Highly efficient alkyne hydroarylation with chelating dicarbene palladium (II) and platinum(II) complexes

Article abstract of DOI:10.1002/adsc.200700271

We report on a novel reaction protocol for the coupling of arenes with alkynes (the Fujiwara reaction), yielding products of formal trans/hydroarylation of the triple bond. The protocol makes use of a chelating N-heterocyclic dicarbene palladium(II) compl

Full text of DOI:10.1002/adsc.200700271

FULL PAPERS
DOI: 10.1002/adsc.200700271
Highly Efficient Alkyne Hydroarylation with Chelating Dicarbene
Palladium(II) and Platinum(II) Complexes[1]
Andrea Biffis,a,* Cristina Tubaro,a Gabriella Buscemi,a and Marino Basatoa
a
Dipartimento di Scienze Chimiche, Università degli Studi di Padova, via Marzolo 1, 35131 Padova, Italy
Phone: (+39)-049-827-5216; fax: (+39)-049-827-5223; e-mail: andrea.biffis@unipd.it
Received: May 30, 2007; Revised: September 17, 2007; Published online: November 23, 2007
Abstract: We report on a novel reaction protocol for albeit limited to electron-rich arenes. We also pres-
the couplingof arenes with alkynes (the Fujiwara re-
ent the results of catalyst optimisation with respect
action), yieldingproducts of formal trans-hydroaryla- to the nature of the nitrogen substituents in the car-
tion of the triple bond. The protocol makes use of a bene units, of the bridging group between the car-
chelatingN-heterocyclic dicarbene palladium(II)
bene units and of the coordinated anionic ligands. Fi-
complex as catalyst and allows us to perform the re- nally, we also discuss the catalytic performance of a
action in a few hours with only 0.1 mol% catalyst related chelatingdicarbene complex of platinum(II).
yieldingthe trans-hydroarylation product in high
yields and with excellent selectivity. We discuss the
applicability of this reaction protocol, which appears Keywords: alkynes; C H activation; hydroarylation;
at present quite general with respect to the alkyne, N-heterocyclic carbenes; palladium; platinum
À
Introduction
usual stereoselectivity: in fact, cis-arylalkenes (i.e.,
the thermodynamically less favoured alkenes) are the
major products. The reaction mechanism was initially
À
Transition metal-catalysed C C couplingreactions
have undoubtedly become one of the most important thought to involve electrophilic arene metallation as
reaction classes in chemical synthesis.[2] In particular, the key reaction step;[6b] however, in recent years val-
Pd-catalysed couplingreactions, most notably the
uable experimental work carried out by Tunge and
Heck and Suzuki reactions, have been developed up Foresee as well as theoretical calculations by Soriano
to a very high degree of synthetic utility.[3] Such reac- and Marco-Contelles have brought evidence in favour
tions allow the transformation of aryl and vinyl hal- of a mechanism going through a Friedel–Crafts-type
ides into more complex organic compounds, through alkenylation.[7]
the substitution of the halide with an organic moiety.
However, their applicability to chemical processes is reaction is arguably one of the most promising C C
often limited by the need to use aryl bromides or io- couplingreactions via C H activation/functionalisa-
dides as reagents, which are more reactive but also tion, since it involves cheap, commercially available
more costly and less widely available than the corre- reagents and it requires neither directing groups on
From the technological point of view, the Fujiwara
À
À
spondingchlorides; furthermore, salt waste is usually
the arene nor oxidising agents to regenerate the cata-
formed as coproduct of the reaction. A process that lyst. However, its possible industrial utilisation cannot
effects the same transformations upon direct function- leave apart a thorough optimisation of the catalyst
À
alisation of an aromatic C H bond would be obvious- and of the reaction conditions. In particular, the origi-
ly preferable from the point of view of both reagent nal Fujiwara reaction protocol implies using1–5
cost and process cleanness, but has yet to be devel- mol% palladium, which heavily affects the cost of the
oped, although there are considerable literature prec- process. Other metal centres such as platinum(II),[8]
edents for reactions of this kind.[4,5]
gold(I) and gold
Simple palladium(II) compounds such as PdACHTRE(UGN OAc)2 ployed as alternative catalysts, but their efficiency ap-
(III)[9] have been successfully em-
ACHTREUNG
in an acidic environment are, for example, known to pears to be lower than that of palladium(II). Further-
promote the couplingreaction of arenes with alkynes
more, use of less-noble, electrophilic metal centres
(the Fujiwara reaction).[6] In this case, products of has been also reported; however, their applicability
formal hydroarylation of the triple bond are formed. appears to be currently limited to arylacetylenes.[10]
The reaction is characterised by a high and quite un-
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A viable solution could be the use of palladium(II)
complexes with suitable ligands, which should in-
crease the stability of the catalyst under reaction con-
ditions without negatively affecting its reactivity. N-
heterocyclic carbene ligands[11] appear particularly
suited to this purpose since it is known that their pal-
ladium(II) complexes possess a high thermal and hy-
drolytic stability, even under strongly acidic condi-
tions.[12] Not surprisingly, therefore, monocarbene pal-
ladium(II) complexes are the only complexes which
have been reported to be active in the Fujiwara reac-
tion in the absence of other promoters, though their
activity is only slightly superior to that of simple
PdACHTREUNG
(OAc)2.[13]
Our research group has been interested for some
time in the synthesis and catalytic application of tran-
sition metal complexes with N-heterocyclic carbene li-
gands in technologically relevant reactions like, for
Scheme 1. Possible reaction products of the Fujiwara hydro-
arylation.
À
À
example, Heck reaction or C N and C O cou-
plings.[14] In particular, we have recently started a sys-
tematic study aimed at critically evaluatingthe reac-
tivity of chelatingdicarbene palladium(II) complexes,
by tuningthe properties of the dicarbene liagnd,
actingon the substituents at the imidazole ringand
thane with 1,2-dichloroethane. Remarkably, under
these conditions, the reaction gave complete conver-
sion and 88% selectivity to the Z product (other
on the nature of the bridging group.[14b] In this contri- products beingthe E isomer b and the product of
bution, we investigate the catalytic efficiency of com- double alkyne insertion c, see Scheme 1). We repeat-
plexes of this kind in the Fujiwara reaction.
ed the experiment usingonly 0.1 mol% catalyst and
again we obtained a very high conversion (96%) with
86% selectivity to the Z product. Finally, determina-
tion of the conversion curve (Figure 2) made it clear
that the reaction proceeded rapidly duringthe first
Results and Discussion
We decided to begin our evaluation of the catalytic five hours, reaching89% conversion with 92% selec-
efficiency of chelatingdicarbene palladium(II) com-
plexes in the Fujiwara hydroarylation by testingcom-
tivity; after this time, the concentration of the Z prod-
uct remained almost constant, eventually decreasing
plex 1 (Figure 1), whose preparation and characterisa- at longer reaction times, while there was a slow but
tion is reported for the first time herein,[15] in a stan- continuous increase in the concentration of the E
dard test reaction between pentamethylbenzene and product, due to a background acid-catalysed isomeri-
ethyl propiolate.
sation process.
At first, we adopted the reaction conditions usually
We also found out that use of excess arene was not
employed by Fujiwara (1 equiv. alkyne, 2 equivs. necessary in order to achieve good results, although
arene, 0.01 equiv. catalyst, solvent CF3COOH:CH2Cl2 the reaction rate was somewhat slower: 68% conver-
4:1, room temperature, 20 h).[6b] However, under sion with 94% selectivity for the Z product were ob-
these conditions the conversion amounted to only a tained after 7 h usingcatalyst 1 with only 1 equiv. of
few percent (albeit with complete selectivity for the Z pentamethylbenzene. On the other hand, excess tri-
product). While attemptingto improve the reactivity
fluoroacetic acid was indeed fundamental, as in Fuji-
of the catalytic system, we chose to increase the reac- waraꢁs case. Usingas solvent CF COOH:1,2-dichloro-
3
tion temperature to 808C and to replace dichlorome- ethane 1:4 the conversion was halved, while the selec-
tivity remained high (94%). In conclusion, at the end
of this preliminary study we found ourselves able to
perform the reaction in high yields at 808C with stoi-
chiometric reagents and using only 0.1 mol% catalyst.
On the basis of the above, the catalytic efficiency of
[6b]
complex 1 appears superior to both PdCATHRE(UGN OAc)2 and
monocarbene palladium(II) complexes[13] which, how-
ever, were tested at lower temperature. Therefore, we
performed some additional experiments to compare
more directly the efficiency of the various catalysts.
Figure 1. Complexes 1 and 2.
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Highly Efficient Alkyne Hydroarylation
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Table 1. Alkyne screeningfor the Fujiwara hydroarylation
with pentamethylbenzene catalysed by complex 1.
Entry Alkyne
Time Product
[h]
Yield
[%][a]
(Z:E)
1
2
5
5
58 (57:1)
56 (18:1)
11 (10:1)
79 (25:1)
28
3
4
5
Figure 2. Yield (%) vs. time diagram for the reaction of pen-
tamethylbenzene and ethyl propiolate catalysed by complex
1.
48
23
5
24
25
6
7
8
5
5
5
5
20
35
39 (7:1)
87
9
[a]
1
Yield [%] determined by GC-MS and/or H NMR. Reac-
tion conditions: see Experimental Section.
Figure 3. Conversion (%) vs. time diagram for the reaction
of pentamethylbenzene and ethyl propiolate catalysed by
complex 1, complex 2 and Pd
A
ported in Table 1 that our catalytic system displays
higher activities and selectivities than Fujiwaraꢁs one,
which invariably requires larger amounts of catalysts
Determination of the conversion curves under our re- and/or longer reaction times.[6b] Electron-poor termi-
action conditions (Figure 3) revealed that complex 1 nal alkynes are very reactive, though internal alkynes
was indeed the most efficient catalyst for this reac- substituted with two electron-withdrawinggroups are
tion, with an initial activity almost two times higher converted more sluggishly. 3-Butyn-2-one (Table 1,
than that of both PdACHTRE(UNG OAc)2 and monocarbene palladi- entry 9) gives the E-product exclusively, and phenyl-
um complex 2,[16] which expectedly forms Nolanꢁs acetylene (Table 1, entry 6) the a-arylated product, as
bis(trifluoroacetate) complex in situ. After five hours it is invariably the case with metal-catalysed hydroar-
of reaction, Pd
92% selectivity for the Z product, while complex 2 quite low, but it has to be remarked that in this case
gave 31% conversion with 94% selectivity. alkyne polymerisation and hydration by traces of
ACHTREUNG
(OAc)2 gave only 37% conversion with ylations;[6–10,13] with the latter substrate the yield is
With this result at hand, we set out to evaluate the water are found to be serious competitive reactions.
generality of our catalytic system, with respect to Clearly, the reaction conditions need in this case to be
both the alkyne and the arene. The results are report- further optimised in order to suppress or at least to
ed in Table 1 and Table 2, respectively.
limit the side reactions. 1-Phenyl-1-propyne, diphenyl-
Differently substituted alkynes were reacted with acetylene and ethyl phenylpropiolate are also hydrat-
pentamethylbenzene under our standard conditions; ed to a small extent under the employed reaction con-
the reaction time was not optimised for any of the al- ditions, thus explainingthe moderate yields observed
kynes employed. It is apparent from the results re- (Table 1, entries 8, 7 and 5, respectively).
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Table 2. Arene screeningfor the Fujiwara hydroarylation
with ethyl propiolate catalysed by complex 1.
ed product. This is evidently the consequence of the
decreased reactivity of the aromatic ringtowards hy-
droarylation as well as of the possible greater inci-
dence of thermal isomerisation through the dipolar
mechanism already proposed by Fujiwara and by
Alper.[6b,17]
Arene
Time [h] Conversion[a]
Yield [%][a]
a[b]
b c d
e
5
24
73
87
64 (5) 1 (1) 2 -
50 (25) 7 (3) 2 -
-
-
A slow reaction was observed with p-dimethoxy-
benzene as well, as reported also by Fujiwara and
Nolan;[6b,13] however, in our case the arene:alkyne 2:1
adduct e was the main reaction product (Scheme 1).
Prolonging the reaction time led to an increase in
conversion but also to the formation of oligomeric
products presumably derivingfrom further electro-
philic substitution reactions (attack of another prod-
uct molecule on the arene rings of the 2:1 adduct). Fi-
nally, the presence of halide substituents had a delete-
rious effect on the reaction, p-bromotoluene being
almost inactive and p-bromoanisole beingonly slowly
converted with poor selectivity. However, it has to be
remarked that arenes substituted with electron-with-
drawinggroups are usually inactive for this reaction:
the only aryl bromide that is known to be converted
is mesityl bromide.[6–10,13]
5
24
58
81
47 (4)
62 (11) 1
-
3 4
3 4
-
-
5
24
74
81
58 (4)
34 (25) 5 (3) 4 10 -
1
3 8
-
5
24
10
30
5 (1)
7 (14)
2
6
2 -
3 -
-
-
5
20
4
-
-
-
8
Havingestablished the high catalytic efficiency in
the Fujiwara reaction of complex 1, we then tested
and compared the efficiency of a series of chelating
dicarbene palladium(II) complexes, already known in
literature,[15,18–22] and characterised by different sets of
dicarbene and anionic ligands (Figure 4). The Fuji-
[a]
1
Yield [%] determined by GC-MS and/or H NMR.
[b]
In parentheses the yield in the hydrolysed product. Reac-
tion conditions: see Experimental Section.
Traces of water may also hydrolyse ester functions wara reaction between pentamethylbenzene and ethyl
under the conditions of the Fujiwara reaction:[6b] lim- propiolate was employed as a standard test reaction
ited hydrolysis of the reaction product was indeed ob- to probe the reactivity of these palladium(II) com-
served, most notably with methyl propiolate (Table 1, plexes.
entry 2), while in the case of ethyl phenylpropiolate
First of all, we evaluated the influence of the
(Table 1, entry 5) the reaction product was a mixture nature of the anionic ligand on the reactivity of our
of the expected ester and of the correspondingdecar-
boxylated compound, presumably derivingfrom ester
hydrolysis with subsequent rapid decarboxylation.
The screeningof arene substrates was performed
upon reactingethyl propiolate with differently sub-
complexes. Determination of the conversion curves
for complexes 1 and 3 (Figure 5) and for complexes
4–6 (Figure 6) revealed that this influence is very
small. Indeed, the catalytic activity of the complexes
is the same within the margin of experimental error,
stitued arenes, both in terms of the numbers of sub- irrespective of the nature of the halid. These results
stituents and of their nature. The results reported in strongly support the hypothesis that the actual cata-
Table 2 show that most methyl-substituted benzenes lytically active species does not contain halide ligands,
are competent substrates irrespective of the number which are probably removed through exchange with
of substituents, although the yield recorded with p- trifluoroacetate anions. Remarkably, DFT calcula-
xylene was unexpectedly low. Hydroarylation yields tions, performed by Strassner et al., have predicted
could be improved by prolonging the reaction time. that replacement of the two halides by trifluoroace-
However, this also resulted in the extensive hydrolysis tate anions takes place at high temperatures, like in
of the ester function of the product (Table 2). We are our reaction conditions, and that the bindingenergy
currently further optimisingthe reaction conditions in
difference between the various halides and the metal
order to suppress hydrolysis as well as the other side is not large enough to produce effects on the kinetic
reactions observed with different alkynes (see above). outcome of the reaction.[23]
The distribution of side products (see Scheme 1)
On the other hand, the nature of the dicarbene
also changes significantly upon going from 1,2,4,5-tet- ligand has an effect on catalytic activity. This effect
ramethylbenzene to p-xylene. While in the former can be appreciated by comparingthe performance of
case the double hydroarylation product is the main a series of complexes with the same anionic ligand
side product, in the latter case it is the E-monoarylat- but different dicarbene ligands (3 and 6–9). Analysis
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Highly Efficient Alkyne Hydroarylation
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Figure 4. Palladium(II) dicarbene complexes employed as catalysts in this study.
Figure 5. Conversion (%) vs. time diagram for the reaction
of pentamethylbenzene and ethyl propiolate catalysed by
complexes 1 and 3.
Figure 7. Conversion (%) vs. time diagram for the reaction
of pentamethylbenzene and ethyl propiolate catalysed by
complex 3 and complexes 6–9.
of the conversion curves (Figure 7) revealed that all
complexes perform in a similar manner over longre-
action times (23 h), reachingcomparable conversions.
However duringthe first 5 h of reaction the catalytic
performances are quite different and, in particular, it
turns out that complexes with the more hindering
NHC ligands 7 and 8 show higher activity. The meth-
ylbenzimidazolin-2-ylidene complex 3 and the corre-
spondingimidazolin-2-ylidene complex 6 also perform
quite well in this reaction, while the reactivity of com-
plex 9 is significantly lower. These results suggest that
the actual catalytically active species still contains the
dicarbene ligand, and that the steric hindrance of the
dicarbene ligand is an important factor in determining
the reactivity of the catalyst. We are now extending
this study in order to further optimise the structure of
the dicarbene ligand as well as to fully identify the
Figure 6. Conversion (%) vs. time diagram for the reaction
of pentamethylbenzene and ethyl propiolate catalysed by
complexes 4–6.
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Table 3. Arene screeningfor the Fujiwara hydroarylation
with ethyl propiolate catalysed by complex 10.
Arene
Time [h] Conversion[a]
Yield [%][a]
a[b]
b
c
d
e
Figure 8. Platinum(II) complex 10.
5
72
63 (6) 1 (1) -
-
-
-
real catalytically active species involved in the reac-
tion.
Finally, we have also started a preliminary evalua-
tion of the catalytic performance of dicarbene platin-
um(II) complex 10 (Figure 8), which has been syn-
5
5
5
56
63
4
47 (4) -
1 4
[24]
thesised followingan already reported strateyg.
45 (4) 1 (1) - 12 -
Simple platinum(II) salts such as PtCl2 were already
tested as catalysts by Fujiwara et al., and found to be
less active but sometimes more selective than palla-
dium(II) compounds, provided they are activated
3 (1)
-
-
-
-
upon halide removal by a proper silver(I) salt of a
[6,8]
non-coordinatinganion.
The results in the arene
and alkyne screeningare reported in Table 3 and
Table 4, respectively.
[a]
1
Yield [%] determined by GC-MS and/or H NMR.
[b]
In parentheses the yield in the hydrolysed product. Reac-
tion conditions: see Experimental Section.
The screeningof arene and alkyne substrates was
performed followingthe protocol used for the palladi-
um complexes, that is, upon reactingethyl propiolate
with differently substitued arenes or upon reacting
pentamethylbenzene with various alkynes. In general,
our catalytic system usingcomplex 10 displays slightly
lower productivities than Fujiwaraꢁs systems (PtCl2/
2AgOAc or PtCl2/2AgOTf) which, however, require
higher catalyst loadings (2.5–5 mol%), longer reac-
tion times (15–24 h), and most notably an Agsalt as
cocatalyst.[6b,8b] Moreover, comparingthe results ob-
tained with platinum complex 10 with those of palla-
dium complex 1, it is evident that conversions and se-
lectivities are quite similar.
Table 4. Alkyne screeningfor the Fujiwara hydroarylation
with pentamethylbenzene catalysed by complex 10.
Alkyne
Time [h]
5
Product
Yield [%][a] (Z:E)
72 (35:1)
5
5
51
45 (7:2)
Therefore, the results of this preliminary study
highlight the fact that Pt-dicarbene complexes can
represent an alternative to Pd complexes in this reac-
tion. Consequently, we are currently investigating also
these complexes in more detail.
[a]
1
Yield [%] determined by GC-MS and/or H NMR.
In parentheses the yield in the hydrolysed product. Reac-
tion conditions: see Experimental Section.
[b]
tron-poor arenes (a class of compounds for which
there is at present no viable catalyst), at runningthe
reaction under milder conditions (lower temperature,
Conclusions
In conclusion, we have developed a novel reaction less acidic environment) as well as at understanding
protocol for the Fujiwara hydroarylation employing the reaction mechanism in greater detail.
chelatingdicarbene palladium(II) and platinum(II)
complexes as catalyst. The protocol allows us to per-
form the reaction with good to excellent yields and
Experimental Section
with high chemo- and stereoselectivity in short times,
with stoichiometric reagents and with only 0.1 mol%
catalyst. The reaction protocol appears to be quite
general with respect to the alkyne, while its applica-
bility to arene substrates is at present limited to elec-
tron-rich molecules. Work currently in progress aims
at expandingthe applicability of this reaction to elec-
General Remarks
All manipulations were carried out usingstandard Schlenk
techniques under an atmosphere of argon or dinitrogen. The
reagents were purchased by Aldrich as high-purity products
and generally used as received. Complexes 2,[16] 4,[18] 5,[19]
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Highly Efficient Alkyne Hydroarylation
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6,[20] 7,[21] 8,[14b] 9,[22] 10[24] and 1,1’-dibenzimidazolylme-
thane[25] were prepared accordingto literature procedures.
Complex 3 was prepared in the same manner as 1 (see
below) usingKI instead of KBr and its spectroscopic charac-
terisation was identical to that reported in the literature.[15]
All solvents were used as received as technical grade sol-
vents. NMR spectra were recorded on a Bruker Avance
300 MHz (300.1 MHz for 1H and 75.5 for 13C); chemical
shifts (d) are reported in units of ppm relative to the residu-
al solvent signals. Elemental analysis were carried out by
the microanalytical laboratory of our department with a
Fisons EA 1108 CHNS-O apparatus.
(4.3 mmol), alkyne (4.3 mmol), catalyst (0.0043 mmol), TFA
(4 mL) and 1,2-dichloroethane (1 mL). For the catalytic
tests reported in Table 3 and Table 4 the molar amounts of
the various reagents were as follows: arene (7.9 mmol),
alkyne (7.9 mmol), catalyst (0.0079 mmol), TFA (4 mL) and
1,2-dichloroethane (1 mL).
References
[1] Reactivity of chelatingdicarbene metal complex cata-
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[2] A. de Meijere, F. Diederichs, (Eds.), Metal-catalyzed
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portions to a suspension of Me3OBF4 (1.43 g, 9.7 mmol) in
dichloromethane (20 mL). The reaction mixture was stirred
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487.99): C 41.84, H 4.54, N 11.48; found: C 41.60, H 3.74, N
À
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11.25%; H NMR (DMSO-d6, 258C): d=4.13 (s, 3H, CH3),
7.39 (s, 1H, CH2), 7.5–8.5 (m, 4H, Ar), 10.10 (s, 1H,
NCHN); 13C{1H} NMR (DMSO-d6, 258C): d=33.7 (CH3),
54.9 (CH2), 113.5, 114.1, 127.2, 127.5, 130.3, 131.8 (C Ar),
144.3 (NCHN).
Synthesis of (1,1’-Dimethyl-3,3’-methylenedibenzimi-
dazolin-2,2’-diylidene)palladium(II) Dibromide (1)
A solution of PdACHTREUNG(OAc)2 (0.12 g, 0.49 mmol), diimidazolium
salt (1a) (0.25 g, 0.50 mmol) and KBr (0.12 g, 1.00 mmol) in
DMSO (10 mL) was heated at 608C for 2 h, 808C for 1 h,
1208C for 1 h; the solvent was removed under vacuum and
the orange residue was washed with acetonitrile (3”5 mL),
filtered and dried under vacuum; yield: 58%; anal. calcd.
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General Procedure for the Catalytic Tests
This is a general procedure for the catalytic tests reported in
Table 1 and for the determination of conversion curves. The
arene (2.65 mmol, if solid) and the palladium(II) complex
(0.00265 mmol) were placed in a 50-mL round-bottomed
flask, previously evacuated and filled with argon. Trifluoro-
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Portions of the solution (0.2 mL) were drawn off from the
1
reaction mixture and analysed by H NMR or GC-MS. For
the catalytic tests reported in Table 2 the molar amounts of
the various reagents were slightly changed as follows: arene
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