(EtO)3SiH-promoted palladium-catalyzed isomerization of olefins: Convenient synthesis of internal alkenes from terminal alkenes

Article abstract of DOI:10.1055/s-0033-1340290

The catalytic activity of an in situ forming palladium catalyst system generated from PdCl₂(PPh₃)₂ and hydrosilane [(EtO)₃SiH] was revealed in the one-carbon migration and isomerization of 4,4-biaryl-substituted 1-butenes, in which the (EtO) ₃SiH plays a key role in the palladium-catalyzed one-carbon migration and subsequent isomerization of terminal alkenes. This catalytic protocol is applied in the synthesis of a key building block of cassumunin C in high yield and promising selectivity. Georg Thieme Verlag Stuttgart. New York.

Full text of DOI:10.1055/s-0033-1340290

LETTER
▌417
letter
(EtO)3SiH-Promoted Palladium-Catalyzed Isomerization of Olefins: Conveni-
ent Synthesis of Internal Alkenes from Terminal Alkenes
Silane-Promoted Palladium-Catalyzed Isomerization
Xing-Feng Bai,a,b Tao Song,a Wen-Hui Deng,a Yun-Long Wei,a Li Li,*a Chun-Gu Xia,b Li-Wen Xu*a,b
a
Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, College of Material, Chemistry and Chemical
Engineering, Hangzhou Normal University, Hangzhou 310012, P. R. of China
Fax +(86)57128868918; E-mail: liwenxu@hznu.edu.cn; E-mail: licpxulw@yahoo.com
b
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics and University of Chinese Academy
of Sciences, Chinese Academy of Sciences, Lanzhou 730000, P. R. of China
Received: 16.09.2013; Accepted after revision: 17.10.2013
Trapp’s11 research groups also demonstrated the catalytic
Abstract: The catalytic activity of an in situ forming palladium cat-
activity of novel palladium catalyst systems for the isom-
alyst system generated from PdCl2(PPh3)2 and hydrosilane
erization of terminal alkenes independently. Notably,
[(EtO)3SiH] was revealed in the one-carbon migration and isomeri-
zation of 4,4-biaryl-substituted 1-butenes, in which the (EtO)3SiH
Wang and coworkers12 reported an ortho-phenol hydroxyl
plays a key role in the palladium-catalyzed one-carbon migration group mediated isomerization of alkenes in the presence
and subsequent isomerization of terminal alkenes. This catalytic
protocol is applied in the synthesis of a key building block of cassu-
munin C in high yield and promising selectivity.
of a palladium and iron-based bimetallic catalyst system,
in which the catalytic isomerization of various 2-(but-3-
enyl)phenol substrates resulted in the formation of conju-
gated arylalkenes in good yields.
Key words: olefin, palladium, isomerization, cassumunin, organo-
silicon
We have recently reported a highly efficient catalytic pro-
tocol for the isomerization of substituted amide-derived
olefins, which was presented successfully using a palladi-
Olefins represent key and basic structural features in nu-
um hydride catalyst system generated from PdCl2(PPh3)2
merous organic compounds and occupy an important po-
and (EtO)3SiH.13 The Z-to-E isomerization was carried
sition in organic synthesis, since it can readily be
out smoothly and resulted in geometrically pure substitut-
transformed into a wide variety of functional molecules.1
ed olefins. This process is simply and can be operated eas-
At present, there are various synthetic methods for the
ily. Although the isomerization mechanism via a
construction of carbon–carbon double bonds, including
palladium(dihydrido)olefin complex in the isomerization
the Wittig reaction,2 Peterson olefination,3 or Takai reac-
of substituted amide-derived olefins required some direct
tion.4 In addition, the isomerization of olefins shares the
evidence, we suggested that the in situ generated palladi-
importance of olefination and is a generally essential route
um hydride complex from PdCl2(PPh3)2 and (EtO)3SiH
in the clean synthesis of E-alkenes.5 The catalytic isomer-
was the key catalyst in this reaction.13 Herein, we wish to
ization of alkenes to form a single isomer of alkenes is
contribute to this important field of research by applica-
both a 100% atom efficient reaction and an extremely use-
tion of this catalyst system to the controllable one-carbon
ful process, either for the preparation of starting materials
migration of terminal alkenes to 2-alkenes. We chose
in the flavor and fragrance industry, or for stereocon-
4,4-biaryl-substituted but-1-enes as benchmark substrates
trolled rearrangement of functionality along the carbon
since they are high-value molecules. More interestingly,
chain.6 Nowadays, transition-metal catalysis exhibited ex-
the hydrosilane play a key role in the isomerization but not
cellent properties in olefin isomerization, in which diverse
in the reduction of terminal olefins. In addition, this cata-
processes with various transition metals (e.g., Pd, Rh, Pt,
lytic protocol could produce a convenient route to the key
Ir, or Ni) for olefin isomerization are known.7 Despite the
building block of cassumunin C.
popularity of ruthenium-based systems,8 which are known
Initially,
to
extend
the
utility
of
the
to be effective catalysts in the isomerization of terminal
alkenes, the use of palladium catalysts presents an attrac-
tive alternative. In this regard, Skrydstrup and Lindhardt
have reported recently that the application of an in situ
generated bulky palladium hydride catalyst obtained from
Pd(dba)2, P(t-Bu)3, and i-PrCOCl provided an efficient
protocol for the cis–trans isomerization and migration of
allylic or terminal alkenes.9 Very recently, Bercaw’s10 and
PdCl2(PPh3)2/(EtO)3SiH catalyst system and with the aim
of establishing a more easily accessible and active system
for the migration–isomerization reaction of terminal al-
kenes than those previously reported, we proceeded to
evaluate its activity in a model isomerization–migration
reaction of 2-(1-phenylbut-3-enyl)phenol (1a). As shown
in Table 1, the catalytic transformation of olefin 1 could
lead to three different products 2a–c because of reduction,
one-carbon migration, and two-carbon migration of the
carbon–carbon double bond as well as isomerization. Un-
der the reaction conditions reported previously13
[PdCl2(PPh3)2 (1 mol%), (EtO)3SiH (1 equiv), THF] the
SYNLETT 2014, 25, 0417–0422
Advanced online publication: 04.12.2013
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9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1340290; Art ID: ST-20136-W0884-L
© Georg Thieme Verlag Stuttgart · New York

418
X.-F. Bai et al.
LETTER
conversion of compound 1a was almost completed. Prod- of 2-(1-phenylbut-3-enyl)phenol (1a). Fortunately, when
uct 2a resulting from the one-carbon migration of the car- the amount of (EtO)3SiH was reduced to 20 mol%, both
bon–carbon double bond and the reduced compound 2c the conversion and chemoselectivity were excellent, and
were obtained in a ratio of 79:21 (Table 1, entry 1). Al- 2a was obtained as sole product in excellent yield (93%
though the reaction result is not good enough in selectivi- isolated yield) from the one-carbon migration of the car-
ty, this preliminary result revealed that the bon–carbon double bond (Table 1, entry 7). However, a
PdCl2(PPh3)2/(EtO)3SiH catalyst system could promote lower amount of (EtO)3SiH resulted in decreased yield
the migration and isomerization of a terminal alkene to an (Table 1, entry 8). Different solvents were also screened,
internal alkene. As expected, 1a almost cannot be isomer- such as 1,2-dichloroethane and ethanol. 1,2-Dichloroeth-
ized to the corresponding compound 2a, 2b, or 2c. Only ane also provided a good result in terms of conversion and
2a was detected in poor conversion (15%, Table 1, entry chemoselectivity, but 1,2-dichloroethane and ethanol ex-
2), which further supported the crucial role of hydrosilane hibited inferior compared to THF (Table 1, entries 9 and
in this isomerization reaction. Interestingly, the use of 10 for DCE and THF, respectively). Encouraged by these
commercial available palladium salts [PdCl2 and results, we continued to investigate the activity of the pal-
Pd(OAc)2] instead of PdCl2(PPh3)2 resulted in complicat- ladium catalyst in the presence of other hydrosilanes, such
ed products, and compound 2c obtained from reduction as Et3SiH and poly(methylhydrosiloxane) (PMHS).14 Un-
was the major product (Table 1, entries 3 and 5). Compa- like (EtO)3SiH, trace of product 2a was observed in the
rably, PdCl2 was an effective catalyst in this isomerization presence of Et3SiH (Table 1, entry 11). The PMHS-
reaction in the absence of (EtO)3SiH. However, the che- promoted palladium-catalyzed one-carbon migration–
moselectivity of the present reaction was not good (Table isomerization reaction of 1a undergoes in promising con-
1, entry 4).
version (69%, Table 1, entry 12), unfortunately, the che-
moselectivity was not good due to the low ratio of 2a/2c
(ca. 2:1). In summary, the expected internal alkene 2a was
formed in an E/Z ratio of 80:20 under the optimized reac-
For further optimization of reaction conditions, we inves-
tigated the effect of the amount of (EtO)3SiH on the
PdCl2(PPh3)2-catalyzed isomerization–migration reaction
Table 1 Optimization of the Palladium-Catalyzed Isomerization–Migration Reaction of 2-(1-Phenylbut-3-enyl)phenol (1a)a
OH
OH
OH
OH
Pd (1 mol%)
hydrosilane ( x equiv)
+
+
THF
1a
2a
2b
2c
Entry
Pd catalyst
Silane
(EtO)3SiH
x
Conversion (%)
2a/2b/2c (%)b
1
2
PdCl2(Ph3P)2
PdCl2(Ph3P)2
PdCl2
1.0
0
>99
15
79:0:21
15:0:0
d
–
c
3
(EtO)3SiH
1.0
0
–
2c >50
56:44:0
2c >80
59:0:22
>99:0:0
>99:0:0
94:6:0e
59:0:0f
3:0:0
d
4
PdCl2
–
>99
c
5
Pd(OAc)2
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
Et3SiH
1.0
0.5
0.2
0.1
0.2
0.2
0.2
0.2
–
6
PdCl2(Ph3P)2
PdCl2(Ph3P)2
PdCl2(Ph3P)2
PdCl2(Ph3P)2
PdCl2(Ph3P)2
PdCl2(Ph3P)2
PdCl2(Ph3P)2
81
>99
70
7
8
9
>99
59
10
11
12
trace
69
PMHS
48:0:21
a Reaction conditions: terminal olefin 1 (0.5 mmol), Pd catalyst (1 mol%), hydrosilane (x equiv), solvent (e.g., THF, 1 mL), 60 °C, 10 h.
b The selectivities were determined by GC and 1H NMR analysis.
c The conversion is not determined because unidentified byproducts were formed and the selectivity could not be detected by GC–MS and NMR
analysis under these reaction conditions.
d No use of hydrosilanes and any additives.
e Solvent: DCE.
f Solvent: EtOH.
Synlett 2014, 25, 417–422
© Georg Thieme Verlag Stuttgart · New York

LETTER
Silane-Promoted Palladium-Catalyzed Isomerization
419
tion conditions [PdCl2(PPh3)2 (1 mol%), (EtO)3SiH (20 of 2-(1-phenylbut-3-enyl)phenol (1a). It was also shown
mol%), 60 °C, THF].
that the position of the substituted phenol group has slight
influence on the Z/E selectivity of the isomerization reac-
tion but the ortho-phenol hydroxyl group was beneficial
to the one-carbon migration of the carbon–carbon double
bond. Interestingly, the further two-carbon migration of
the carbon–carbon double bond did not occur during the
isomerization of 4,4-diarylbut-1-enes, in which it is dif-
ferent from that of palladium-iron-catalyzed isomeriza-
tion of terminal alkenes.12
With the optimized reaction conditions in hand, the scope
of the regioselective (EtO)3SiH-mediated PdCl2(Ph3P)2-
catalyzed one-carbon migration–isomerization of the car-
bon–carbon double bond of various 4,4-diarylbut-1-enes
was evaluated. As shown in Scheme 1, all of the substitut-
ed 4,4-diarylbut-1-enes tested in this work afforded the
desired products 3 though one-carbon migration of the
carbon–carbon double bond and further isomerization in
good to excellent yields (up to 94% isolated yield). Inter- In view of internal olefin as a salient structural feature in
estingly, various substituted alkenes with a methyl group many bioactive natural products, we next turned our atten-
or a fluorine atom at the ortho position of the aryl rings, tion to the construction of such backbone. For example,
instead of a hydroxyl group, also gave the desired prod- cassumunins that contain internal olefin backbones are a
ucts in good isolated yields (81–88%) with a high E/Z ra- family of curcuminoids, and their activity was shown to
tio (e.g., 86:14 for 3a and 83:17 for 3k). When 4-(1- be stronger than curcumin.15 In the past years, total syn-
phenylbut-3-enyl)phenol containing a phenol group at the theses of cassumunin A and cassumunin B have been
para position was used as substrate in this reaction, the achieved by Masuda et. al.16 However, the synthesis of
yield of the desired product 3e was slightly lower than that
Pd(Ph3P)2Cl2 (1 mol%)
(EtO)3SiH (20 mol%)
R1
R2
R1
R2
THF, 60 °C, 10 h
1
2a, 3a–m
OH
OMe
OMe
2a
93% yield
E/Z = 80:20
3a
88%yield
E/Z = 86:14
3b
3c
92% yield
E/Z = 88:12
92% yield
E/Z = 84:16
Cl
OH
F
OMe
3d
94% yield
E/Z = 83:17
3e
3f
76% yield
E/Z = 84:16
74% yield
E/Z = 87:13
MeO
OMe Cl
OMe
OMe
3g
3h
3i
74% yield
E/Z= 84:16
68% yield
E/Z = 76:24
81% yield
E/Z = 84:16
OMe
F
OMe
OMe
OMe
3j
3k
3l
3m
82% yield
E/Z = 77:23
83% yield
E/Z = 85:15
82% yield
E/Z = 83:17
90% yield
E/Z = 89:11
Scheme 1 Regioselective (EtO)3SiH-mediated PdCl2(Ph3P)2-catalyzed isomerization–migration reaction of various substituted 4,4-diarylbut-1-enes
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 417–422

420
X.-F. Bai et al.
LETTER
cassumunin C that has both antioxidant and anti-inflam- Initially, the commercially available o-vanillin (6) was
matory activities has not been reported up now.
used as the starting material in the protection of 6 with
TBSCl in DCE, in which the desired product 7 was ob-
tained in high yield (95%). Then 1,2-nucleophilic addition
of (3,4-dimethoxyphenyl)lithium to 7 proceeded smooth-
ly at –78 °C in THF. Deprotection of 8 by TBAF (1.0
equiv) for 0.5 hours led to the formation of 2-[(3,4-dime-
thoxyphenyl)(hydroxy)methyl]-6-methoxyphenol. And
then, the introduction of an allylic group to this molecule
was carried out through a FeCl3-catalyzed allylation with
allyltrimethylsilane,17 in which the important intermediate
4 was obtained in promising yield. The next task was the
conversion of compound 4 into the desired product 5. The
optimized conditions for isomerization of compound 4
were PdCl2(PPh3)2 (1 mol%) and (EtO)3SiH (20 mol%) in
THF (Scheme 3, step e), which afforded the desired prod-
uct successfully in 92% yield. Although the pure E isomer
of 5 has not been obtained through this procedure (92%
isolated yield with an E/Z ratio up to 81:19), the applica-
tion of this hydrosilane-mediated palladium-catalyzed mi-
gration–isomerization protocol provide a simple and
efficient strategy for the synthesis of an internal alkene
from a terminal alkene (Scheme 3).
In this work, we tried to apply the palladium-catalyzed
isomerization reaction to the construction of the key inter-
mediate of cassumunin C. The natural product cassu-
munin C is equipped with a 4,4-diarylbut-2-ene, therefore,
the molecule (E)-2-[1-(3,4-dimethoxyphenyl)but-2-
enyl]-6-methoxyphenol (5) could be a key intermediate
framework of cassumunin C (Scheme 2). In this synthetic
scenario, we hypothesized that the catalyst system of
PdCl2(Ph3P)2 and (EtO)3SiH could promote the isomeri-
zation of 2-[1-(3,4-dimethoxyphenyl)but-3-enyl]-6-meth-
oxyphenol (4) to the synthesis of compound 5 through
one-carbon migration and isomerization of the carbon–
carbon double bond.
O
O
MeO
HO
OMe
OMe
OH
OMe
Although the primary aim of this work is to disclose this
new methodology for the migration–isomerization of ter-
minal alkenes and the true mechanism was not clear at
present, the possible mechanism is shown in Scheme 4 on
the basis of experimental results. This mechanism in-
volves the formation of palladium hydride complexes via
a repeated olefin addition and a β-hydride elimination.13,18
In our proposed mechanism of the palladium-catalyzed
one-carbon migration–isomerization of olefins 1, an equi-
librium between intermediate II and palladium catalyst
(palladium hydride or palladium dihydride complex I),
possibly generated in situ from PdCl2(Ph3P)2 and
(EtO)3SiH, in which the hydride addition of the palladium
hydride complex I to terminal alkenes through a four-
membered transition state led to the formation intermedi-
ate II. Then β-hydride elimination occurred to complete
the one-carbon migration and further cis-trans isomeriza-
tion of the carbon–carbon double bond (Scheme 4). Nota-
bly, the conversion of 2-alkenes into 3-alkenes did not
occur under the reaction conditions, possibly owing to the
cassumunin C
OMe
OHC
OMe
OMe
OH
OMe
OH
OMe
OMe
5
OMe
OH
OMe
OMe
4
Scheme 2 The possible synthetic strategy for cassumunin C through
catalytic isomerization of allylic intermediate 4
OTBS OH
OH
6
OTBS
MeO
OMe
OMe
MeO
CHO
a
MeO
CHO
b
95%
69%
7
8
OH
OH
MeO
OMe
OMe
MeO
OMe
OMe
c, d
e
40% for the
two step
92%
4
5
E/Z = 80:20
Scheme 3 Synthesis of the key building block of cassumunin C. Reagents and conditions: (a) TBSCl (1.5 equiv), Et3N (2.0 equiv), DMAP
(5 mol%); (b) THF, (3,4-dimethoxyphenyl)lithium (1.2 equiv), –78 °C to r.t.; (c) TBAF, r.t.; (d) FeCl3 (5 mol%), allyltrimethylsilane (1.5
equiv), DCE, 50 °C; (e) PdCl2(Ph3P)2 (1 mol%), (EtO)3SiH (20 mol%), THF, 60 °C, 12 h.
Synlett 2014, 25, 417–422
© Georg Thieme Verlag Stuttgart · New York

LETTER
Silane-Promoted Palladium-Catalyzed Isomerization
421
(EtO)3SiH
PdCl2(PPh3)2
[Pd]-H
R1
R2
I
H
H
TM
[Pd]
R1
R2
R1
R2
H
II
1
H
[Pd]
R1
R2
transition state
Scheme 4 Plausible reaction mechanism
weak activity of palladium hydride complex I in the hy-
dride addition with 2-alkenes. It is also a further indication
for this mechanism involving a hydride addition–elimina-
tion procedure instead of a π-allylic pathway.19
References and Notes
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dium catalyst system, generated from PdCl2(PPh3)2 and
(EtO)3SiH, is a very useful catalyst for the one-carbon mi-
gration and subsequent isomerization of 4,4-biaryl-substi-
tuted 1-butenes, in which the hydrosilane [(EtO)3SiH]
plays a key role in the palladium-catalyzed isomerization
of terminal alkenes. Various terminal alkenes tested in
this work afforded the desired products though one-car-
bon migration of the carbon–carbon double bond and sub-
sequent isomerization in good to excellent yields (up to
94% isolated yield) and good diastereoselectivity (E/Z ra-
tio is up to 89:11).20 Furthermore, this method provides a
convenient access to the key building block of natural
product cassumunin C in high yield and good diastereose-
lectivity (E/Z ratio = 4:1). Further studies on expanding
the substrate scope with higher diastereoselectivity and
exploring their applications in the synthesis of high value
molecules are ongoing.
Acknowledgment
Financial support by the National Natural Science Foundation of
China (NSFC Grant No. 21173064), Zhejiang Provincial Natural
Science Foundation of China (ZPNSFC, Q12B020037), and Pro-
gram for Excellent Young Teachers in Hangzhou Normal Universi-
ty (HNUEYT, JTAS 2011-01-014) is appreciated. XLW thank Dr.
W.F. Chen of Hangzhou Normal University for helpful discussions.
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© Georg Thieme Verlag Stuttgart · New York
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4,4-Diarylbut-1-enes (0.5 mmol), (EtO)3SiH (0.1 mmol,
18.5 μL, 0.2 equiv) and a catalytic amount of Pd(Ph3P)2Cl2
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mixture was solved with dry THF (1 mL). The resulting
mixture was stirred at 60 °C for 10 h. The mixture was then
cooled to r.t., quenched by the addition of H2O, and
extracted by the addition of CH2Cl2. The organic layer was
washed with H2O, dried over Na2SO4, and concentrated. The
residue was purified by flash column chromatography to
give corresponding 4,4-diarylbut-1-enes. All the products
were confirmed by GC–MS, NMR and IR spectrosopy.
These data are provided in the Supporting Information.
Analytical Data for 1-Methyl-2-(1-phenylbut-2-
enyl)benzene (3a)
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1H NMR (400 MHz, CDCl3): δ = 7.32 (t, J = 6.8 Hz, 2 H),
7.23 (J = 6.8 Hz, 3 H), 7.14 (d, J = 6.8 Hz, 2 H), 6.87 (d, J =
6.8 Hz, 2 H), 5.94 (dd, J = 13.6, 7.6 Hz, 1 H), 5.74–5.68 (m,
0.14 H), 5.47 (dq, J = 14.8, 6.4 Hz, 0.88 H), 5.02 (d, J = 9.6
Hz, 0.14 H), 4.66 (d, J = 7.2 Hz, 0.86 H), 3.81 (s, 3 H), 1.77
(d, J = 6.4 Hz, 3 H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 158.0, 144.6, 136.5, 133.9, 129.5, 128.5, 128.3, 126.7,
126.1, 113.8, 55.3, 53.3, 18.0 ppm. GC–MS: m/z calcd for
C17H18 [M]+: 222.3; found: 222.1. IR (KBr): νmax = 3412,
3059, 3025, 3000, 2957, 2931, 2853, 2835, 1653, 1608,
1510, 1493, 1448, 1302, 1250, 1178, 1150, 1034, 971, 830,
700, 560 cm–1.
Synlett 2014, 25, 417–422
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