Hydrido-cobalt catalyst as a selective tool for the dimerisation of arylacetylenes: Scope and theoretical studies

Article abstract of DOI:10.1002/adsc.201300486

A simple hydrido-cobalt complex efficiently catalyses the highly regio- and stereoselective dimerisation of various terminal arylacetylenes under mild conditions. The corresponding (E)-1,4-enynes are obtained as sole isomers with good to excellent yields. DFT calculations revealed that the reaction proceeds via a C-H activation/hydrocobaltation pathway. Copyright

Full text of DOI:10.1002/adsc.201300486

FULL PAPERS
DOI: 10.1002/adsc.201300486
Hydrido-Cobalt Catalyst as a Selective Tool for the Dimerisation
of Arylacetylenes: Scope and Theoretical Studies
Sandrine Ventre,a Etienne Derat,a Muriel Amatore,a,* Corinne Aubert,a,*
and Marc Petita,*
a
UPMC Univ Paris 06, UMR CNRS 7201, IPCM, 4, place Jussieu, 75252 Paris Cedex 05, France
Fax : (+33)-1-4427-7360; phone: (+33)-1-4427-3932; e-mail: muriel.amatore@upmc.fr or corinne.aubert@upmc.fr or
marc.petit@upmc.fr
Received: June 4, 2013; Published online: September 10, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300486.
À
Abstract: A simple hydrido-cobalt complex efficient- proceeds via a C H activation/hydrocobaltation
ly catalyses the highly regio- and stereoselective di- pathway.
merisation of various terminal arylacetylenes under
mild conditions. The corresponding (E)-1,4-enynes
À
are obtained as sole isomers with good to excellent Keywords: catalysis; C H activation; enynes; hydri-
yields. DFT calculations revealed that the reaction do-cobalt complex; (E)-selective coupling
Introduction
catalytic systems such as CoBr
CoCl2·6H2O/Dipimp/Zn or CoCl2·6H2O/dppe/Zn.[15]
(dppe)/Mg[14] and
2ACHTUNGTRENNUNG
The transition metal-catalysed dimerisation of alkynes However, in the first report devoted mainly to
is a very simple and atom-economical strategy to [2+2+2]cyclotrimerisation of alkynes, only one exam-
access 1,3-diynes and conjugated 1,4-enynes.[1] The ple is detailed with a moderate 47% yield using phe-
latter represent important precursors in the fields of nylacetylene and no further optimisation has been
organic chemistry, medicinal chemistry and materials conducted. The second report displays a broader
sciences.[2] Nowadays, various transition metals[3–8] and scope but this in situ generated catalytic system is
f-elements[9] are able to catalyse efficiently these mainly applied to the cross-dimerisation of trimethyl-
transformations, either as homodimerisation or as se- silylacetylene to internal alkynes. Reactivity of some
lective cross-dimerisation.[10] However, many of the cobalt hydride complexes has also been investigated
reported catalytic systems afford mixtures of the toward alkynes with narrow substrate scopes, moder-
regio- and stereoisomeric 1,4-enynes, higher oligomers ate yields and low regioselectivities.[16] To the best of
and in some particular cases the corresponding
[2+2+2]adducts (Scheme 1). Although efforts have
been devoted to understand the origins of selectivi-
ty[11] for the development of highly selective methods,
the design of new clean, selective and economical
processes is still challenging.
Recently we reported that the simple HCoACTHUNTRGNE(UNG PMe3)4
catalyst[12] efficiently catalysed the [2+2+2]cycloaddi-
tion of various enediynes.[13] In contrast with previous
works based on cobalt catalysis, these reactions occur
for the first time under catalytic (5 mol%) and mild
conditions. In the course of our investigations into the
reactivity of HCoACHTNUTRGNEUNG(PMe3)4, we found that this cobalt
catalyst could be applicable to terminal alkynes with
a switch of selectivity from cyclotrimerisation to di-
merisation affording conjugated enynes. A similar
Scheme 1. General transition metal-catalysed oligomerisa-
tuning in reactivity has been reported with the use of tion of terminal alkynes.
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Hydrido-Cobalt Catalyst as a Selective Tool for the Dimerisation of Arylacetylenes
our knowledge, besides these brief reports, no other to be highly reactive (entry 4) for this dimerisation
cobalt complexes exhibit such a reactivity toward the process, its clean preparation remained tedious[18] and
selective dimerisation of alkynes.
we thus retained HCoACHTUNGTERUNN(G PMe3)4 to further explore this
reaction (entry 5). Interestingly, the catalyst loading
could be reduced to 1 mol% albeit with a concomitant
yield decrease (entries 5–7). Further experiments
Results and Discussion
ACHUTNGREN[NUG HCoACHTUNGTRNE(NUGN PMe3)4 5 mol%] revealed that the selectivity of
Herein, we describe the highly selective and catalytic the reaction was not compromised under refluxing
formation of (E)-conjugated enynes using a simple conditions (entry 8) and that a complete control could
cobalt catalyst HCo
(Scheme 2).
ACHTUNGTRENNUNG
In optimization studies, phenylacetylene 1a was covered when no cobalt catalyst was used in a control
used as a model substrate (Table 1). After a rapid experiment (with PMe3 as sole hypothetic organocata-
survey of solvents, tetrahydrofuran was found to be lyst, entry 10). The viability of a direct cobalt-cata-
the most effective at room temperature in terms of re- lysed synthesis of (E)-enynes using a catalytic mixture
action times and conversions. In all cases, the dimeri- of CoACHTNUTRGNEUNG(acac)2/PMe3/NaBH4 was investigated and
sation of 1a selectively afforded the desired (E)- showed a slow and moderate conversion to the de-
enyne 2a. Related cobalt complexes were also tested sired product, demonstrating that the use of defined
under standard conditions (entries 1–5).[17] Cobalt(I) preformed catalysts allows higher control and effi-
analogue complexes such as MeCo
ClCo(PMe3)3 turned out to be less efficient (entries 1 reaction was not affected by addition of a radical trap
and 2). The hydride complex HCo(N2)(PPh3)3 was (TEMPO), experimental evidence for a non-radical
tested as well but in that case only higher oligomers pathway with H-abstraction (entry 12).[19]
ACHTUNGTREN(NNUG PMe3)4 and ciency (entry 11). Finally, this cobalt hydride-catalysed
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
were formed (entry 3). Although Co
(PMe3)4 proved
Subsequently, reactions of various substituted phe-
nylacetylene derivatives 1a–v were conducted and the
results are depicted in Table 2. Interestingly, in most
cases, the substitution pattern on the aromatic moiety
did not affect the regio- and stereoselectivities and
the desired (E)-enynes were obtained as sole isomers.
Substitution in para-, meta- and ortho-positions was
tolerated with only slight decreases in yields for the
more hindered ortho-substituted derivatives 1n–p (en-
Scheme 2. (E)-Selective cobalt-catalysed dimerisation of
arylacetylene derivatives.
Table 1. Dimerisation of phenylacetylene 1a in the presence of cobalt catalysts.
Entry
[Co] cat. (x mol%)
MeCo(PMe3)4 (5 mol%)
Time [h]
Ratio 2a/3a[a]
Yield of 2a [%]
1
2
3
4
5
6
7
8
G
8[b]
4
1
0.5
2
2
100:0
96:4
nd
97:3
90:10
93:7
>99:1
89:11
100:0
no reaction
100:0
90:10
31
84
0
93
97
93
62
92
41
–
ClCo(PMe3)3 (5 mol%)
HCo(N2)(PPh3)3 (5 mol%)
Co(PMe3)4 (5 mol%)
ACHTUNGTRENNUNG
R
N
HCo
HCo
HCo
HCo
HCo
N
6
0.03
12[b]
8
12
0.75
9
10
11
12
“HCo
N
55
87
HCo(PMe3)4/TEMPO (5 mol%)
AHCTUNGTRENNUNG
[a]
1
2a/3a ratios were determined by H NMR analysis of the crude reaction mixture.
No full conversion of 1a could be observed.
[b]
[c]
[d]
Reaction was carried out in the sole presence of 20 mol% PMe3.
Reaction was carried out using a 5 mol% mixture of CoACTHNUTRGNE(UNG acac)2/4PMe3/2NaBH4.
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Table 2. Cobalt-catalysed dimerisation of terminal arylacetylenes 1.
Entry
Ar
Time
[h]
Ratio of
Yield of
2 [%]
Entry
Ar
Time
[h]
Ratio of
Yield of
2 [%]
2/3[a]
2/3[a]
1
2
3
4
5
6
7
8
C6H5 1a
2
6
90:10
>99:1
100:0
100:0
100:0
100:0
100:0
>99:1
100:0
>99:1
91:9
97
98
95
78
98
50
14
15
16
17
18
19
20
21
22
23
24
25
26
m-MeOC6H4 1l
m-FC6H4 1m
o-MeC6H4 1n
2
2
4
4
12
1
4
2
2
4
2
4
24
94:6
>99:1
91:9
96
90
84
88
nd[d]
85[e]
81
63
71
44
98
61
0
p-MeC6H4 1b
p-MeOC6H4 1c
p-NMe2C6H4 1d
p-FC6H4 1e
p-ClC6H4 1f
p-BrC6H4 1g
p-IC6H4 1h
p-CF3C6H4 1i
p-CO2MeC6H4 1j
m-MeC6H4 1k
m-MeC6H4 1k
m-MeC6H4 1k
4
6[b]
2
o-MeOC6H4 1o
o-MeOC6H4 1o
o-MeOC6H4 1o
o-FC6H4 1p
3,5-diMeOC6H4 1q
3,5-diFC6H4 1r
2,4,6-triMeC6H4 1s
3-thiophene 1t
3-pyridine 1u
83:17
85:15
83:17
>99:1
100:0
100:0
>99:1
95:5
4
4
4
2
2
2
12
1
58
<15[c]
94
9
10
11
12
13
66
94
95:5
88:12
nd[d]
89[e]
100:0
nd
2-pyridine 1v
[a]
1
2/3 ratios were determined by H NMR analysis of the crude reaction mixture.
[b]
[c]
[d]
[e]
Due to the poor solubility of the corresponding enyne in the medium, reaction was carried out at 408C in 4 mL THF.
Enyne 1h was isolated with a low yield and in non-pure state.
No full conversion with the use of 1 mol% HCo
Reaction was carried out with 5 mol% Co(PMe3)4.
ACHTUNGTRNE(NUNG PMe3)4.
AHCTUNGTRENNUNG
tries 16, 17 and 20). Whereas in the case of phenylace- good yields (entries 24 and 25). However, dimerisa-
tylene 1a the reaction selectivity could be improved tion of substrate 1v with the 2-pyridyl substituent was
using lower catalyst loadings or CoACTHNUTRGNE(UNG PMe3)4 (Table 1, not successful as the starting material remained un-
entries 4, 6 and 7), we found that similar conditions touched under a variety of reaction conditions
did not provide considerable variations with substitut- (entry 26) suggesting a catalyst deactivation due to
ed terminal alkynes 1k or 1o (Table 2, entries 12, 13, a strong and concomitant coordination of the metal to
18 and 19). Although the reaction was slowed down the nitrogen and the close triple bond.
using 2,4,6-trimethylphenylacetylene 1s as substrate,
Since arylacetylene derivatives proved to be effi-
the corresponding coupling product 2s could be ob- cient partners for this coupling, the reactivity of vari-
tained in a satisfactory 44% yield and an excellent ous aliphatic terminal alkynes was also evaluated.
(E)-selectivity (entry 23). The outcome of the reaction Rapid screening of reaction conditions resulted
seemed not to depend on the nature of the aromatic mainly in inseparable mixtures of enyne regioisomer
substituents. Electron-donating groups allowed to adducts in addition with non-identified compounds,
form the corresponding (E)-enynes in good yields certainly issued from oligomerisation (Scheme 3).
(entries 2–4, 11, 14, 16, 17 and 21). Among electron-
withdrawing groups, fluorinated substituents gave the was introduced in the medium, the catalyst
best results (entries 5, 9, 15, 20, and 22). Other halide HCo(PMe3)4 exhibited a novel reactivity towards ali-
However, when additionnal diphenylacetylene 8
AHCTUNGTRENNUNG
sources could be employed but the following arylace- phatic alkynes as the corresponding cross-coupling
tylene derivatives 1f–h (X=Cl, Br and I, entries 6–8) products 9 could be observed in some cases as major
were converted only moderately to the conjugated compounds next to homocoupling products (Table 3).
coupling products and non-identified compounds
could be detected. These results would confirm the
trend for cobalt (I or 0) species to undergo competi-
[20]
À
tive oxidative addition into the C X bonds.
The
ester-substituted phenylacetylene 1j exhibited a lower
reactivity toward the catalyst, probably due to a com-
petitive chelation effect (entry 10). Finally, terminal
alkynes 1t and 1u bearing an heteroaromatic group
such as thiophene or pyridine moieties reacted
smoothly under these conditions with moderate to Scheme 3. Dimerisation of aliphatic terminal alkynes.
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Hydrido-Cobalt Catalyst as a Selective Tool for the Dimerisation of Arylacetylenes
Table 3. Cross-dimerisation of aliphatic terminal alkynes 4 with diphenylacetylene 8.
Entry
R
Temperature [8C]
Time [h]
Yield of 9 [%]
1
2
3
4
5
6
7
8
n-Bu (4a)
n-Bu (4a)
n-Bu (4a)
A
N
25
40
66
25b
40
66
66
66
48
24
24
48
24
24
24
24
29
45
48
0
51
U
68
SiMe3 (4c)
CO2Me (4d)
nd[a]
nd[a]
[a]
The corresponding heterocoupling products 9 were detected as minor products in the crude NMR but could not be sepa-
rated from complex mixtures.
Terminal alkynes such as 1-hexyne 4a or silyl ether 4b first alkyne being p-coordinated while the second one
À
afforded the corresponding enynes under refluxing is a s-adduct through the pre-activated C H bond.
conditions with yields up to 68% (entries 3 and 6). Two isomer-adducts can be found and characterised
Under similar conditions, trimethylsilylacetylene 4c or according to the relative position of the two phenyl
methyl propiolate 4d led only to small amounts of groups, either in a trans conformation (A) or in a syn
heterocoupling products that could not be separated conformation (A’). The conformer A will ultimately
from complex mixtures (entries 7 and 8). Work re- give the (E)-head-to-head product while A’ will
garding this cross-dimerisation process is now under- afford the minor gem-product 3a. Due to pre-activa-
À
way in our laboratory using cobalt complex analogues, tion of the C H bond in complex A, the barrier to
in an attempt to obtain a more controlled reaction obtain the dihydro complex
B is rather small
and to extend this preliminary scope.
(0.66 kcalmolÀ1 in DG). On the other hand, starting
Our current mechanistic assumptions regarding this
new catalytic process are summarised in Scheme 4.
DFT calculations were also performed and will be dis-
cussed in the following paragraph in order to assess
the mechanism. Classically, after prior insertion of the
À
cobalt centre into the alkyne C H bond to form inter-
mediate B, two mechanistic pathways can be consid-
ered for this dimerisation process based either on hy-
drocobaltation or carbocobaltation (Scheme 4). In
both cases, subsequent reductive elimination would
afford the desired enyne with regeneration of the
active cobalt species A after a favoured ligand ex-
change. The high selectivities observed would be then
related to the geometry of the different cobalt com-
plexes involved in the catalytic cycle thus minimising
steric effects, as also suggested by Ogoshi et al, under
nickel catalysis.[7b] To gain information about the fav-
oured pathway, the DFT computational studies have
been performed using Gaussian 09 (Figure 1). Follow-
ing previous studies made in our group,[21] the chosen
level of theory is the classical B3LYP functional in
conjunction with the SVP basis set, which also incor-
porates polarisation on all hydrogen atoms. Since hy-
drido-cobalt species are involved in the catalytic
mechanism under scrutiny, this point is non-negligible.
The starting point is an adduct A between the cobalt
Scheme 4. Proposed mechanism for the cobalt-catalysed di-
centre and two phenylacetylene molecules 1a, the merisation of terminal arylacetylene derivatives.
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Figure 1. Calculated potential energy surfaces (in kcalmolÀ1) of the cobalt-catalysed dimerisation of arylacetylene deriva-
tives. Values in normal, italic and bold fonts correspond respectively to the electronic, enthalpy and Gibbs free energies.
from complex A’, a barrier of 2.31 kcalmolÀ1 was cal- etry to a square pyrimidal adduct D (barrier: 1.98 kcal
culated. Using the Arrhenius equation at 258C, the molÀ1 in DG, TS-CD). The last transition state (TS-
theoretical ratio between 2a and 3a is thus 96:4. This DF) on the hydrocobaltation pathway corresponds to
ratio is fully compatible with the experimental obser- the reductive elimination furnishing F and is in fact
vations using lower catalyst loadings (Table 1, en- the rate-determining step (9.96 kcalmolÀ1, DG). On
tries 5–7). Modification of the reaction temperature the other hand, for the carbocobaltation pathway, the
also confirms this tendancy as the (E)-enyne 2a is ob- reductive elimination is found to be much easier with
tained as sole isomer at 08C (Table 1, entries 5, 8 and a barrier of 1.78 kcalmolÀ1, without inclusion of zero-
9). Starting from complex B, and as explained above point and entropic factors (TS-EF). The common
(Scheme 4), two mechanistic possibilities can be en- final product F is found to be a square planar com-
visaged: hydrocobaltation and carbocobaltation. The plex, thus able to easily undergo exchanges with coor-
two corresponding transition states were located, re- dinating ligands or substrates in order to release the
spectively TS-BC and TS-BE. The barrier for the hy- desired enyne 2a. As a conclusion, the relative energy
drocobaltation is found to be 1.51 kcalmolÀ1 (DG difference existing between TS-BC and TS-BE would
value) while for the carbocobaltation the barrier is confirm
a
preferred hydrocobaltation pathway
14.81 kcalmolÀ1. With such a difference between the through prior C H activation of the terminal alkyne.
two mechanistic possibilities and despite the error
À
range associated to the B3LYP functional (4–5 kcal
molÀ1), one can safely admit that the hydrido transfer Conclusions
À
is favoured over the C C bond formation starting
from complex B. The hydrocobaltation pathway af- In summary, we provide a novel and general cobalt-
fords a first intermediate C in which the ligands are catalysed procedure for the highly regio- and stereo-
coordinated around the metal in a trigonal bipyramid selective dimerisation of terminal arylacetylene deriv-
fashion. To undergo the final reductive elimination, atives. These mild catalytic conditions are tolerant of
the complex C first switches from the previous geom- a large range of functionalised aromatic moieties and
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Hydrido-Cobalt Catalyst as a Selective Tool for the Dimerisation of Arylacetylenes
tered over a silica pad, washed with diethyl ether and the
solvents were concentrated under vacuum. Column chroma-
tography of the residue afforded the desired (E)-enynes.
the corresponding linear (E)-isomers are obtained
with excellent selectivities and yields over the linear
(Z)-isomers, the branched ones and [2+2+2]adducts.
To the best of our knowledge, this method represents
the first general (E)-selective dimerisation of terminal
arylacetylenes under cobalt catalysis. Another note-
Acknowledgements
worthy outcome of this work is that HCoACHTNUTRGNE(UNG PMe3)4
proved to be a very active complex for the efficient
activation of C H acetylenic bonds and subsequent
This work was supported by CNRS, MRES, UPMC and
Agence Nationale de la Recherche (ANR-12-BS07-0031-
01COCACOLIGHT) which we gratefully acknowledge.
À
selective hydrocobaltation. The full mechanism of this
cobalt-catalysed dimerisation of alkynes has been in-
vestigated computationally and results clearly confirm
that reactions proceed through a hydrocobaltation
pathway (rather than carbocobaltation). As well,
a presumed agostic interaction between an ortho-H of
the aromatic group and the cobalt centre is not re-
sponsible for the high regioselectivities observed.
Moreover, DFT calculations are in perfect agreement
with experimental observations. Finally, the use of ter-
minal aliphatic alkynes has been briefly illustrated
and demonstrates in that case the high tendency of
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Experimental Section
All dimerisation reactions were carried out under argon at-
mosphere using standard Schlenk techniques. HCoACTHUNTRGNE(UNG PMe3)4
was stored in a glovebox. Appropriate samples were taken
off the glovebox in a Schlenk flask. THF was degassed using
argon bubbling for 15 min prior to running the experiments.
General Procedure
On a vacuum line, an oven-dried Schlenk flask charged with
HCoACHTUNGTRENNUNG(PMe3)4 (5 mol%, 0.05 mmol, 18.3 mg) was purged with
argon one time and stored under vacuum. During this time,
THF was degassed using argon bubbling for 15 min. A solu-
tion of the appropriate alkyne (1 equiv., 1 mmol) in previ-
ously degassed THF (2 mL) was prepared in a round-pear
flask and degassed using argon bubbling for 5 additional mi-
nutes. Under argon, the alkyne solution was then rapidly
added to the Schlenk flask by the mean of a syringe (de-
pending on the reaction time, the mixture turned generally
from dark yellow to dark brown). The Schlenk flask was
tightly sealed and allowed to stir at room temperature under
an argon atmosphere until completion as monitored by
TLC. After reaction completion, the crude mixture was fil-
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FULL PAPERS
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AHCTUNGTRENNUNG
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À
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À
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