Base-mediated direct arylation of pyrrole derivatives

Article abstract of DOI:10.1002/adsc.201000723

It appears that transition metal catalysts are not necessary to perform the direct arylation of electron-rich heterocycles with aryl iodides and bromides. Lithium tert-butoxide in DMF promotes this reaction for a variety of N-alkyl- and N-arylpyrroles as

Full text of DOI:10.1002/adsc.201000723

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DOI: 10.1002/adsc.201000723
Base-Mediated Direct Arylation of Pyrrole Derivatives
Olena Vakuliuk,a Beata Koszarna,a and Daniel T. Grykoa,b,*
a
Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224, Warsaw, Poland
Fax : (+48)-22-343-2036; e-mail: daniel@icho.edu.pl
Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
b
Received: September 22, 2010; Revised: December 13, 2010; Published online: April 12, 2011
In memory of Keith Fagnou.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000723.
Abstract: It appears that transition metal catalysts
are not necessary to perform the direct arylation of
electron-rich heterocycles with aryl iodides and bro-
mides. Lithium tert-butoxide in DMF promotes this
reaction for a variety of N-alkyl- and N-arylpyrroles
as well as for benzofuran and some other electron-
rich aromatic compounds and provides the desired
products in moderate to high yields. In contrast to
all previous reports on the Pd-catalyzed direct ary-
lation of indolizine, the reaction mediated by lithi-
um tert-butoxide proceeds selectively at position 5.
tion of one of the coupling partners, that is, direct ary-
lation, a process which has undergone significant de-
velopment in the last decade.[7]
One of the fundamental heterocyclic skeletons is
that of pyrrole and its importance is difficult to over-
estimate. Over the last few years, our team[8] as well
as other researchers,[9] have developed Pd-based con-
ditions for the direct coupling of pyrrole derivatives
with aryl halides. The importance of the pyrrole skele-
ton combined with the lack of an environmentally
friendly methodology prompted us to further focus on
this problem. Therefore, the central theme of our re-
search is the development of new competitive systems
for the direct arylation of pyrroles.
À
À
Keywords: aryl halides; C C coupling; C H activa-
tion; heterocycles; pyrroles
Recently, there has be some development in copper-,
iron- and nickel-based procedures for the arylation of
various heterocycles.[10,11] Amongst these discoveries,
it was found that copper-mediated coupling reactions
Recently, much attention has been paid towards demonstrated efficiency for both C- and N-arylation
searching for new and environmentally friendly alter- of various aromatic skeletons.[10] This approach ap-
natives for the construction of biaryl linkages between peared to be a valuable alternative to Pd-catalyzed
various homo- and heterocyclic units, which are ubiq- direct arylation. A model reaction was investigated
uitous as medicinal compounds.[1] These are also fun- with N-methylpyrrole (1) and 4-iodobenzonitrile (2)
damental building blocks in the synthesis of more using
complex molecules which possess interesting photo- (Table 1).
physical and biological properties.[2]
a variety of published catalytic systems
Conditions revealed by Do and Daugulis for the
Although, currently, numerous synthetic procedures direct arylation of benzoxazoles[12] offered only 2% of
are extensively used,[3] most of them utilize expensive the desired product. Reactions performed according
transition metal catalysts and require preactivation of to the procedure described by Miura and co-workers,
both reaction partners. Consequently, due to the lack consisting of a copper-based methodology for the
of greener alternatives, synthetic chemists have start- direct arylation of 1,3,4-oxadiazole and 1,2,4-triazole
ed to pay significant attention towards the use of aryl cores with aryl iodides,[13] showed traces of both
[4,5]
À
C H bonds as functional groups. Such oxidative in- mono- and bis-arylated products. Other screened cat-
termolecular coupling can be attractive and the sim- alytic systems provided only miniscule amounts of the
plest alternative to classical methods such as Suzuki– desired product 3. Finally, a methodology originally
Miyaura, Stille and Negishi reactions. This process, published for the N-arylation of pyrazole,[14] based on
however, is unfavourable from a thermodynamic per- copper(I) oxide and Cs2CO3, provided the desired
spective due to the strength of the aryl C H bond.
An interesting solution to this issue can be preactiva-
[6]
À
compound in 5% yield. Replacing Cs2CO3 with t-
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Olena Vakuliuk et al.
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Table 1. Influence of the reaction conditions on the coupling
of N-methylpyrrole (1) with 4-iodobenzonitrile (2).[a]
Compound 3 was not produced when the reaction
was performed without any base (Table 1, entry 7).
Changing the amount of base (Table 1, entries 8 and
9) also did not led to an increase in yield. Unexpect-
edly, the yield of the model reaction dropped to 8%
when an aged solution of t-BuOLi (previously used
for convenience) was replaced with the addition of
that base as a freshly prepared solution (Table 1,
entry 10). The decrease of the yield could be ex-
plained either by the formation of a ꢁcomplexꢂ be-
tween DMF and the base or by the decomposition of
t-BuOLi in solution. To provide evidence supporting
one of these concepts, additional experiments were
conducted. The addition of either Li2CO3 or LiOH re-
sulted in a decrease in the yield of product 3 (Table in
the Supporting Information). However, simple pre-
heating the solution of t-BuOLi in DMF provided us
with the desired product in a 31% yield. With this
result in hand, we focused on screening other polar
aprotic solvents; however, no improvement in yield
was observed (Table 1, entries 11–14). When an ex-
periment was performed in neat N-methylpyrrole, no
Entry Catalyst Base
Solvent Yield [%]
1
2
3
4
5
6
7
8
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
none
none
none
none
none
none
none
none
none
none
none
none
none
none
t-BuOLi
n-BuLi
LDA
DMF
DMF
DMF
DMF
DMF
DMF
DMF
8
4
4
DBU
trace
0
31
0
13
31
5
NaOH
t-BuOLi[b]
none
t-BuOLi[c] (1 equiv.) DMF
9
t-BuOLi[c] (4 equiv.) DMF
10
11
12
13
14
15[e]
16[f]
17[g]
18[h]
19[i]
t-BuOLi[d]
t-BuOLi[c]
t-BuOLi[c]
t-BuOLi[c]
t-BuOLi[c]
t-BuOLi[c]
t-BuOLi[c]
t-BuOLi[c]
t-BuOLi[c]
t-BuOLi[c]
DMF
DMSO 23
DMA
dioxane
NMP
None
DMF
DMF
DMF
DMF
13
7
11
0
15
25
49
87
evidence of compound
3
was found (Table 1,
entry 15). Other parameters, such as time and temper-
ature were also altered, but did not offer any im-
provement (Table 1, entries 16 and 17). Finally, by
using a larger excess of one of the coupling partners
(Table 1, entry 18 and 19), the desired product 3 was
isolated in a satisfactory yield. Other modifications to
the reaction conditions did not bring any further im-
provement (Table in the Supporting Information).
After establishing the optimal conditions (entry 19,
dry DMF, 2 equiv. of t-BuOLi solution prepared by
heating the base in DMF at 808C for 16 h, 15 equiv.
of pyrrole) for the desired transformation, the scope
and limitations of this approach were investigated by
synthesizing a broad range of 2-arylpyrroles and test-
ing a variety of iodoarenes with N-substituted pyr-
roles (Table 2).
[a]
If not noted otherwise, reactions were performed under
the following conditions: 4-iodobenzonitrile (1 equiv.), N-
methylpyrrole (5 equiv.), base (2 equiv.) catalyst (20 mol%),
1458C, 17 h; yields were determined by HPLC.
t-BuOLi was used as an aged solution.
t-BuOLi was used as preheated stock solution.
t-BuOLi was used as a freshly prepared solution.
The reaction was performed in the absence of DMF with
20 equiv. of N-methylpyrrole.
The reaction was performed at 1008C.
Yield was measured after 5 h.
For 10 equiv. of N-methylpyrrole.
For 15 equiv. of N-methylpyrrole.
[b]
[c]
[d]
[e]
[f]
[g]
[h]
[i]
BuOLi increased the yield of compound 3 up to 8%
(Table 1, entry 1).
Aryl iodides with various substituents were reacted
with N-methyl-, N-benzyl- and N-phenylpyrrole, lead-
Inspired by these preliminary results, we initiated a ing to products 13–21 in yields ranging from 53% to
systematic study of the reaction conditions. The first 92%. In all cases, complete regioselectivity was ob-
experiments were aimed towards optimization; specif- served. The reaction was found to be more effective
ically, by focusing on the type and amount of base when electron-withdrawing substituents were present
used. Bases such as n-BuLi, LDA, DBU and NaOH on the aryl iodide. Interestingly, aryl bromides turned
showed less efficiency in comparison to t-BuOLi out to be very efficient substrates for the described
(Table 1, entries 2–5). Other easily available sources system (Table 2). However, direct coupling of 4-
of copper were also investigated (Table in the Sup- chloro
ACHTUNGTRENbNUNG enzonitrile with N-methylpyrrole (1) yielded
porting Information), but a significant difference in only 16% of the desired product 3.
catalytic activity was not noted. Subsequently, other
Unfortunately, pyrrole itself and its derivatives
metal oxides were examined as catalysts (Table in the bearing N-electron-withdrawing protecting groups
Supporting Information). To our great surprise, yields (Ts, Boc), as well as N-TIPS-pyrrole, were unreactive,
remained consistent and led us to hypothesize that probably due to the lability of these groups under the
Cu2O does not participate in the catalytic cycle. An reaction conditions.
experiment where only a solution of t-BuOLi was
used supported our theory (Table 1, entry 6).
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Base-Mediated Direct Arylation of Pyrrole Derivatives
Table 2. Scope of the optimized conditions for the arylation
of pyrrole derivatives.[a]
Figure 1. Structures of products 22–26.
Taking into consideration that trace amounts of tran-
sition metals, which are known to catalyze direct ary-
lation, can be present in t-BuOLi, some additional ex-
periments were done to examine this issue. No differ-
ence in yield was noted in experiments when t-BuOLi
from different sources was used. Additionally, to elim-
inate this possibility, a detailed analysis of lithium
tert-butoxide via ICP-AES (inductively coupled
plasma-atomic emission spectrometry) was conducted.
The results clearly showed that the concentrations of
most of the transition metals (Pd, Fe, Cu, Rh and Ru)
in the t-BuOLi employed in our study were in the
range of 0.07–0.13 ppm (see the Supporting Informa-
[a]
If not noted otherwise, reactions were performed under
the following conditions: iodide (1 equiv.), N-methylpyr-
role (15 equiv.), base (2 equiv.), DMF.
Yields of isolated products after column chromatography.
Yields in brackets correspond to bromides.
4-Iodo(phenylethynyl).
[b]
[c]
[d]
It is worth mentioning that, for the reaction be- tion for details). Among metals which are known to
tween N-phenylpyrrole (4) and 4-iodobenzonitrile (2), catalyze certain coupling reactions, only nickel was
we were able to recover 85% of 4.
present at the level of 9 ppm. Experiments with the
Intrigued by the fact that base alone could mediate addition of nickel, palladium, copper and iron salts to
direct arylation of certain N-substituted pyrrole deriv- the model reaction showed no improvement in the
atives, we resolved to attempt the same conditions for yield of product 3. No difference was observed when
other electron-rich compounds.
the model reaction was performed in brand-new glass-
Various aromatic compounds were tested in reac- ware. Still, in view of the fundamental study by Lead-
tions with 4-iodobenzonitryle, under optimized reac- beater[18] showing that Suzuki coupling is catalyzed by
tion conditions. It was found that reactions with such palladium, present in sodium carbonate at the level of
substrates as benzene, 1,2-dimethoxybenzene, anthra- 50 ppb, a definitive conclusion cannot be drawn as to
cene and benzothiophene did not lead to products of whether or not the studied process is metal-free. A
direct arylation. On the other hand, more electron- very recent report by Shi and co-workers has de-
rich substrates like benzofuran, N-methylindole 1,3,5- scribed the direct arylation of benzene and some
trimethoxybenzene, indolizine gave the products 22– other aromatic compounds with aryl iodides and bro-
26, although the yields were rather moderate mides, mediated by t-BuOK and 1,10-phenanthro-
(Figure 1).
line.[19] In conjunction with our findings, a significant
It is noteworthy to emphasize that the arylation of excess of arene had to be used in order to achieve
indolizine proceeded selectively at position 5, leading good yields. It is notable, however, that unlike in our
to the strongly fluorescent product 25. This is very in- case, electron-deficient aryl iodides displayed dimin-
teresting, since all previous Pd-catalyzed arylations of ished reactivity. Although the role of the latter is un-
this heterocycle proceeded selectively at position 3.[15] clear at this moment, the authors claim that this re-
The direct arylation of 1,3,5-trimethoxybenzene with markable transformation was not catalyzed by transi-
aryl halides has never been reported before, although tion metals.
similar reactions have been developed.[16]
Four recent reports have described the metal-free
It is known that impurities present in commercially arylation as well as alkylation of electron-poor
available metal salts can catalyze various reactions.[17] hetero
cyclic compounds and benzene, which accord-
AHCTUNGTRENNUNG
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Olena Vakuliuk et al.
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ing to the authors, proceeds via a radical mecha- halides, but they may also open the door to practical
nism.[19,20] Initially, we assumed that, in the case of ar- applications and become a convenient pathway to a
ylation of N-alkylpyrroles, the same mechanism was variety of compounds bearing aryl-aryl linkages.
valid as well. The addition of tert-butyl peroxide and
tert-butyl peroxybenzoate, sometimes used as radical
scavengers, stopped the model reaction only when Experimental Section
they were used in an equimolar ratio to N-methylpyr-
role. The control experiments clearly showed that Representative Procedure
when N-methylpyrrole alone was heated with these
radical scavengers in a 1:1 ratio at 1458C, complete
Iodoarene (0.6 mmol, 1 equiv.) and pyrrole (9 mmol,
15 equiv.) were placed into a glass tube, flushed with argon.
decomposition occurred within 15 min. The utilization
of another radical scavenger such as 2,2,6,6-tetrame-
thylpiperidine-1-oxyl (TEMPO) did not completely
hinder the reaction either. The decrease of the yield
of 3 to 23% with 3 equiv. of TEMPO was probably
due to side reactions of the same type. In further ex-
periments, it turned out that N-phenylpyrrole was
more resistant to oxidation promoted by TEMPO (in
comparison to N-methylpyrrole). In the reaction of
N-phenylpyrrole with 4-iodobenzonitrile (2) in the
presence of 3 equiv. of TEMPO, the yield of product
20 decreased, but only to 30%. Finally, the influence
of galvinoxyl on the rate of reaction was studied. The
model reaction carried out in the presence of 3 equiv.
of galvinoxyl did not afford the desired product.
Again, however, some side reactions took place. The
Subsequently 750 mL of stock solution of the base in DMF
(containing 1.2 mmol of t-BuOLi) were added. The resulting
reaction mixture was stirred overnight at 1458C. Subse-
quently, it was evaporated under reduced pressure and puri-
fied by column chromatography.
Acknowledgements
This study was financed by the European Community [RTN
Network (R) Evolutionary Catalysis MRTN-CT-2006-
035866]. We thank Dorota Gryko for useful discussion and
valuable suggestions, Jan Klajn for sample of indilizine and
Keith O. Proinsias for the amending the manuscript.
analogous reaction with N-phenylpyrrole was also References
quenched with 3 equiv. of galvinoxyl and N-phenyl-
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