Highly active and selective semihydrogenation of alkynes with the palladium nanoparticles-tetrabutylammonium borohydride catalyst system

Article abstract of DOI:10.1002/adsc.200900721

Palladium nanoparticles are prepared from palladium(II) acetate and 2 equivalents of potassium tert-butoxide in the presence of 4-octyne. The palladium nanoparticles-tetrabutylammonium borohydride system shows excellent catalytic activity and selectivity in the semihydrogenation of alkynes to the [(Z)-]alkenes. The hydrogenation of 4-octyne is conducted with the catalyst system at a substrate-to-palladium molar ratio of 10,000-200,000 under 8 atm of hydrogen to give (Z)-4-octene in > 99% yield. Isomerization and over-reduction of the Z-alkene are very slow even after consumption of the alkyne.

Full text of DOI:10.1002/adsc.200900721

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DOI: 10.1002/adsc.200900721
Highly Active and Selective Semihydrogenation of Alkynes with
the Palladium Nanoparticles-Tetrabutylammonium Borohydride
Catalyst System
Junichi Hori,b Kunihiko Murata,b Toshiki Sugai,c,d Hisanori Shinohara,c
Ryoji Noyori,c Noriyoshi Arai,a Nobuhito Kurono,a and Takeshi Ohkumaa,*
a
Division of Chemical Process Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628,
Japan
Fax : (+81)-11-706-6598; e-mail: ohkuma@eng.hokudai.ac.jp
Central Research Laboratory, Technology and Development Division, Kanto Chemical Co., Inc., Soka, Saitama 340-0003,
b
Japan
c
Department of Chemistry, Graduate School of Science and Research Center for Materials Science, Nagoya University,
Chikusa, Nagoya 464-8602, Japan
Present address: Department of Chemistry, Faculty of Science, Toho University, 2-2-1 Miyama, Funabashi 274-8510, Japan
d
Received: August 20, 2009; Revised: October 16, 2009; Published online: December 8, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900721.
Abstract: Palladium nanoparticles are prepared
from palladium(II) acetate and 2 equivalents of po-
tassium tert-butoxide in the presence of 4-octyne.
The palladium nanoparticles-tetrabutylammonium
borohydride system shows excellent catalytic activi-
ty and selectivity in the semihydrogenation of al-
kynes to the [(Z)-]alkenes. The hydrogenation of 4-
octyne is conducted with the catalyst system at a
substrate-to-palladium molar ratio of 10,000–
200,000 under 8 atm of hydrogen to give (Z)-4-
octene in>99% yield. Isomerization and over-re-
duction of the Z-alkene are very slow even after
consumption of the alkyne.
yield in the best cases. However, most of the hydroge-
nation catalysts have an inherent drawback: The cata-
lytic hydrogenations must be strictly stopped when
one equivalent of H2 is absorbed, because over-reduc-
tion of the [(Z)-]alkene and isomerization occur after
consumption of the alkyne substrate with the same
catalysts. The heterogeneous Pd/C,[4d] Pd clusters/phe-
nanthroline/TiO2,[5c] Ni2B,[6d] and Pd/BER[4h] catalysts
[8a]
and the homogeneous [RuH{PACHTNURTGNE(UNG CH3)2C6H5}5]PF6
and [(arene)Cr(CO)3][11] were reported as exceptional
catalysts which do not hydrogenate and/or isomerize
the olefinic products in the absence of acetylenic sub-
strates. However, the reactivity of these catalysts, for
which the turnover number (TON) is <100, is not
enough for practical use.
Keywords: (Z)-alkenes; alkynes; hydrogenation;
nanoparticles; palladium
In recent years, Pd nanoparticles (Pd NPs) have
been utilized as catalysts for the hydrogenation of un-
saturated organic compounds.[5,12] The cluster consist-
ing of a limited number of Pd atoms seems to have a
homogeneous surface, which can be modified to a
The semihydrogenation of internal alkynes to the (Z)- “uni-functionalized catalyst” that suppresses unde-
alkenes and terminal alkynes to the alkenes is an effi- sired side reactions. We here describe the rapid and
cient and fundamental transformation in synthetic or- selective semihydrogenation of alkynes to the [(Z)-]-
ganic chemistry.[1] The importance of the olefin forma-
ACHTUNGRTENGNUNalkenes with a novel Pd NPs-(n-C4H9)4NBH4 catalyst
tion is increased by its combination with metathesis system. The reaction was conducted under 8 atm of
chemistry.[2] The development of a highly active and H2 with a substrate-to-palladium molar ratio (S/Pd) of
chemo- and stereoselective catalyst for this reaction 10,000–200,000 to afford the desired alkenes in >99%
would be highly desirable from a practical point of content at 100% conversion in the best cases. The
view.[3] Heterogeneous catalysts based on Pd[4,5] and average turnover frequency (TOFav, defined as S/Pd
Ni[6] metals as well as homogeneous Rh,[7] Ru,[8] and per time-to-full conversion) reached up to
Pd[9,10] catalysts have been studied extensively for this 12,500 minÀ1. Furthermore, isomerization and over-re-
purpose. The desired olefins are obtained in >99% duction of the [(Z)-]alkene products were very slow,
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Junichi Hori et al.
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6.5 ppm, suggesting formation of the alkyne oligo-
mers. Mass spectroscopic measurements of 1a pre-
pared from 3-hexyne instead of 4-octyne showed sig-
nals at m/z=246 and 328, indicating the presence of
hexaethylbenzene (trimer) and octaethylcyclooctate-
1
traene (tetramer). The H and 13C NMR signals of Pd
NPs prepared in the presence of di-tert-butylacety-
lene, which is not readily oligomerized, instead of 4-
octyne are the same as those of the acetylene itself,
showing that the interaction between the particles
and the acetylene is rather weak and reversible.
Scheme 1. Preparation of Pd NPs 1.
even after consumption of the alkyne substrates These particles aggregated after 2 to 3 weeks. These
under the pressurized H2 conditions. results suggested that Pd NPs 1 are stabilized by
The Pd NPs (1) were prepared by the reaction of alkyne molecules and the oligomeric compounds.
Pd(OCOCH3)2 (98.3 mg, 0.438 mmol) and 2 equiva- The catalyst efficiency of Pd NPs 1a was examined
ACHTUNGTRENNUNG
lents of t-C4H9OK (1a),[13] NaBH4 (1b), HCO2H (1c), in the semihydrogenation of 4-octyne (2a) (Figure 2).
or HCO2NH4 (1d) with 10 equivalents of 4-octyne A stock solution of 1a in DMF (10.0 mM) was used in
(483 mg, 4.38 mmol) in DMF (43.8 mL) at 20–258C portions for each experiment. A mixture of alkyne 2a
for 12 h (see Scheme 1 and the Experimental Sec- (1.10 g, 10.0 mmol), THF (10 mL), and 1a (10.0 mM
tion). Unsupported Pd NPs are known to readily ag- DMF solution, 50 mL, S/Pd=20,000) in a 100-mL
gregate, but NPs 1 in DMF (10.0 mM) were homoge- glass autoclave with a sampling needle connected to a
neously dispersed without observation of light scatter- stop valve was vigorously stirred (1000 rpm) at 308C
ing even after 1-year storage at ambient temperature. under 8 atm of H2. Aliquots were taken from the re-
The TEM image of 1a in DMF and the histogram are action mixture and analyzed by GC. Figure 2 shows
shown in Figure 1. The particles with a relatively typical reaction profiles. With its high catalytic activi-
small size distribution have the highest population in ty, 1a achieved 85% conversion of 2a in 2 min, and
the histogram at 1.8 nm diameters. The XRD pattern then it was quantitatively consumed by 5 min. The re-
suggested that particles 1a are face-centered cubic action selectively gave (Z)-4-octene [(Z)-3a] in 97%
(fcc) Pd(0) with an average size of 1.6 nm.[14]
content [content=(yield of (Z)-3a)/(conversion of
The homogeneous features of 1a in DMF suggested 2a)] until ca. 90% conversion of 2a; however, the con-
that 4-octyne and/or its derivatives stabilized the NPs tent was gradually decreased to 87% in 5 min (100%
1
to prevent them from aggregating. H NMR measure- conversion), and 81% in 10 min. The isomerization
ments of 1a prepared in DMF-d7 showed broad and compounds (E)-3a, 2-octene, and 3-octene, and over-
weak signals in the region d=0.7–1.2, 2.5, and 5.5– reduced octane (4a) were observed as by-products.
Figure 1. TEM image of Pd NPs 1a with a histogram for the particle size.
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Highly Active and Selective Semihydrogenation of Alkynes
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the reaction without the additive. Thus, alkyne 2a was
consumed in 5 min, affording (Z)-3a in 99.7% con-
tent. A content of 99.4% was observed after reaction
for 10 min. This means that the borohydride additive
selectively inhibited the hydrogenation and isomeriza-
tion of the Z-alkene, but did not affect the alkyne re-
action.[15]
Both amount and concentration of (n-C4H9)4NBH4
affected the reaction rate and yield of the Z-alkene.
The TOFav and yield of (Z)-3a at full conversion in
the hydrogenation of 2a catalyzed by 1a with various
amounts and concentrations of the borohydride are
shown in Table 1. The reaction with an S/Pd=2000 re-
quired a (n-C4H9)4NBH4/Pd ratio of >2.5 or [(n-
C4H9)4NBH4] >0.63 mM to obtain a yield of (Z)-3a
of >98.5% (entries 1–3). On the other hand, a (n-
C4H9)4NBH4/Pd ratio of <5.0 or [(n-C4H9)4NBH4]
<1.25 mM was required to maintain a high reaction
rate (entries 1–3). The reaction with (n-C4H9)4NBH4/
Pd=2.5 and [(n-C4H9)4NBH4]=0.25 mM gave a (Z)-
3a yield of 99.0% (entry 4), but the yield was de-
creased to 96.4% under the conditions of (n-
C4H9)4NBH4/Pd=1.0 and [(n-C4H9)4NBH4]=0.63 mM
(entry 5). An even higher (Z)-alkene yield of 99.3%
was obtained at (n-C4H9)4NBH4/Pd=5.0 and [(n-
C4H9)4NBH4]=0.63 mM (entry 6). However, a lower
yield of 97.9% was observed in the reaction under (n-
Figure 2. Effect of a borohydride additive on hydrogenation
of 2a. : % conversion of 2a without additive; : % conver-
~
*
*
sion of 2a with 10 equivalents of (n-C4H9)4NBH4 to Pd;
:
~
% content of (Z)-3a without additive; : % content of (Z)-
3a with 10 equivalents of (n-C4H9)4NBH4 to Pd. The content
of (Z)-3a is defined as [yield of (Z)-3a]/(conversion of 2a).
This result shows that the use of Pd NPs 1a alone C4H9)4NBH4/Pd=5.0 and [(n-C4H9)4NBH4]=0.21 mM
cannot inhibit isomerization and over-reduction of the conditions (entry 7). A high (Z)-alkene yield of
Z-alkene.
99.7% was again obtained with (n-C4H9)4NBH4/Pd=
This problem was overcome by addition of catalytic 10.0 and [(n-C4H9)4NBH4]=0.42 mM (entry 8). These
amounts of (n-C4H9)4NBH4 to the reaction system. results suggested that the (n-C4H9)4NBH4/Pd ratio of
When 2a was hydrogenated with 1a in the presence of >2.5 (preferably >5.0) and [(n-C4H9)4NBH4]=ca.
(n-C4H9)4NBH4 {S/Pd/ACHTNUTRGNEU(NG n-C4H9)4NBH4 =20,000:1:10, 0.5 mM conditions are crucial to achieve a high reac-
[(n-C4H9)4NBH4]=0.42 mM} under otherwise identi- tion rate and (Z)-alkene yield. (n-C4H9)4NBH4 ap-
cal conditions, isomerization and over-reduction of peared to reversibly interact with the Pd NP surface
(Z)-3a were almost completely suppressed (Figure 2). to avoid reaction of the Z-alkene, although the mech-
It is worth noting that the rate of the alkene hydroge- anism of the additive effect is unclear.[16]
nation was only slightly diminished relative to that of
Table 1. Semihydrogenation of 2a with the 1a-(n-C4H9)4NBH4 system.[a]
Entry
n-(C4H9)4NBH4/Pd
E
S/Pd[b]
TOFav [minÀ1][c]
(Z)-3a [%][d]
1
2
1.0
2.5
5.0
2.5
1.0
5.0
5.0
10.0
0.25
0.63
1.25
0.25
0.63
0.63
0.21
0.42
2000
2000
2000
2000
800
4000
20000
20000
700
700
70
1000
400
800
4000
4000
91.2
98.8
98.7
99.0
96.4
99.3
97.9
99.7
3
4[e]
5
6
7
8
[a]
Unless otherwise stated, reactions were conducted at 308C using 4.0–10.0 mmol of 2a in THF (10.0–19.2 mL) containing
1a and (n-C4H9)4NBH4.
[b]
[c]
[d]
[e]
Substrate/Pd molar ratio.
S/Pd per time-to-full conversion.
Yield of (Z)-3a at full conversion.
[2a]0 =0.20M.
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Junichi Hori et al.
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stabilizers of NPs, and aluminum hydrides were not
effective as additives.[12e]
The novel Pd NPs 1a-(n-C4H9)4NBH4 catalyst
system was applied to the semihydrogenation of a ter-
minal alkyne, 1-pentyne (5a) (Figure 4). Addition of a
greater amount of (n-C4H9)4NBH4 {5a/Pd/
ACHTUNGTRENNUNG(n-
C4H9)4NBH4 =20,000:1:100, [(n-C4H9)4NBH4]=
4.2 mM} than that used in the reaction of internal
alkyne 2a was required to get 1-pentene (6a) in high
yield. The reaction in THF at 308C under 8 atm H2
was completed in 7.5 min (TOFav =2700 minÀ1) with
98.1% content of 6a. The high alkene content of
94.1% was maintained after 30 min reaction. The re-
action without the additive under otherwise identical
conditions was relatively slow and less selective. The
reaction was completed in 25 min with 93.2% content
of 6a. The alkene content was then quickly decreased
to 67.9% in 5 min after the complete conversion of
the alkyne. The addition of (n-C4H9)4NBH4 efficiently
accelerated the hydrogenation of alkyne 5a, and sup-
pressed the reaction of alkene 6a at the same time.
A series of simple and functionalized internal al-
kynes 2 was hydrogenated to give the Z alkenes (Z)-3
selectively (Table 2). The reactions were conducted
with the Pd NPs 1 and (n-C4H9)4NBH4 [2/1/ACHTUNGTRENNUNG(n-
C4H9)4NBH4 =10,000–20,000:1:10] in THF at 308C
under 8 atm H2. Hydrogenation of 4-octyne (2a) was
completed in 5 min to give the Z-alkene in 99.7%
yield. Diphenylacetylene (2c), which is a difficult sub-
strate to hydrogenate cis-selectively, was converted to
*
Figure 3. Competitive hydrogenation of 2b and (Z)-3a. : %
~
conversion of 2b without additive; : % conversion of 2b
with 25 equivalents of (n-C4H9)4NBH4 to Pd; : % conver-
sion of (Z)-3a without additive; : % conversion of (Z)-3a
*
~
with 25 equivalents of (n-C4H9)4NBH4 to Pd.
Figure 3 shows the profile of competitive reaction
of 3-hexyne (2b) and (Z)-3a. A 1:1 mixture of 2b and
(Z)-3a in THF was hydrogenated using NPs 1a with
(n-C4H9)4NBH4 {2b/Pd/ACHTUNTGRNEUNG(n-C4H9)4NBH4 =25,000:1:25,
[(n-C4H9)4NBH4]=0.44 mM, 8 atm H2, 308C}. The
alkyne 2b was consumed in 2 min (TOFav =
12,500 minÀ1), and (Z)-3a was converted at a rate of
only 0.03%. The conversion of (Z)-3a after reaction
for 30 min was 0.78%. The ratio of the reaction rate
between alkyne 2b and alkene (Z)-3a was estimated
to be >3000:1. The hydrogenation without (n-
C4H9)4NBH4 addition under otherwise identical con-
ditions resulted in the lower reactivity and selectivity.
After reaction for 40 min, the conversion of 2b and
(Z)-3a were 100% and 5.4%, respectively. The ratio
of the reaction rates was 18:1.
The catalyst efficiency of 1b–1d was basically the
same as that of 1a. Other borohydrides, including
LiBH4, NaBH4, and KBH4 showed additive effects
similar to that of (n-C4H9)4NBH4, whereas BH3·THF,
Figure 4. Effect of a borohydride additive on hydrogenation
of 5a. : % conversion of 5a without additive; : % conver-
~
*
*
sion of 5a with 100 equivalents of (n-C4H9)4NBH4 to Pd;
:
~
% content of 6a without additive; : % content of 6a with
100 equivalents of (n-C4H9)4NBH4 to Pd. The content of 6a
LiBACHTUNGTRENNUNG(C2H5)3H, and NaBH3CN had no effect at all. Tet-
raalkylammonium salts, which are known as effective is defined as (yield of 6a)/(conversion of 5a).
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Table 2. Semihydrogenation of alkynes 2 and 5.[a]
Yield [%]
E-Alkene
[b]
Alkyne
1
S/Pd/
G
TOFav [minÀ1][c]
Z-Alkene
Alkane
2a
2c
2d
2e
2f
2g
2h
2i
1a
1a
1a
1b
1d
1a
1a
1c
1a
1a
20000:1:10
10000:1:10
10000:1:10
10000:1:10
10000:1:10
10000:1:10
10000:1:10
10000:1:10
20000:1:100
10000:1:100
4000
200
100
99.7
98.0
92.0
91.5
96.6
99.8
99.6
99.6
98.1[g]
93.7[g]
0.3
trace[d]
0
1.7
4.3
1.1
0.1
0
0
0.2
6.3
2.0
5.7[e]
4.2
300
1000
1000
5000
3300
2700
300
1.9[f]
0.1
0.4
0.4
[h]
5a
5b
–
–
[a]
Unless otherwise stated, reactions were conducted at 308C using 10.0 mmol of alkyne in THF (10.0 mL) containing 1 and
(n-C4H9)4NBH4.
Substrate/Pd/ACHTUNGTRENNUNG(n-C4H9)4NBH4 molar ratio.
[b]
[c]
[d]
[e]
[f]
S/Pd per time-to-full conversion.
Trace amount of octane and 3-octene were observed.
Regio-isomers were observed in 0.6% yield.
Regio-isomers were observed in 0.4% yield.
Yield of terminal alkenes 6.
[g]
[h]
Regio-isomers were observed in 1.7% yield.
(Z)-3c in 98.0% yield.[17] The reaction of 1-(trimethyl- completed in 55 min to give (Z)-3a in 99.2% yield
silyl)-1-hexyne (2e) gave (Z)-3e in 92.0% yield with (TOFav =3600 minÀ1; see the Supporting Information).
(E)-3e (5.7%) and the alkane (1.7%). The bulkiness
In summary, we have reported the highly reactive
of the trimethylsilyl group appeared to cause the high and selective semihydrogenation of alkynes to the
content of the E-alkene. The Z-alkene yield of hydro- [(Z)-]alkenes that is catalyzed by a new Pd NPs-(n-
genation of a propargylic alcohol 2e was 91.5%, but C4H9)4NBH4 system. The reaction of 4-octyne can be
this yield was increased to 96.6% by using the acetate conducted with an S/Pd ratio as high as 200,000:1
2f as a substrate. The reaction of a propargylic amine under 8 atm of H2 to give (Z)-4-octene in >99% con-
2g gave (Z)-3g in 99.8% yield. A homopropargylic al- tent at complete conversion. A TOFav of 12,500 minÀ1
cohol 2h and the acetate 2i were smoothly converted is achieved in the best cases. It is noteworthy that
to (Z)-3h and (Z)-3i in 99.6% yield. Aliphatic and ar- over-reduction as well as isomerization of the alkenes
omatic terminal alkynes, 5a and 5b, were also hydro- are efficiently inhibited even after consumption of the
genated with 1a and (n-C4H9)4NBH4 [5/Pd/ACTHNUGTRNEUNG(n- alkyne substrates under H2 pressure. The inhibition
C4H9)4NBH4 =10,000–20,000:1:100] in THF at 308C effect is seriously decreased in the absence of (n-
under 8 atm H2 to afford the alkenes 6a and 6b in C4H9)4NBH4. The reaction mechanism is currently
98.1% and 93.7% yield, respectively.
being studied.
The highly catalytically active Pd NPs 1a-(n-
C4H9)4NBH4 system achieved a TON of 200,000 in
the hydrogenation of 2a (2.21 g, 20.0 mmol) in THF
(20 mL) with 1a (1.0 mM DMF solution, 100 mL, S/
Pd=200,000) and (n-C4H9)4NBH4 {2.6 mg, 10 mmol,
Experimental Section
Preparation of Pd NPs 1a
(n-C4H9)4NBH4/Pd=100,
[(n-C4H9)4NBH4]=
0.50 mM} at 308C under 8 atm H2. The reaction was
Pd
ACHTUNGRTEN(UNNG OCOCH3)2 (98.3 mg, 0.438 mmol) was placed in a 50-mL
Schlenk flask. After replacing the air in the flask with
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Junichi Hori et al.
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argon, degassed DMF (43.8 mL) was added, and the mixture
was stirred at 258C for 10 min to give a deep orange solu-
tion. To this solution were added 2a (483 mg, 4.38 mmol)
and t-C4H9OK (98.3 mg, 0.876 mmol), and the mixture was
stirred at 258C for 12 h, resulting in a dark brown solution
of 1a (10.0 mM) which was used as a stock catalyst solution.
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Typical Procedure for the Hydrogenation of 2a
Alkyne 2a (1.10 g, 10.0 mmol), THF (10 mL), (n-
C4H9)4NBH4 (1.29 mg, 5.0 mmol), and 1a (10.0 mM in DMF,
50 mL, 0.50 mmol; S/Pd=20,000:1) were placed in a 100-mL
glass autoclave equipped with a Teflon-coated magnetic stir-
ring bar. The vessel was placed into a water bath controlled
at 308C. Hydrogen was introduced into the autoclave at a
pressure of 8 atm, and the reaction mixture was vigorously
(1000 rpm) stirred for 5 min. After careful venting of the hy-
drogen gas, the reaction mixture was analyzed by GC:
column, CP Sil PONA CB (0.25 mmꢁ100 m, film thick-
ness=0.50 mm, Varian); carrier gas: helium (228 kPa);
column temperature: 658C; injection temperature: 2308C;
retention time (tR) of (Z)-3a: 29.9 min (99.7%), tR of (E)-3a:
29.0 min (0.3%), tR of 3-octene: 29.5 min (trace), tR of 2-
octene: 30.8 min and 32.6 min (0%), tR of octane: 30.0 min
(trace).
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Acknowledgements
This work was partly supported by a Grant-in-Aid from the
New Energy and Industrial Technology Development Organ-
ization (NEDO) (Support Program for Technology Develop-
ment on the Basis of Academic Findings).
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t-C4H9OK with moisture probably reduced the
À
Pd
N
[16] Metal complex–BH4 interaction has been observed;
odor of amine. This system could provide the reductant
gradually.
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also fcc Pd(0) with average sizes (nm) as follows: 1b:
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3149