Efficient iridium-catalyzed decarbonylation reaction of aliphatic carboxylic acids leading to internal or terminal alkenes

Article abstract of DOI:10.1021/om1009268

Vaska's complex, IrCl(CO)(PPh₃)₂, when combined with KI as an additive, served as an excellent catalyst for the decarbonylation of long-chain aliphatic carboxylic acids to give internal alkenes with high selectivity. On combination with KI and Ac₂O as additives under controlled temperatures, decarbonylation proceeded to give terminal alkenes with high selectivity.

Full text of DOI:10.1021/om1009268

ARTICLE
pubs.acs.org/Organometallics
Efficient Iridium-Catalyzed Decarbonylation Reaction of Aliphatic
Carboxylic Acids Leading to Internal or Terminal Alkenes
Shinji Maetani,† Takahide Fukuyama,*,† Nobuyoshi Suzuki,‡ Daisuke Ishihara,†,‡ and Ilhyong Ryu*,†
†Department of Chemistry, Graduate School of Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
‡Kao Corporation, Wakayama, Wakayama 640-8580, Japan
S Supporting Information
b
ABSTRACT: Vaska’s complex, IrCl(CO)(PPh3)2, when com-
bined with KI as an additive, served as an excellent catalyst for
the decarbonylation of long-chain aliphatic carboxylic acids to
give internal alkenes with high selectivity. On combination with
KI and Ac2O as additives under controlled temperatures,
decarbonylation proceeded to give terminal alkenes with high
selectivity.
’ INTRODUCTION
’ RESULTS AND DISCUSSION
In conjunction with a rapid consumption of earth’s fossil fuel
reservoirs, innovative solutions for future resources are required.
Recently the importance of oleochemicals has been recognized
for many chemical industries, as they are derived from renewable,
environmentally friendly, easily available, low-toxicity raw
materials.1 Transition-metal-catalyzed decarbonylation of long-
chain aliphatic carboxylic acids, which are abundant in vegetable
oils and animal fats, is a useful tool for alkene synthesis.2 In this
regard, catalytic systems that are based on palladium and
rhodium complexes have been developed for this type of
degradation reaction. For example, Miller and co-workers pre-
viously reported on palladium- and rhodium-catalyzed decarbo-
nylation of aliphatic carboxylic acids, which allowed for selective
isolation of the terminal alkenes via continuous distillation in a
reactor.2d,e Recently, Goossen and co-workers have developed
a catalytic system that uses PdCl2/DPE-phos (DPE-phos = bis-
(2-diphenylphosphinophenyl) ether) and pivalic anhydride to
effectively produce terminal alkenes under mild conditions.2f
Although these previous studies focused on the conversion of
carboxylic acids to terminal alkenes, the potential of the same
system as a means to synthesize internal alkenes is also important
from an industrial viewpoint. A mixture of internal alkenes can be
used as paper-sizing compositions, lubricant oils, and synthetic
intermediates for the surfactant agents, just to name a few.3
Herein, we report that iridium complexes, such as Vaska’s
complex4 when combined with KI as an additive, catalyzed both
the decarbonylation of aliphatic carboxylic acids to give terminal
alkenes and their subsequent isomerization to internal alkenes.5
It was also found that when the iridium catalyst system was
combined with KI and Ac2O as additives, the decarbonylation
proceeded at lower temperature to give terminal alkenes with
high selectivity (Scheme 1).6
We surveyed a variety of iridium catalysts and additives for
decarbonylation of stearic acid (1a), which is present in palm oil,
as a model compound (Table 1). When the reaction was carried
out in the presence of a catalytic amount of [IrCl(cod)]2 ([Ir] = 2
mol %, cod = 1,5-cyclooctadiene) and PPh3 (4 mol %) at 250 °C
for 3 h under solvent-free conditions, decarbonylation proceeded
smoothly to give a mixture of heptadecenes (2a and 3a) in 80%
yield (entry 1). The ratio of the internal alkenes 2a and the
terminal alkene 3a was 94/6, suggesting that a very rapid
isomerization reaction took place in this iridium-catalyzed sys-
tem. Vaska’s complex, IrCl(CO)(PPh3)2, also afforded a 96/4
mixture of 2a and 3a in 68% yield (entry 2). Interestingly, the use
of KI as an additive gave 2a in 91% yield with perfect internal
selectivity (>99%) (entry 3). When IrI(CO)(PPh3)2 was used, a
similar result was obtained (entry 4),7a which suggested that the
addition of KI caused ligand exchange between Cl and I.7 In
contrast, the addition of KBr did not improve the reaction (entry
5). The pronounced effect of KI was also observed when IrCl-
(CO)3/PPh3 and [IrCl(cod)]2/PPh3 were used as the catalyst
(entries 6 and 7). The reaction at 200 °C using either KI or Ac2O
gave poor results (entries 8 and 9); interestingly, however,
addition of both KI and Ac2O improved the product yield to
66% (entry 10). Eventually, by raising the catalyst loading from 2
to 5 mol % and lowering the temperature from 200 to 160 °C,
terminal alkene 3a was obtained in 84% yield with high selectivity
(entry 11). This result suggests that, in our iridium-catalyzed
system, temperature control in the isomerization reaction plays a
critical role in distinguishing the products 2a and 3a. The
reaction in the absence of phosphine ligands using
Received: September 27, 2010
Published: February 22, 2011
r
2011 American Chemical Society
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|

Organometallics
ARTICLE
Scheme 1. Iridium-Catalyzed Synthesis of Terminal or In-
ternal Alkenes via Decarbonylation of Aliphatic Carboxylic
Acids
Table 1. Decarbonylation of Stearic Acid (1a) by Ir Catalysts
Combined with Additives under Various Conditionsa
Figure 1. GC trace of the reaction mixture following addition
of DMDS.
entry
cat.
additive temp (°C) yield, %b 2a/3ac
1
[IrCl(cod)]2/PPh3
IrCl(CO)(PPh3)2
IrCl(CO)(PPh3)2
Irl(CO)(PPh3)2
250
250
250
250
250
250
250
200
200
200
160
200
160
80
68
94/6
2
96/4
3
KI
(91)
87
>99/1
>99/1
90/10
>99/1
98/2
4
Figure 2. Alkene distribution ratio resulting from the decarbonylation
of 1a.
5
IrCl(CO)(PPh3)2
KBr
73
6
[IrCl(cod)]2/PPh3 KI
(86)
(87)
<1
7
IrCl(CO)3/PPh3
IrCl(CO)(PPh3)2
IrCl(CO)(PPh3)2
IrCl(CO)(PPh3)2
IrCl(CO)(PPh3)2
[IrCl(cod)]2
KI
(Figure 2). These results indicate that the distribution of the
double bond in the heptadecenes was random at the internal
positions of the carbon chain. Since it is known that the
disulfide adducts were obtained stereospecifically via anti
addition,8 the E/Z ratio was determined for 2-, 3-, and 4-alkenes
(2-alkene, E/Z = 66/34; 3-alkene, 75/25; 4-alkene, 74/26)
(Figure 1).
8
KI
n.d.
9
Ac2O
12
74/26
20/80
2/98
10
11d
12
13
KI, Ac2O
KI, Ac2O
KI, Ac2O
KI, Ac2O
66
(84)
97
95/5
[IrCl(cod)]2
54
52/48
The iridium-catalyzed decarbonylation reaction that used the
combination of IrCl(CO)(PPh3)2 and KI was tested on various
carboxylic acids 1 at 250 °C. The results are summarized in
Table 2. The decarbonylation of margaric acid (1b, C17), palmitic
acid (1c, C16), pentadecanoic acid (1d, C15), and myristic acid
(1e, C14) gave the expected internal alkenes 2b (C16), 2c (C15),
2d (C14), and 2e (C13), respectively, in high yield and selectivity
(entries 2-5). Similarly, the reaction of arachidic acid (1f, C20)
and behenic acid (1g, C22) gave the C19 internal alkenes 2f and
C21 internal alkenes 2g in 94% and 92% yields, respectively
(entries 6 and 7). The secondary carboxylic acid 2-methylhex-
adecanoic acid (1h) was decarbonylated to give the correspond-
ing internal alkenes 2b in 90% yield (entry 8). 2-Hexyldecanoic
acid (1i) also worked well (entry 9). The reaction of cyclodo-
decanecarboxylic acid (1j) gave cyclododecene (2j) in 84% yield
(E/Z = 69/31, entry 10). Interestingly, the reaction of the R,β-
unsaturated carboxylic acid 2-hexadecenoic acid (1k) gave
pentadecenes (2c), formal decarboxylation products, in 77%
yield with 93/7 selectivity for internal/terminal alkenes (entry
11). Using the same catalyst system, the reaction of 1-naphtha-
lenecarboxylic acid (1l) also proceeded smoothly to give the
decarboxylation product naphthalene (4) (entry 12).9
a Reaction conditions: 1a (0.5 mmol), catalyst (2 mol % as metal), PPh3
(4 mol % for entries 1, 6, and 7), additive (KI or KBr, 20 mol % for entries
3, 5-8, 10, 12, and 13; Ac2O, 100 mol % for entries 9, 10, 12, and 13),
1
160-250 °C, 3 h under nitrogen. b Yield based on H NMR analysis
relative to anisole as an internal standard. Total yields of isomers after
silica gel chromatography are in parentheses. c Selectivity was deter-
mined by 1H NMR analysis. d IrCl(CO)(PPh3)2 (5 mol %), KI (50 mol
%), Ac2O (200 mol %) for 5 h.
[IrCl(cod)]2 also proceeded with good selectivity for 2a at
200 °C but low selectivity at 160 °C (entries 12 and 13).
The double-bond positions were determined by transforming
the C17-alkene mixtures (Table 1, entry 3) into disulfide adducts
according to the established method.8 The adducts were
prepared in a single step: heptadecenes were treated with
dimethyl disulfide (DMDS) in the presence of I2, which served
as the catalyst (eq 1). The resulting adducts were analyzed using
GC and GC-MS (Figure 1). Cleavage of the methylthio-sub-
stituted carbons yielded sets of key fragment ions that revealed
the original positions of the double bonds in the carbon chain.
A mixture of adducts included diastereomers and the adducts
generated at the C2-C8 positions in nearly equal proportion
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ARTICLE
Table 2. Iridium-Catalyzed Degradation Reaction of Various
Carboxylic Acids 1a
Table 3. Synthesis of Terminal Alkenes 3 from Various
Carboxylic Acids 1a
a Reaction conditions: carboxylic acid (0.5 mmol), IrCl(CO)(PPh3)2
(5 mol %), KI (50 mol %), Ac2O (200 mol %), 160 °C, 5 h under
nitrogen. b Total yield of isomers after silica gel chromatography.
c Selectivity was determined by 1H NMR analysis. d Yield based on 1H
NMR analysis relative to anisole as an internal standard.
of 4-phenylbutyric acid (1m) and 5-phenylvaleric acid (1n)
gave 3-phenyl-1-propene (3m) and 4-phenyl-1-butene (3n) in
good yield, respectively, although terminal selectivity was
slightly decreased compared with those of 1b-g. 5-Cyclohex-
ylpentanoic acid (1o) also worked well to give the correspond-
ing terminal alkene 3o. Decarbonylation of sebacic acid
monomethyl ester (1p) gave methyl 8-nonenate (3p), showing
that the ester group was tolerated under these reaction
conditions.
The proposed mechanism for the decarbonylation and iso-
merization of long-chain aliphatic carboxylic acids is shown
in Scheme 2. In the first step, oxidative addition of the C(O)-
O bond of the acid anhydride,10 which is generated in situ from
two molecules of carboxylic acid or carboxylic acid and Ac2O by
condensation,11 gives the acyl iridium complex A. The acyl
iridium complex A then undergoes decarbonylation10a to afford
the alkyl iridium complex B. β-Hydride elimination from B
generates a terminal alkene and the iridium hydride complex
C. Then, the reductive elimination of carboxylic acid from C
regenerates the iridium(I) complex. At the same time, this
iridium hydride C catalyzes isomerization to internal alkenes
a Reaction conditions: carboxylic acid (0.5 mmol), IrCl(CO)(PPh3)2
(2 mol %), KI (20 mol %), 250 °C, 3 h under nitrogen. b Total yield of
isomers after silica gel chromatography. c Selectivity was determined by
1H NMR analysis. d Reaction time 5 h. e E/Z ratio. f GC yield.
We then examined the synthesis of terminal alkenes 3 from
various aliphatic carboxylic acids 1 in the presence of IrCl(CO)-
(PPh3)2, KI, and Ac2O at 160 °C. The results are summarized in
Table 3. The long-chain carboxylic acids 1b-g were decarbo-
nylated to give the corresponding terminal alkenes 3b-g
in good yield and high selectivity (entries 2-7). The reaction
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Organometallics
ARTICLE
Scheme 2. Possible Reaction Mechanism
Scheme 3. Decarbonylation of the Hydroxycarboxylic Acids
1q,r
of pentadecenes (2c). It is conceivable that decarbonylation
of 1q would give the enol 6, which leads to ketone 5. High
temperature might cause partial dehydration of 1q
to give the dehydrated carboxylic acid 1k, which would be
decarboxylated to give 2c. On the other hand, the reaction of
R-hydroxyhexadecanoic acid (1r) gave tetradecane (7), with the
loss of two carbon atoms, in 64% yield, which was estimated by
GC. The formation of 7 is interesting, and we hypothesize that
the first decarbonylation would give a pentadecanal (9) via an
enol (8), which would undergo Ir-catalyzed decarbonylation to
give 7.14
’ CONCLUSIONS
In conclusion, upon the proper choice of reaction conditions,
internal alkenes or terminal alkenes were selectively obtained by
iridium-catalyzed decarbonylation of aliphatic carboxylic acids.
When we applied the reaction system to R- and β-hydroxy-
substituted carboxylic acids, the major reaction pathways di-
verged to give alkanes with two-carbon degradation and ketones
with one-carbon degradation, respectively.
’ EXPERIMENTAL SECTION
General Information. 1H NMR spectra were recorded with a
JEOL JMN ECP-500 (500 MHz) spectrometer in CDCl3 and are
referenced at 7.26 ppm for CHCl3. Chemical shifts are reported in parts
per million (δ). 13C NMR spectra were recorded with a JEOL JMN-500
(125 MHz) spectrometer in CDCl3 and are referenced at 77.00 ppm for
CDCl3. GC analysis was carried out with a Shimadzu GC-2014 instru-
ment equipped with an FID detector using a J&W Scientific DB-5
column and a Hewlett-Packard HP6890 instrument equipped with an
FID detector using a Frontier Lab Ultra-Alloy-1 column. GC-MS analysis
was carried out with a Agilent 6890 equipped with a SGE BPX-35
column. IrI(CO)(PPh3)2 was prepared according to the literature
procedure.1 All other reagents were received from commercial sources
and used without further purification. The products were purified by flash
chromatography on silica gel (Kanto Chem. Co. Silica Gel 60N
(spherical, neutral, 40-50 μm)).
via the alkyl iridium complex D.12,13 In the case of 1k,l, probably
the absence of a β-hydrogen in A would lead to a different course,
hydrolysis by carboxylic acid of A to form an intermediate
analogous to C, which undergoes decarboxylation to form vinyl
and aryl iridium hydride and subsequent reductive elimination to
give 2c and 4, respectively.
We also examined decarbonylation of hydroxycarboxylic acids
(Scheme 3). When the reaction of β-hydroxyhexadecanoic acid
(1q) was performed using a [IrCl(cod)]2/PPh3 ([Ir] = 2 mol %,
PPh3 = 4 mol %) and KI (20 mol %) catalyst system at 250 °C,
2-pentadecanone (5) was formed in 83% yield together with 14%
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ARTICLE
Typical Procedure for Ir-Catalyzed Decarbonylation Reac-
tion of Carboxylic Acids. To a screw-capped test tube were added
stearicacid(142.2mg, 0.5mmol), IrCl(CO)(PPh3)2 (7.8mg, 0.01mmol),
and KI (16.6 mg, 0.1 mmol) and a magnetic stirring bar. The test tube was
evacuated and subsequently flushed with nitrogen. The reaction mixture
was heated at 250 °C by a salt bath (eutectic mixture of inorganic salts:
potassium nitrate 53%, sodium nitrite 40%, and sodium nitrate 7%) for 3 h.
After cooling, the mixture was purifiedby columnchromatography on silica
gel (hexane) to give heptadecenes (108.5 mg, 91%, 2a/3a = >99/1)
Determination of the Double-Bond Position of Heptade-
cenes. To a screw capped test tube were added heptadecenes (2a, 60.3
mg, 0.25 mmol), I2 (29.8 mg, 0.12 mmol), and dimethyl disulfide (1 mL)
and a magnetic stirring bar. The test tube was flushed with nitrogen and
the reaction mixture was stirred for 2 h at room temperature. After the
reaction, 30% NaHSO3(aq) was added to reduce I2, and then the
mixture was extracted with hexane/Et2O (1/1). The organic phase was
subjected to GC (initial temperature 60 °C, initial time 10 min, rate
2 °C/min, final temperature 350 °C, final time 15 min) and GC-MS
(initial temperature 60 °C, rate 2 °C/min, final temperature 300 °C, final
time 5 min) analysis.
Heptadecenes (2a). Obtained as an inseparable mixture (internal/
terminal = >99/1); colorless oil. 1H NMR (500 MHz, CDCl3): δ 5.38-
5.47 (m. 2H), 1.95-2.05 (m, 4H), 1.26-1.38 (m, 22H), 0.87-0.98
(m, 6H).
Hexadecenes (2b). Obtained as an inseparable mixture (internal/
terminal = >99/1); colorless oil. 1H NMR (500 MHz, CDCl3): δ 5.32-
5.47 (m. 2H), 1.95-2.03 (m, 4H), 1.25-1.40 (m, 20H), 0.88-0.97
(m, 6H).
δ 5.77-5.86 (m, 1H), 4.91-5.01 (m, 2H), 1.94-2.06 (m, 2H), 1.25-
1.39 (m, 22H), 0.88 (t, J = 6.9 Hz, 3H).
1-Tetradecene (3d).17. Obtained as an inseparable mixture
(internal/terminal = 4/96); colorless oil. 1H NMR (500 MHz, CDCl3):
δ 5.78-5.86 (m, 1H), 4.92-5.01 (m, 2H), 1.95-2.06 (m, 2H), 1.26-
1.39 (m, 20H), 0.88 (t, J = 6.9 Hz, 3H).
1-Tridecene (3e).18. Obtained as an inseparable mixture (internal/
terminal = 4/96); colorless oil. 1H NMR (500 MHz, CDCl3): δ 5.76-
5.86 (m, 1H), 4.91-5.02 (m, 2H), 2.01-2.09 (m, 2H), 1.26-1.39
(m, 18H), 0.88 (t, J = 6.9 Hz, 3H).
1-Nonadecene (3f).19. Obtained as an inseparable mixture
(internal/terminal = 3/97); colorless oil. 1H NMR (500 MHz, CDCl3):
δ 5.77-5.86 (m, 1H), 4.91-5.01 (m, 2H), 2.02-2.06 (m, 2H), 1.25-
1.39 (m, 30H), 0.88 (t, J = 6.9 Hz, 3H).
1-Henicosene (3g).20. Obtained as an inseparable mixture (internal/
terminal = 2/98); white solid; mp 32.3 °C (lit.21 mp 35.5 °C). 1H NMR
(500 MHz, CDCl3): δ 5.77-5.86 (m, 1H), 4.92-5.01 (m, 2H), 2.02-
2.06 (m, 2H), 1.25-1.39 (m, 34H), 0.88 (t, J = 6.9 Hz, 3H).
Methyl 8-Nonenate (3p).22. Obtained as an inseparable mixture
(internal/terminal = 5/95); colorless oil. 1H NMR (500 MHz, CDCl3):
δ 5.76-5.84 (m, 1H), 4.91-5.01 (m, 2H), 3.66 (s, 3H), 2.30 (t, J = 7.6
Hz, 2H), 2.01-2.08 (m, 2H), 1.59-1.65 (m, 2H), 1.31-1.41 (m, 6H).
2-Pentadecanone (5). White solid; mp 39.0 °C (lit.23 mp 38.9 °C).
1H NMR (500 MHz, CDCl3): δ 2.41 (t, J=7.8 Hz, 2H), 2.13 (s, 3H),
1.55-1.58 (m, 2H), 1.25-1.31 (m, 20H), 0.88 (t, J = 6.9 Hz, 3H). The
NMR data were identical with those of a commercial sample of
2-pentadecanone.
Pentadecenes (2c). Obtained as an inseparable mixture (internal/
terminal = 99/1); colorless oil. 1H NMR (500 MHz, CDCl3): δ 5.32-
5.47 (m. 2H), 1.95-2.03 (m, 4H), 1.25-1.38 (m, 18H), 0.88-0.98
(m, 6H).
Tetradecenes (2d). Obtained as an inseparable mixture (internal/
terminal =99/1); colorless oil. 1H NMR (500 MHz, CDCl3): δ 5.32-
5.47 (m. 2H), 1.95-2.05 (m, 4H), 1.26-1.40 (m, 16H), 0.87-0.98
(m, 6H).
Tridecenes (2e). Obtained as an inseparable mixture (internal/
terminal = >99/1); colorless oil. 1H NMR (500 MHz, CDCl3): δ
5.35-5.45 (m. 2H), 1.96-2.02 (m, 4H), 1.26-1.38 (m, 14H), 0.87-
0.98 (m, 6H).
Nonadecenes (2f). Obtained as an inseparable mixture (internal/
terminal = 99/1); colorless oil. 1H NMR (500 MHz, CDCl3): δ 5.32-
5.41 (m. 2H), 1.96-2.02 (m, 4H), 1.25-1.36 (m, 26H), 0.86-0.98
(m, 6H).
’ ASSOCIATED CONTENT
S
Supporting Information. Text and figures giving experi-
b
mental procedures and characterization data for the compounds.
This material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*To whom correspondence should be addressed. Tel and fax:
þ82 72 254 9695. E-mail: fukuyama@c.s.osakafu-u.ac.jp (T.F.);
ryu@c.s.osakafu-u.ac.jp (I.R.).
’ ACKNOWLEDGMENT
Henicosenes (2g). Obtained as an inseparable mixture (internal/
terminal = >99/1); colorless oil. 1H NMR (500 MHz, CDCl3): δ
5.32-5.45 (m. 2H), 1.95-2.03 (m, 4H), 1.25-1.38 (m, 30H), 0.87-
0.98 (m, 6H).
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas (No. 2105) and a Grant-in-Aid for
Young Scientists from the MEXT.
Cyclododecene (2j).15. Obtained as an inseparable mixture (E/Z =
69/31); colorless oil. E isomer: 1H NMR (500 MHz, CDCl3) δ 5.37 (m,
2H), 2.05-2.06 (m, 4H), 1.40-1.47 (m, 4H), 1.25-1.36 (m, 12H). Z
isomer: 1H NMR (500 MHz, CDCl3) δ 5.37 (m, 2H), 2.05-2.06 (m,
4H), 1.40-1.47 (m, 4H), 1.25-1.36 (m, 12H).
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dx.doi.org/10.1021/om1009268 |Organometallics 2011, 30, 1389–1394