NH-Heterocyclic Aryliodonium Salts and their Selective Conversion into N1-Aryl-5-iodoimidazoles

Article abstract of DOI:10.1002/anie.201602569

The synthesis of N-arylimidazoles substituted at the sterically encumbered 5-position is a challenge for modern synthetic approaches. A new family of imidazolyl aryliodonium salts is reported, which serve as a stepping stone on the way to selective formation of N1-aryl-5-iodoimidazoles. Iodine acts as a “universal” placeholder poised for replacement by aryl substituents. These new λ³-iodanes are produced by treating the NH-imidazole with ArI(OAc)₂, and are converted to N1-aryl-5-iodoimidazoles by a selective copper-catalyzed aryl migration. The method tolerates a variety of aryl fragments and is also applicable to substituted imidazoles.

Full text of DOI:10.1002/anie.201602569

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Communications
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International Edition: DOI: 10.1002/anie.201602569
German Edition: DOI: 10.1002/ange.201602569
Arylation Reagents
NH-Heterocyclic Aryliodonium Salts and their Selective Conversion
into N1-Aryl-5-iodoimidazoles
Yichen Wu, Susana Izquierdo, Pietro Vidossich, Agustí Lledós,* and Alexandr Shafir*
Abstract: The synthesis of N-arylimidazoles substituted at the
sterically encumbered 5-position is a challenge for modern
synthetic approaches. A new family of imidazolyl aryliodo-
nium salts is reported, which serve as a stepping stone on the
way to selective formation of N1-aryl-5-iodoimidazoles. Iodine
acts as a “universal” placeholder poised for replacement by
aryl substituents. These new l3-iodanes are produced by
treating the NH-imidazole with ArI(OAc)2, and are converted
to N1-aryl-5-iodoimidazoles by a selective copper-catalyzed
aryl migration. The method tolerates a variety of aryl frag-
ments and is also applicable to substituted imidazoles.
Scheme 1. Examples of common imidazole N-arylation strategies (A)
and the relay arylation (B) proposed here.
I
midazole is a ubiquitous heterocyclic core present in a wide
a hypervalent iodoarene fragment[8] serves as a trampoline for
aryl transfer to the proximal NH site (Scheme 1B). We
reasoned that if the iodane I could be generated, it can then
undergo a phenyl transfer to produce II, perhaps akin to the
intramolecular O- and N-arylation observed in iodonium
ylides.[9] Somewhat surprisingly, the NH-heterocyclic
l3-iodanes have only received limited attention beyond the
early work by Neiland et al. in the 1970ꢀs.[10,11] However,
recent reports highlight the promise of hypervalent iodine
reactivity in azole functionalization, including via heterocyclic
l3-iodanes.[12]
range of biologically relevant molecules.[1] Although the
synthesis of imidazole derivatives is commonly accomplished
through a variety of cyclization routes, it is often desirable to
obtain a particular derivative starting from a preformed
heterocyclic ring. For this reason, imidazole derivatization has
been the focus of attention from a number of laboratories.
A particularly common challenge is the selective construction
of the 1,4- and 1,5-disubstituted imidazoles. Thus, the
NH-arylation of an imidazole substituted at the C4(5)
position tends to produce a mixture of isomers favoring the
sterically less encumbered NH position, and thus, the
1,4-substitution pattern.[2,3] This bias was recently perfected
by Buchwald et al. using a highly bulky biaryl phosphine
In particular, we found only a single precedent for an
imidazolyl-l3-iodane derived from unprotected imidazole;[13]
this species, however, was described as an imidazole fragment
bound to iodine through the nitrogen atom.[13a] A reaction
between PhI(O2CCF3)2 and imidazole (2 equiv) in acetoni-
trile at room temperature produced a white precipitate
identified as [PhI(Imid)][TFA] salt, 1a (58%; TFA = tri-
fluoroacetate). However, the presence of just two imidazolic
ligand in palladium-catalyzed imidazole N-arylation.[3b]
similar preference for the less encumbered NH position can
also be seen in the oxidative Chan–Lam N-arylation of
imidazole (Scheme 1A).[4]
However, a challenge remains to selectively access the
corresponding 1,5-disubstituted imidazoles. Progress made in
recent years includes the use of well-designed protection/
deprotection strategies,[5] and the C5-selective CH-boryla-
tion[6] and CH-arylation[7] reactions.
Herein, we present a new route to a versatile class of
precursors for 1,5-disubstituted imidazoles. Specifically, the
N1-aryl-5-iodoimidazoles are produced via a relay in which
A
1
resonances in H NMR (1H each) strongly suggested a CH
rather than NH functionalization of the imidazole. Accord-
ingly, X-Ray crystallography revealed a classical T-shaped
diaryliodonium environment, with the imidazole bound to the
iodine through the C4(5) carbon atom (Scheme 2). An
analogous acetate salt 2a was obtained by employing
PhI(OAc)2. A DFT analysis confirmed that both the C2 and
the N-bound isomer are higher in energy than the observed
C4(5) isomer. An N-bound species was found unlikely even as
an intermediate on the way to 1a; rather, the reaction
appeared to proceed through a Wheland-type intermediate
(see Supporting Information).
[*] Y. Wu, Dr. S. Izquierdo, Dr. A. Shafir
Institute of Chemical Research of Catalonia (ICIQ), Barcelona
Institute of Science and Technology
Av. Països Catalans 16, 43007 Tarragona (Spain)
E-mail: ashafir@iciq.es
While sparingly soluble in CDCl3, 1a and 2a dissolved
well in MeOH and water. They also underwent a facile
deprotonation into zwitterionic 3, for which both the solid
state and DFT structures show an essentially “normal” single
Dr. P. Vidossich, Prof. Dr. A. Lledós
Departament de Química, Universitat Autònoma de Barcelona
08193 Cerdanyola del Vall›s (Spain)
E-mail: agusti@klingon.uab.es
À
Cimid I bond (2.051 and 2.076 Š, respectively, vs. 2.091 Š
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201602569.
observed for 1a). We quickly discovered that the desired
iodine-to-nitrogen phenyl transfer does not take place upon
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However, CH3CN remained convenient for large scale
applications because of favorable product precipitation, as
seen in the synthesis of a 23 g batch of 2a (Supporting
Information). All the aryl(imidazolyl)-l3-iodanes, 2, exhibited
the corresponding Ar-I(Imid)+ cation in the HR (ESI +) mass
spectra. These species were subsequently transformed into
the N1-aryl-5-iodoimidazole, 4, with good selectivities. As
previously observed for 4a, in all cases a characteristic 13C
resonance at 71–73 ppm was observed for the 13C-I unit in 4,
which is approximately 10 ppm lower than in the correspond-
ing 1,4 species 5 (82–85 ppm). Given the synthetic potential of
4a, the method was extended to structurally diverse
aryl(imidazolyl)-l3-iodanes (Table 2). The most robust pro-
tocol involves the use of 20 mol% of N-Me-benzimidazole in
combination with 5 mol% of Cu(OTf)2.
Scheme 2. Formation and structures of the imidazole-based l3-iodanes
and of the neutral (betaine) 3. Gibbs Energies (kcalmolÀ1) in CH3CN.
heating 1a, 2a, or 3 in CH2Cl2, with or without Cs2CO3.
The improved selectivity achieved with these additives is
likely to be due to the formation of copper-heterocycle
complexes. Indeed, best results were achieved by pre-mixing
Cu(OTf)2 with the additive and base for 20 min, presumably
favoring complex formation. We observed that, while
Cu(OTf)2 alone did not dissolve in HFIP, a green solution
formed upon addition of N-Me-benzimidazole.
Consistently, only
a
high energy transition state
(35.6 kcalmolÀ1) could be identified for the direct (non-
catalyzed) iodine-to-nitrogen 1,3 phenyl migration in 3
(Scheme 3).
Both electron-donating and mildly electron-withdrawing
substituents were well tolerated on the aryl fragment (4b–i,
Table 2). In fact, even a di-ortho substitution was tolerated, as
illustrated in the successful synthesis of the highly hindered
N-mesityl-5-iodoimidazole, 4j. We were particularly pleased
with the successful incorporation of a second heterocycle, as
in the 2- and 3-thienyl derivatives 4k and 4l. The
4-iodobiphenyl and 2-iodonaphthalene derivatives could
also be obtained in 70% and 74% yield, respectively (4m
and 4n). In the case of the 4-Me-imidazolyl iodane 2o, a 13:1
4/5 selectivity was achieved, affording the target 4o in an 87%
yield. In this case, selectivity evidently benefited from
hindrance at the competing N-site. The aryl transfer in the
2-Me derivative 2p was less efficient, providing 4p in 31%
yield. The method was also applied to produce an 82% of the
4,5-diiodo derivative 4q. In general, chromatographic sepa-
ration between 4 and 5 proved straightforward.
As mentioned earlier (see Scheme 1), the high selectivity
towards 4 stems from an intramolecular aryl migration from
iodine to the proximal nitrogen.[15] Accordingly, a crossover
experiment between 2a-d2 and 2c revealed a predominant
formation of 4a-d2 and 4c, as expected for an intramolecular
process (Scheme 4A).[16] Small amounts of the 1,4 isomers
were also produced, for which full aryl/imidazole scrambling
was observed, indicating their origin in a bimolecular process.
Indirect support for an intramolecular process was also
obtained from the poor performance of the pyrazole-derived
iodane 6 (< 15% yield, Scheme 4B), which lacks a proximal
NH site.
Scheme 3. Reaction path modelled for uncatalyzed 1,3 phenyl
migration.
Gratifyingly, the addition of 5 mol% of Cu(OTf)2 did
allow for the formation of two regioisomeric N-phenyl
iodoimidazoles, and a moderate selectivity for the more
hindered 4a was achieved in fluorinated alcohols (Table 1,
runs 1–3; both isomers confirmed by X-Ray diffraction). The
use of Cs2CO3 in hexafluoroisopropanol (HFIP) led to
a combined yield of 86% with a 4:1 ratio in favor of 4a
(run 4). This ratio was further improved by employing
catalytic amounts of certain heterocyclic additives (runs
5–7). For example, the use of 20 mol% of N-Me-benzimid-
azole (run 6) led to an 8:1 selectivity and a 93% yield.
It was subsequently found that the highest yields of 2 were
achieved in trifluoroethanol[14] and, notably, MeOH solvents.
Table 1: Copper-catalyzed iodine-to-nitrogen phenyl transfer in 2a.[a]
Run Base
Solvent
Additive
Yield [%][b] 4a/5a[b]
1
2
3
4
5
6
7
–
–
–
CH2Cl2
CF3CH2OH
HFIP
–
–
–
–
39
51
53
86
90
90
0.1:1
1.5:1
4.2:1
4.1:1
7.3:1
8.4:1
8.0:1
We envisaged that 3 (formed upon deprotonation of 2),
binds a CuI-OTf fragment through N1 (Scheme 5).[17,18]
Indeed, despite employing a CuII precatalyst, the true
catalytic species is likely a CuI center.[18,19] The inclusion of
MeOH in the coordination sphere of copper (as a stand-in
solvent molecule) was found to be beneficial to properly
describe the copper intermediate, and given that the process
was already moderately selective (up to 4:1) in the absence of
Cs2CO3 HFIP
Cs2CO3 HFIP
Cs2CO3 HFIP
Cs2CO3 HFIP
4-methylimidazole
benzimidazole
N-Me-benzimidazole 93
[a] Using 2a (0.5 mmol), Cu(OTf)2 (5 mol%), and base (1.6 equiv, if any)
in solvent (2.6 mL). [b] Total yield (%4a+%5a) and the ratio, as
determined by GC.
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Table 2: Scope of the relay synthesis of N1-aryl-5-iodoimidazoles 4.
Scheme 5. A DFT profile for the CuI-catalyzed aryl migration. Relative
Gibbs energies in methanol (kcalmolÀ1).
an additive, this initial DFT study was performed in the
absence of an added heterocycle. In the first step, the phenyl
group in A is transferred from iodide to copper, leading to
a formal CuIII-phenyl intermediate B.[19,20] This step features
an activation barrier of 26.2 kcalmolÀ1 (ts-1). A Localized
Orbital analysis supports the change in copper oxidation state
and allows visualization of the flow of electrons (see small
green spheres of ts-1 in Scheme 5 and Supporting Informa-
À
tion). The final C N bond is formed through an essentially
barrierless reductive elimination step (Scheme 5, ts-2). Given
the energetic proximity between B and ts-2, the mechanism
resembles
a copper-guided concerted iodine-to-nitrogen
phenyl migration. A preliminary investigation also revealed
that the coordination of N-Me-benzimidazole to the CuI
center may disfavor the binding of two molecule of 3 to the
same copper center, hence enforcing an intramolecular
phenyl transfer.[21]
In agreement with Scheme 5, the preformed zwitterionic 3
was also an excellent substrate even in the absence of a base
[Eq. (1)].
[a] 1H NMR yield (isolated yield). [b] Yield of isolated products. [c] 4/5
ratio determined by GC. [d] Benzimidazole (20 mol%) as additive.
[e] 4-methylimidazole (20 mol%) as additive. [f] Ar-I(imid)+OAcÀ was
added before injection of the solvent; no additive was used.
The reason for the poor performance of solvents such as
CH2Cl2 is likely two-fold. The deprotonation of 2 in CH2Cl2
appears sluggish, which negatively affects the selectivity,
giving rise to bimolecular crossover events (see Supporting
Information). Additionally, while the use of 3 in CH2Cl2 does
render the reaction moderately selective, the rate remains
low.
Iodine introduced at the C5 position enabled the synthesis
of a wide spectrum of 1,5-imidazole derivatives (Scheme 6).
Thus, the 5-alkynyl and 5-aryl derivatives 7 and 8 were
À
prepared by palladium-catalyzed C C coupling reactions.
À
Additionally, copper-catalyzed C N bond formation was
readily accomplished to give 9.[22] The 5-iodoimidazole 2a
was also readily converted into an organomagnesium
species,[23] which served as a precursor to the 5-formyl and the
5-borylderivatives 10 and 11.[23b,c]
Scheme 4. Crossover experiment (A), and the assay with pyrazole (B).
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and aryliodine diacetate, ArI(OAc)2, serves as an excellent
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complements common existing methods known to produce
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Acknowledgements
This work was funded by Fundació ICIQ, MINECO
(CTQ2013-46705-R, CTQ2014-54071-P and 2014-2018
Severo Ochoa Excellence Accreditation SEV-2013-0319)
and the Generalitat de Catalunya (2014 SGR 1192). The
CELLEX Foundation is gratefully acknowledged for a post-
doctoral contract to S.I. and for support through the
CELLEX-ICIQ HTE platform.
À
À
Keywords: C H functionalization · C N coupling ·
copper catalysis · hypervalent iodine · imidazoles
How to cite: Angew. Chem. Int. Ed. 2016, 55, 7152–7156
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Received: March 13, 2016
Published online: May 3, 2016
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