Nickel-Catalyzed Decarbonylation of Aromatic Aldehydes

Article abstract of DOI:10.1021/acs.joc.7b00284

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 toward homocoupled esters.

Full text of DOI:10.1021/acs.joc.7b00284

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Note
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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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
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‡
Rigaku Americas, The Woodlands, TX 77381, United States
keying.ding@mtsu.edu
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 homoꢀcoupled esters.
Decarbonylation of aldehydes as an important organic transformation has recently
received increasing research attention with the tremendous progress of biomass
1
conversion and natural product synthesis in the chemical and pharmaceutical industries.
2
Since the first discovery of this reaction, decarbonylation of aldehydes and related
3
4
5
6
reactions have been accomplished by catalysts containing Rh, Ir, Ru and Pd. However,
harsh reaction conditions such as high temperatures over 180 °C are normally required.
7
Although significant improvements have appeared recently, 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
8
,9
ones based on earthꢀabundant metals. Nevertheless, nonꢀprecious 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
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challenge for decarbonylation reaction. Dong and coꢀworkers studied a stoichiometric
decarbonylation of 2ꢀnaphthaldehyde by Ni(COD) /IPr (COD = 1,5ꢀcyclooctadiene, IPr =
2
1
,3ꢀbisꢀ(2,6ꢀdiisopropylphenyl)imidazolꢀ2ꢀylidene) with trifluoroacetophenone as an
1
1
additive, 38% yield of naphthalene was obtained. The only known catalytic example
used heterogeneous Ni on charcoal (Ni/C) catalyst in a flow reactor at 10 Pa, but the
12
substrate scope is very limited. Hence, it is highly desirable to develop a general
methodology of homogeneous Ni catalyzed decarbonylation of aldehydes under mild
conditions using simple setꢀups. Very recently, one of us reported a study on Ni catalyzed
dehydrative decarbonylation of carboxylic acids to alkenes by continuous distillation
1
3a
process at 180 °C.
Decarbonylation of ketones was just reported with Ni/Nꢀ
Herein, we report results from the systematic study of Ni
1
3b
heterocyclic carbene.
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) . In order to optimize decarbonylation reaction
2
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 PCy was selected as the ligand in cyclohexane at 140 °C after 24 hours
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4
a
3
(Table 1, entry 6). Bidentate phosphine ligands have shown success in Ir and Rh
mediated decarbonylation of aldehydes, however, significantly improved yields resulted
when monodentate ligands are used instead with Ni (Table 1, entries 1ꢀ5). There are
minor decarbonylated products when IPr or IMes was used (Table 1, entries 12ꢀ13).
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n
P Bu exhibited excellent yield of furan in 90% comparable to PCy (Table 1, entry 2).
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n
However, PCy outperforms P Bu with many other substrates, therefore PCy was
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chosen as the ligand for the rest of study.
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Table 1. Ligand and Solvent Optimization
of Ni-catalyzed Decarbonylation of
Furfural
a
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b
entry ligand
solvent
yield
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1
PPh3
cyclohexane
n
2
P Bu
cyclohexane
cyclohexane
cyclohexane
cyclohexane
cyclohexane
toluene
90
36
41
54
3
3
DPPE
DPPP
DCPE
PCy3
4
5
c
6
92 (96 )
7
PCy3
67
8
PCy3
diglyme
18
9
PCy3
PCy3
PCy3
IPr
mꢀxylene
30
10
dioxane
63
1
1
2
diphenylethane 7
1
cyclohexnae
cyclohexane
cyclohexane
11
13
IMes
N/A
4
0
14
a
Reaction conditions: 1a (0.25 mmol),
cyclohexane (0.7 mL), Ni(COD) (15 mol%),
2
b
c
ligand (30 mol%), 140 °C. NMR yield. GC
yield.
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It was observed that inorganic additives (K PO KI, Cs CO , K CO ) have little
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3
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3
impact on the reaction (see Supporting Information). Molecular sieves were reported to
greatly enhance Pd catalyzed decarbonylation of aldehydes due to its capability to
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7cꢀe
remove trace amount of water, 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 3ꢀfuraldehyde and 5ꢀ
methylfurfural were nicely converted to the corresponding decarbonylated products
(
Table 2, 2b and 2c) in good to excellent yields. Then, a series of paraꢀsubstituted
benzaldehydes were tested for decarbonylation with our Ni/PCy system. Interestingly,
3
aldehydes bearing electron donating substituents perform significantly better than those
with electron withdrawing ones (Table 2, 2dꢀi). The highest decarbonylation yield (83%)
was achieved with 4ꢀmethoxybenzaldehyde (Table 2, 2d). Meanwhile, moderate yields
were observed for 4ꢀfluorobenzaldehyde and 4ꢀtrifluoromethylbenzaldehyde (Table 2,
2h-i). Besides, 1ꢀnaphthaldehyde was tested and a moderate yield of 31% was obtained
(
Table 2, 2j). No conversion was detected for 4ꢀchlorobenzaldehyde 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 byꢀproducts. Interestingly, when the reaction was performed at reflux
1
4
condition under Ar with 4ꢀmethoxybenzaldehyde as the substrate, only 16% anisole
e
product was formed (Table 2, 2d ).
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Table 2. Ni-catalyzed Decarbonylation of
Aromatic Aldehydes
a
0
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a
Reaction conditions: aldehyde 1b-m (0.25
mmol), cyclohexane (0.7 mL), Ni(COD)2
(
15 mol%), PCy (30 mol%), 140 °C, 24 h.
GC yield. NMR yield. isolated yield.
3
b
c
d
e
reflux at 140 °C for 24 h under Ar
f
atmosphere. isolated 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 homoꢀcoupled
1
5
ester (Table 3, 3) known as the Tishchenko reaction.
For paraꢀsubstituted
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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 electronꢀwithdrawing groups on the para position (Table 3, entries 1ꢀ
4
). 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 4ꢀ
c
trifluoromethylbenzaldehyde (Table 3, entry 1 ). 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
1
5a
the other with the aid of the catalyst,
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 2ꢀ
trifluoromethylbenzaldehyde was used to afford trifluoromethylbenzene (Table 3, entry
6
). Desirably, 2ꢀmethylbenzaldehyde (Table 3, entry 8) shows even better selectivity than
4ꢀmethylbenzaldehyde (Table 3, entry 3).
Surprisingly, 2ꢀfluorobenzaldehyde 1p (Table 3, entry 7) without a bulky substituent
also shows excellent selectivity (87% in yield) compared to 4ꢀfluorobenzaldehyde 1h and
benzaldehyde 1n (Table 3, entries 2 and 5), suggesting electronic interactions may also
play a role. Correspondingly, 2ꢀmethoxybenzaldehyde 1r (Table 3, entry 9) and furfural
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a (Table 1, entry 6) have better yields than 2ꢀmethylbenzaldehyde 1q (Table 3, entry 8)
and 3ꢀfuraldehyde 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 inꢀdepth studies. Decarbonylation of metaꢀsubstituted
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 orthoꢀsubstituted substrates.
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Scheme 1. Proposed Mechanism for
Ortho Effects" Observed in
Decarbonylation of Aldehydes
"
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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 Ni-
Catalyzed Decarbonylation of Aryl Aldehydes
a
0
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b
yield (%)
entry R
conv (%)
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3
c
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d
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c
1
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7
8
4ꢀCF , 1i
>99, 79
22, 28
40
78, 51
60
3
4ꢀF, 1h
>99
4ꢀCH , 1e
96
55
41
3
d
d
4ꢀOMe, 1d 95, 75
86, 69
36
9, 6
4ꢀH, 1n
>99
90
64
0
2ꢀCF , 1o
90
3
d
e
d
2ꢀF, 1p
93, 78 , 5 87, 75
6, 3
2ꢀCH , 1q
87
75
85
12
5
3
9
2ꢀOMe, 1r 90
a
Reaction conditions: aryl aldehyde (0.25 mmol),
cyclohexane (0.7 mL), Ni(COD) (15 mol%), PCy
2
3
b
c
(
30 mol%), 140 °C, 24 h. NMR yield. cyclohexane
d
e
(1 mL). filtration experiment. Hg test
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We next performed an initial mechanistic study. Addition of furfural to the red
solution of Ni(COD) and PCy in toluene at room temperature resulted in the color
2
3
change to orange. Upon removal of solvent, a red orange powder was obtained, which
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2
was later characterized as nickel η ꢀaldehyde complex 4 (Figure S5, Supporting
2
15a,b 31
Information) with an η interaction between Ni and −C=O.
P NMR spectrum of 4
displays two doublets for the PCy ligand. Slow diffusion of pentane into toluene solution
3
of 4 afforded red crystals which were analyzed by single crystal Xꢀray diffraction (Figure
S9, Supporting Information). The nickel is formally threeꢀcoordinate with two PCy3
2
ligands and furfural in a η fashion. Complex 4 shows an elongated C=O bond distance of
2
1
.321(4) Å and is consistent with those reported nickel η ꢀcarbonyl complexes (1.32ꢀ1.34
1
5cꢀf
Å).
Heating 4 at 75 °C in deuterated cyclohexane in a JꢀYoung NMR tube for 60
hours resulted in the decarbonylated product furan in 78% NMR yield (Figure S5,
3
1
Supporting Information). P NMR of the product mixture detects a known nickel
1
6,17
carbonyl complex Ni(CO) (PCy ) at 41.7 ppm.
Whether the active species 4 is a true
2
3 2
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,
d
d
filtration experiments (Table 3, entries 4 and 7 ) were performed and the results suggest
e
a possible homogeneous reaction (see Supporting Information). Hg test (Table 3, entry 7 )
shows only 5% conversion, however, the results are not reliable because Ni(COD) is
2
1
8
known to react with Hg.
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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 orthoꢀsubstituted aldehydes.
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EXPERIMENTAL SECTION
General Methods. All reactions were performed in MBraun glovebox under an
atmosphere of N or using standard Schlenk techniques. Anhydrous pentane and toluene
2
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. C D and C D were purchased from Cambridge Isotope
6
6
6
12
Lab and dried over sodium/benzophenone before being distilled and degassed three times
cycles. CDCl was purchased from Cambridge Isotope Lab and dried over molecular
3
sieves (4 Å). Furfural was distilled over CaH before use. Ni(COD) and all organic
2
2
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
31
on either a JEOL Unity 300 or 500 MHz spectrometer. P NMR spectra were referenced
to 85% H PO at 0 ppm. UVꢀVis spectra were recorded on HewlettꢀPackard Model
3
4
8452A with diode array. Analysis of products from catalytic experiments was performed
using a HewlettꢀPackard 5890 GC (HPꢀ5MS 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 graphiteꢀ
monochromated Mo K radiation.
α1
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Typical Procedure for Reaction Optimization of Decarbonylation of Furfural. A
solution of Ni(COD) (10.3 mg, 0.0375 mmol, 15 mol%) and ligand (0.075 mmol, 30
2
mol%) in cyclohexane (0.7 mL) was loaded into a reaction tube (see Supporting
Information) and stirred at room temperature for 15 min under N atmosphere. Furfural
2
(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
1
20–140 °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 CDCl , and
3
filtered through a silica gel plug (30−40 mg silica). CDCl (0.2 mL × 2) was added to the
3
1
silica plug to rinse the mixture completely. 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. GCꢀMS (m/z): 68.1 [M] .
Typical Procedure for Ni(COD) /PCy Catalyzed Decarbonylation of Aldehydes.
2
3
A solution of Ni(COD) (10.3 mg, 0.0375 mmol, 15 mol%), and PCy (21.0 mg, 0.075
2
3
mmol, 30 mol%) in cyclohexane (0.7 mL) was loaded into a reaction tube and stirred at
room temperature for 15 min under N atmosphere. Aldehyde (0.25 mmol) was added to
2
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 1,4ꢀdioxane
(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 CDCl , filtered through silica gel plug (30−40 mg
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silica) and further rinsed with CDCl (0.2 mL × 2) for H NMR analysis. For GC yield
3
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 3ꢀfuraldehyde (Table 2, 1b). Decarbonylation was done by general
procedure with 3ꢀfuraldehyde (21.6 µL, 0.25 mmol), Ni(COD) (10.3 mg, 0.0375 mmol)
2
and PCy (21.0 mg, 0.075 mmol). Yield was determined by gas chromatography using
3
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%. GCꢀMS (m/z): 68.1 [M] .
Decarbonylation of 5ꢀmethylfurfural (Table 2, 1c). Decarbonylation was done by
general procedure with 5ꢀmethylfurfural (24.9 µL, 0.25 mmol), Ni(COD) (10.3 mg,
2
0
.0375 mmol), and PCy (21.0 mg, 0.075 mmol). Yield was determined by gas
3
chromatography using naphthalene (32.1 mg, 0.25 mmol) as internal standard. GC yield
0%. In a repeated reaction, the yield was determined by NMR spectroscopy using
9
nitromethane (13.4 µL, 0.25 mmol) as internal standard. NMR yield 89%. GCꢀMS (m/z):
+
82.1 [M] .
Decarbonylation of 4ꢀmethoxybenzaldehyde (Table 2, 1d). Decarbonylation was done
by general procedure with 4ꢀanisaldehyde (30.4 µL, 0.25 mmol), Ni(COD) (10.3 mg,
2
0.0375 mmol), and PCy (21.0 mg, 0.075 mmol). Yield was determined by gas
3
chromatography using naphthalene (32.1 mg, 0.25 mmol) as internal standard. GC yield
8
3%. In a repeated reaction, the yield was determined by NMR spectroscopy using 1,4ꢀ
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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
1
with yield of 37.3 mg, 69%. H NMR (300 MHz, CDCl ) δ: 7.26 (t, J =8.7 Hz, 2H), 6.92
3
1
3
(
t, J =7.1 Hz, 1H), 6.90 (d, J =8.7 Hz, 2H), 3.75 (s, 3H). C NMR (75 MHz, CDCl ) δ:
3
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11
12
13
14
15
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19
20
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1
59.7, 129.5, 120.7, 114.0, 55.1. GC–MS (m/z): 108.1 [M]+.
Decarbonylation of pꢀtolualdehyde (Table 2, 1e). Decarbonylation was done by
general procedure with pꢀtolualdehyde (29.5 µL, 0.25 mmol), Ni(COD) (10.3 mg,
2
0
.0375 mmol), and PCy (21.0 mg, 0.075 mmol). Yield was determined by gas
3
chromatography using naphthalene (32.1 mg, 0.25 mmol) as internal standard. GC yield
8%. In a repeated reaction, the yield was determined by NMR spectroscopy using 1,4ꢀ
dioxane (8.5 µL, 0.1 mmol) as internal standard. NMR yield 55%. GC–MS (m/z): 92.1
M]+.
5
[
Decarbonylation of 4ꢀethylbenzaldehyde (Table 2, 1f). Decarbonylation was done by
general procedure with 4ꢀethylbenzaldehyde (34.3 µL, 0.25 mmol), Ni(COD) (10.3 mg,
2
0.0375 mmol), and PCy (21.0 mg, 0.075 mmol). Yield was determined by gas
3
chromatography using naphthalene (32.1 mg, 0.25 mmol) as internal standard. GC yield
6
9%. In a repeated reaction, the yield was determined by NMR spectroscopy using 1,4ꢀ
dioxane (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) (10.3 mg,
2
0
.0375 mmol), and PCy (21.0 mg, 0.075 mmol). Yield was determined by gas
3
chromatography using naphthalene (32.1 mg, 0.25 mmol) as internal standard. GC yield
6%. In a repeated reaction, the yield was determined by NMR spectroscopy using 1,4ꢀ
7
dioxane (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
1
with yield of 37.1 mg, 62%. H NMR (300 MHz, CDCl ) δ: 7.29 (t, J = 7.8 Hz, 2H), 7.23
3
13
(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). C
NMR (75 MHz, CDCl ) δ: 148.8, 128.4, 126.4, 125.8, 34.2, 24.0. GC–MS (m/z): 120.1
3
[
M]+.
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Decarbonylation of 4ꢀfluorobenzaldehyde (Table 2, 1h). Decarbonylation was done by
general procedure with 4ꢀfluorobenzaldehyde (26.8 µL, 0.25 mmol), Ni(COD) (10.3 mg,
2
0.0375 mmol), and PCy (21.0 mg, 0.075 mmol). NMR yield was determined using 1,4ꢀ
3
dioxane (8.5 µL, 0.1 mmol) internal standard to be 40%. GC–MS (m/z): 96.1 [M]+.
0
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Decarbonylation of 4ꢀ(trifluoromethyl)benzaldehyde (Table 2, 1i). Decarbonylation
was done by general procedure with 4ꢀ(trifluoromethyl)benzaldehyde (34.2 µL, 0.25
mmol), Ni(COD) (10.3 mg, 0.0375 mmol), and PCy (21.0 mg, 0.075 mmol). Yield was
2
3
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 1,4ꢀdioxane (8.5 µL, 0.1 mmol) as internal standard. NMR yield 22%.
GC–MS (m/z): 146.1 [M]+.
Decarbonylation of 1ꢀnaphthaldehyde (Table 2, 1j). Decarbonylation was done by
general procedure with 1ꢀnaphthaldehyde (34.0 µL, 0.25 mmol), Ni(COD) (10.3 mg,
2
0
.0375 mmol), and PCy (21.0 mg, 0.075 mmol). Yield was determined by gas
3
chromatography using naphthalene (32.1 mg, 0.25 mmol) as internal standard. GC yield
3
1%. In a repeated reaction, the product was isolated by silica gel chromatography using
1
hexane as an eluent with yield of 12.1 mg, 19%. H NMR (300 MHz, CDCl ) δ: 7.85 (m,
3
1
3
J = 3.6 Hz, 4H), 7.48 (m, J = 3.6 Hz, 4H). C NMR (75 MHz, CDCl ) δ: 133.5, 127.8,
3
125.8. GC–MS (m/z): 128.2 [M]+.
Decarbonylation of 4ꢀchlorobenzaldehyde (Table 2, 1k). Decarbonylation was done by
general procedure with 4ꢀchlorobenzaldehyde (35.2 mg, 0.25 mmol,), Ni(COD) (10.3
2
mg, 0.0375 mmol), and PCy (21.0 mg, 0.075 mmol). No chlorobenzene product was
3
1
observed by H NMR.
Decarbonylation of hydrocinnamaldehyde (Table 2, 1l). Decarbonylation was done by
general procedure with hydrocinnamaldehyde (32.9 µL, 0.25 mmol), Ni(COD) (10.3 mg,
2
0
.0375 mmol), and PCy (21.0 mg, 0.075 mmol). Yield was determined by gas
3
chromatography using naphthalene (32.1 mg, 0.25 mmol) as internal standard. GC yield
4%. 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) (10.3 mg,
2
0.0375 mmol), and PCy (21.0 mg, 0.075 mmol). Yield was determined by gas
3
chromatography using naphthalene (32.1 mg, 0.25 mmol) as internal standard. GC yield
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%. 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) (10.3 mg, 0.0375
2
mmol), and PCy (21.0 mg, 0.075 mmol). Yield was determined by NMR spectroscopy
3
using 1,4ꢀdioxane (8.5 µL, 0.1 mmol) as internal standard. NMR yield 36%. GC–MS
(m/z): 78.1 [M]+
Decarbonylation of 2ꢀtrifluoromethyl benzaldehyde 1o (Table 3, entry 6).
Decarbonylation was done by general procedure with 2ꢀtrifluoromethyl benzaldehyde (33
µL, 0.25 mmol), Ni(COD) (10.3 mg, 0.0375 mmol), and PCy (21.0 mg, 0.075 mmol).
2
3
Yield was determined by NMR spectroscopy using 1,4ꢀdioxane (8.5 µL, 0.1 mmol) as
internal standard. NMR yield 90%. GC–MS (m/z): 146.1 [M]+
Decarbonylation of 2ꢀfluorobenzaldehyde 1p (Table 3, entry 7). Decarbonylation was
done by general procedure with 2ꢀfluorobenzaldehyde (26.3 µL, 0.25 mmol), Ni(COD)2
(10.3 mg, 0.0375 mmol), and PCy (21.0 mg, 0.075 mmol). Yield was determined by
3
NMR spectroscopy using 1,4ꢀdioxane (8.5 µL, 0.1 mmol) as internal standard. NMR
yield 87%. GC–MS (m/z): 96.1 [M]+.
Decarbonylation of 2ꢀmethylbenzaldehyde 1q (Table 3, entry 8). Decarbonylation was
done by general procedure with 2ꢀmethylbenzaldehyde (29.0 µL, 0.25 mmol), Ni(COD)2
(10.3 mg, 0.0375 mmol), and PCy (21.0 mg, 0.075 mmol). Yield was determined by
3
NMR spectroscopy using 1,4ꢀdioxane (8.5 µL, 0.1 mmol) as internal standard. NMR
yield 75%. GC–MS (m/z): 92.1 [M]+.
Decarbonylation of 2ꢀmethoxybenzaldehyde 1r (Table 3, entry 9). Decarbonylation
was done by general procedure with 2ꢀmethoxybenzaldehyde (30.2 µL, 0.25 mmol),
Ni(COD) (10.3 mg, 0.0375 mmol), and PCy (21.0 mg, 0.075 mmol). Yield was
2
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determined by NMR spectroscopy using 1,4ꢀdioxane (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
0
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atmosphere, a 10 mL round bottom flask was loaded with Ni(COD) (20.6 mg, 0.075
2
mmol, 15 mol%), PCy (42.0 mg, 0.15 mmol, 30 mol%), 4ꢀmethoxybenzaldehyde (60 µL,
3
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 CDCl and filtered
3
1
through silica gel for H 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 4ꢀ
methoxybenzaldehyde (121 µL, 1 mmol), Ni(COD) (42.0 mg, 0.15 mmol), PCy (84.0
2
3
mg, 0.3 mmol) and cyclohexane (2.8 mL). The vessel was closed with teflon thread cap
fitted with FETFE Oꢀring 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) (20.6 mg, 0.075 mmol, 15 mol%), and PCy (42.0 mg, 0.15 mmol,
2
3
3
0 mol%) in cyclohexane (1.4 mL) was loaded into a reaction tube and stirred at room
temperature for 5 min under N atmosphere. 4ꢀMethoxybenzaldehyde (60 µL, 0.5 mmol)
2
or 2ꢀfluorobenzaldehyde (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. 1,4ꢀDioxane (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
10
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40 °C for 24 h. After the reaction is finished, the reaction mixture was added 1,4ꢀ
dioxane (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 PCy (42.0 mg, 0.15 mmol, 30 mol%) in
3
cyclohexane (1.4 mL) was loaded into a reaction pressure vessel and stirred at room
temperature for 15 min under N atmosphere. 2ꢀFluorobenzaldehyde (0.5 mmol) and
2
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
1
0
.7 mL CDCl and filtered through silica gel for H NMR analysis.
3
Synthesis and Characterization of Complex 4. In N filled glovebox, to a stirred
2
solution of Ni(COD) (275.1 mg, 1 mmol) and PCy (560.8 mg, 2 mmol) in toluene (10
2
3
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
ꢀ1
ꢀ
toluene solution of 4. mp 210ꢀ221 ºC (decomp.). UV−Vis (THF) λmax, nm (ε, L mol cm
1
1
)
: 214 sh (16,164), 270 (14,420), 314 (15,480), 424 (852). H NMR (300 MHz, C D ): δ
6 12
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4
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.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,
3
1
6H), 1.81−1.67 (m, 12H), 1.31−1.15 (m, 12H). P NMR (122 MHz, C D ): δ 44.9 (d, J
6
12
13
=
44 Hz), 36.6 (d, J = 44 Hz) . C NMR (76 MHz, C D ): δ 164.4, 137.2, 110.8, 99.9,
6 12
7
0.0, 36.1, 34.6, 30.7, 30.1, 28.0, 26.8. Anal. Calcd. for C H NiO P : C, 68.81; H, 9.86;
41 70 2 2
N, 0.00. Found: C, 68.53; H, 9.99; N, 0.00.
Decarbonylation of Complex 4. A JꢀYoung NMR tube was loaded with 10 mg of
complex 4 and 0.5 mL C D in N filled glovebox, then sealed with teflon valve. The
6
12
2
1
31
tube was heated at 75 °C for 60 h, and the sample was analyzed by H and P NMR
spectroscopy.
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SUPPORTING INFORMAITON
The Supporting Information is available free of charge on the ACS Publications website.
Experimental Setꢀup
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Reaction Optimizations for Ni catalyzed Decarbonylation of Furfural
Characterization of Complex 4
Xꢀray Crystallographic Data Collection and Refinement of the Structure of 4
Copies of NMR Spectra of Crude Decarbonylation Reaction Mixtures and Isolated
Products
GCꢀFID Calibrations and GCꢀFID Results
ACKNOWLEDGEMENTS
Support for this work was provided by MTSU Faculty Research and Creative Activity
Committee (FRCAC) (16ꢀ16ꢀ201), MTSU Clean Energy Fee Fund (8.001.F13) and
MTSU startꢀup fund. We thank Professors Norma Dunlap (MTSU) and William Tolman
(University of Minnesota) for helpful discussions.
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