Light-Induced C-H Arylation of (Hetero)arenes by in Situ Generated Diazo Anhydrides

Article abstract of DOI:10.1002/chem.201502357

Diazo anhydrides (Ar-N=N-O-N=N-Ar) have been known since 1896 but have rarely been used in synthesis. This communication describes the development of a photochemical catalyst-free C-H arylation methodology for the preparation of bi(hetero)aryls by the one-pot reaction of anilines with tert-butyl nitrite and (hetero)arenes under neutral conditions. The key step in this procedure is the in situ formation and subsequent photochemical (>300 nm) homolytic cleavage of a transient diazo anhydride intermediate. The generated aryl radical then efficiently reacts with a (hetero)arene to form the desired bi(hetero)aryls producing only nitrogen, water, and tert-butanol as byproducts. The scope of the reaction for several substituted anilines and (hetero)arenes was investigated. A continuous-flow protocol increasing selectivity and safety has been developed enabling the experimentally straightforward preparation of a variety of substituted bi(hetero)aryls within 45 min of reaction time.

Full text of DOI:10.1002/chem.201502357

DOI: 10.1002/chem.201502357
Communication
&
Flow Chemistry
Light-Induced CÀH Arylation of (Hetero)arenes by In Situ
Generated Diazo Anhydrides
David Cantillo,[a] Carlos Mateos,[b] Juan A. Rincon,[b] Oscar de Frutos,*[b] and
C. Oliver Kappe*[a]
abundant catalysts. In this context, the synthesis of bi(het-
Abstract: Diazo anhydrides (ArÀN=NÀOÀN=NÀAr) have
been known since 1896 but have rarely been used in syn-
thesis. This communication describes the development of
a photochemical catalyst-free CÀH arylation methodology
for the preparation of bi(hetero)aryls by the one-pot reac-
tion of anilines with tert-butyl nitrite and (hetero)arenes
under neutral conditions. The key step in this procedure is
the in situ formation and subsequent photochemical
(>300 nm) homolytic cleavage of a transient diazo anhy-
dride intermediate. The generated aryl radical then effi-
ciently reacts with a (hetero)arene to form the desired
bi(hetero)aryls producing only nitrogen, water, and tert-
butanol as byproducts. The scope of the reaction for sev-
eral substituted anilines and (hetero)arenes was investigat-
ed. A continuous-flow protocol increasing selectivity and
safety has been developed enabling the experimentally
straightforward preparation of a variety of substituted
bi(hetero)aryls within 45 min of reaction time.
ero)aryl motifs by direct arylation of (hetero)arenes has re-
ceived significant attention during the past decade owing to
the high atom-economy of CÀH activation reactions and the
broad availability of unfunctionalized aromatic compounds.[5]
Most recently, two elegant light-mediated methods based
on the formation of aryl radicals from diazonium salts by
means of photoredox catalysis have been described by the
groups of Kçnig[6] and Sanford.[7] In these protocols, a photore-
dox catalyst ([Ru(bpy)3Cl2] or Eosin Y) is excited by visible light
and subsequently—by single-electron transfer from the excited
photocatalyst to the diazonium salt—the corresponding aryl
radicals are generated which then react with the arene or het-
eroarene (Scheme 1a).[8] In view of results described in older lit-
The synthesis of bi(hetero)aryl moieties arguably is one of the
most important transformations in organic chemistry due to
the omnipresence of this structural motif in active pharmaceut-
ical ingredients, agrochemicals, and organic materials.[1] The
emergence of Pd-catalyzed cross-coupling chemistry in the
1980s has undoubtedly revolutionized bi(hetero)aryl synthesis,
broadening the scope and increasing the selectivity and effi-
ciency for the preparation of these types of structures.[2,3] De-
spite the extraordinary success of cross-coupling chemistry, the
search for novel methodologies for the preparation of bi(het-
ero)aryls continues.[4] These efforts are generally motivated by
the ambition to utilize more atom-economic and sustainable
starting materials (taking into account both the organometallic
species and the aryl halide), and to replace the generally re-
quired Pd catalyst/ligand system with less expensive and more
Scheme 1. Photoredox catalysis and catalyst-free procedures for the CÀH ar-
ylation of arenes and heteroarenes.
erature on aryl radical chemistry associated to the homolytic
cleavage of diazo species,[9] it appeared to us that an even sim-
pler CÀH arylation route starting directly from anilines avoiding
the use of a photoredox catalyst could be conceived. In 1962
Müller and Haiss have described the radical decomposition of
dimeric forms of aromatic nitrosamine/diazo hydroxide deriva-
tives,[10] termed diazo anhydrides (ArÀN=NÀOÀN=NÀAr) when
first isolated by Bamberger in 1896.[11] Notably, when the radi-
cal fragmentation is performed in aromatic solvents, such as
benzene or chlorobenzene, the formation of biaryl products
[a] Dr. D. Cantillo, Prof. Dr. C. O. Kappe
Institute of Chemistry, University of Graz, NAWI Graz
Heinrichstrasse 28, 8010 Graz (Austria)
E-mail: oliver.kappe@uni-graz.at
Homepage: http://www.maos.net
[b] Dr. C. Mateos, Dr. J. A. Rincon, Dr. O. de Frutos
Centro de Investigación Lilly S. A.
Avda. de la Industria 30, 28108 Alcobendas-Madrid (Spain)
E-mail: de_frutos_oscar@lilly.com
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502357.
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Communication
was observed.[10] A related radical decomposition performed
under mildly basic conditions had previously been utilized by
Gomberg and Bachman for the generation of biaryls from di-
azonium salts (Gomberg–Bachmann reaction).[12] The proposed
mechanism for this transformation also involves a diazo anhy-
dride species,[13] in close analogy to the structures described
by Müller and Haiss.[10] The potential of this somewhat under-
developed diazo anhydride chemistry attracted our attention,
as diazo anhydrides have been described as colored spe-
cies,[11,14] and therefore could likely decompose more efficiently
when exposed to light.
was only 63%, with chlorobenzene being the main side prod-
uct observed by GCMS analysis (~20%). Thermal byproducts
such as diarylamine and diazo compounds were also observed.
When employing Eosin Y as the catalyst, the conversion and
selectivity decreased dramatically (entry 2) compared to the
ruthenium catalyst. We next evaluated the reactivity using only
catalytic amounts of BF3·Et2O. It has been shown that reactions
involving the formation of diazonium salts can proceed under
these conditions when an acidic proton is released during the
catalytic cycle.[15] With 2 mol% of acid (entry 3) 71% conver-
sion with improved selectivity was observed after 3 h of irradi-
ation. Notably, when the reaction was carried out without
a photocatalyst (entry 4) or acid (entry 5) analogous results
were obtained. Under these conditions formation of a nitrosa-
mine intermediate instead of a diazonium salt can be expect-
ed.[9] In the absence of light significantly lower conversions
(17%) were obtained after 3 h (entry 7). After 24 h in the dark,
83% of the starting material had been converted although
only 58% of the obtained product corresponded to the de-
sired bi(hetero)aryl (entry 8). The results of these experiments
in the absence of light are in agreement with the radical de-
composition of diazo anhydrides already observed by Müller
and Haiss.[10] However, the data collected in Table 1 clearly
demonstrate that light significantly enhances the reaction.
To further intensify this photochemical transformation
(Table 1) a continuous-flow process was developed. By using
transparent capillary tubing in a microreactor setup very in-
tense irradiation of the reaction mixture can be achieved.[16]
The flow setup consisted of two separate feeds, containing
a solution of the aniline and the
Herein we present a light-induced, catalyst-free room tem-
perature CÀH arylation methodology for the preparation of
bi(hetero)aryls by a one-pot reaction of anilines with tBuONO
and (hetero)arenes under neutral conditions (Scheme 1b). Ani-
lines are initially transformed to nitrosamines; upon dimeriza-
tion the corresponding photoactive diazo anhydrides are
formed which under irradiation with UV light (>300 nm) de-
compose to aryl radicals. In the presence of the (hetero)arene
the desired bi(hetero)aryls are formed. To further intensify this
method and to improve the safety profile, the process has
been translated to a continuous-flow procedure providing in-
creased selectivities and a reduction of the overall reaction
time to 45 min.
Initially, we performed a set of batch experiments conceived
to evaluate the reactivity of in situ generated diazonium salts
or nitrosamines (and their corresponding diazo anhydrides) to-
wards CÀH arylation of arenes in the presence of light
(Table 1). For this purpose, the coupling of 4-chloroaniline (1a)
arene
in
MeCN
(feed A)
Table 1. Preliminary batch experiments on the reactivity of in situ generated diazonium salts and nitrosamine-
(0.8 mmol scale), and the alkyl
nitrite (1.2 equiv) in MeCN
(feed B), respectively (Scheme 2).
The solutions were pumped to
the system and mixed using
a standard T-mixer before enter-
ing the photoreactor (Vapourtec
UV-150 E series), which consisted
of transparent fluorinated ethyl-
ene propylene (FEP) tubing
(1 mm i.d., 10 mL volume) coiled
s.[a]
Entry
Additive (equiv)
Photocat. (mol%)
Light Irradiation [h]
Conversion [%][b,c]
Selectivity [%][c]
1
2
3
4
5
6
7
8
BF3·Et2O (1)
BF3·Et2O (1)
BF3·Et2O (2)
BF3·Et2O (2)
–
–
–
–
[Ru(bpy)3Cl2] (1)
Eosin Y (1)
Eosin Y (1)
–
–
–
–
–
3
3
3
3
3
5
>95
24
71
69
74
81
17
83
63
35
70
70
71
70
63
58
around
a
power-adjustable
no light, 3 h
no light, 24 h
medium pressure Hg lamp (max.
150 W). The aniline and nitrite
were mixed shortly before enter-
ing the photoreactor minimizing
potential side reactions outside
of the photoreactor (a virtually
spontaneous orange coloration
[a] Conditions: 1a (0.4 mmol), solvent (2 mL), 10 mL Pyrex vial, tBuONO (1.2 equiv). [b] Conversion referred to
the disappearance of the diazo intermediate that is rapidly formed upon addition of tBuONO to the aniline so-
lution. [c] Determined by HPLC and GCMS analysis.
with thiophene was selected as a model. Reaction mixtures in
which an acid (BF3·Et2O) was added and therefore diazonium
tetrafluoroborate salts were present were also evaluated for
comparison purposes.[6,7] In the presence of [Ru(bpy)3Cl2]
(bpy=2,2’-bipyridyl) excellent conversion was obtained after
3 h irradiation using a 60 W cool-white compact fluorescent
lamp (CFL>360 nm) (Table 1, entry 1). The selectivity, however,
of solutions of 1a after addition of tBuONO indicated rapid for-
mation of the diazo intermediate). A UV filter with a 350 nm
cutoff was installed in the system providing an emission spec-
trum analogous to that for the white CFL lamp making batch
and flow results comparable (the emission spectra of the fil-
tered Hg lamp and CFL are shown in the Supporting Informa-
tion). Gratifyingly, under continuous-flow conditions, the reac-
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residence times strongly suggest that the reaction is second-
order kinetics with respect to the aniline, thus supporting the
formation of a dimeric species (see the Supporting Information
for details). DFT calculations on the relative stability of 3, 3’,
and 4 suggest that these species are in equilibrium (with an
energy gap <2 kcalmolÀ1) and can thus be readily intercon-
verted. Notably, the intermediacy of diazonium salts in the re-
action mechanism appears rather unlikely (in contrast to the
Gomberg–Bachmann reaction).[12,13] Performing an experiment
Scheme 2. Continuous-flow setup (schematic).
À
in which 4-ClC6H4N2+BF4 was prepared independently and
used in the photochemical process under identical reaction
conditions provided a mixture of products consistent with the
formation of an aryl carbocation (see the Supporting Informa-
tion), in agreement with reports showing that diazonium salts
photochemically (350–400 nm) decompose to carbocations in-
stead of radicals.[17]
tion time could be reduced to ~45 min (>99% conversion,
compare Table 1, entry 6 for the batch result) and selectivity
improved to 85%. Apart from the high irradiation intensity
achievable in a capillary tubing in a microreactor, the reaction
enhancement is also partially due to the higher efficiency of
the medium pressure Hg lamp compared to the household
CFL utilized for the batch experiments. The selectivity enhance-
ment is probably due to the fact that the fast photochemical
reaction prevents competing thermal processes from occur-
ring.
When unsubstitued aniline (1b, R=H) was employed as
a substrate instead of 4-chloroaniline (1a) poor conversions
(35%) were obtained employing identical reaction conditions
(see Table S1 in the Supporting Information). Assuming that
the mechanism outline in Scheme 3 is correct, we ascribe this
significant difference in reactivity to inefficient absorption of
light by the diazo anhydride 4 in the case of aniline (R=H).
Theoretical calculations (DFT) of the UV/Vis spectra of the
diazo anhydrides of aniline and 4-chloroaniline reveal that the
lmax in both cases is below 350 nm (335 nm for R=Cl, and
322 nm for R=H). As the lowest
The proposed mechanism for this transformation is based
on the formation of diazo anhydrides 4 from the self-conden-
sation of nitrosamine 3, generated in situ from aniline and
alkyl nitrite (Scheme 3). The formation of this type of dimeric
(and highly explosive)[11,14] diazo anhydride structure is well es-
wavelength emission peak ob-
tained with the UV filter installed
in the photoreactor is at 360 nm,
a less than optimal interaction of
light with the photochemically
active diazo anhydride species 4
can be expected (for details and
figure on the theoretical UV/Vis
spectra and overlap plots with
the lamp emission spectra, see
the Supporting Information). We
thus decided to fit the UV reac-
tor with
a lower wavelength
cutoff filter (>300 nm) to en-
hance irradiation efficiency. Uti-
lizing the new filter conversion
for the reaction of aniline (1b,
R=H) with thiophene dramati-
cally increased. In the case of
Scheme 3. Proposed mechanism for the light-induced CÀH arylation of arenes with in situ generated
nitrosamines.
tablished in the literature.[9–11,14] Fragmentation of the diazo an-
hydride, which can take place spontaneously[10] (thus explain-
ing that the reaction also proceeds in the dark), is accelerated
in the presence of light providing the corresponding aryl radi-
cal 5, an azoxy radical 6, and N2 gas. Reaction of aryl radical 5
with the (hetero)arene followed by single-electron and proton
transfer to 6 results in the desired bi(hetero)aryl, recovering
a molecule of nitrosamine (diazo hydroxide 3’), which can fur-
ther react (Scheme 3). Although direct evidence for the pres-
ence of an unstable diazo anhydride intermediate 4 could not
be established, continuous-flow experiments involving variable
4-chloroaniline (1a, R=4-Cl), the lamp power could be re-
duced from 150 to 75 W maintaining excellent conversion
and selectivity, providing an 80% isolated yield of
2-(4-chlorophenyl)thiophene 2a after column chromatography.
Using the continuous-flow protocol described above a collec-
tion of substituted anilines and (hetero)arenes were employed
as building blocks for bi(hetero)aryl synthesis (Scheme 4). Thus,
11 different anilines were used as substrates and reacted with
thiophene as a model arene (Scheme 4a). Moderate to good
isolated product yields were obtained after column chroma-
tography for ortho- (2c), meta- (2d), and para-substituted (2e)
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terials. However, with pyridine N-oxide the reaction worked
smoothly, resulting in an orange reaction mixture containing
the desired 2-(4-chlorophenyl)pyridine N-oxide (2q).
The method has further been evaluated for the synthesis of
two specific bi(hetero)aryls that are important building blocks
in the preparation of active pharmaceutical ingredients
(Scheme 5). 2-(4-Fluorophenyl)thiophene (2r) is a key inter-
mediate in the synthesis of Canagliflozin, a drug used for the
Scheme 5. Application of the light-induced CÀH activation method towards
the preparation of important bi(hetero)aryl scaffolds.
treatment of diabetes.[18] Its bi(hetero)aryl moiety 2r is typically
prepared by cross-coupling chemistry using Pd catalysts, phos-
phine ligands, and boronic acid starting materials.[18] Using the
light-mediated catalyst-free radical protocol developed herein
2r was prepared in 74% isolated yield using inexpensive
4-fluoroaniline and thiophene as starting materials. Similarly,
the reaction of 4-nitroaniline with furfural under the same con-
ditions provided 2-(4-nitrophenyl)furaldehyde (2s, 56%), an in-
termediate in the synthesis of Dantrolene, an important
muscle relaxant.[19] Preparation of 2s is typically carried out by
starting from the diazonium salt of 4-nitroaniline in a copper-
catalyzed Meerwein arylation.[19] The photochemical continu-
ous-flow method avoids the handling of hazardous diazonium
salts and eliminates the use of a metal catalyst. The isolated
yields obtained using this protocol are similar to those ob-
tained by Kçnig’s procedure using Eosin Y as the catalyst in
DMSO. Importantly, one of the advantages of working under
flow conditions is that the reaction scale (0.8 mmol) can be
readily increased by simply operating the reactor for longer
periods.
Scheme 4. Reaction scope of anilines and (hetero)arenes under continuous
flow conditions. Product 2m was isolated as an 8:1:1 mixture of the possible
isomers and 2p was isolated as a 5:3:1:1 mixture of isomers. All reactions
were carried out on a 0.8 mmol scale. Boc=tert-butoxycarbonyl.
haloanilines. The reaction also exhibited compatibility with
other functional groups including esters (2 f), nitriles (2g), and
nitro groups (2h), and also proceeded with 2-aminobenzothia-
zol (2i). As a limitation of this methodology, electron-rich ani-
lines such as anisidine provided poor results (2j), leading to
formation of polymeric byproducts. In these cases, the aryl rad-
icals 5 (Scheme 3) formed during the process probably react
preferentially with the unreacted aniline substrates 1 instead
of the (hetero)arene. The presence of a bulky group at the
ortho-position to the amino group (2-aminobiphenyl) also pre-
vented the reaction from proceeding in high conversion (2k).
Conversely, several arenes and heteroarenes were tested for
the reaction using 4-chloroaniline as the starting material
(Scheme 4b). Apart from thiophene used as a model substrate,
the reaction proceeded satisfactorily with furan (2l) and N-Boc-
pyrrole (2o), as well as with unactivated arenes such as ben-
zene (2n) and anisole (2m). Reaction with pyridine was unsuc-
cessful and led to the rapid decomposition of the starting ma-
In summary, this communication has described the develop-
ment of a light-induced, catalyst-free CÀH arylation methodol-
ogy for the preparation of bi(hetero)aryls from the direct reac-
tion of anilines with arenes and heteroarenes. The anilines are
reacted with tBuONO and transformed to nitrosamines which,
in equilibrium with their corresponding diazo anhydrides, de-
compose under light irradiation (>300 nm) at room tempera-
ture to aryl radicals. In the presence of (hetero)arenes this radi-
cal fragmentation results in the formation of bi(hetero)aryls,
producing N2 gas, H2O, and tBuOH as the only byproducts. In
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this way hazardous nitrosamine and diazo-type intermediates
are generated in situ and are immediately consumed within
the reactor avoiding the handling of these substances. In order
to make this novel method preparatively useful, a continuous
flow protocol was developed enabling overall reaction times
of 45 min. We believe that this mild and highly atom-economic
procedure has the potential to be developed into a powerful
alternative to more traditional methods of bi(hetero)aryls for-
mation.
Acknowledgements
This work was supported by Eli Lilly and Company through the
Lilly Research Award Program (LRAP). We thank Vapourtec Ltd
for the provision of a UV-150 photochemical E-series flow reac-
tor.
[13] C. Rüchardt, E. Merz, Tetrahedron Lett. 1964, 5, 2431–2436.
[14] T. Kauffmann, H. O. Friestad, H. Henkler, Liebigs Ann. Chem. 1960, 634,
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Keywords: CÀH arylation · continuous-flow chemistry · diazo
compounds · photochemistry · radical chemistry
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Received: June 18, 2015
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Published online on August 3, 2015
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