Microwave-accelerated Ru-catalyzed hydrovinylation of alkynes and enynes: A straightforward approach toward 1,3-dienes and 1,3,5-trienes

Article abstract of DOI:10.1002/chem.201300790

Quick, but not dirty! Microwave heating was found to have a significant effect on the Ru-catalyzed hydrovinylations of alkynes. A broad range of different terminal alkynes were coupled to methyl acrylate within just 30min in good to excellent yields (see scheme). This new protocol was transferred to the hydrovinylation of enynes as novel coupling partners in C-H-activation chemistry giving different 1,3,5-trienes as the sole products. Copyright

Full text of DOI:10.1002/chem.201300790

DOI: 10.1002/chem.201300790
Microwave-Accelerated Ru-Catalyzed Hydrovinylation of Alkynes and
Enynes: A Straightforward Approach toward 1,3-Dienes and 1,3,5-Trienes
Tobias Schabel and Bernd Plietker*[a]
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Chem. Eur. J. 2013, 19, 6938 – 6941

COMMUNICATION
Table 1. Influence of the solvent and temperature.[a]
Recently we were able to show that based upon Muraiꢁs
À
seminal contribution in the field of Ru-catalyzed C H acti-
vations[1] [Ru(CO)
(Ph3P)3]HCl (1) or its corresponding dihy-
drido species are able to catalyze the hydrovinylation of al-
kynes with a,b-unsaturated carbonyl compounds.[2] The ad-
vantages of this transformation are obvious, the catalyst is
readily available in multigram quantities as an air-stable
Entry
3 (equiv)
Solvent
t [h]
T [8C]
4/5 [%][b]
1
2
3
4
5
6
7
8
9
2
5
DMF
DMF
DMF
DMF
DMF
DMF
–
24
24
24
24
100
100
100
100
120
21:23
38:14
47:6
62:–
44:–
65:–
73:–
79:–
75:–
[3]
10
20
20
20
20
20
20
complex in just one step from RuCl3 and the transforma-
tion is atom economic.[4] However, rather long reaction
times of 18 to 48 h and elevated reaction temperatures of
1008C were required.[2] Furthermore, within an extensive in-
vestigation on scope and limitation we observed a serious
limitation when functionalized aryl acetylenes were em-
ployed in this transformation. Instead of the desired hetero-
coupling product,[5] the homocoupling product[6–8] was ob-
tained in good to excellent yields. These findings clearly
point into the direction of two competing mechanistic mani-
folds to be operating under thermal conditions
(Scheme 1).[2,9]
24
0.5
0.5
0.5
0.5
100 (MW)
100 (MW)
80 (MW)
60 (MW)
–
–
[a] The reactions were performed under a N2 atmosphere on a 0.5 mmol
scale by using 5 mol% catalyst. [b] Determined by GC integration by
using dodecane as an internal standard.
ically under the standard conditions (DMF, 1008C, 24 h,
Table 1).
The product ratio changed significantly into the desired
direction; however, the long heating periods led to a severe
decomposition of the acrylate. At this point we changed the
experimental setup and decided to employ microwave heat-
ing. This kind of energy supply has been shown to solve
problems that are connected with simple thermal heating, in
which the energy is absorbed by the solvent and eventually
transferred from the solvent to the reaction components.[11]
Microwave absorption is only possible by polar molecules.
Hence, microwave irradiation of a catalytic reaction, in
which usually polar intermediates are involved, in a nonpo-
lar solvent, allows for a directed energy transfer into the re-
active intermediates.[11] As a consequence of this directed
“heating” shorter reaction times and in certain cases higher
yields due to the avoidance of undesired thermal side reac-
tions are observed. We were surprised to find that the con-
cept of microwave irradiation turned out to be a key to the
solution of our chemical problem. After only 30 min full
conversion was observed by using an excess of acrylate.
Under these conditions the addition of DMF as a solvent
was not necessary.
A variety of terminal acetylenes were transferred into the
corresponding 1,3-dienes without formation of the undesired
homocoupling product (Table 2). Good to excellent isolated
yields were obtained. The reaction is compatible with func-
tional groups, such as halides, ethers, nitriles, and even het-
erocycles. Furthermore, no ring opening of the cyclopropyl
moiety was observed.
From a mechanistic point of view, the E/Z-configured 1,3-
diene should be the primary product.[4] Due to the thermal
conditions we envisioned a subsequent Ru-catalyzed p-bond
isomerization into the all-E-configured diene to account for
the predominant formation of the latter product.[12] The mi-
crowave irradiation allows for a decrease in the reaction
temperature. From these data it is obvious that the isomeri-
zation of the E/Z into the E/E-configured product strongly
depends on the reaction temperature. To underline the abili-
Scheme 1. Allenylidene versus hydrovinylation mechanism.
Apparently the insertion of the Ru catalyst into the
À
C(sp) H bond becomes predominant with increasing acidity
of this bond.[10] Under standard conditions, this undesired
side reaction can be suppressed by employing two equiva-
lents of the acrylate. From a mechanistic point of view the
Ru hydride species resulting from the direct insertion into
À
the acrylate b-C H bond is more reactive in the subsequent
hydrometallation of the alkyne. Hence, we speculated that a
change in the stoichiometry and a more efficient heating
procedure might offer the chance to outcompete the unde-
sired homodimerization of the alkyne. At the outset of our
investigations we increased the amount of acrylate systemat-
[a] T. Schabel, Prof. Dr. B. Plietker
Institut fꢂr Organische Chemie, Universitꢃt Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
Fax : (+49)711-68564285
E-mail: bernd.plietker@oc.uni-stuttgart.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300790.
Chem. Eur. J. 2013, 19, 6938 – 6941
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6939

B. Plietker and T. Schabel
Table 2. Microwave-accelerated hydrovinylation of terminal alkynes.[a]
Table 3. Microwave-accelerated hydrovinylation of enynes.[a]
Entry
R
Product
(E,E)/ (E,Z)
Yield [%]
Entry
R
Product
(E,E,E)/
G
Yield [%]
1
2
3
4
5
6
Ph
6
7
8
9
10
11
12
13
14
15
16
91:9
92:8
79
82
76
82
83
91
85
79
66
90
74
1
2
3
4
Ph
17
18
19
20
91:9
92:8
86:14
81:19
79
93
76
82
p-MeOC6H4
p-ClC6H4
p-F3CC6H4
p-MeC6H4
C5H11
PhOCH2
4-Cl-C4H8
4-NC-C4H8
cyclopropyl
3-pyridyl
p-MeO-C6H4
p-tBu-C6H4
p-F-C6H4
86:14
81:19
81:19
76:24
75:25
74:26
70:30
81:19
74:26
5
21
76:24
82
6[b]
7
8[c]
9
[a] The reactions were performed under a N2 atmosphere on a 0.5 mmol
scale by using 5 mol% catalyst 808C.
10[d]
[a] The reactions were performed under a N2 atmosphere on a 0.5 mmol
scale by using 5 mol% catalyst 808C. [b] 5 mol% catalyst, 1008C, 1 h.
[c] 5 mol% catalyst, 1008C, 1 h, DMF (1 mL). [d] 10 mol% catalyst,
1008C, 30 min.
ing trienes by using microwave irradiation (Table 3). A mix-
ture of isomers was observed; however, whereas the geome-
try of the a,b-double bond displays the thermodynamic
equilibrium mixture (Scheme 1) the g,d- and e,f-double
bond geometry was only E in all cases. Undesired side reac-
tions, such as, for example, benzannulations,[13] were not ob-
served under these conditions.[7–9]
ty of the Ru catalyst to isomerize p-bonds, isomerically pure
E/Z-configured product 5 was subjected to the reaction con-
ditions at different temperatures (Figure 1).
In the present manuscript, we report on the beneficial in-
fluence of microwave irradiation in Ru-catalyzed hydroviny-
lation. The interplay of stoichiometry on the one side and
the more efficient (“target-oriented”) energy transfer by mi-
crowaves on the other allows the fast and high yielding
atom-economic coupling of monosubstituted alkynes and
acrylates to give the corresponding 1,3-dienes. In further-
ance of these studies, enynes were employed as a new class
of reactive alkynes. The microwave-accelerated hydrovinyla-
tion of these compounds provided a straightforward access
to 1,3,5-trienes in good yields. This reaction was not possible
by using conventional heating technologies. It is our hope
that the beneficial influence of microwave irradiation will
[14]
À
open up a new perspective in C H-activation chemistry.
Our future investigations will concentrate on this aspect.
Figure 1. Ru-catalyzed diene isomerization.
Experimental Section
General procedure: A 10 mL-microwave vial was charged at room tem-
perature with [RuCl(CO)(H)ACHTUNGTRENUNG(PPh3)3] (5–10 mol%) under a N2 atmos-
From these data it is obvious that the stereoselective
course of the hydrovinylation is the consequence of a ther-
mal isomerization rather than of a different mechanism.
After having found suitable reaction conditions that allow
for the fast hydrovinylation of different alkynes, we finally
set out to employ enynes as starting materials (Table 3).
These starting materials are of interest for several reasons.
On the one hand, and from a synthetic point of view, these
compounds would allow a preparation of 1,3,5-trienes. On
the other hand, and from a mechanistic point of view, the
use of a vinylogous alkyne opposes a challenge to the regio-
selectivity of the hydride transfer. Gratifyingly, this com-
pound class was efficiently transferred into the correspond-
phere. Methyl acrylate (900 mL) and alkyne (0.5 mmol) were added. The
tube was immediately closed and heated for 30 min to the given tempera-
ture. After cooling to room temperature the excess acrylate was evapo-
rated and the residue was directly purified by silica gel column chroma-
tography.
Acknowledgements
Financial support by the Deutsche Forschungsgemeinschaft (SFB 706) is
gratefully acknowledged.
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Ru-Catalyzed Hydrovinylation of Alkynes and Enynes
COMMUNICATION
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Keywords: catalysis
ruthenium
· coupling · diene · microwave ·
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Received: February 28, 2013
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Published online: April 19, 2013
Chem. Eur. J. 2013, 19, 6938 – 6941
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6941