Nickel Catalysts for the Dehydrative Decarbonylation of Carboxylic Acids to Alkenes

Article abstract of DOI:10.1021/acs.organomet.6b00415

Combining high-throughput experimentation with conventional experiments expedited discovery of new first-row nickel catalysts for the dehydrative decarbonylation of the bioderived substrates hydrocinnamic acid and fatty acids to their corresponding alkenes. Conventional experiments using a continuous distillation process (180 °C) revealed that catalysts composed of Ni^II or Ni⁰ precursors (NiI₂, Ni(PPh₃)₄) and simple aryl phosphine ligands were the most active. In the reactions with fatty acids, the nature of the added phosphine influenced the selectivity for α-alkene, which reached a maximum value of 94%. Mechanistic studies of the hydrocinnamic reaction using Ni(PPh₃)₄ as catalyst implicate a facile first turnover to produce styrene at room temperature, but deactivation of the Ni(0) catalyst by CO poisoning occurs subsequently, as evidenced by the formation of Ni(CO)(PPh₃)₃, which was isolated and structurally characterized. Styrene dimerization is a major side reaction. Analysis of the reaction mechanism using density functional theory supported catalyst regeneration along with alkene formation as the most energetically demanding reaction steps.

Full text of DOI:10.1021/acs.organomet.6b00415

Article
pubs.acs.org/Organometallics
Nickel Catalysts for the Dehydrative Decarbonylation of Carboxylic
Acids to Alkenes
†
,§
†,§
†,§
sra Dereli, Manuel A. Ortun
o,†
†
Alex John, Maria O. Miranda, Keying Ding, Bu
̈
̃
,
‡
,‡
,†
,†
Anne M. LaPointe,* Geoffrey W. Coates,* Christopher J. Cramer,* and William B. Tolman*
†Department of Chemistry, Center for Sustainable Polymers, Chemical Theory Center, and Minnesota Supercomputing Institute,
University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States
‡
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States
*
S Supporting Information
ABSTRACT: Combining high-throughput experimentation
with conventional experiments expedited discovery of new
first-row nickel catalysts for the dehydrative decarbonylation of
the bioderived substrates hydrocinnamic acid and fatty acids to
their corresponding alkenes. Conventional experiments using a
continuous distillation process (180 °C) revealed that catalysts
II
0
composed of Ni or Ni precursors (NiI , Ni(PPh ) ) and
2
3 4
simple aryl phosphine ligands were the most active. In the
reactions with fatty acids, the nature of the added phosphine
influenced the selectivity for α-alkene, which reached a
maximum value of 94%. Mechanistic studies of the hydro-
cinnamic reaction using Ni(PPh ) as catalyst implicate a facile
3
4
first turnover to produce styrene at room temperature, but deactivation of the Ni(0) catalyst by CO poisoning occurs
subsequently, as evidenced by the formation of Ni(CO)(PPh ) , which was isolated and structurally characterized. Styrene
3
3
dimerization is a major side reaction. Analysis of the reaction mechanism using density functional theory supported catalyst
regeneration along with alkene formation as the most energetically demanding reaction steps.
INTRODUCTION
acrylates, and acrylonitrile, respectively, in high yields (>80%)
using a palladium-phosphine catalyst in combination with
pivalic anhydride (Scheme 1). Related Pd catalysts with acetic
■
In efforts to develop alternative routes to important polymers,
6
new synthetic routes to high-volume and high-value monomers1
anhydride as additive were used to convert fatty acids to
from renewable resources are key targets of current research.
7
terminal alkenes, and in recent work, anhydride additive use
In view of the prevalence of carboxylic acid functional groups in
biomass-derived feedstocks, catalytic dehydrative decarbon-
ylation to yield alkenes is an attractive approach for the
was circumvented via Pd-catalyzed decarbonylation of p-
8
nitrophenol esters.
Although precious metals are effective catalysts, they are also
expensive and rare. The use of inexpensive and readily available
first-row transition metals is desirable, but there have been few
reports of their use for the dehydrative decarbonylation of
carboxylic acids. Recently, an iron-based catalyst was reported
synthesis of bioderived monomers (Scheme 1, inset). Building2
upon previous work using precious metals such as palladium,
2
a
3
rhodium, and iridium, we recently reported the catalytic
dehydrative decarbonylation of the renewable substrates
4
hydrocinnamic acid (from cinnamaldehyde ), monoalkyl
succinates (from biobased succinic acid), and 3-cyanopropanoic
acid (from the amino acid glutamic acid ) to styrene, alkyl
for the dehydrative decarbonylation of long chain fatty acids to
9
terminal alkenes. Using 10 mol % FeCl , 20−40 mol % of an
5
2
added phosphine ligand, 100 mol % of the additives KI and
acetic anhydride, and 20 psi of CO, high yields of terminal
alkene products were obtained. In a noncatalytic route, alkenes
were prepared in up to 56% yield from cyclic anhydrides and
thioanhydrides using stoichiometric amounts of (PPh ) Ni-
Scheme 1. Catalytic Dehydrative Decarbonylation of
Carboxylic Acids to Alkenes (Inset) and Specific Example of
Conversion of Hydrocinnamic Acid to Styrene Using Pivalic
Anhydride as Additive
3
2
(
CO) , Fe (CO) , or (PPh ) RhCl; however, side reactions
2 2 9 3 3
were observed for anhydrides containing abstractable hydro-
10
gens. In addition, a low-valent nickel species generated in situ
from NiCl and zinc dust was shown to be effective for the
2
Received: May 24, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00415
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Figure 1. Ligands and metal precursors used in the first set of high-throughput screening reactions.
Figure 2. Ligands and metal precursors used in the second set of high-throughput screening reactions.
1
1
dehydrative decarbonylation of 2-pyridyl thioates. The
dehydrative decarbonylation of thioesters under similar
conditions generated thioethers from unsaturated or aromatic
selectivity for linear alpha alkenes were discovered. Further
experimental and theoretical studies provided mechanistic
insights useful for guiding future catalyst development.
12
substrates and alkenes from aliphatic substrates. A disadvant-
age of these approaches is that 2-pyridyl thioates and thioesters
must be prepared from the carboxylic acid prior to dehydrative
decarbonylation. In the patent literature, nickel, cobalt, and iron
precursors were reported to be effective dehydrative decarbon-
ylation catalysts for carboxylic acids in the presence of KI or by
using an iodide precursor, although conversions are lower than
RESULTS AND DISCUSSION
■
Initial Catalyst Screening. A series of catalyst screening
experiments was performed in order to identify effective metal
precursors, ligands, and reaction conditions for the dehydrative
decarbonylation of hydrocinnamic acid. All reagents were
dispensed robotically. Metal−ligand combinations were
screened in parallel in 96 × 1 mL arrays, and thin-layer
chromatography (TLC) with UV light detection was used as a
rapid, qualitative measure of reactivity due to its sensitivity (<1
mg). Validation of the high-throughput workflow as a suitable
13
those observed with palladium or rhodium precursors.
Inspired by literature precedent that suggested that the first-
row metals could catalyze dehydrative decarbonylation
9
−13
reactions,
we were interested in screening a broad range
of first-row transition metal−ligand combinations for the
dehydrative decarbonylation of bioderived carboxylic acids.
The conversion of hydrocinnamic acid to styrene was selected
as a convenient and illustrative test case (Scheme 1). Herein we
report the results of these studies, using a combination of high-
throughput experimentation (HTE) and conventional batch
catalyst assessment. HTE was used to expedite the initial
discovery process, and conventional catalyst screening was used
to explore active systems in more detail. Several new types of
catalysts were identified for styrene production, their use in the
dehydrative decarbonylation of fatty acids to produce long-
chain alkenes was examined, and conditions that yielded high
primary screen came from the observation of styrene formation
6
by previously described Pd/PPh catalyst systems, which were
3
included for validation of the workflow. Metal precursors and
ligands screened in the first set of experiments are shown in
Figure 1. Subsequent rounds of screening using the compounds
shown in Figure 2 focused on the most promising Ni(II) and
Fe(II) precursors as well as additional zerovalent compounds
containing these metals under varying conditions (reaction
time, ligand/metal loading, added CO or CO , varying amounts
2
of KI), with monitoring of reaction products via GC in some
cases. Detailed experimental descriptions and screening results
are presented in the Supporting Information (Figures S1−S4,
B
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Article
a
Table 1. Effect of Nickel Precursor, KI, and PPh /Ni Ratio for Dehydrative Decarbonylation of Hydrocinnamic Acid
3
entry
Ni precursor (mol %)
PPh (mol %)
KI (mol %)
styrene yield (%)
ethylbenzene yield (%)
PivOH yield (%)
3
1
2
3
4
5
6
7
8
9
NiI (5)
10
20
20
20
40
40
8
0
0
13
13
23
19
19
13
7.0
29
13
27
30
2.6
2.9
4.7
5.2
2.9
2.6
0.4
2.6
1.1
4.1
3.2
43
45
80
81
48
45
7.4
71
25
80
81
2
NiI (5)
2
NiI (10)
0
2
NiI (10)
100
0
2
NiI (10)
2
NiI (10)
100
100
100
100
0
2
Ni(PPh ) (2)
3
4
Ni(PPh ) (5)
10
20
20
20
3
4
Ni(PPh ) (5)
3
4
1
1
0
1
Ni(PPh ) (10)
3 4
Ni(PPh ) (10)
100
3
4
a
Reactions performed at 180 °C for 2 h, under a flow of N or Ar using a distillation method. Styrene, ethylbenzene, and PivOH yields determined
2
by GC-MS analysis of the distillate.
a
Table 2. Variation of Ni Source, Phosphine, and KI for Dehydrative Decarbonylation of Hydrocinnamic Acid
entry
Ni precursor (mol %)
ligand (mol %)
KI mol %
time (h)
styrene yield (%)
ethylbenzene yield (%)
PivOH yield (%)
1
2
3
4
5
6
7
8
9
NiI (10)
Davephos (20)
DPEphos (10)
DPPB (10)
DPPF (10)
DPPB (20)
Davephos (10)
DPEphos (5)
DPPB (5)
100
100
100
100
100
100
100
100
100
0
2
2
20
21
34
15
28
27
34
23
30
24
28
4.2
2.5
8
71
61
81
58
83
85
90
60
78
44
75
2
NiI (10)
2
NiI (10)
2
2
NiI (10)
2
2.7
6.9
2.7
3.0
2.4
2.6
4.5
2.6
2
NiI (10)
24
2
2
Ni(PPh ) (5)
3
4
Ni(PPh ) (5)
2
3
4
Ni(PPh ) (5)
2
3
4
Ni(PPh ) (5)
DPPF (5)
2
3
4
1
1
0
1
Ni(PPh ) (5)
DPPB (5)
2
3
4
Ni(PPh ) (5)
DPPB (5)
0
18
3
4
a
Reactions performed at 180 °C, under a flow of N or Ar using a distillation method. Styrene, ethylbenzene, and PivOH yields determined by GC-
2
MS analysis of the distillate.
Tables S1−S8). While styrene formation was observed in many
cases using a wide range of catalyst precursors, it should be
noted that the TLC screen is very sensitive and can detect low-
catalyst conditions that generate the highest yields of styrene
(entries 3, 4, 8, 10, and 11).
Using the optimized ligand-to-metal ratios and metal
precursor loadings, the effect of ligand identity on the
dehydrative decarbonylation efficiency and styrene yield was
activity systems (1% conversion). Upon scale up, the best
0
results were obtained using NiI or Ni precursors with a variety
2
of phosphine ligands in a 2:1 P/Ni ratio. These systems were
selected for follow-up studies. Ethylbenzene was identified as a
coproduct, with a [styrene]/[ethylbenzene] ratio typically
ranging from 1:1 to 4:1. Product formation was inhibited
then explored (Table 2). Using NiI , the highest yields were
2
obtained with DPPB (entry 3), with a styrene yield of 34%,
although higher yields of ethylbenzene were also observed
(8%). Allowing the reaction to proceed for 24 h did not afford
an increase in styrene yield (entry 5). Although the bidentate
phosphine DPPB gave the highest conversions, reactions with
other bidentate phosphines did not show improved perform-
when the reaction was performed under CO or CO pressure.
2
Batch Reactions of Hydrocinnamic Acid. In order to
determine whether the general trends observed under high-
throughput conditions were replicated under conventional
batch conditions on a larger scale, reactions using the most
ance relative to PPh . Davephos (entry 1) and DPEphos (entry
3
2) resulted in comparable yields of styrene and PivOH to those
II
0
promising Ni and Ni precursors, NiI and Ni(PPh ) , and
observed with PPh , and DPPF (entry 4) had the lowest
2
3 4
3
PPh were run using a previously described experimental setup
using 1.0 g (6.7 mmol) of hydrocinnnamic acid. In agreement
performance of the ligands tested with Ni(II). In general,
3
6
Ni(PPh ) was more efficacious than NiI with the ligands
3
4
2
with the HTE results, the optimal ligand-to-metal ratio was 2:1
for both Ni and Ni precursors, as seen in Table 1 (entries 3
tested, although this may be due to catalysis by Ni(PPh3)n
intermediates. Addition of KI was found to enhance conversion
and reduce ethylbenzene formation, although styrene yield was
nearly unchanged (entries 8 and 10). Extending the reaction
time to 18 h resulted in only a modest increase in styrene yields
(entries 10 and 11).
From the data in Tables 1 and 2, a discrepancy between
styrene and PivOH yields is evident; PivOH yields (or
hydrocinnamic acid conversion) were higher than those of
styrene in the distillate. These findings can be partly explained
by the presence of styrene dimers and/or oligomers (such as
the species shown in Figure 3) observed by GC-MS and NMR
II
0
and 8). Using NiI as the metal precursor, the highest yield for
2
styrene was found with 10 mol % NiI and 20 mol % PPh3
2
(
23%, entry 3). Reactions using NiI precursors generally
2
required higher loadings, with an optimal loading of 10 mol %.
For Ni(PPh ) , an optimal loading of 5 mol % was determined;
3
4
higher styrene yields were not observed at 10 mol % (entries 8
and 11). Addition of KI enhances styrene yield slightly for
Ni(PPh ) (entries 10 and 11) and has minimal effect on the
3
4
yield for NiI (entries 3−6). Despite relatively low styrene
2
yields, high yields of PivOH (70−80%) are observed for the
C
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the hydrocinnamic acid anhydride (A′), formed by reaction of
A with another equivalent of hydrocinnamic acid (Scheme 3).
Scheme 3. Formation of Anhydrides A and A′
Figure 3. Styrene dimers isolated from the reaction mixture.
spectroscopy after the reaction had been worked up (Figures
14
15
16
17
S5−S17). Metal hydride (M = Ni, Pd, Ir, Ru ) species
are known to facilitate the dimerization of styrene (and other
alkenes) to give alkene- and alkane-containing dimers via a
variety of mechanisms; typically, pathways analogous to those
for alkene polymerization are invoked, although alternatives
15a,16,17
have been proposed.
Mechanistic Analysis of Hydrocinnamic Acid Reac-
tion. a. Experiments. With the intent of gaining further
understanding of the Ni-catalyzed dehydrative decarbonylation
reaction and identifying the rate-limiting step, we performed a
series of experiments designed to test various aspects of the
The ratio of the anhydride components in the mixture was
found to be unchanged upon heating the mixture at 90 °C for 1
h or overnight. On the other hand, a higher proportion of the
hydrocinnamic anhydride (A′, 51%; A, 35%) was generated
under the reaction conditions (180 °C, 2 h, distillation of
PivOH). These data suggest that under catalytic conditions the
reaction mixture is enriched in A′.
2b,6
generally accepted mechanism for the process (Scheme 2).
Scheme 2. Proposed Mechanism for Ni-Catalyzed
Dehydrative Decarbonylation
In previous investigations of the feasibility of anhydride
oxidative addition to low-valent phosphine-nickel complexes,
facile oxidative addition was followed by decarbonylation to
yield stable nickelalactones (from cyclic anhydrides) or aryl-
18
nickel complexes (from aromatic carboxylic anhydrides). We
found that addition of 20 mol % Ni(PPh ) to the mixture
3
4
formed upon adding pivalic anhydride to hydrocinnamic acid in
benzene-d at room temperature resulted in an instantaneous
6
reaction, as indicated by an immediate color change from deep
red-brown to a golden-yellow. Surprisingly, analysis of the
resulting solution by 1H NMR spectroscopy showed that
styrene was generated under these conditions (20%, 1
turnover). This result indicates that alkene generation from
the initially generated nickel-alkyl species is also facile at room
temperature. The resulting [Ni-PPh ] species showed a single
3
resonance at δ 32.4 ppm in the 31P NMR spectrum. Further
turnovers were not observed at room temperature, but upon
heating the reaction mixture at 100 °C for 1 h, the styrene yield
increased to 30% (1.5 turnovers) with the generation of a
31
second [Ni-PPh ] species ( P NMR peak at δ 33.5 ppm).
3
However, no further increase in styrene yield was observed
even upon continued heating for 2 days.
In order to probe the identity of the first [Ni-PPh ] species
δ 32.4 ppm), an alternate route was sought that would
3
(
generate a highly volatile alkene such as 1-propene, facilitating
isolation of the Ni complex. Addition of stoichiometric
amounts of Ni(PPh ) to a solution of n-butyric anhydride in
3
4
According to this mechanism, the carboxylic acid substrate
reacts with the anhydride additive to yield new anhydrides (A
and/or A′), which oxidatively add to the metal center to yield
an acyl-carboxylate complex (B). Deinsertion to yield an alkyl
complex (C) is then followed by β-hydride elimination,
affording the product alkene and a metal hydride (D).
Subsequent loss of carboxylic acid and CO regenerates the
catalyst.
In initial experiments, we examined the activation of
hydrocinnamic acid by the anhydride additive. The formation
of the mixed anhydride, A, was observed to occur
instantaneously upon combining equimolar amounts of hydro-
benzene-d at room temperature with stirring for 30 min
6
resulted in the formation of 1-propene (75% by NMR) along
3
1
with butyric acid and a species with a single peak in the
P
NMR spectrum with the same chemical shift (δ 32.4 ppm) as
that observed in the hydrocinnamic acid reactions. This species
was isolated in 57% yield after recrystallization from a 1:1
mixture of toluene/pentane at −40 °C and was identified as
Ni(CO)(PPh ) by X-ray crystallography (Figure S18 and
3
3
19
Table S9). The [Ni-PPh ] species obtained after the second
3
31
turnover was identified as Ni(PPh ) (CO) ( P NMR = 33.5
3
2
2
ppm) by comparison with the 3 P NMR spectrum of an
1
authentic sample purchased from Strem Chemicals. Similar
31
cinnamic acid and pivalic anhydride in benzene-d at room
[Ni-PPh ]-containing species ( P δ 32.4 and 33.5 ppm) were
6
3
1
temperature. However, as judged by H NMR spectroscopy, it
observed during experiments conducted starting with an
independently prepared sample of hydrocinnamic anhydride
was generated only to the extent of ∼21%, along with ∼15% of
D
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(
A′) at 100 °C. Addition of other exogenous ligands {e.g.,
quantum chemical calculations at the density functional level of
theory (DFT; see Methods section for details). Since
experiments suggest that Ni(PPh ) is one of the most efficient
bidentate phosphines such as DPPE, DCYPE (1,2-bis-
dicyclohexylphosphino)ethane), DPPF, or an N-heterocyclic
(
3
4
carbene (IPr)} did not result in any improvements in styrene
yield, and no further turnovers were obtained.
precatalysts (Table 1, entry 10), we explored the reaction as
depicted in Scheme 5. All reported energies correspond to
The aforementioned inability of the second complex,
Ni(PPh ) (CO) , to catalyze dehydrative decarbonylation of
Scheme 5. Modeled Reaction for the Ni-Catalyzed
Dehydrative Decarbonylation of A
3
2
2
hydrocinnamic acid at 100 °C suggests that the regeneration of
the active Ni(0) species (presumably by ligand loss) is the rate-
limiting step in the sequence at lower temperatures. An
independent catalytic run conducted using Ni(CO) (PPh ) as
2
3 2
the catalyst (10 mol %) in the presence of DPPB (10 mol %) at
80 °C yielded 31% styrene (3 turnovers) in the distillate.
1
Thus, at higher temperatures (180 °C), ligand loss is promoted
and more turnovers can be attained.
−
1
Gibbs free energies (kcal mol ) in butanoic acid (BA; chosen
to mimic a solvent environment rich in pivalic acid) at 190 °C
(463.15 K). The fully separated species Ni(PPh ) , hydro-
We also examined the process by which the side products
may be formed (ethylbenzene, styrene dimers). Ethylbenzene
may be formed if Ni alkyl intermediate D is protonated by
pivalic acid. Alternatively, it may be generated directly from
hydrocinnamic acid by decarboxylation under the reaction
conditions. When neat hydrocinnamic acid was heated in the
presence of NiI (20%), PPh (40%), and KI (1 equiv) at 180
3
4
cinnamic acid, and pivalic anhydride are used to define the zero
of energy.
Following reaction of hydrocinnamic acid with pivalic
anhydride to form the mixed anhydride A, the proposed
22
mechanism includes four elementary steps, namely, oxidative
2
3
23
°
C, ethylbenzene was obtained as the major product, albeit in
addition of the mixed anhydride to the metal, decarbonylation
of the acyl group, formation of styrene, and loss of CO to
regenerate the catalyst (Scheme 2).
low yield (9%), with an ethylbenzene to styrene ratio of 11:1. A
similar study performed using Ni(PPh ) (20%) as the catalyst
3
4
did not yield any deoxygenated products, suggesting that
ethylbenzene formation takes place exclusively from the Ni(II)
oxidation state. The feasibility of ethylbenzene originating from
decarboxylation of hydrocinnamic acid is further supported by
the observation that a higher amount of ethylbenzene is
obtained in the distillate when half an equivalent of pivalic
anhydride is used.
A complete reaction coordinate is shown in Figure 4 (L =
PPh
[NiL
NiL ] with a free energy change of −7.7 kcal mol .
). Initially, one phosphine ligand dissociates from the
3
] complex to generate a more stable trisphosphine species
4
−
1
[
3
Subsequently, the mixed anhydride A coordinates to [NiL ],
3
−1
forming 1 (2.2 kcal mol ). Dissociation of one phosphine
ligand from [NiL ] generates the active catalyst 2 (−0.3 kcal
mol ), which undergoes oxidative addition. In the oxidative
3
−1
Styrene dimers are another side product observed in the
reaction mixture. We hypothesize that the dimers result from
subsequent Ni-catalyzed reactions of the initially formed
styrene. Consistent with our hypothesis, when styrene or 4-
methylstyrene was heated at 180 °C in the presence of
Ni(PPh ) (10%) and pivalic acid (1−5 equiv relative to
addition step (Scheme 6), C−O bond activation takes place via
−
1
TS2−3 at 9.3 kcal mol . The resulting acyl−Ni intermediate
(
B in Scheme 2) can rearrange one phosphine ligand from its
−1
orientation cis in cis-3 (−5.9 kcal mol ) to the more stable
−1
trans configuration in trans-3 (−14.7 kcal mol ). Ligand loss
3
4
−1
from either cis- or trans-3 leads to 4 (−7.3 kcal mol ), from
which the decarbonylation step (Scheme 6) proceeds via TS4−
alkene), the respective dimeric products (Figure 3) were
1
obtained in ∼40% yield ( H NMR, identity confirmed by GC-
−1
5
−
at 0.2 kcal mol to generate alkylmetal intermediate 5 at
MS). Multiple pathways for production of these compounds
may be envisioned involving initial coordination of styrene to
5.4 kcal mol− (C in Scheme 2).
1
2
0
Intermediate 5 can follow three alternative pathways leading
Ni(0) and/or reaction with an intermediate Ni−H species
2
1
to the formation of styrene (Scheme 7): β-hydride elimination
followed by reductive elimination (Figure 4, blue line) and
metal-assisted deprotonation via either the monophosphine
Figure 4, orange line) or phosphine-free (Figure 4, gray line)
intermediates. Alternative pathways involving bisphosphine
intermediates are less favored (see SI, Figure S19).
(
Scheme 4). We are unable to distinguish among those
pathways with the available evidence.
b. Theory. To shed light on the details of the hydrocinnamic
acid dehydrative decarbonylation mechanism, we turned to
(
Scheme 4. Possible Routes for Generation of Styrene
Dimers: (i) Styrene Coordination; (ii) Oxidative Coupling;
Considering first β-hydride elimination, TS structure TS5−6
−1
is found at 14.5 kcal mol , which represents a free energy of
activation of 29.2 kcal mol− relative to the preceding free
energy sink trans-3. Owing to the basic nature of the pivalate
group, intramolecular deprotonation of the C−H bond of the
alkyl group by the pivalate ligand is also possible. A
corresponding TS structure, TS5−7, was found at 11.1 kcal
(
iii) β-Hydride Elimination; (iv) Reductive Elimination; (v)
1
Protonation; (vi) Styrene Insertion
−1
mol , which reduces the free energy of activation to 25.8 kcal
mol . In this TS structure the metal center assists the proton
−1
transfer via a β-agostic interaction (Ni···H = 1.840 Å in TS5−
7
). The deprotonation pathway was also studied in the absence
of the phosphine ligand. A phosphine-free metal-assisted
deprotonation TS structure, TS8−9, is found at 10.5 kcal
−1
mol , and its free energy of activation relative to trans-3 is thus
E
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Article
−1
Figure 4. Gibbs free energy along the reaction coordinate (kcal mol ) for Ni-catalyzed dehydrative decarbonylation of hydrocinnamic acid.
a
Scheme 6. Oxidative Addition and Dehydrative
Decarbonylation Steps
Scheme 7. Alkene Formation Step
a
a
ΔGBA in kcal mol−1.
ligands generates complex [NiL (CO)] (E in Scheme 2) at
3
−
24.0 kcal mol− relative to initial reactants (i.e., the reaction to
1
E is quite exergonic). The stability of E is consistent with its
a
ΔGBA in kcal mol−1.
experimental observation in reaction mixtures (vide supra). To
reenter the catalytic cycle, regeneration of [NiL ] requires 26.2
4
kcal mol− (the free energy difference associated with replacing
1
−1
2
5.2 kcal mol . Thus, both monophosphine and phosphine-
CO by PPh ), and the complete reaction cycle is then predicted
3
−1
free deprotonation routes dominate reactivity.
to be endergonic by 2.2 kcal mol . However, that is a standard-
state free energy associated with taking CO to have a standard-
state pressure of 1 atm. Assuming that CO is swept from the
system upon dissociation from Ni to a pressure of 10− atm, for
instance, the free energy for a single turnover would then be
After formation of the Ni-hydride intermediate 6 (D in
Scheme 2) via β-hydride elimination, a subsequent reductive
elimination step can occur via TS6−10, having a free energy of
5
−1
activation of 36.4 kcal mol relative to free energy sink trans-3.
−1
However, this route to eliminate pivalic acid and intermediate
1
−8.4 kcal mol ; that is, the catalytic cycle should proceed
0 has a substantially higher barrier than do the deprotonation
when CO is removed from the reaction medium.
pathways, which by virtue of generating coordinated pivalic acid
intermediate 7 or 9 directly, need only dissociate the carboxylic
acid to generate 10.
The final catalyst regeneration step requires recoordination
of phosphine ligands and the dissociation of styrene and CO.
Styrene release from 10 and recoordination of phosphine
For the proposed full reaction mechanism(s) for the Ni-
catalyzed dehydrative decarbonylation of hydrocinnamic acid,
the lowest standard-state free energy species in the cycle is
carbonyl compound E ([NiL (CO)]) and the turnover-limiting
3
free energy of activation may derive from the sum of the CO
dissociation free energy (which is CO pressure dependent) and
F
DOI: 10.1021/acs.organomet.6b00415
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
the energy required to form intermediate 2 from regenerated
catalyst. If CO pressure is kept sufficiently low, however, this
sum may become sufficiently small to make the turnover-
limiting free energy of activation correspond to that for alkene
to other reported catalytic processes using Pd (80−99%), Fe
2d,3,7,24
(91−98%), or Ir (90−98%) catalysts.
The yield of
octene was adversely affected when a particularly strongly
coordinating ligand such as 1,2-bis(dicyclohexylphosphino)-
ethane (dcype, entry 9) was used. Increasing the ligand:Ni ratio
(entry 8, L:Ni = 1.5) showed a slight improvement in the α-
selectivity (94%) but at the expense of the overall alkene yield
(56%). Lowering the catalyst loading (1 mol %, entry 11)
resulted in reduced yields with a slight improvement in the α-
selectivity (96%). In conclusion, we have identified Ni-based
catalysts for the dehydrative decarbonylation of a representative
long-chain fatty acid to a terminal α-alkene with high selectivity.
Future computational efforts will focus on discerning the
mechanism of the process and in particular the factors that
underlie the high α-selectivity.
−1
formation from trans-3, via either TS5−7 (25.8 kcal mol ) or
−1
TS8−9 (25.2 kcal mol ). This is fully consistent with the
experimental observation that CO poisons the Ni catalyst
through formation of the very stable [Ni−CO] species E. Along
this line, a second catalytic cycle starting from E required larger
activation barriers (see SI, Figure S20).
Our previous studies for Pd-catalyzed dehydrative decarbon-
ylation of hydrocinnamic acid display a similar reactivity trend,
22
albeit with larger activation barriers. Regarding the catalyst
regeneration, the bisphosphine Pd−acyl intermediate and
[
[
PdL (CO)] were close in energy, whereas the species
3
NiL (CO)] E is greatly favored and acts as a free energy sink.
3
Batch Reactions of Fatty Acids. We then investigated the
SUMMARY AND CONCLUSIONS
■
reaction of the Ni catalysts with long-chain aliphatic carboxylic
acids, many of which are derived from natural sources. It is
particularly desirable to invent new methods for the synthesis of
odd-numbered linear α-olefins (LAOs) by dehydrative
decarbonylation of appropriate saturated fatty acids in view of
the high cost of odd-numbered LAOs that are not accessible via
ethylene oligomerization or ethenolysis of unsaturated fatty
Using a combination of high-throughput and conventional
experiments, we determined that a combination of nickel
precursors and arylphosphine ligands catalyze the dehydrative
decarbonylation of hydrocinnamic acid to form styrene.
Optimal ligand-to-metal ratios were determined to be 2:1,
with a decrease in ligand-to-metal ratio resulting in a decreased
yield of styrene. The reaction is inhibited by CO. A variety of
phosphines were studied, and simple bidentate phosphines such
as DPPB and DPEphos were found to increase the styrene
3
b,7
acids.
Nonanoic acid was used as a convenient model
substrate for the dehydrative decarbonylation of straight-chain
aliphatic fatty acids, as its selective dehydrative decarbonylation
would also yield industrially important 1-octene. We discovered
that under the standard reaction conditions (Table 2, entry 3),
nonanoic acid could be decarbonylated to octene(s) in 61%
yield (Table 3, entry 1). In contrast to the reactions involving
yield, although they were only modestly better than PPh . A
3
discrepancy between the observed styrene and pivalic acid
yields has been attributed mostly to the generation of styrene
dimers under the reaction condition. Mechanistic studies
suggest that regeneration of the active [Ni-PPh ] catalytic
3
species via CO dissociation is the rate-limiting step in the
sequence. Computational investigation of the catalytic mech-
anism supports these experimental results and identifies catalyst
regeneration along with alkene formation as the most energy-
demanding steps. The catalytic reaction conditions developed
could be expanded to include other substrates. Specifically, we
have demonstrated that fatty acids such as nonanoic acid can be
efficiently decarbonylated to octene(s) under identical
conditions. Ni(0) precursors in conjunction with bisphosphine
ligands were found to be superior at achieving high α-selectivity
Table 3. Nickel-Catalyzed Dehydrative Decarbonylation of
Nonanoic Acid
a
KI
yield
(%)
α-selectivity
entry
[Ni] (%)
ligand (%)
dppb (10)
(%)
(%)
1
2
3
4
5
6
7
8
9
NiI (10)
100
100
100
61
63
55
53
64
50
78
56
4
37
61
52
91
91
83
90
94
76
90
96
2
NiI (10)
PPh (20)
2
3
NiI (10)
DPEPhos (10)
2
Ni(PPh ) (5) DPEPhos (5)
3
4
Ni(COD) (5) DPEPhos (5)
2
(
up to 94%) in these dehydrative decarbonylation reactions.
Ni(COD) (5) dppe (5)
2
Since octene is less reactive than styrene and has a lower boiling
point, secondary dimerization reactions were not observed,
resulting in high overall yields (up to 78%). Ongoing work in
our laboratories is aimed at developing a more efficient process
that would address (a) the need for high catalyst loading, (b)
avoiding the use of an anhydride activator, and (c) achieving
high α-selectivity in the decarbonylation of fatty acids.
Ni(COD) (5) dppf (5)
2
Ni(COD) (5) DPEPhos (7.5)
2
Ni(COD) (5) dcype (5)
2
1
1
0
1
Ni(COD) (5) XantPhos (5)
43
10
2
Ni(COD) (1) DPEPhos (1)
2
a
Temperature: 180−190 °C, under a flow of N using a distillation
2
method, 2 h. Octene(s) yield and α-selectivity (1-octene) were
determined by H NMR spectroscopy using 1,3,5-trimethoxybenzene
1
as an internal standard and confirmed by GC-MS.
EXPERIMENTAL SECTION
Materials and Methods. All experiments were carried out in an
inert atmosphere unless otherwise noted. Metal precursors were
purchased from Strem Chemicals or Sigma-Aldrich and used without
■
hydrocinnamic acid, analysis of the mixture did not show the
presence of appreciable amounts of dimers in the reaction
mixture. The nature of the phosphine ligand was observed to
influence 1-octene (α-alkene) selectivity relative to other
further purification. Ni(PPh ) was synthesized according to literature
3 4
2
5
procedures. Pivalic anhydride was purchased from Aldrich and dried
over molecular sieves. Hydrocinnamic acid purchased from Sigma-
Aldrich was recrystallized from dry pentane and sublimed prior to use.
KI was crushed in a mortar and pestle, then dried in an oven at 120 °C
internal alkene isomers (Table 3, entries 2 + 3) using NiI as
2
the catalyst (Figure S21). Use of the Ni(0) catalysts, Ni(PPh3)4
for 3 days prior to use. PPh was recrystallized from ethanol prior to
3
and Ni(COD) , in the presence of added bidentate phosphine
2
drying under vacuum. Bipy was recrystallized from hexane prior to
ligand was found to improve the reaction efficiency (yields 50−
drying under vacuum. TMEDA and N-methylimidazole were dried
2
6
26
8
0%) and 1-octene selectivity (83−94%) significantly. The α-
over CaH and vacuum distilled prior to use. IMes, IPr, phenyl
2
27 28 29
selectivity observed with our Ni(0)/L system is notable relative
bis(imine),
pyridyl bis(imine),
α-diimine,
and bis-
G
DOI: 10.1021/acs.organomet.6b00415
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
3
0
(
aminomethylphenol) ligands were synthesized according to
filtration and the hexane was removed under vacuum. The residual
material was analyzed by GC-MS (Figures S7−S11). To the hexane-
literature procedures. Phenoxyimine and salph ligands were prepared
by condensation of 1 or 2 equiv of 3,5-di-tert-butylsalicylaldehyde,
respectively, and aniline (for phenoxyimine) or o-phenylenediamine
insoluble material was added 125 mL of CHCl with 100 mL of
3
deionized water, and the mixture was allowed to stir for 30 min until
everything dissolved in the chloroform/water phases. The layers were
allowed to separate, and the yellow chloroform layer was washed once
with 100 mL of NaOH (0.23 M), yielding a colorless solution, then
once with 1 M aqueous HCl (20 mL). The chloroform was removed
under vacuum to yield a yellow solid, which was also subjected to GC-
MS analysis (Figures S12−S16). Both the hexane-soluble and hexane-
insoluble GC-MS traces revealed that styrene dimers were the majority
of the components present in the mixture.
(
for salph) in ethanol. All other organic compounds were purchased
from Sigma-Aldrich or Strem and used as received. All GC-MS and
GC experiments were conducted on an Agilent Technologies 7890A
GC system with a 5975C VL MSD and an HP-5 ms silica column (30
m × 0.25 mm) or a Hewlett-Packard 6890 series gas chromatograph
using a flame ionization detector and a Supelco 2-4304 beta-Dex 120
fused silica capillary column (30 m × 0.25 mm i.d., 0.25 μm film
thickness). The standard method (with He as carrier gas) used for all
runs involved an initial oven temperature of 50 °C (held for 2 min)
Mass Balance in the Dehydrative Decarbonylation Reac-
tions. A reaction was carried out using conditions mentioned in Table
2, entry 3, at 0.5 g scale of hydrocinnamic acid. The reaction was
allowed to run for 30 min, by which time distillation had ceased. Both
the distillate and residue from the reaction were analyzed directly by
−1
followed by a 20 °C min ramp to 70 °C (held for 6 min), followed
−
1
by a final 20 °C min ramp to 230 °C (held for 3 min).
High-Throughput Screening. All high-throughput experiments
were performed using a Freeslate Core Module 3 (CM3) under a N2
atmosphere in an MBraun glovebox. Reactions were performed in an
Al rack containing 96 × 1 mL or 48 × 2 mL glass vials. Ligands were
dispensed as stock solutions in THF or as neat liquids, and metal
precursors were dispensed as solids or as solutions or slurries in THF.
The reaction mixtures were allowed to complex for 1 h at room
temperature, and then solvent was removed in vacuo using a
ThermoSavant Speedvac vacuum centrifuge. Equimolar amounts of
1
H NMR spectroscopy using 1,3,5-trimethoxybenzene (1,3,5-TMB) as
an internal standard. The distillate showed exclusively styrene (22%)
and pivalic acid and minute traces of ethylbenzene. The residue was
diluted with CDCl (1 mL), and then an aliquot was analyzed using
3
internal standard. The residue showed residual hydrocinnamic acid
(20%), styrene (3%), and styrene dimer {(E/Z)-but-1-ene-1,3-
diyldibenzene, 20%, E/Z = 7.6}. The saturated dimer, 1,3-
1
hydrocinnamic acid and of Piv O were added robotically. KI was
diphenylbutane, was hard to identify/quantify in the H NMR
2
added via automated solid addition. The plates were sealed and heated
to either 100 or 180 °C and allowed to react for between 30 and 120
min, depending on the plate. At the end of the experiment, the arrays
were removed from the glovebox, cooled, and analyzed for the
presence of styrene using thin-layer chromatography (silica on glass as
stationary phase, hexanes as mobile phase, imaged using UV light) or
GC. TLC visualized with UV light is a very sensitive qualitative
detection method for styrene; even very low amounts of styrene (<1
mg) can be detected.
spectroscopy. By GC-MS, it was observed to be present only to the
same extent as (Z)-but-1-ene-1,3-diyldibenzene. Based on residual
hydrocinnamic acid quantified, the conversion of this reaction was
80%. The identified/quantified components account for ∼70% of the
hydrocinnamic acid used for the reaction.
Mechanistic Studies. Inside a N -filled glovebox to a stirred
2
solution of hydrocinnamic acid (0.020 g, 0.013 mmol) in benzene-d6
(ca. 1 mL) was added pivalic anhydride (0.025 g, 0.013 mmol) and
1,3,5-trimethoxybenzene (0.004 g, 0.002 mmol). The mixture was
transferred to a J-Young NMR tube, brought outside the box, and
Scale-up Reactions under CO/CO /N . In an inert-atmosphere
2
2
1
glovebox, hydrocinnamic acid (1.0 g, 6.6 mmol or 0.50 g, 3.3 mmol),
Piv O (1.35 mL, 6.66 mmol or 0.68 mL, 3.3 mmol), Ni(PPh ) (368
analyzed by H NMR spectroscopy. The NMR tube was set in an oil
bath preheated to 90 °C and then analyzed again after 1 h. Following
2
3 4
mg, 0.333 mmol or 154 mg, 0.139 mmol), and PPh (175 mg, 0.666
this, it was then taken back inside the glovebox, and Ni(PPh ) (0.029
3
3 4
mmol or 77 mg, 0.29 mmol) were added to a Fisher-Porter bottle,
which was sealed and removed from the glovebox. The reactor was
g, 0.002 mmol) was added to it. The reaction mixture was then
allowed to stir for 2 min at room temperature, over which the color of
the solution changed from deep red-brown to a yellow-green. The
mixture was again sealed in the NMR tube and brought outside to be
analyzed by NMR spectroscopy.
pressurized to 80 psi of CO or CO gas or left at ambient pressure
2
under N . The reaction was heated to 180 °C and allowed to react for
2
1.5 h. After the reaction time had finished, the reaction was cooled and
diluted with 10 mL of hexanes. A 200 μL aliquot was taken for GC
analysis and diluted to 1.5 mL using a solution of mesitylene in
hexanes (201.2 mg in 100 mL of hexanes). Yields were determined
from a calibration curve using mesitylene as an internal standard and
are reported in Table 2.
Dehydrative Decarbonylation of Butyric Anhydride. Inside a
N -filled glovebox, Ni(PPh ) (0.029 g, 0.002 mmol) and 1,3,5-
2
3 4
trimethoxybenzene (0.004 g, 0.002 mmol) were dissolved in benzene-
d6 (ca. 0.5 mL). The deep red-brown solution was transferred to a J-
Young NMR tube, and a solution of n-butyric anhydride (0.004 g,
Large-Scale (1 g) Dehydrative Decarbonylation Reactions.
In a nitrogen-filled glovebox, hydrocinnamic acid (1.0 g, 6.7 mmol), Ni
precursor (2−10 mol %), pivalic anhydride (1.35 mL, 6.66 mmol), KI
0.002 mmol) in benzene-d (ca. 0.5 mL) was added. The mixture was
6
agitated for even mixing, and within 5 min the color changed to
yellow-green and it was analyzed by 31P NMR. For isolation of the
Ni(CO)(PPh ) species the reaction was performed using 0.135 g of
(
0−100 mol %), and phosphine ligand (5−20 mol %) were loaded
3
3
into an oven-dried 10 mL round-bottom flask equipped with a Teflon
stir bar. Not all of the reagents were soluble at room temperature,
resulting in a heterogeneous mixture. The round-bottom flask was
capped with a rubber septum, removed from the glovebox, and quickly
attached to an oven-dried short-path distillation apparatus under a flow
of argon gas. The reaction flask was lowered into a 160 to 170 °C oil
bath and allowed to continue to heat until the oil bath reached 180 °C.
Once the reaction mixture reached about 175 °C, it began to bubble,
indicating loss of carbon monoxide. The reaction was allowed to
proceed for the indicated time, during which time a distillate was
collected. After 2 h, heating was ceased and the reaction was exposed
to air. The distillate was analyzed using GC-MS, using mesitylene as an
internal standard (representative GC-MS traces in Figures S5 and S6).
Workup of Reaction Mixture to Isolate Styrene Dimers. To
the reaction with conditions in Table 2, entry 5, was added 200 mL of
hexane to the reaction flask after it had cooled to room temperature to
dissolve any hexane-soluble species. After stirring for 30 min, the
hexane was separated from the hexane-insoluble components by
Ni(PPh ) (0.12 mmol) and 0.019 g of n-butyric anhydride (0.12
3 4
mmol). Ni(CO)(PPh ) crystallized out of the reaction mixture (0.059
3
3
g, 57%).
Dehydrative Decarbonylation Reaction of Nonanoic Acid. In
a nitrogen-filled glovebox, nonanoic acid (0.58 mL g, 3.3 mmol), Ni
precursor (5−10 mol %), pivalic anhydride (0.67 mL, 3.3 mmol), KI
(0−100 mol %), and phosphine ligand (5−20 mol %) were loaded
into an oven-dried 10 mL round-bottom flask equipped with a Teflon
stir bar. Not all of the reagents were soluble at room temperature,
resulting in a heterogeneous mixture. The round-bottom flask was
capped with a rubber septum, removed from the glovebox, and quickly
attached to an oven-dried short-path distillation apparatus under a flow
of argon gas. The reaction flask was lowered into a 160 to 170 °C oil
bath and allowed to continue to heat until the oil bath reached 180 °C.
Once the reaction mixture reached about 175 °C, it began to bubble,
indicating loss of carbon monoxide. The reaction was allowed to
proceed for the indicated time, during which time a distillate was
collected. After 2 h, heating was ceased and the reaction was exposed
H
DOI: 10.1021/acs.organomet.6b00415
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
to air. The distillate was analyzed using H NMR and GC-MS, using
,3,5-TMB as an internal standard. Total alkene content in the
Energy (DE-FG02-05ER15687) for financial support of this
research. The authors acknowledge the Minnesota Super-
computing Institute (MSI) at the University of Minnesota for
providing resources that contributed to the research results
reported within this paper.
1
distillate was estimated by using integrations of the alkene protons
from the terminal and internal octene isomers vs 1,3,5-TMB. α-
Selectivity was determined by comparing the ratio of terminal protons
to internal protons. GC-MS analysis corroborated NMR analysis.
3
1
Computational Methods. All DFT calculations were performed
32
33,34
with Gaussian 09 using the M06-L functional.
was motivated by a survey of a variety of functionals compared to
Choice of M06-L
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AUTHOR INFORMATION
Corresponding Authors
E-mail: lapointe@cornell.edu.
E-mail: coates@cornell.edu.
E-mail: cramer@umn.edu.
E-mail: wtolman@umn.edu.
■
*
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, A.; Millar, S. L.; Prashar, S. Tetrahedron 2005, 61, 9799−
A. John, M. O. Miranda, and K. Ding contributed equally to
the experimental work.
9
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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We thank the National Science Foundation (Graduate
Research Fellowship under Grant No. 00006595 to M.O.M.),
the Center for Sustainable Polymers, a National Science
Foundation supported Center for Chemical Innovation
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