategies that can suppress the Tishchenko reaction may facilitate decarbonylation. To test whether lowering substrate concentration can disfavor the Tishchenko reaction, 1 mL (instead of 0.7 mL) cyclohexane was used for 4trifluoromethylbenzaldehyde (Table 3, entry 1c). However, only slight improvement of selectivity was obtained at a cost of much lower substrate conversion (79%). Knowingly, one crucial step of the Tishchenko reaction is the hydride transfer from one aldehyde to the other with the aid of the catalyst,15a and we envision that the relatively bulky substituent at the ortho position may hinder hydride transfer and enhance decarbonylation selectivity (Scheme 1A). Gratifyingly, significant improvement was achieved when 2trifluoromethylbenzaldehyde was used to afford trifluoromethylbenzene (Table 3, entry 6). Desirably, 2-methylbenzaldehyde (Table 3, entry 8) shows even better selectivity than 4-methylbenzaldehyde (Table 3, entry 3). Surprisingly, 2-fluorobenzaldehyde 1p (Table 3, entry 7) without a bulky substituent also shows excellent selectivity (87% in yield) compared to 4-fluorobenzaldehyde 1h and benzaldehyde 1n (Table 3, entries 2 and 5), suggesting electronic interactions may also play a role. Correspondingly, 2-methoxybenzaldehyde 1r (Table 3, entry 9) and furfural



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1a (Table 1, entry 6) have better yields than 2-methylbenzaldehyde 1q (Table 3, entry 8) and 3-furaldehyde 1b (Table 2), respectively. One possible explanation is that the nearby F and O atoms with lone pair electrons may coordinate to the nickel center during the catalytic cycle, providing enough steric hindrance to suppress the Tishchenko reaction (Scheme 1B). We can’t exclude the possibility that bulk ortho sustituents may tend to prevent nickel from coordinating with two aldehydes, thus limiting the Tischenko reaction, which requires further in-depth studies. Decarbonylation of meta-substituted substrates is unsuccessful possibly due to the diminished steric and electronic effects. This "ortho effects" greatly increase the substrate scopes, making our method more general. To the best of our knowledge, it is the first time that high selectivity towards decarbonylation is achieved with nickel catalyst using ortho-substituted substrates.


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Scheme 1. Proposed Mechanism for "Ortho Effects" Observed in Decarbonylation of Aldehydes

A B Steric hindrance generated by a bulky group (gray ball in A) or coordination of Ni with electron donor atom (symbol E in B) at the ortho position is proposed to suppress hydride transfer to the carbonyl carbon.



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Table 3. Electronic and Steric Effects on NiCatalyzed Decarbonylation of Aryl Aldehydesa

entry

R

conv (%)b

1

4-CF3, 1i

2

4-F, 1h

>99, 79 >99

3 4 5 6

4-CH3, 1e 4-OMe, 1d 4-H, 1n 2-CF3, 1o

96 95, 75d >99 90

c

yield (%)b 2 22, 28c 40

3 78, 51c 60

55 86, 69d 36 90

41 9, 6d 64 0

7 2-F, 1p 93, 78d, 5e 87, 75d 6, 3d 8 2-CH3, 1q 87 75 12 9 2-OMe, 1r 90 85 5 a Reaction conditions: aryl aldehyde (0.25 mmol), cyclohexane (0.7 mL), Ni(COD)2 (15 mol%), PCy3 (30 mol%), 140 °C, 24 h. bNMR yield. ccyclohexane (1 mL). dfiltration experiment. eHg test


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We next performed an initial mechanistic study. Addition of furfural to the red solution of Ni(COD)2 and PCy3 in toluene at room temperature resulted in the color change to orange. Upon removal of solvent, a red orange powder was obtained, which was later characterized as nickel η2-aldehyde complex 4 (Figure S5, Supporting Information) with an η2 interaction between Ni and −C=O.15a,b 31P NMR spectrum of 4 displays two doublets for the PCy3 ligand. Slow diffusion of pentane into toluene solution of 4 afforded red crystals which were analyzed by single crystal X-ray diffraction (Figure S9, Supporting Information). The nickel is formally three-coordinate with two PCy3 ligands and furfural in a η2 fashion. Complex 4 shows an elongated C=O bond distance of 1.321(4) Å and is consistent with those reported nickel η2-carbonyl complexes (1.32-1.34 Å).15c-f Heating 4 at 75 °C in deuterated cyclohexane in a J-Young NMR tube for 60 hours resulted in the decarbonylated product furan in 78% NMR yield (Figure S5, Supporting Information).

31

P NMR of the product mixture detects a known nickel

carbonyl complex Ni(CO)2(PCy3)2 at 41.7 ppm.16,17 Whether the active species 4 is a true intermediate of the reaction is subjected to further theoretical study. Efforts to identify possible nickel hydride intermediates through control experiments at various temperatures were unsuccessful. In order to identify whether the reaction system is homogeneous or heterogeneous, filtration experiments (Table 3, entries 4d and 7d) were performed and the results suggest a possible homogeneous reaction (see Supporting Information). Hg test (Table 3, entry 7e) shows only 5% conversion, however, the results are not reliable because Ni(COD)2 is known to react with Hg.18



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In conclusion, we have reported the first systematic study on Ni catalyzed decarbonylation of aldehydes. Aromatic aldehydes with electron donating groups generally have excellent conversions towards decarbonylated products. For challenging substrates with electron withdrawing groups, the competing and undesirable Tischenko reaction is efficiently suppressed using ortho-substituted aldehydes. EXPERIMENTAL SECTION General Methods. All reactions were performed in MBraun glovebox under an atmosphere of N2 or using standard Schlenk techniques. Anhydrous pentane and toluene were deoxygenated by sparging with dinitrogen and dried by passing through activated alumina columns of a Pure Solv solvent purification system. Anhydrous cyclohexane was purchased from Aldrich, dried over sodium/benzophenone, and stored over molecular sieves (4 Å) in the glovebox. C6D6 and C6D12 were purchased from Cambridge Isotope Lab and dried over sodium/benzophenone before being distilled and degassed three times cycles. CDCl3 was purchased from Cambridge Isotope Lab and dried over molecular sieves (4 Å). Furfural was distilled over CaH2 before use. Ni(COD)2 and all organic ligands were purchased from Strem Chemicals and used as received. All other chemicals were purchased from Sigma Aldrich and used as received. NMR spectra were recorded on either a JEOL Unity 300 or 500 MHz spectrometer. 31P NMR spectra were referenced to 85% H3PO4 at 0 ppm. UV-Vis spectra were recorded on Hewlett-Packard Model 8452A with diode array. Analysis of products from catalytic experiments was performed using a Hewlett-Packard 5890 GC (HP-5MS column, 30 m) with a flame ionization detector. Gas chromatography/mass spectral analyses were performed on a Shimadzu QP2010S with an installed auto sampler. Elemental analysis was performed by Atlantic


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Microlab. Crystallography data were collected by Rigaku MM007HF Mo rotating anode equipped with a Pilatus P200K hybrid photon counting detector, at 100 K using graphitemonochromated Mo Kα1 radiation. Typical Procedure for Reaction Optimization of Decarbonylation of Furfural. A solution of Ni(COD)2 (10.3 mg, 0.0375 mmol, 15 mol%) and ligand (0.075 mmol, 30 mol%) in cyclohexane (0.7 mL) was loaded into a reaction tube (see Supporting Information) and stirred at room temperature for 15 min under N2 atmosphere. Furfural (0.25 mmol, 21 µL) was added to the vessel which was then tightened with a screw cap fitted with PTFE septa (see Supporting Information). The reaction was carried out at 120–140 °C for 24–36 h, then the reaction vessel was cooled to 0 °C by ice bath. Nitromethane (0.25 mmol, 13.4 µL) was added to the reaction mixture as internal standard. An aliquot of mixture (0.2 mL) was taken out, mixed with 0.3 mL CDCl3, and filtered through a silica gel plug (30−40 mg silica). CDCl3 (0.2 mL × 2) was added to the silica plug to rinse the mixture completely.

1

H NMR analysis was used to identify the

products and determine product yields. For the reaction described in Table 1, entry 6, the yield was also determined by gas chromatography using naphthalene (32 mg, 0.25 mmol) as internal standard. GC-MS (m/z): 68.1 [M]+. Typical Procedure for Ni(COD)2/PCy3 Catalyzed Decarbonylation of Aldehydes. A solution of Ni(COD)2 (10.3 mg, 0.0375 mmol, 15 mol%), and PCy3 (21.0 mg, 0.075 mmol, 30 mol%) in cyclohexane (0.7 mL) was loaded into a reaction tube and stirred at room temperature for 15 min under N2 atmosphere. Aldehyde (0.25 mmol) was added to the vessel which was then tightened by a screw cap fitted with a PTFE septa. The reaction was carried out at 140 °C for 24 h, then the reaction vessel was cooled to 0 °C by



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ice bath. For NMR yield calculation, nitromethane (13.4 µL, 0.25 mmol) or 1,4-dioxane (8.5 µL, 0.1 mmol) was added to the reaction mixture as internal standard. About 0.2 mL of mixture was mixed with 0.3 mL CDCl3, filtered through silica gel plug (30−40 mg silica) and further rinsed with CDCl3 (0.2 mL × 2) for 1H NMR analysis. For GC yield calculation, the reaction mixture was diluted by 1 mL hexane. Unless specified, naphthalene (32.1 mg, 0.25 mmol) was added as internal standard. The reaction mixture was filtered through silica gel plug (30−40 mg silica) for GC analysis. Decarbonylated products with boiling point above 150 °C were isolated by silica gel chromatography using hexane as an eluent. Decarbonylation of 3-furaldehyde (Table 2, 1b). Decarbonylation was done by general procedure with 3-furaldehyde (21.6 µL, 0.25 mmol), Ni(COD)2 (10.3 mg, 0.0375 mmol) and PCy3 (21.0 mg, 0.075 mmol). Yield was determined by gas chromatography using naphthalene (32.1 mg, 0.25 mmol) as internal standard. GC yield 69%. In a repeated reaction, the yield was determined by NMR spectroscopy using nitromethane (13.4 µL, 0.25 mmol) as internal standard. NMR yield 68%. GC-MS (m/z): 68.1 [M]+. Decarbonylation of 5-methylfurfural (Table 2, 1c). Decarbonylation was done by general procedure with 5-methylfurfural (24.9 µL, 0.25 mmol), Ni(COD)2 (10.3 mg, 0.0375 mmol), and PCy3 (21.0 mg, 0.075 mmol). Yield was determined by gas chromatography using naphthalene (32.1 mg, 0.25 mmol) as internal standard. GC yield 90%. In a repeated reaction, the yield was determined by NMR spectroscopy using nitromethane (13.4 µL, 0.25 mmol) as internal standard. NMR yield 89%. GC-MS (m/z): 82.1 [M]+. Decarbonylation of 4-methoxybenzaldehyde (Table 2, 1d). Decarbonylation was done by general procedure with 4-anisaldehyde (30.4 µL, 0.25 mmol), Ni(COD)2 (10.3 mg, 0.0375 mmol), and PCy3 (21.0 mg, 0.075 mmol). Yield was determined by gas chromatography using naphthalene (32.1 mg, 0.25 mmol) as internal standard. GC yield 83%. In a repeated reaction, the yield was determined by NMR spectroscopy using 1,4-


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dioxane (8.5 µL, 0.1 mmol) as internal standard. NMR yield 86%. In another repeated reaction, the product was isolated by silica gel chromatography using hexane as an eluent with yield of 37.3 mg, 69%. 1H NMR (300 MHz, CDCl3) δ: 7.26 (t, J =8.7 Hz, 2H), 6.92 (t, J =7.1 Hz, 1H), 6.90 (d, J =8.7 Hz, 2H), 3.75 (s, 3H).13C NMR (75 MHz, CDCl3) δ: 159.7, 129.5, 120.7, 114.0, 55.1. GC–MS (m/z): 108.1 [M]+. Decarbonylation of p-tolualdehyde (Table 2, 1e). Decarbonylation was done by general procedure with p-tolualdehyde (29.5 µL, 0.25 mmol), Ni(COD)2 (10.3 mg, 0.0375 mmol), and PCy3 (21.0 mg, 0.075 mmol). Yield was determined by gas chromatography using naphthalene (32.1 mg, 0.25 mmol) as internal standard. GC yield 58%. In a repeated reaction, the yield was determined by NMR spectroscopy using 1,4dioxane (8.5 µL, 0.1 mmol) as internal standard. NMR yield 55%. GC–MS (m/z): 92.1 [M]+. Decarbonylation of 4-ethylbenzaldehyde (Table 2, 1f). Decarbonylation was done by general procedure with 4-ethylbenzaldehyde (34.3 µL, 0.25 mmol), Ni(COD)2 (10.3 mg, 0.0375 mmol), and PCy3 (21.0 mg, 0.075 mmol). Yield was determined by gas chromatography using naphthalene (32.1 mg, 0.25 mmol) as internal standard. GC yield 69%. In a repeated reaction, the yield was determined by NMR spectroscopy using 1,4dioxane (8.5 µL, 0.1 mmol) as internal standard. NMR yield 62%. GC–MS (m/z): 106.2 [M]+. Decarbonylation of cuminaldehyde (Table 2, 1g). Decarbonylation was done by general procedure with cuminaldehyde (37.9 µL, 0.25 mmol), Ni(COD)2 (10.3 mg, 0.0375 mmol), and PCy3 (21.0 mg, 0.075 mmol). Yield was determined by gas chromatography using naphthalene (32.1 mg, 0.25 mmol) as internal standard. GC yield 76%. In a repeated reaction, the yield was determined by NMR spectroscopy using 1,4dioxane (8.5 µL, 0.1 mmol) as internal standard. NMR yield 66%. In another repeated reaction, the product was isolated by silica gel chromatography using hexane as an eluent with yield of 37.1 mg, 62%. 1H NMR (300 MHz, CDCl3) δ: 7.29 (t, J = 7.8 Hz, 2H), 7.23 (d, J = 7.8 Hz, 2H), 7.18 (t, J = 7.2 Hz, 1H), 2.90 (sep, J = 7.0 Hz, 1H), 1.25 (s, 6H). 13C NMR (75 MHz, CDCl3) δ: 148.8, 128.4, 126.4, 125.8, 34.2, 24.0. GC–MS (m/z): 120.1 [M]+.



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Decarbonylation of 4-fluorobenzaldehyde (Table 2, 1h). Decarbonylation was done by general procedure with 4-fluorobenzaldehyde (26.8 µL, 0.25 mmol), Ni(COD)2 (10.3 mg, 0.0375 mmol), and PCy3 (21.0 mg, 0.075 mmol). NMR yield was determined using 1,4dioxane (8.5 µL, 0.1 mmol) internal standard to be 40%. GC–MS (m/z): 96.1 [M]+. Decarbonylation of 4-(trifluoromethyl)benzaldehyde (Table 2, 1i). Decarbonylation was done by general procedure with 4-(trifluoromethyl)benzaldehyde (34.2 µL, 0.25 mmol), Ni(COD)2 (10.3 mg, 0.0375 mmol), and PCy3 (21.0 mg, 0.075 mmol). Yield was determined by gas chromatography using naphthalene (32.1 mg, 0.25 mmol) as internal standard. GC yield 24%. In a repeated reaction, the yield was determined by NMR spectroscopy using 1,4-dioxane (8.5 µL, 0.1 mmol) as internal standard. NMR yield 22%. GC–MS (m/z): 146.1 [M]+. Decarbonylation of 1-naphthaldehyde (Table 2, 1j). Decarbonylation was done by general procedure with 1-naphthaldehyde (34.0 µL, 0.25 mmol), Ni(COD)2 (10.3 mg, 0.0375 mmol), and PCy3 (21.0 mg, 0.075 mmol). Yield was determined by gas chromatography using naphthalene (32.1 mg, 0.25 mmol) as internal standard. GC yield 31%. In a repeated reaction, the product was isolated by silica gel chromatography using hexane as an eluent with yield of 12.1 mg, 19%. 1H NMR (300 MHz, CDCl3) δ: 7.85 (m, J = 3.6 Hz, 4H), 7.48 (m, J = 3.6 Hz, 4H). 13C NMR (75 MHz, CDCl3) δ: 133.5, 127.8, 125.8. GC–MS (m/z): 128.2 [M]+. Decarbonylation of 4-chlorobenzaldehyde (Table 2, 1k). Decarbonylation was done by general procedure with 4-chlorobenzaldehyde (35.2 mg, 0.25 mmol,), Ni(COD)2 (10.3 mg, 0.0375 mmol), and PCy3 (21.0 mg, 0.075 mmol). No chlorobenzene product was observed by 1H NMR. Decarbonylation of hydrocinnamaldehyde (Table 2, 1l). Decarbonylation was done by general procedure with hydrocinnamaldehyde (32.9 µL, 0.25 mmol), Ni(COD)2 (10.3 mg, 0.0375 mmol), and PCy3 (21.0 mg, 0.075 mmol). Yield was determined by gas chromatography using naphthalene (32.1 mg, 0.25 mmol) as internal standard. GC yield 14%. GC–MS (m/z): 106.2 [M]+


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Decarbonylation of cinnamaldehyde (Table 2, 1m). Decarbonylation was done by general procedure with cinnamaldehyde (31.5 µL, 0.25 mmol), Ni(COD)2 (10.3 mg, 0.0375 mmol), and PCy3 (21.0 mg, 0.075 mmol). Yield was determined by gas chromatography using naphthalene (32.1 mg, 0.25 mmol) as internal standard. GC yield 9%. GC–MS (m/z): 104.1 [M]+ Decarbonylation of benzaldehyde 1n (Table 3, entry 5). Decarbonylation was done by general procedure with benzaldehyde (25.4 µL, 0.25 mmol), Ni(COD)2 (10.3 mg, 0.0375 mmol), and PCy3 (21.0 mg, 0.075 mmol). Yield was determined by NMR spectroscopy using 1,4-dioxane (8.5 µL, 0.1 mmol) as internal standard. NMR yield 36%. GC–MS (m/z): 78.1 [M]+ Decarbonylation of 2-trifluoromethyl benzaldehyde 1o (Table 3, entry 6). Decarbonylation was done by general procedure with 2-trifluoromethyl benzaldehyde (33 µL, 0.25 mmol), Ni(COD)2 (10.3 mg, 0.0375 mmol), and PCy3 (21.0 mg, 0.075 mmol). Yield was determined by NMR spectroscopy using 1,4-dioxane (8.5 µL, 0.1 mmol) as internal standard. NMR yield 90%. GC–MS (m/z): 146.1 [M]+ Decarbonylation of 2-fluorobenzaldehyde 1p (Table 3, entry 7). Decarbonylation was done by general procedure with 2-fluorobenzaldehyde (26.3 µL, 0.25 mmol), Ni(COD)2 (10.3 mg, 0.0375 mmol), and PCy3 (21.0 mg, 0.075 mmol). Yield was determined by NMR spectroscopy using 1,4-dioxane (8.5 µL, 0.1 mmol) as internal standard. NMR yield 87%. GC–MS (m/z): 96.1 [M]+. Decarbonylation of 2-methylbenzaldehyde 1q (Table 3, entry 8). Decarbonylation was done by general procedure with 2-methylbenzaldehyde (29.0 µL, 0.25 mmol), Ni(COD)2 (10.3 mg, 0.0375 mmol), and PCy3 (21.0 mg, 0.075 mmol). Yield was determined by NMR spectroscopy using 1,4-dioxane (8.5 µL, 0.1 mmol) as internal standard. NMR yield 75%. GC–MS (m/z): 92.1 [M]+. Decarbonylation of 2-methoxybenzaldehyde 1r (Table 3, entry 9). Decarbonylation was done by general procedure with 2-methoxybenzaldehyde (30.2 µL, 0.25 mmol), Ni(COD)2 (10.3 mg, 0.0375 mmol), and PCy3 (21.0 mg, 0.075 mmol). Yield was



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determined by NMR spectroscopy using 1,4-dioxane (8.5 µL, 0.1 mmol) as internal standard. NMR yield 85%. GC–MS (m/z): 108.1 [M]+. Decarbonylation of 4-Methoxybenzaldehyde Under Reflux Condition. Under Ar atmosphere, a 10 mL round bottom flask was loaded with Ni(COD)2 (20.6 mg, 0.075 mmol, 15 mol%), PCy3 (42.0 mg, 0.15 mmol, 30 mol%), 4-methoxybenzaldehyde (60 µL, 0.5 mmol) and cyclohexane (1.4 mL). A reflux condenser was attached to its top and the flask was heated under Ar flow for 24 h at 140 °C. Then the reaction flask was cooled to 0 °C by ice bath. Nitromethane (26.8 µL, 0.5 mmol) was added to the reaction mixture as internal standard. An aliquot of mixture was mixed with 0.7 mL CDCl3 and filtered through silica gel for 1H NMR analysis. NMR yield 16%. Decarbonylation of 4-Methoxybenzaldehyde (1 mmol scale). A 35 mL Ace pressure vessel (see Supporting Information) was loaded inside the glovebox with 4methoxybenzaldehyde (121 µL, 1 mmol), Ni(COD)2 (42.0 mg, 0.15 mmol), PCy3 (84.0 mg, 0.3 mmol) and cyclohexane (2.8 mL). The vessel was closed with teflon thread cap fitted with FETFE O-ring and brought out of box. The vessel bottom part is submerged into oil bath deep enough to cover the reaction mixture. A safety shield is used for protection from overpressure. The vessel is heated at 140 ºC for 48 hours. Anisole product (78 mg) is isolated by silica gel column chromatography using hexane as eluent in 73% yield. Filtration Experiment as a Test for Homogeneity of the Reaction System. A solution of Ni(COD)2 (20.6 mg, 0.075 mmol, 15 mol%), and PCy3 (42.0 mg, 0.15 mmol, 30 mol%) in cyclohexane (1.4 mL) was loaded into a reaction tube and stirred at room temperature for 5 min under N2 atmosphere. 4-Methoxybenzaldehyde (60 µL, 0.5 mmol) or 2-fluorobenzaldehyde (53 µL, 0.5 mmol) was added to the tube which was then


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tightened by screw cap. The reaction mixture was stirred at 140 ºC for 5 min, then cooled down. 1,4-Dioxane (17 µL, 0.2 mmol) internal standard was added to the mixture, and an aliquot was taken for NMR analysis. Yield of anisole product is 25%. Yield of fluorobenzene is 16%. A parallel reaction was set up for each aldehyde, and terminated at 5 minutes respectively. The reaction tube was kept sealed, brought into glovebox, and reaction mixture was filtered through Celite. The clear filtrate was further heated at 140 °C for 24 h. After the reaction is finished, the reaction mixture was added 1,4dioxane (17 µL, 0.2 mmol) internal standard for NMR analysis. Decarbonylation yield of anisole is 69%. Yield of fluorobenzene 75%. These results suggest that our system is presumably homogeneous. Mercury Test for Homogeneity of the Reaction System. A solution of Ni(COD)2 (20.6 mg, 0.075 mmol, 15 mol%), and PCy3 (42.0 mg, 0.15 mmol, 30 mol%) in cyclohexane (1.4 mL) was loaded into a reaction pressure vessel and stirred at room temperature for 15 min under N2 atmosphere. 2-Fluorobenzaldehyde (0.5 mmol) and mercury (1.5 g, 7.5 mmol) were added to the vessel which was then tightened by a teflon screw cap. After the reaction mixture was stirred at 140 °C for 24 h, nitromethane (0.5 mmol, 26.8 µL) was added as internal standard. An aliquot of mixture was mixed with 0.7 mL CDCl3 and filtered through silica gel for 1H NMR analysis. Synthesis and Characterization of Complex 4. In N2 filled glovebox, to a stirred solution of Ni(COD)2 (275.1 mg, 1 mmol) and PCy3 (560.8 mg, 2 mmol) in toluene (10 mL) was added furfuryl (83.0 µL, 1 mmol) slowly. The reaction mixture changed to the color of orange, then back to red again. The reaction mixture was allowed to stir for 6 h at room temperature, and the volatiles were pumped down by vacuum to give a red orange



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powder (644 mg, 90%). Crystals of 4 were grown by slow evaporation of pentane into a toluene solution of 4. mp 210-221 ºC (decomp.). UV−Vis (THF) λmax, nm (ε, L mol-1 cm1

): 214 sh (16,164), 270 (14,420), 314 (15,480), 424 (852). 1H NMR (300 MHz, C6D12): δ

7.16 (d, J = 1.2 Hz, 1H), 6.08 (d, J = 1.2 Hz, 2H), 5.11 (t, J = 4.9 Hz, 1H), 2.09−1.93 (m, 6H), 1.81−1.67 (m, 12H), 1.31−1.15 (m, 12H). 31P NMR (122 MHz, C6D12): δ 44.9 (d, J = 44 Hz), 36.6 (d, J = 44 Hz) . 13C NMR (76 MHz, C6D12): δ 164.4, 137.2, 110.8, 99.9, 70.0, 36.1, 34.6, 30.7, 30.1, 28.0, 26.8. Anal. Calcd. for C41H70NiO2P2: C, 68.81; H, 9.86; N, 0.00. Found: C, 68.53; H, 9.99; N, 0.00. Decarbonylation of Complex 4. A J-Young NMR tube was loaded with 10 mg of complex 4 and 0.5 mL C6D12 in N2 filled glovebox, then sealed with teflon valve. The tube was heated at 75 °C for 60 h, and the sample was analyzed by 1H and spectroscopy.


31

P NMR

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SUPPORTING INFORMAITON The Supporting Information is available free of charge on the ACS Publications website. Experimental Set-up Reaction Optimizations for Ni catalyzed Decarbonylation of Furfural Characterization of Complex 4 X-ray Crystallographic Data Collection and Refinement of the Structure of 4 Copies of NMR Spectra of Crude Decarbonylation Reaction Mixtures and Isolated Products GC-FID Calibrations and GC-FID Results ACKNOWLEDGEMENTS Support for this work was provided by MTSU Faculty Research and Creative Activity Committee (FRCAC) (16-16-201), MTSU Clean Energy Fee Fund (8.001.F13) and MTSU start-up fund. We thank Professors Norma Dunlap (MTSU) and William Tolman (University of Minnesota) for helpful discussions. REFERENCES 1 For books and reviews: (a) Modak, A.; Maiti, D. Org. Biomol. Chem. 2016, 14, 21-35; (b) Dermenci, A., Dong, G. Sci China Chem. 2013, 56, 685–701; (c) Patra, T.; Manna, S.; Maiti, D. Angew. Chem. Int. Ed. 2011, 50, 12140–12142; (d) Hartwig, J. F. Organotransition Metal Chemistry, from Bonding to Catalysis, University Science Books, New York, 2010; (e) Tsuji, J. Handbook of Organopalladium Chemistry for Organic Synthesis, Negishi, E. I. (ed.), Wiley, NY, 2002, 2, 2648–2653; (f) Doughty, D. H., Pignolet, L. H. Homogeneous Catalysis with Metal Phosphine Complexes, Pignolet, L. H. (ed.), Springer US, 1983, 343–375; (g) Tsuji, J., Ohno, K. Synthesis 1969, 4, 157–169. 2 Eschinazi, H. E. La. Bull soc chim France 1952, 967–969. 3 (a) Murphy, S. K., Park, J.-W., Cruz, F. A., Dong, V. M. Science, 2015, 347, 5660; (b) Fristrup, P., Kreis, M., Palmelund, A., Norrby P. O., Madsen, R. J. Am.



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Nickel-Catalyzed Decarbonylation of Aromatic Aldehydes - The

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