Copper-catalyzed N-arylation of azoles and diazoles using highly functionalized trivalent organobismuth reagents

Article abstract of DOI:10.1039/c4ra02467b

The N-arylation of indoles, indazoles, pyrroles, and pyrazoles using highly functionalized trivalent arylbismuth reagents is reported. The reaction is promoted by a substoichiometric amount of copper acetate, and it tolerates a wide diversity of functional groups on the azole and the organobismuth reagent. The method is also applied to the N-arylation of tryptophan derivatives. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02467b

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Copper-catalyzed N-arylation of azoles and
diazoles using highly functionalized trivalent
Cite this: RSC Adv., 2014, 4, 22255
organobismuth reagents†
Received 20th March 2014
Accepted 17th April 2014
*
Pauline Petiot, Julien Dansereau and Alexandre Gagnon
DOI: 10.1039/c4ra02467b
www.rsc.org/advances
The N-arylation of indoles, indazoles, pyrroles, and pyrazoles using However, the method was applied exclusively to the transfer of
highly functionalized trivalent arylbismuth reagents is reported. The an unsubstituted phenyl group and required the use of a
reaction is promoted by a substoichiometric amount of copper pentavalent bismuth species, which is less stable than its
acetate, and it tolerates a wide diversity of functional groups on the trivalent counterpart. More importantly, C3-arylation was
azole and the organobismuth reagent. The method is also applied to observed in most cases, except when the C3 position was
the N-arylation of tryptophan derivatives.
blocked by an alkyl group or when an ester was present at the C2
position of indole.
In 1996, Chan published a variation of a protocol disclosed
by Barton and Finet,15 in which trivalent bismuthanes are used
to N-arylate nitrogenated compounds (Scheme 1).16 While this
method is very useful, it suffers from multiple limitations as it
requires up to 2.0 equivalents of the organobismuth reagent, 1.5
equivalents of the copper catalyst and reaction times that are as
long as 72 hours. Moreover, the method was not applied to
indoles, indazoles, pyrazoles, or pyrroles, but rather to type C
compounds, in which the nitrogen atom is connected to a
carbonyl functional group. Finally, functional group tolerance
was not demonstrated, as only simple arylbismuthanes were
coupled with substrates bearing no functionalities.
Azoles and diazoles are common scaffolds that are frequently
used in medicinal chemistry to project key pharmacophores
along different vectors inside the binding pocket of a biological
target.1,2 N-Arylation of these nitrogenated compounds makes it
possible to screen for new interactions around an inhibitor3 and
to modify its biophysical properties.4 N-Arylazoles have also
found numerous applications in material and polymer
sciences.5 N-Arylazoles and diazoles are commonly prepared via
metal-catalyzed N-arylation6 of N–H heteroarenes using aryl
halides,7 arylboronic acids,8 or aryllead9 reagents. However, lead
reagents are highly toxic, and other methods sometimes require
extended reaction times, super-stoichiometric amounts of
catalyst or costly ligands to obtain good yields. Consequently,
new procedures that lead to the facile installation of densely
functionalized aryl groups on azoles and diazoles are still
desirable.
Over the past years, we have disclosed a portfolio of reactions
for the formation of C–C,10 C–N,11 and C–O12 bonds using
organobismuth reagents. Organobismuthanes can be prepared
from inexpensive and non-toxic bismuth salts and offer mild
reactivity that tolerates the presence of numerous functional
groups on both the substrate and the organometallic species.13
Barton and Finet reported in 1988 the use of triphenylbismuth-
bis-triuoroacetate in the arylation of indole A (Scheme 1).14
´
´
´
` ´
a Montreal, C.P. 8888, Succ.
Departement de chimie, Universite du Quebec
´
´
Centre-Ville, Montreal, Quebec, H3C 3P8, Canada. E-mail: gagnon.alexandre@
uqam.ca
† Electronic supplementary information (ESI) available. CCDC 986210. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4ra02467b
Scheme 1 Comparison of our N-arylation reaction with precedents in
the literature.
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We report herein the rst method for the N-arylation of improvement in the yield of the reaction was observed on
azoles and diazoles E using highly functionalized trivalent conducting the reaction in acetonitrile (entry 5). We recently
bismuth reagents and promoted by catalytic amounts of copper reported the benecial effect of oxygen during our studies on
acetate. The procedure exclusively generates the N-arylation the O-arylation of phenols using trivalent organobismuthanes.12
product and shows exceptional functional group tolerance on Therefore, we next investigated the use of oxygen as the reaction
both coupling partners. The procedure is also applied to the N- atmosphere and observed a near quantitative yield for the
arylation of tryptophan derivatives to afford N-arylindolyl formation of 2 (entry 6). To further validate the importance of
products.
oxygen, we performed the reaction under argon and observed a
We began by optimizing the conditions for the phenylation yield similar to when the reaction was run under ambient air,
of methyl indole-5-carboxylate 1 using the Barton–Finet–Chan thus conrming the positive effect of oxygen on the reaction
protocol (Table 1).15,16 Upon reacting 1.0 equivalent of triphe- (entry 7 vs. 3). Encouraged by this observation, we then gradu-
nylbismuth with indole 1 in the presence of 1.5 equivalents of ally lowered the amount of catalyst and found that the yield was
copper acetate and 3.0 equivalents of pyridine under air, we retained upon using 10 mol% of copper acetate (entry 8).
obtained the N-arylated product 2 regioselectively in quantita- However, reducing the loading further was not tolerated and
tive yield (entry 1). This result is interesting and important for produced only a modest yield of the desired N-phenyl indole 2
two reasons. First, to the best of our knowledge, this is the rst (entry 9). Although Barton reported that the exclusion of oxygen
time that trivalent organobismuth reagents are used directly in led to a negative impact on the arylation of amines with trivalent
the N-arylation of indoles, eliminating the need to prepare the organobismuthanes,15 the possibility of lowering the copper
less stable bis-triuoroacetate species. Second, in contrast with acetate loading by performing the reaction under pure oxygen
triphenylbismuth-bis-triuoroacetate, the process involving was not demonstrated. It is likely that the role of the oxygen is to
triphenylbismuth leads exclusively to N-arylation, as opposed to oxidize the copper species with lower valency, which are
C3-arylation. Using 0.7 equivalent of the organobismuth species generated during the reaction, to the +2 oxidation state. To
resulted in a 25% decrease in the yield of the reaction, sug- minimize the amount of base, we performed the reaction using
gesting that only one phenyl group can be transferred from the 1.0 equivalent of pyridine and still obtained a near quantitative
organobismuth species during the reaction (entry 2).
yield of product 2 (entry 10). The exploration of non-haloge-
In order to develop a more efficient protocol, we next sought nated solvents (entries 11–13) led to the identication of THF as
to reduce the catalyst loading. Unfortunately, the use of a stoi- the best alternative to dichloromethane. Using our optimal
chiometric amount of copper acetate led to a signicant conditions, we next explored the impact of varying the steric
decrease in the yield of the reaction (entry 3), motivating us to and electronic properties of the organobismuthane on the yield
revisit other parameters of the reaction. Changing the base to of the arylation reaction. The functionalized organobismuthanes
triethylamine proved inconsequential (entry 4), but an
Table 1 Optimization of reaction conditions for the N-phenylation of
methyl-indole-5-carboxylate 1 using triphenylbismuth
Ph3Bi
Cu(OAc)2
Entry (x equiv.) (y equiv.) Base
Solvent Atm. Yielda (%)
1
2
3
4
5
6
7
8
1.0
0.7
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.5
1.5
1.0
1.0
1.0
1.0
1.0
0.1
0.05
0.1
0.1
0.1
0.1
Pyridine CH2Cl2 Air
Pyridine CH2Cl2 Air
Pyridine CH2Cl2 Air
99
74
45
45
76
96
47
99
45
96
82
57
54
Et3N
CH2Cl2 Air
Pyridine CH3CN Air
Pyridine CH2Cl2 O2
Pyridine CH2Cl2 Ar
Pyridine CH2Cl2 O2
Pyridine CH2Cl2 O2
Pyridine CH2Cl2 O2
9
10b
11
12
13
Pyridine THF
Pyridine DME
O2
O2
Pyridine CH3CN O2
a
Isolated yield of pure product 2. b Reaction performed with 1.0 equiv.
Fig. 1 Functionalized organobismuthanes 3a–q used in the N-aryla-
of base.
tion of azoles and diazoles.
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needed for this study (Fig. 1) were prepared according to proce- lipophilicity (log D) of arylated products that are generated in
dures that we previously reported.10b,12
the next step. A Horner–Emmons–Wadsworth reaction was also
Our studies show that the transfer of para and meta-tolyl performed on 3m to furnish the cinnamyl ester 3t. This func-
groups proceeds efficiently, delivering the corresponding tional group is also frequently found in numerous bioactive
products 4a and 4b in excellent yields (Scheme 2). The transfer compounds.
of the more sterically hindered ortho-tolyl group proved more
The scope of the reaction was then studied by arylating
challenging and required a higher catalyst loading to provide different azoles and diazoles 5 using highly functionalized tri-
the desired product 4c in an acceptable yield. While it is difficult arylbismuthanes 3a–t (Scheme 4). Our investigations reveal that
to establish a clear correlation between the electronic properties the reaction proceeds smoothly on indoles (6a–o), pyrroles (6p,
of the organobismuthane and the yield of the arylation reaction, q), and pyrazoles18 (6r, s) to afford the desired N-arylated
the results obtained to date demonstrate that good yields are products in good to excellent yields. These results also suggest
afforded by using triarylbismuthanes substituted with electron- that the substitution pattern of the indole has little impact on
donating (4a, b, d) and electron-withdrawing (4e–h) groups. the outcome of the reaction. However, using our conditions, we
Interestingly, a 2,6-dimethylphenyl unit was installed on indole were not able to arylate 7-methylindole, possibly due to the
1
using our conditions, although with
(compound 4i). Note that very few methods exist for the transfer formation of the product. In the case of pyrazole 6s, the product
of this highly hindered group.17
of arylation on the nitrogen distal to the tolyl group was
a modest yield development of a strong 1,3-allylic-type strain during the
In order to further expand the functional group tolerance, we obtained, as conrmed by NMR studies. Importantly, the
prepared three new bismuth reagents by performing a func- procedure tolerates a wide variety of functional groups on the
tional group manipulation directly on selected organo- azole such as aldehydes (6a–c), nitriles (6d–f), O-acetates (6g),
bismuthanes. As illustrated in Scheme 3, triarylbismuthanes 3r nitro groups (6m), amides (6n, o), and ketones (6p, q). Note that
and 3s, bearing alcohol functional groups, were synthesized by
Grignard addition on the ester 3p and on the aldehyde 3m,
respectively. These groups are important in medicinal chem-
istry as the presence of an alcohol group controls the
Scheme 2 Steric and electronic effects of organobismuthane on the
a
N-arylation of 1.
Alternative conditions were employed for
compound 4c: Ar3Bi (1.0 equiv.), Cu(OAc)2 (1.0 equiv.), pyridine (3.0
equiv.), CH2Cl2, O2, 50 ꢀC, 12 h.
Scheme 4 N-Arylation of azoles and diazoles 5 using functionalized
triarylbismuthanes 3a–t. a Condition A: Ar3Bi (1.0 equiv.), Cu(OAc)2 (0.1
equiv.), pyridine (1.0 equiv.), CH2Cl2, O2, 50 ꢀC; condition B: Ar3Bi (1.0
equiv.), Cu(OAc)2 (0.1 equiv.), pyridine (3.0 equiv.), CH2Cl2, O2, 50 ꢀC;
Scheme 3 Preparation of highly functionalized organobismuthanes condition C: Ar3Bi (1.0 equiv.), Cu(OAc)2 (1.0 equiv.), pyridine (3.0
by functional group manipulation.
equiv.), CH2Cl2, O2, 50 ꢀC.
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pyrazoles that proceeds directly using highly functionalized
trivalent organobismuth reagents. The transformation is
promoted by catalytic amounts of copper acetate and tolerates
an exceptional diversity of functional groups on both coupling
partners, giving access to highly functionalized azoles. The
protocol was also applied to the N-arylation of tryptophan
derivatives. Application of this methodology to other arylation
reactions, including amino-acids and tryptophan-containing
peptides, is in progress in our laboratory and the results will be
reported in due course.
a
Scheme 5 N-Arylation of 1H-indazole-6-carbaldehyde 7. Condi-
tions: (p-FC6H4)3Bi (1.0 equiv.), Cu(OAc)2 (1.0 equiv.), pyridine (3.0
b
equiv.), DCM, 50 ꢀC, O2, o.n.; ORTEP diagram of compound 8b:
thermal ellipsoids are shown at the 50% probability level.
Acknowledgements
This work was supported by the Natural Sciences and Engi-
neering Research Council of Canada (NSERC) and the Uni-
`
´
´
`
´
versite du Quebec a Montreal (UQAM). We thank Pr. Xavier
Ottenwaelder, Mohammad S. Askari (Concordia University),
Scheme 6 N-Arylation of tryptophan derivatives 9a, b.
´
´
´
´
and Francine Belanger-Gariepy (Universite de Montreal) for the
crystallographic analysis of compound 8b and Dr Alexandre
`
Arnold (UQAM) for NMR analysis of compound 6s. P.P. thanks
bromides (6h, i), and iodides (6k), which could be expected to
interfere in the direct N-arylation of azoles with aryl halides,
were found to be inert under these conditions. Numerous
functional groups can also be present on the organobismuth
reagents, such as acetals (6a, m, n), uorides (6b, c), aldehydes
(6f, p), esters (6i), ethers (6q, s) and a,b-unsaturated esters (6k).
Interestingly, we found that an alcohol can be present on the
substrate (6j, l) or on the arylbismuth reagent (6r) without
undergoing O-arylation; moreover, a secondary amide is not
`
UQAM for a FARE scholarship. J.D. thanks the Faculty of
Sciences of UQAM for an undergraduate research scholarship.
A.G. thanks Dr Hugo Lachance for useful discussions.
`
Notes and references
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´
arylated under these conditions (6n). Lastly,
a cyclo-
´
propylphenyl (6e) and a triuoromethylphenyl (6g) group were
efficiently transferred using this method. These groups are
important in medicinal chemistry as they show increased
metabolic stability compared to typical alkyl groups.19
The N-arylation of indazoles was next evaluated, keeping in
mind that a mixture of regioisomers resulting from arylation at
N1 and N2 is oen obtained with other methods.18,20 When 1H-
indazole-6-carbaldehyde 7 was subjected to our conditions, the
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lished by X-ray crystallography.
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chemistry as these species are involved in numerous biological
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peptides containing tryptophan residues has also found appli-
cations in chemical biology.22 To the best of our knowledge, very
few methods for the N-arylation of tryptophan derivatives have
been reported in the literature.23 Notably, Boc- and Fmoc-pro-
tected tryptophan derivatives 9a and 9b were smoothly N-ary-
lated using our protocol to selectively provide the N-indolyl
derivatives 10a, b in good yield (Scheme 6).24
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22258 | RSC Adv., 2014, 4, 22255–22259
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´
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