Copper-catalyzed highly selective semihydrogenation of non-polar carbon-carbon multiple bonds using a silane and an alcohol

Article abstract of DOI:10.1002/adsc.201200200

A copper catalyst bearing a suitable Xantphos derivative or NHC ligand was found to be highly efficient for the selective semihydrogenation of non-polar unsaturated compounds using a mixture of a silane and an alcohol as reducing agent. The catalytic system was useful for the selective semihydrogenation of internal alkynes to (Z)-alkenes with suppression of overreduction to the corresponding alkanes. Furthermore, semihydrogenations of terminal alkyne, 1,2-diene, 1,3-diene, 1,3-enyne and 1,3-diyne systems were also achieved selectively. Copyright

Full text of DOI:10.1002/adsc.201200200

FULL PAPERS
DOI: 10.1002/adsc.201200200
Copper-Catalyzed Highly Selective Semihydrogenation of
Non-Polar Carbon-Carbon Multiple Bonds using a Silane and an
Alcohol
Kazuhiko Semba,a Tetsuaki Fujihara,a Tinghua Xu,a Jun Terao,a
and Yasushi Tsujia,*
a
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510,
Japan
Fax : (+81)-75-383-2514; phone: (+81)-75-383-2515; e-mail: ytsuji@scl.kyoto-u.ac.jp
Received: March 12, 2012; Published online: May 15, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200200.
Abstract: A copper catalyst bearing a suitable Xant- pression of overreduction to the corresponding alk-
phos derivative or NHC ligand was found to be
ACHTUNGTRENaNGNU nes. Furthermore, semihydrogenations of terminal
highly efficient for the selective semihydrogenation alkyne, 1,2-diene, 1,3-diene, 1,3-enyne and 1,3-diyne
of non-polar unsaturated compounds using a mixture systems were also achieved selectively.
of a silane and an alcohol as reducing agent. The cat-
alytic system was useful for the selective semihydro- Keywords: alkynes; copper; homogeneous catalysis;
genation of internal alkynes to (Z)-alkenes with sup- reduction
Introduction
uptake may be strictly monitored to prevent the over-
reduction to undesired alkanes. On the other hand,
Copper compounds are highly valuable reagents in or- several homogeneous catalysts showed good selectivi-
ganic synthesis.[1] Among them, copper hydrides, typi- ty in the semihydrogenation of alkynes.[9,10,11] Recent-
cally [CuH
duction of a,b-unsaturated carbonyl compounds.[2b,c] developed.[9] During the course of our studies on
These reductions also can be carried out catalytically copper-catalyzed hydrosilylation of bulky ketones[12a]
ACHTUNGTRENNUNG
(PPh3)]6,[2a,b] are powerful tools for 1,4-re- ly, homogeneous palladium catalysts were intensively
,
using various reducing reagents.[2d,e,3] Especially, si- hydrocarboxylation of alkynes using carbon dioxi-
lanes are widely applied in the catalytic reduction of de[12b] and hydroboration of alkynes[12c] we found that
polar unsaturated bonds such as C=O, C=N, and C=C non-polar carbon-carbon multiple bonds were effi-
conjugated with polar functionalities (viz., Michael ciently semihydrogenated by a homogeneous copper
acceptors).[3] However, it is quite surprising and frus- catalyst. As reported herein, a copper complex bear-
trating that reduction of non-polar carbon-carbon ing a suitable bidentate phosphane or N-heterocyclic
multiple bonds such as simple alkynes cannot be car- carbene (NHC) ligand shows high catalytic activity
ried out catalytically with copper complexes. To date, and excellent selectivity by using a mixture of a silane
there have been six precedents[4] using an excess and an alcohol as a reducing agent.
amount of copper reagents in the reduction of al-
kynes. However, such an important class of transfor-
mations[5] should be performed catalytically.
Results and Discussion
Semihydrogenation of internal alkynes is a crucial
methodology to provide (Z)-alkenes which are often Optimization of the Reaction Conditions
found in many biologically active compounds.[6] Vari-
ous heterogeneous catalysts are effective in this trans- The semihydrogenation of 1-phenyl-1-propyne (1a)
formation.[7] Especially, the Lindlar catalyst[8] is best was carried out at room temperature (Table 1). As
known and most efficient, but it often suffers from a reducing reagent, a mixture of polymethylhydrosi-
Z/E isomerization, low chemoselectivity, and poor re- loxane (PMHS) and t-BuOH was employed. Using
producibility. In the catalytic reactions, hydrogen CuACTHNUTRGEN(UNG OAc)2·H2O as a catalyst precursor without any
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Adv. Synth. Catal. 2012, 354, 1542 – 1550

Copper-Catalyzed Highly Selective Semihydrogenation of Non-Polar Carbon-Carbon Multiple Bonds
Table 1. Semihydrogenation of 1-phenyl-1-propyne (1a) with
various catalysts.[a]
Entry Cu catalyst
Ligand (P/Cu=4.0) Yield [%][b]
(Z)-2a 3a
Figure 1. The structures of Xantphos derivatives.
1
2
3
4
5
6
7
8
Cu
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
(OAc)2·H2O
(OAc)2·H2O
(OAc)2·H2O
(OAc)2·H2O
(OAc)2·H2O
(OAc)2·H2O
G
none
PPh3
PCy3
dppe
dppp
rac-BINAP
dppbz
Xan
Xan
CF3Ar-Xan
MeAr-Xan
t-Bu-Xan
CF3Ar-Xan
CF3Ar-Xan
2
6
9
2
3
8
4
34
75
99
80
2
0
0
0
0
0
0
0
1
9
<1
10
0
0
0
overreduction to alkanes.[15] With a Xantphos deriva-
tive bearing 3,5-xylyl moieties (MeAr-Xan[16]), (Z)-2a
was obtained in 80% yield with considerable forma-
tion of 3a in 10% yield (entry 11). A Xantphos deriv-
ative bearing t-Bu on the phosphorus atoms (t-Bu-
Xan) was not effective at all (entry 12). Thus, the
electron-deficient aryl moieties on the phosphorus
atoms would be preferable. Both PMHS and t-BuOH
are indispensable components in the reducing agent.
When PMHS was removed from the reaction mixture,
no hydrogenation occurred (entry 13). Removing t-
BuOH from the system decreased the yield of (Z)-2a
to 10% (entry 14). PMHS is a by-product of the sili-
con industry, and a cheap, easy-to-handle, and envi-
ronmentally friendly reducing agent.[17] In place of
PMHS in entry 10, other silanes such as (EtO)3SiH,
Ph2SiH2, (EtO)2MeSiH, PhMe2SiH, and Et3SiH af-
forded (Z)-2a in 93%, 82%, 61%, 55%, and 0%
yields, respectively. t-BuOH can be replaced with i-
PrOH and MeOH in entry 10, and (Z)-2a was ob-
tained in 99% and 94% yields, respectively. As for
9[c]
10
11
12
13[d]
14[e]
15
16
Cu
Cu
Cu
Cu
Cu
Cu
0
10
43
54
0
0
CuF
N
CF3Ar-Xan
[a]
1-Phenyl-1-propyne
(0.010 mmol, 2.0 mol%), ligand (P/Cu=4.0), PMHS
(2.0 mmol as the Si H unit, 4.0 equiv.), t-BuOH
(1.0 mmol, 2.0 equiv.), THF (0.50 mL), hexane (0.50 mL),
at room temperature, for 17 h.
GC yields by the internal standard method.
At 658C.
Without PMHS.
Without t-BuOH.
(1a, 0.50 mmol),
Cu
cat.
À
[b]
[c]
[d]
[e]
[f]
catalyst precursors, CuCl/t-BuONa and CuFACHTUNGTRENNUNG(PPh3)3
were not so effective (entries 15 and 16).
To confirm the coordination ability of CF3-ArXan
to copper, we successfully isolated the chlorocop-
per(I) complex with CF3-ArXan as a ligand and the
CuCl (0.010 mmol) and t-BuONa (0.060 mmol).
added ligands resulted in very low conversion of 1a structures of copper(I) chloride complexes with Xan
(entry 1). With added monodentate phosphanes (P/ and CF3-ArXan were determined by X-ray crystallog-
Cu=4.0) such as PPh3 and PCy3,[13] the conversions raphy. The complex bearing a CF3-ArXan was ob-
were also low and the corresponding (Z)-alkene [(Z)- tained as a dimeric form (Figure 2x, a). The copper
2a] was formed only in 6% and 9% yields, respective- atom has a distorted tetrahedral geometry with two
ly (entries 2 and 3). Bidentate phosphanes such as phosphorus atoms and two chlorine atom. Two
dppe, dppp, rac-BINAP and dppbz[13] were not effec- copper atoms were bridged by two chlorine atoms. In
tive in the reaction (entries 4–7). In contrast, Xant- contrast, the complex bearing Xan was obtained as
phos[13] (Xan) as a ligand afforded the product in 34% a monomeric form (Figure 2, b). The copper atom has
yield (entry 8). When the reaction temperature was a trigonal-planar coordination geometry with two
raised to 658C with Xan, the yield of (Z)-2a increased phosphorus atoms and chlorine atom. Generally,
to 75%, but the undesired alkane (3a) via the overre- a bulkier ligand supresses the formation of mutinu-
duction was formed in considerable amounts (9% clear complexes.[18] However, the present results are
yield, entry 9). Gratifyingly, a Xantphos derivative interesting since the bulkier CF3-ArXan gives a dinu-
bearing 3,5-bis(trifluoromethyl)phenyl moieties on clear complex. Electron-deficiency of the ligand
the phosphorus atoms (CF3Ar-Xan[14], Figure 1) was would favor formation of the dimeric structure.
found to be much more effective, giving (Z)-2a in
99% yield without the formation of (E)-2a (entry 10).
This excellent (Z)-selectivity is noteworthy because
such selective semihydrogenation of aromatic alkynes
was often difficult owing to Z/E isomerization and
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Kazuhiko Semba et al.
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Semihydrogenation of Various Internal Alkynes
The scope of the catalytic reaction was examined
using various internal alkynes (1b–x), and the corre-
sponding (Z)-alkenes [(Z)-2b–x] were isolated in high
yields [Eq. (1) and Table 2]. In the reaction of 1-
phenyl-1-hexyne (1b), the (Z)-alkene [(Z)-2b] was
isolated in 92% yield with concomitant overreduction
to hexylbenzene in 2% yield (entry 1). Fortunately,
however, in all the other reactions in Table 2 (en-
tries 2–24), the overreduction to undesired alkanes (3)
did not occur at all. Various functionalities such as hy-
droxy (entry 2), siloxy (entry 3), phthalimido
(entry 4), chloro (entry 5), cyano (entry 6) and vinylsi-
lane (entry 7) were tolerated in the reactions. An
alkyne bearing a thienyl moiety also afforded the cor-
responding (Z)-alkene [(Z)-2i] exclusively (entry 8).
Diaromatic internal alkynes also afforded the corre-
sponding (Z)-alkenes stereoselectively without the
formation of alkanes. Diphenylacetylene (1j) was re-
duced to stilbene (2j, Z/E=98/2) in 93% isolated
yield without formation of bibenzyl (entry 9), while
the conventional Lindlar catalyst afforded a considera-
ble amount of the alkane: selectivity of (Z)-2j/(E)-2j/
bibenzyl=93/2/5.[7d] It is noteworthy that even
0.10 mol% catalyst loading afforded a satisfactory
result at 508C after 79 h (entry 10). Both electron-rich
(entries 12 and 13) and electron-poor (entries 14–19)
Figure 2. The crystal structures of (a) [(CF3-ArXan)CuCl]2
and (b) (Xan)CuCl.
Table 2. Semihydrogenation of various internal alkynes.[a]
Entry 2: Product
Temp. [8C] Yield [%][b] Entry 2: Product
Temp. [8C] Yield [%][b]
(Z/E ratio)
(Z/E ratio)
1[c]
65
65
65
92 (100/0)
13
50
r.t.
65
86 (100/0)
94 (>99/1)
98 (100/0)
2
3
>99 (100/0) 14
98 (100/0)
15
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Copper-Catalyzed Highly Selective Semihydrogenation of Non-Polar Carbon-Carbon Multiple Bonds
Table 2. (Continued)
Entry 2: Product
Temp. [8C] Yield [%][b] Entry 2: Product
(Z/E ratio)
Temp. [8C] Yield [%][b]
(Z/E ratio)
4[d]
65
65
96 (100/0)
90 (100/0)
16[d]
17
65
90 (100/0)
86 (100/0)
5[e]
r.t.
6
65
65
99 (100/0)
72 (95/5)
18
r.t.
65
97 (100/0)
86 (100/0)
7[f]
19[i]
8
50
r.t.
50
88 (100/0)
93 (98/2)
96 (98/2)
20
65
65
65
98 (100/0)
98 (100/0)
78 (100/0)
9[g]
21
10[g,h]
22[e,j]
11
65
50
94 (100/0)
23[k]
65
65
87 (100/0)
83 (100/0)
12
98 (>99/1) 24 L]
[a]
Internal alkyne (0.50 mmol), Cu
(2.0 mmol as the Si H unit, 4.0 equiv.), t-BuOH (1.0 mmol, 2.0 equiv.), solvent [1.0 mL, hexane:THF=1:1 (v/v)], for 20 h.
Isolated yields. In the case of 1c and 1p, 2c and 2p’ were isolated after hydrolysis by adding 1.0M HCl/MeOH.
Hexylbenzene was afforded in 2% yield.
Hexane:THF=1:2 (v/v).
Hexane:THF=10:1 (v/v).
Hexane (1.0 mL).
G
À
[b]
[c]
[d]
[e]
[f]
[g]
[h]
[i]
THF (1.0 mL).
Cu
ACHTUNGTRENNUNG(OAc)2·H2O (0.50 mmol, 0.10 mol%), CF3Ar-Xan (1.0 mmol, 0.20 mol%), for 79 h.
Hexane:THF=3:1 (v/v).
[j]
CuACHTUNGTRENNUNG(OAc)2·H2O (0.020 mmol, 4.0 mol%), CF3Ar-Xan (0.040 mmol, 8.0 mol%).
Cl
[k]
2w (1.0 mmol), ( IPr)CuCl (0.040 mmol, 4.0 mol%), t-BuONa (0.12 mmol, 12 mol%), PMHS (4.0 mmol as the Si H unit,
4.0 equiv.), t-BuOH (2.0 mmol, 2.0 equiv.), hexane (2.0 mL).
[l]
Cl
2x (1.0 mmol), ( IPr)CuCl (0.060 mmol, 6.0 mol%) and t-BuONa (0.18 mmol, 18 mol%), PMHS (4.0 mmol as the Si H
unit, 4.0 equiv.), t-BuOH (2.0 mmol, 2.0 equiv.), hexane (2.0 mL).
diaromatic alkynes afforded the (Z)-alkenes selective- (entry 14) and amide (entry 15) were intact after
ly. Functionalities on the phenyl rings such as ester the reaction, while the acetyl moiety of 4-
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Kazuhiko Semba et al.
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Scheme 1. Competitive reaction of diphenylacetylene (1j) and 5-decyne (1y) employing copper catalysts or the Lindlar cata-
lyst.
CH3CO-C6H4C CC6H5 (1p) was reduced to the corre- Table 3. Semihydrogenation of various terminal alkynes.[a]
ꢀ
sponding hydroxy moiety (2p’, entry 16). To our de-
light, bromo and iodo moieties on the aromatic ring
remained intact (2t and 2u, entries 20 and 21), which
must undergo the oxidative addition reaction with
most low-valent transition metal catalyst centers such
as Pd(0).[9] 6-Dodecyne (1v) was smoothly converted
to the corresponding (Z)-alkene [(Z)-2v] in good
yield with perfect selectivity (entry 22). However,
other internal aliphatic alkynes such as 1w and 1x
were
not
converted
completely
with
the
Cu(OAc)2·H2O/CF3Ar-Xan catalyst system which was
ACHTUNGTRENNUNG
highly efficient in most entries in Table 2. In these
cases, (ClIPr)CuCl/t-BuONa catalyst system (for ClIPr,
see Figure 2) was much more effective, and (Z)-2w
and (Z)-2x were isolated in high yields with perfect
selectivities (entries 23 and 24).
One of the remarkable features of the present
copper-catalyzed semihydrogenation is that diaromat-
ic alkynes can be more easily reduced than dialkyl-
[a]
Terminal alkyne (0.50 mmol), (ClIPr)CuCl (0.010 mmol,
2.0 mol%), t-BuONa (0.060 mmol, 12 mol%), PMHS
ACHTUNGTRENNUNGalkynes (entries 9 vs. 22 in Table 2). Such preferential
À
(2.0 mmol as the Si H unit, 4.0 equiv.), t-BuOH
reduction was confirmed in a competitive reaction
with an equimolar mixture of diphenylacetylene (1j)
and 5-decyne (1y) (Scheme 1). Notabely, 1j was selec-
tively reduced to 2j in the presence of 1y. In sharp
contrast, employing the Lindlar catalyst, non-selective
reduction was observed.
(1.0 mmol, 2.0 equiv-), THF (0.50 mL), hexane
(0.50 mL), at 408C, for 20 h.
[b]
[c]
Isolated yields. The numbers in the parentheses show GC
yields determined by the internal standard method.
CuACHTNURGTNENUG(OAc)2·H2O (0.010 mmol, 2.0 mol%) and CF3Ar-Xan
(0.020 mmol, 4.0 mol%) as the catalyst.
[d]
[e]
[f]
(IPr)CuCl in place of (ClIPr)CuCl.
(
MeIPr)CuCl in place of (ClIPr)CuCl.
Semihydrogenation of Terminal Alkynes
At 508C.
[g]
(
MeIPr)CuCl
(0.12 mmol, 24 mol%), THF (1.0 mL), hexane (1.0 mL).
(0.020 mmol,
4.0 mol%),
t-BuONa
Selective semihydrogenation of terminal alkynes is
known to be difficult due to the overreduction to the
corresponding alkanes.[7b,9e] The Cu
ACTHNUTRGEN(UGN OAc)2·H2O/
CF3Ar-Xan catalyst system, which was efficient for in-
ternal alkynes (Table 2), was not active enough to re-
alize complete conversion of terminal alkynes. Ac-
tually, phenylacetylene (3a) afforded styrene (4a)
only in 13% yield with the CuACHTNUGTRNE(UNG OAc)2·H2O/CF3Ar-
Xan catalyst system (entry 1, Table 3), even overre-
duction to ethylbenzene did not occur at all. For ter-
minal alkynes, NHC ligands such as IPr, MeIPr, and
ClIPr (Figure 3) were more effective than the Xan de- Figure 3. The structures of IPr derivatives.
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Copper-Catalyzed Highly Selective Semihydrogenation of Non-Polar Carbon-Carbon Multiple Bonds
Table 4. Semihydrogenation of conjugated carbon-carbon
Semihydrogenation of Conjugated Carbon-Carbon
Unsaturated Compounds
unsaturated compounds.[a]
Entry Substrate
Cat.[b]/
Temp.
Product
AHCTUNGTRENNUNG
(Yield [%][c])
Besides simple alkynes, conjugated non-polar carbon-
carbon unsaturated compounds such as 1,2-diene, 1,3-
diene, 1,3-enyne and 1,3-diyne systems were selective-
ly semihydrogenated to products (6a–e) in high yields
(Table 4). No overreductions occurred in all the cases
in Table 4. From a 1,2-diene (5a), the terminal alkene
(6a) was selectively afforded in 81% yield (entry 1).
1-Phenyl-1,3-butadiene (5b) provided the correspond-
ing (Z)-alkene [(Z)-6b] exclusively (entry 2).[19] Often
semihydrogenation of 1,3-enynes was not so selective.
As for 5c, considerable overreduction occurred with
the Lindlar catalyst (selectivity: 5c/6c/ethylcyclohex-
ene=6/86/8)[8c] or a complex mixture of various iso-
mers was obtained in transfer semihydrogenation with
an NHC Pd(0) catalyst.[9d] In the present reaction, 5c
and 5d were semihydrogenated to the corresponding
1,3-dienes (6c and 6d) in high selectivities (entries 3
and 4). 1,3-Diynes are also known to be difficult sub-
strates in selective semihydrogenation.[9d] However,
1
2
3
A/658C
B/508C
B/408C
4
B/408C
with the CuACTHNUGTRNEUNG(OAc)2/CF3Ar-Xan catalyst system, a 1,3-
diyne (5e) was reduced to the corresponding 1,3-
5[d]
A/r.t.
enyne [(Z)-6e] exclusively (entry 5).
[a]
Substrate (0.50 mmol), Cat. A or B, PMHS (2.0 mmol as
À
the Si H unit, 4.0 equiv.), t-BuOH (1.0 mmol, 2.0 equiv.),
THF (0.50 mL), hexane (0.50 mL) for 20 h.
Reaction Mechanisms
[b]
Cat. A: Cu
ACHTUNGTRENNUNG(OAc)2·H2O (0.010 mmol, 2.0 mol%), CF3Ar-
To gain insights into the reaction mechanism, a deute-
rium labeling experiment was carried out [Eq. (2)].
Employing non-deuterated PMHS and t-BuOD (98
atom% D) in the semihydrogenation of diphenylace-
tylene (1j), a monodeuterated stilbene (2j-d1, Z/E=
98/2) was selectively formed, and dideuterated stil-
bene (2j-d2) was not detected at all, which was con-
Xan (0.020 mmol, 4.0 mol%). Cat. B: (ClIPr)CuCl
(0.010 mmol,
12 mol%).
2.0 mol%),
t-BuONa
(0.060 mmol,
[c]
[d]
Isolated yields. The numbers in the parentheses show GC
yields determined by the internal standard method.
PMHS (1.0 mmol as the Si H unit, 2.0 equiv.), for 18 h.
À
1
firmed by H NMR (see Figure S1 in Supporting In-
formation) and GC-MS. Furthermore, several stoi-
chiometric reactions relevant to each step in the cata-
rivatives (entries 2–4). Especially, the (ClIPr)CuCl/t- lytic cycle were carried out (Scheme 2). In the reac-
BuONa catalyst system was highly effective to afford tions of (ClIPr)CuCl with t-BuONa[20] and successively
4a in 92% yield without the formation of ethylben- with PMHS,[21] 1H resonances of the reaction mixtures
zene (entry 4). With the conventional Lindlar catalyst, (see Figure S2 in Supporting Information) indicated
considerable overreduction occurred (selectivity: 4a/ that the reactions were very clean and the corre-
ethylbenzene=89/11).[8a] Other aromatic and aliphatic sponding copper hydride, (ClIPr)CuH, was afforded
terminal alkynes were also selectively semihydrogen- quantitatively as shown by the diagnostic 1H reso-
[12b]
ated using (ClIPr)CuCl/t-BuONa (entries 5–9) or nance of Cu H at 2.4 ppm
(step i in Scheme 2).
À
(
MeIPr)CuCl/t-BuONa (entries 10 and 11) catalyst The resulting (ClIPr)CuH easily underwent syn-addi-
system without the overreduction to the correspond- tion[22] to C6H5C C
ACHTUNGTRENNUNG
ꢀ
ing alkanes.
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Kazuhiko Semba et al.
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Scheme 2. Stoichiometric reactions relevant to the mecha-
nism.
Scheme 3. A possible catalytic cycle.
which was isolated in pure form in 70% yield[12b] (step
ii). The isolated 7 smoothly and cleanly reacted with
t-BuOH at room temperature for 1 h and the corre- internal alkynes to the corresponding (Z)-alkenes
sponding (Z)-alkene (2z) was quantitatively afforded with suppression of both Z/E isomerization and over-
1
(step iii) as judged by H NMR (see Figure S3 in the reduction to alkanes.
Supporting Information) and GC-MS analysis.
With these results obtained in Eq. (2) and
Scheme 2, a possible catalytic cycle for the present Experimental Section
copper-catalyzed semihydrogenation of alkynes is
shown in Scheme 3. A copper(I) hydride species (A) General Procedure for Copper-Catalyzed
is generated by the reaction of the catalyst precursors Semihydrogenation of 1a (Table 1)
with a silane. Addition of A to alkynes (1) must be
CuACTHNUTRGNE(UGN OAc)2·H2O (2.0 mg, 0.010 mmol, 2.0 mol%) and a ligand
much faster than to alkenes, and affords a copper al-
kenyl intermediate (B) stereoselectively via syn-addi-
tion[22] (step a). Successively, protonation of B with t-
BuOH provides (Z)-alkenes (2) selectively with the
(P/Cu=4.0) were placed in an oven-dried, 20-mL Schlenk
flask. The vessel was evacuated and backfilled with argon
three times. THF (0.50 mL) and hexane (0.50 mL) were
added, and the mixture was stirred for 15 min at room tem-
perature under an argon atmosphere. To the resulting solu-
tion, 1a (63 mL, 0.50 mmol), PMHS (130 mL, 2.0 mmol as the
concomitant formation of LCuOACHTUNTRGNEUNG(t-Bu) (C) (step b).
Finally, s-bond metathesis between C and a silane re-
generates A and the catalytic cycle is closed (step
c).[21]
À
Si H unit, 4.0 equiv.) and t-BuOH (96 mL, 1.0 mmol,
2.0 equiv.) were added and the mixture was stirred at room
temperature for 17 h. After the reaction, the yield of the
product was determined by GC analysis relative to an inter-
nal standard (tridecane).
Conclusions
General Procedure for Copper-Catalyzed
Semihydrogenation of Internal Alkynes (Table 2) –
CuACHTNUTGRNEG(UN OAc)2·H2O/CF3Ar-Xan as Catalyst
Non-polar unsaturated compounds such as internal
alkyne, terminal alkyne, 1,2-diene, 1,3-diene, 1,3-
enyne, and 1,3-diyne systems were semihydrogenated
selectively. A copper catalyst bearing a suitable Xant-
phos derivative or an NHC ligand was highly efficient
in the semihydrogenation. Especially, the present cat-
CuACHTNURGTNENUG(OAc)2·H2O (2.0 mg, 0.010 mmol, 2.0 mol%) and CF3Ar-
Xan (22 mg, 0.040 mmol, 4.0 mol%) were placed in an oven-
dried, 20-mL Schlenk flask. The vessel was evacuated and
alytic system was useful for the semihydrogenation of backfilled with argon three times. THF and hexane were
1548
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Copper-Catalyzed Highly Selective Semihydrogenation of Non-Polar Carbon-Carbon Multiple Bonds
added, and the mixture was stirred for 15 min at room tem-
perature under an argon atmosphere. Alkyne (0.50 mmol),
PMHS (130 mL, 2.0 mmol as the Si H unit, 4.0 equiv.) and t-
BuOH (96 mL, 1.0 mmol, 2.0 equiv.) were added, and the
mixture was stirred at indicated temperature for 20 h. After
the reaction, isolated yields were determined after purifica-
tion by silica gel column chromatography typically with
a mixture of pentane and CH2Cl2 as eluent.
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À
Using (ClIPr)CuCl/t-BuONa as Catalyst (entry 23,
Table 2)
(ClIPr)CuCl (22 mg, 0.040 mmol, 4.0 mol%) and t-BuONa
ACHTUNGTRENNUNG
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(12 mg, 0.12 mmol, 12 mol%) were placed in an oven-dried,
20-mL Schlenk flask. The vessel was evacuated and backfil-
led with argon three times. Hexane (2.0 mL) was added and
the mixture was stirred for 15 min at room temperature
under an argon atmosphere. The alkyne (1w, 190 mL,
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À
1.0 mmol), PMHS (260 mL, 4.0 mmol as the Si H unit,
4.0 equiv.) and t-BuOH (190 mL, 2.0 mmol, 2.0 equiv.) were
added, and the resulting mixture was stirred at 658C for
20 h. After the reaction, (Z)-2w was isolated by silica gel
column chromatography with pentane as an eluent; yield:
150 mg (0.87 mmol, 87%).
General Procedure for Copper-Catalyzed
Semihydrogenation of Terminal Alkynes (Table 3)
(ClIPr)CuCl (5.6 mg, 0.010 mmol, 2.0 mol%) and t-BuONa
ACHTUNGTRENNUNG
(5.8 mg, 0.060 mmol, 12 mol%) were placed in an oven-
dried, 20-mL Schlenk flask. The vessel was evacuated and
backfilled with argon three times. THF (0.50 mL) and
hexane (0.50 mL) were added, and the mixture was stirred
for 15 min at room temperature under an argon atmosphere.
À
Alkyne (0.50 mmol), PMHS (130 mL, 2.0 mmol as the Si H
unit, 4.0 equiv.) and t-BuOH (96 mL, 1.0 mmol, 2.0 equiv.)
were added, and the resulting mixture was stirred at the in-
dicated temperature for 20 h. After the reaction, isolated
yields were determined after the purification by silica gel
column chromatography with a mixture of pentane and
CH2Cl2 as eluent or Kugelrohr distillation.
Acknowledgements
[8] a) J. Rajaram, A. P. S. Narula, H. P. S. Chawla, S. Dev,
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Li, Synthesis 1973, 457–468; c) E. N. Marvell, J. Tashiro,
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[9] Pd-catalyzed semihydrogenation of alkynes, see: a) R.
Shen, T. Chen, Y. Zhao, R. Qiu, Y. Zhou, S. Yin, W.
Wang, M. Goto, L.-B. Han, J. Am. Chem. Soc. 2011,
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This work was supported by Grant-in-Aid for Scientific Re-
search on Innovative Areas (“Organic synthesis based on re-
action integration” and “Molecular activation directed
toward straightforward synthesis”) from MEXT, Japan, and
in part by the Mitsubishi Foundation. K. S. is grateful to a Re-
search Fellowship of JSPS for Young Scientists. T. F. ac-
knowledges The Naito Foundation Natural Science Scholar-
ship.
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4,5-bis(diphenylphosphino)-9,9-dimethylxan-
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bis(diphenylphosphino)-1,1’-binaphthyl, dppbz: 1,2-di-
phenylphosphinobenzene.
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N
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Adv. Synth. Catal. 2012, 354, 1542 – 1550