Stereodivergent ruthenium-catalyzed transfer semihydrogenation of diaryl alkynes

Article abstract of DOI:10.1002/chem.201003662

[Ru₃(CO)₁₂]-catalyzed transfer semihydrogenation of various functionalized diaryl alkynes with N,N-dimethylformamide (DMF) and water as hydrogen source affords cis- and trans-stilbenes. The stereodivergent approach can be switched by the use of acetic (HOAc) or trifluoroacetic (TFA) acid as additives. The catalytic processes can be applied to the synthesis of analogues of natural products such as cis-combretastatin A-4 and trans-resveratrol. Copyright

Full text of DOI:10.1002/chem.201003662

DOI: 10.1002/chem.201003662
Stereodivergent Ruthenium-Catalyzed Transfer Semihydrogenation of Diaryl
Alkynes
Jie Li and Ruimao Hua*[a]
Abstract: ACHTUNGTRENNUNG[Ru3(CO)12]-catalyzed transfer semihydrogenation of various functional-
ized diaryl alkynes with N,N-dimethylformamide (DMF) and water as hydrogen
source affords cis- and trans-stilbenes. The stereodivergent approach can be
switched by the use of acetic (HOAc) or trifluoroacetic (TFA) acid as additives.
The catalytic processes can be applied to the synthesis of analogues of natural
products such as cis-combretastatin A-4 and trans-resveratrol.
Keywords: alkynes · alkenes · hy-
drogenation · ruthenium · stereose-
lectivity
Introduction
efficient transfer semihydrogenation of internal alkynes
catalyzed by Pd(OAc)2 through the use of N,N-dimethylfor-
AHCTUNGTRENNUNG
Alkenes are not only a very important and fundamental
class of compounds for organic transformations, but also
occur in many natural products. As exemplified in
Scheme 1, cis-combretastatin A-4[1] and trans-resveratrol,[2]
mamide (DMF) and potassium hydroxide as hydrogen
source to afford cis-alkenes in high yields with excellent ste-
reoselectivity.[4] With the aim of developing stereocontrolla-
ble semihydrogenation of internal alkynes, we herein report
our new finding of a novel and practical [Ru3(CO)12]-cata-
lyzed transfer semihydrogenation of internal alkynes using
DMF/H2O as hydrogen source under acidic conditions to
afford trans- and cis-alkenes in a stereochemically controlled
manner in which the geometry of the product can be
switched by the use of acetic (HOAc) or trifluoroacetic acid
(TFA) (Scheme 2).
Scheme 1. Structures of cis-combretastatin A-4 (left) and trans-resvera-
trol (right).
as well as their analogues, have been reported to have a va-
riety of interesting physiological and biological properties.
Although there are various methods to access to trans- or
cis-alkenes, the semihydrogenation of internal alkynes is the
simplest and most straightforward approach, because the
transfer semihydrogenation of internal alkynes through the
use of hydrogen donors has clear advantages for safe opera-
tion and high control of the chemoselective formation of al-
kenes. Thus, many attempts have been made to develop effi-
cient catalyst systems that can achieve such transfer semihy-
drogenation reactions.[3] Recently, we have developed an
Scheme 2. Stereodivergent ruthenium-catalyzed transfer semihydrogena-
tion of diarylalkyne.
Results and Discussion
The choice of [Ru3(CO)12] as catalyst for the transfer semi-
hydrogenation of internal alkynes was based on the known
ability of low-valent ruthenium complexes to catalyze trans-
fer hydrogenation of carbonyl compounds,[5] alkenes,[6] and
imines.[7] After screening the reaction conditions for the
transfer semihydrogenation of diphenylacetylene (1a), it
was found that [Ru3(CO)12] (3.0 mol%) showed high cata-
lytic activity in the presence of triphenylphosphane (PPh3;
5 mol%), HOAc (1.5 equiv), and H2O (2.0 equiv) in DMF.
After 24 h heating at 1458C, stilbenes were isolated in 93%
yield with a Z/E ratio of 96:4, implying that the transfer
semihydrogenation occurred with excellent stereoselectivity
for the formation of the cis-isomer (2a; Table 1, entry 1). In
the absence of either PPh3 (conv. 72%, Z/E ratio 90:10) or
[a] J. Li, Prof. R. Hua
Department of Chemistry, Tsinghua University
Key Laboratory of Organic Optoelectronics
& Molecular Engineering of Ministry of Education
Beijing 100084 (P. R. of China)
Fax : (+86)10-62771149
E-mail: ruimao@mail.tsinghua.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003662. It contains the
general experimental procedure, characterization data, and copies of
1H and 13C NMR spectra of all products, including those of the inter-
mediate [Ru2(CO)4(m-CH3COOÀ)
2ACHTNUTRGNE(UNG PCy3)2].
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FULL PAPER
Table 1.
[Ru3(CO)12]-catalyzed transfer semihydrogenation of diarylal-
Table 2.
ACHTNUGTRNEN[NUG Ru3(CO)12]-catalyzed transfer semihydrogenation of diarylal-
kynes affording (Z)-stilbenes.[a]
kynes affording (E)-stilbenes.[a]
Alkyne
Alkene
Yield Z/E[c]
Alkyne
1a
Stilbene
Yield [%][b]
E/Z[c]
>99:1
>99:1
[%][b]
1
2
91
93
1
93
40
84
96:4
>99:1
97:3
1c
2[d]
3[e]
3
4
1e
1g
88
87
>99:1
4
5
92
93
>99:1
>99:1
97:3
[d]
5
1h
1i
85
86
–
6[e]
98:2
6
7
95
92
>99:1
7
89
>99:1
95:5
[a] Reaction conditions: alkyne (0.5 mmol), TFA (0.75 mmol), H2O
(1.0 mmol), PPh3 (0.025 mmol), [Ru3(CO)12] (0.005 mmol), DMF
(1.0 mL), sealed tube, 1458C, 3 h, under N2. [b] Total isolated yield.
[c] Determined by GC analysis of the reaction mixture. [d] No Z product
was isolated. [e] Reaction time was 6 h.
8
9
96
98
99:1
75:25
[a] Reaction conditions: alkyne (0.5 mmol), HOAc (0.75 mmol), H2O
(1.0 mmol), PPh3 (0.025 mmol), [Ru3(CO)12] (0.015 mmol), DMF
(1.0 mL), sealed tube, 1458C, 24 h, under N2. [b] Total isolated yield.
[c] Determined by GC analysis of the reaction mixture. [d] Conversion
was 44% (GC analysis). [e] Conversion was 87% (GC analysis).
(1.0 mol%), PPh3 (5.0 mol%), TFA (1.5 equiv), and H2O
(2.0 equiv), 1a underwent transfer semihydrogenation at
1458C completely within only 3 h (Table 2, entry 1). Dif-
ferent diarylated alkenes could be obtained in high yield
under similar reaction conditions with excellent trans-ste-
reoselectivity (Table 2, entries 2–7).
HOAc (conv. 39%, Z/E ratio 77:23), both the conversion of
1a and the stereoselectivity of the transfer semihydrogena-
tion were unsatisfactory.
As summarized in Tables 1 and 2, the present stereodiver-
gent ruthenium-catalyzed transfer semihydrogenation of in-
ternal alkynes occurs with excellent chemoselectivity. For
À
With the aim of extending the transfer semihydrogenation
to other internal alkynes, various substrates were examined
under the same reaction conditions. As shown in Table 1, al-
though the reductive reaction of 5-decyne (1b) produced
cis-5-decene in only 40% isolated yield with 44% conver-
sion of 1b (Table 1, entry 2), diaryl alkynes with a variety of
substituents on the aromatic rings underwent the transfer
semihydrogenation smoothly to afford the corresponding
stilbenes in excellent yields (Table 1, entries 3–9). With the
exception of 1h, which showed relative low stereoselectivity,
probably due to the existence of the coordinative thienyl
group (Table 1, entry 9), all diaryl alkynes underwent trans-
fer semihydrogenation with excellent stereoselectivity to
afford cis-isomers with ratios higher than 96%.
When the effect of organic acids on the catalytic activity
was examined, it was surprising to find that the use of TFA
instead of HOAc not only resulted in an acceleration in the
rate of transfer semihydrogenation, but also switched the
stereochemical behavior to afford the trans-isomer predomi-
nantly. As shown in Table 2, in the presence of [Ru3(CO)12]
example, the C O bond of the benzyl ether in 1e was not
reductively cleaved, nor was concomitant double bond shift
isomerization observed in 2b. Furthermore, the carbonyl
group in 1g was tolerated. In the case of 1,4-bis(phenylethy-
nyl)benzene (1h), both the Z,Z isomer (2h; Table 1,
entry 8) and the E,E isomer (3h; Table 2, entry 5) were ste-
reoselectively formed in excellent yield.
The synthetic usefulness of the present stereodivergent
transfer semihydrogenation was further developed in the ef-
ficient synthesis of analogues of cis-combretastatin A-4 and
trans-resveratrol. As shown in Equations (1) and (2), under
two different standard reaction conditions, the transfer semi-
hydrogenation of 1-(4-methoxyphenyl)-2-(3,4,5- trimethoxy-
phenyl)acetylene (1k) afforded the corresponding alkenes
in high yields with the formation of cis-isomer 2k with 93%
selectivity, and trans-isomer 3k in 98% isomeric purity, re-
spectively.
Moreover, because the p-cyclohexyl-substituted stilbenes
are potential liquid crystalline compounds,[8] the transfer
semihydrogenation of 1-[4-(4-trans-n-propylcyclohexyl)-
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J. Li and R. Hua
HCOOÀ)
2ACHTNUTRGNE(UGN PPh3)2] was recently reported by Wills and co-
workers,[11c] the mechanism of the present stereodivergent
transfer semihydrogenation of diaryl alkynes requires fur-
ther clarification.
The selective formation of trans-stilbenes through the use
of TFA was then considered in more detail. According to
the following experimental results, the formation of trans-
stilbenes could arise from isomerization of cis-stilbenes
under suitable reaction conditions: 1) In a separate experi-
ment, the transfer semihydrogenation of 1a under similar
reaction conditions to those indicated in entry 1 of Table 2
for 1 h, resulted in 50% conversion of 1a and a 2a/3a ratio
of 3:1, however, the final selectivity for 3a increased to
more than 99% after 3 h (Table 2, entry 1), indicating that
isomerization of 2a to 3a also occurred during the transfer
semihydrogenation of 1a; 2) Product 2a could be converted
into 3a when subjected to the reaction conditions given for
entry 1 of Table 2 [Eq. (4)]; 3) As shown in Equation (4), in
the absence of [Ru3(CO)12], TFA did not promote the iso-
merization of 2a to 3a at 1458C within 3 h.
phenyl]-2-(4-ethyl)acetylene (1l) under the reaction condi-
tions given in Table 2 was carried out; the reaction afforded
the expected trans-isomer (3l) in 93% isolated yield
[Eq. (3)].
It should be noted that although there were some reports
on the cis-to-trans isomerization of stilbene under acidic
conditions, such as in the presence of acidic zeolites[13] or
H2SO4 (50–60%),[14] a series of careful experiments con-
firmed that, in DMF at 1458C, the isomerization of 2a to 3a
did not occur in the presence of TFA in the absence of
[Ru3(CO)12]. The isomerization of cis- to trans-alkenes cata-
lyzed by a ruthenium complex has been also reported re-
cently by Cho et al.[15]
DMF is not only a commonly used polar solvent, it has
also been used as a versatile precursor under different reac-
tion conditions in organic transformations.[9] In the present
transfer semihydrogenation, formic acid (HCOOH), gener-
ated in situ from the hydrolysis of DMF,[10] is considered to
be the hydrogen source.[11] The reaction of 1a under similar
reaction conditions to those given in Table 1 but in the ab-
sence of water, resulted in low conversion of 1a (26% by
GC), indicating the presence of water is crucial for the semi-
Conclusion
hydrogenation. In addition, use of [Ru(CO)2ACHTNUGTRENUNG(PPh3)3]
(5 mol% under the reaction conditions given in Table 1),
which is a mononuclear Ru0 complex that is possibly formed
under the reaction conditions, as an appropriate precatalyst
showed low catalytic activity and stereoselectivity, affording
stilbenes in 37% GC yield with a 2a/3a ratio of 74:26.
Therefore, attempts were made to isolate the active catalytic
species from the reaction mixtures. Fortunately, a dirutheni-
um bis(m-acetate) species, [Ru2(CO)4(m-CH3COOÀ)2-
We have developed an efficient [Ru3(CO)12]-catalyzed trans-
fer semihydrogenation of diaryl alkynes with DMF/H2O as a
hydrogen source. The stereoselectivity of cis- and trans-stil-
benes can be easily achieved simply by the use of HOAc or
TFA as additives. Application of HOAc selectively favors
the production of cis-alkenes, and the use of TFA favors the
formation of trans-alkenes. The observation that cis-stilbene
forms in a higher ratio during the early stages of the semihy-
drogenation in the [Ru3(CO)12]/TFA catalytic system, and
that the isomerization of cis- to trans-stilbene occurs in the
presence of [Ru3(CO)12] or [Ru3(CO)12]/TFA, suggests that
the formation of trans-stilbene arises from the isomerization
of cis-stilbene. It is very clear that the [Ru3(CO)12]/HOAc
ACHTUNGTRENNUNG(PCy3)2], was isolated from the catalytic reaction mixture
when PCy3 was used instead of PPh3; as expected, this diru-
thenium complex showed catalytic activity for the transfer
semihydrogenation of 1a.[12] Although the formation of hy-
drogen from the reaction of HCOOH with [Ru2(CO)4ACTHNUGRTNEUNG(m-
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Semihydrogenation of Diaryl Alkynes
FULL PAPER
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Experimental Section
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Acknowledgements
This project (20573061) was supported by the National Natural Science
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given in Table 1, the transfer semihydrogenation of 1a also occurred
smoothly to afford cis-2a and trans-3a in 97% yield with a ratio of
87:13. After the reaction mixture was cooled to room temperature,
[Ru2(CO)4(m-CH3COOÀ)
2ACHTNUTRGNE(UNG PCy3)2] could be obtained as crystals in
56% yield (relative to ruthenium). This compound showed compa-
rable catalytic activity to that obtained with the combination of
[Ru3(CO)12] and PCy3, to give cis-2a and trans-3a in 93% yield with
a ratio of 89:11, when used in 5 mol% relative to 1a. CCDC-798851
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Received: December 20, 2010
Revised: March 31, 2011
Published online: June 8, 2011
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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