Visible light C-H amidation of heteroarenes with benzoyl azides

Article abstract of DOI:10.1039/c4sc02365j

Benzoyl azides were used for the direct and atom economic C-H amidation of electron rich heteroarenes in the presence of phosphoric acid, a photocatalyst and visible light. Hetero-aromatic amides are obtained in good yields at very mild reaction conditions with dinitrogen as the only by-product. The reaction allows the use of aryl-, heteroaryl- or alkenyl acyl azides and has a wide scope for heteroarenes, including pyrroles, indole, furan, benzofuran and thiophene giving good regio-selectivities and yields.

Full text of DOI:10.1039/c4sc02365j

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DOI: 10.1039/C4SC02365J
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Visible Light C-H Amidation of Heteroarenes with
Benzoyl Azides
Cite this: DOI: 10.1039/x0xx00000x
E. Brachet,a T. Ghosh,a I. Ghosha and B. König*a
Benzoyl azides were used for the direct and atom economic CH amidation of electron rich
heteroarenes in the presence of phosphoric acid, a photocatalyst and visible light. Hetero-
aromatic amides are obtained in good yields at very mild reaction conditions with dinitrogen as
the only by-product. The reaction allows the use of aryl-, heteroaryl- or alkenyl acyl azides and
has a wide scope for heteroarenes, including pyrroles, indole, furan, benzofuran and thiophene
giving good regioselectivities and yields.
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
a. Buchwald-Hartwig amidation
H
Introducing amide moieties into organic molecules is a key
transformation in organic chemistry due to the ubiquitous
presence of this functional group in natural products, pharma-
ceuticals or functional materials.1 Amidation methods have
been improved constantly in terms of functional group
tolerance and conditions. Using transition metal catalysis, e.g.
in Buchwald-Hartwig amidations, proved to be particularly
advantageous.2 However, some limitations still exist: Catalytic
systems can be expensive, and often high reaction temperatures
and pre-functionalized substrates, e.g. halides or pseudo-halides
(Scheme 1, a.) are required. More recently, different groups
have developed C-H amidations for heteroarenes or arenes with
X
TM, ligand
TM, ligand
N
R
R
H2N
R
R
het
het
het
O
O
O
b. Oxidative C-H amidation
H
N
H
H2N
het
O
DG
DG
DG = directing group
c. This work : Visible light C-H amidation
Ru(bipy)3Cl2,
or without the use of a directing group (Scheme 1, b.).3
A
H
N
drawback of this attractive strategy are the often required harsh
reaction conditions. With the aim to improve direct C-H
amidations towards milder conditions and a broader reaction
scope we have developed a photocatalytic4 variant of the
reaction (Scheme 1, c.).
H
R
N3
R
H3PO4, DMSO
X
455 nm
blue LED
O
X
O
Scheme 1. Catalytic aromatic amidation reactions
Very few examples of photocatalyzed C-H amidations have
been reported in the literature. MacMillan and coworkers5
described in 2013 the amidation of enamines yielding α-amino
aldehydes with good yields and excellent stereocontrol. A
second example was reported by Sanford et al.6 exploring the
As aminating agents we choose benzoyl azides, which are
easily accessible and allow for the introduction of simple benz-
amides into a substrate, which was so far not possible using
visible light photocatalysis. Dinitrogen is the only stoichio-
metric by-product. Benzoyl azides have been used previously in
transition metal catalysis, but the reaction requires the presence
of a directing group.10 Exposed to UV irradiation11 benzoyl
azides give oxazolines, dioxazoles or aziridines, but no
conversion of heterocycles or formal amidation was observed
under these conditions.12 We activate benzoyl azides using
visible light and a photocatalyst allowing a direct C-H
amidation of heteroarenes.
reactivity of
N-acyloxyphtalimides under visible light to
generate phthalimide radicals that add to arenes and hetero-
arenes with good to excellent yields. This methods has been
recently extended to
N-chlorophthalimide as aminating agent
under photocatalysis.7 Yu et al. used activated hydroxylamines
introducing amine groups into heteroarenes.8 The reported
methods require a functionalized aminating reagent5,8 or yield
phtalimide moieties.6,7
Inspired by this work we developed an alternative photo-
catalytic method for the functionalization of heteroarenes with
aromatic amides, which may be particularly useful in synthesis.
Many bioactive compounds show this structural motif and
typical examples are found among analgestic and anti-parasitic
compounds or hepatitis C drugs.9 A key advantage of our
method is the introduction of the amide at a late stage of a
synthesis of a more complex structure.
Results and discussion
Optimization of the reaction conditions
The reaction of benzoyl azide 1a with N-methylpyrrole 2a was
used to establish and optimize the conditions. The results of the
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optimization are summarized in Table 1. Using ruthenium(II)- position 2 as confirmed by the crystal structure analysis of
trisbipyridine dichloride (
A
) as the photosensitizer in DMSO compound 3b (Table 2, entry 2). Both benzoyl azides bearing
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with blue light irradiation did not yield any amidation product. electron donating groups or electron withdrawing groups are
Part of the benzoyl azide is converted into the corresponding tolerated. The sterically hindered 1-naphthoyl azide 1c reacts, albeit
DOI: 10.1039/C4SC02365J
isocyanate by Curtius rearrangement and benzamide, as the with a lower yield of 47%. Sensitive groups like cyano or ester
nitrene hydrogen abstraction product (see Supporting moieties are well tolerated (compound 3e and 3f respectively). It was
Information for GC-MS analyses). The addition of an acid as reported that benzoyl azides react under light irradiation with
additive is essential for the formation of the desired amidation cyanide groups to give oxadiazole,11-12 but in our conditions we only
product. Isocyanates are not stable under these conditions and obtained the desired product in 63% yield. Product 3g bearing a
only their decomposition products are observed. Initial low chlorine atom allows for subsequent functionalization by transition
amidation product yields of 10% were improved to 40% after metal mediated cross-coupling 13 and the use of heteroaryl acyl azide
12h of irradiation using phosphoric acid, while changing the 1h yields diheteroaryl amide 3h in 49%. However, alkyl, phenyl,
photocatalyst or the solvent had no significant effect on the diphenylphosphoryl or benzyl acyl azides do not react under the
product yield. Working at lower concentrations gave 65% photocatalytic conditions (entry 9). We explain this selectivity and
product yield already after 4h of blue light irradiation. Control limitation in substrate scope by the available energy of the long-lived
experiments showed that the photocatalyst, irradiation and acid [Ru(bpy)32+]* triplet state (46 kcal/mol), which is sufficient for
are required to get full conversion of the starting material energy transfer to acyl azides (first electronically excited triplet state
(entries
1, 14 and 15). The amount of the ruthenium catalyst ~ 41 kcal/mol), but not for e.g. phenyl azide (68 kcal/mol) (vide
can be reduced to 1 mol% still providing an acceptable product infra for mechanistic proposal and Supporting Information for data
yield of 51%.
and references).
Table 1. Optimization of the reaction conditions
Table 2. Scope of benzoyl azides for the photocatalyzed C-H
amidation.[a]
Entry
1
Benzoyl Azide
Product
Yield [%][b]
65%
N
CO2H
Br
N
N
N
N
N
Br
Ru2+
Ir
N
HO
O
O
N
N
2 Cl-
Br
Br
.
C : Eosin Y
A : Ru(bpy)3Cl2 6 H2O
B : Ir(ppy)3
Entry
1
Cat[a]
A
Additive [b]
Solvent
Conv. [%]
~35%
100
Yield[c]
[d]
-
DMSO (0.34M)
DMSO (0.34M)
DMSO (0.34M)
DMSO (0.34M)
DMSO (0.34M)
DMSO (0.34M)
DMSO (0.34M)
DMSO (0.34M)
DMSO (0.34M)
DMF (0.34M)
-
2
10%[a]
30%
8%
10%
0%
40%
-
71%
2
A
(BnO)2P(O)OH
(BnO)2P(O)OH
Benzoic acid
Acetic acid
PTSA
3
A
100
4
A
100
5
A
100
6
A
A
B
C
A
A
A
A
A
-
100
100
60
75
60
20
100
100
95
0
7
H3PO4
3
4
47%
54%
8
H3PO4
9
H3PO4
-
10
11
12
13
14
15
16
H3PO4
-
H3PO4
ACN (0.34M)
-
H3PO4
H3PO4
H3PO4
H3PO4
H3PO4
DMSO (0.09M)
DMSO (0.09M)
DMSO (0.09M)
DMSO (0.09M)
DMSO (0.09M)
65%[e]
51%[f]
30%[g]
-
5
6
7
63%
46%
61%
[h]
A
0
-
[a] 2.5 mol% of catalyst. [b] 2 eq. of additive with respect to 1a. [c] Isolated yields. [d]
Phenylisocyanate and benzamide are obtained. [e] Reaction conditions : 1a (0.34
mmol, 1 eq.), 2a (1.7 mmol, 5 eq.), additives (0.68 mmol, 2 eq.) and photocatalyst (8.5
mol, 2.5mol%) in dry solvent (0.09M) under N2 was irradiated with blue light during
4h. [f] 1 mol% of Ru catalyst used. [g] 1 eq. of N-methylpyrrole. [h] Without light.
8
9
49%
0%
Scope of the reaction
Having established the reaction conditions we explored the
scope of benzoyl azides (Table 2). Benzoyl azides (1a-i) are readily
obtained from the corresponding benzoyl chlorides and sodium
azide. Gratifyingly, differently substituted benzoyl azides react with [a] Reaction conditions : 1a-i (0.34 mmol, 1 eq.), 2a-i (1.7 mmol, 5 eq.), H3PO4 (0.68
.
mmol, 2 eq.) and Ru(bipy)3Cl2 6H2O (8.5 mol, 2.5mol%) in dry DMSO (0.09M) were
N-methylpyrrole 2a in good yields and a perfect selectivity for
2 | J. Name., 2012, 00, 1‐3
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irradiated with blue light under N2. [b] Isolated yield, average of two reactions.
We then explored the scope of the reaction towards substituted
pyrroles and other heterocycles like indole, furan, thiophene and
benzofuran (Table 3). Substituted pyrroles; either with electron
withdrawing or electron donating groups gave the corresponding
products in moderate to good yields of up to 88%.
10
11
59%
View Article Online
DOI: 10.1039/C4SC02365J
35%
Protection of the N-H moiety of pyrrole is not necessary, which is
advantageous for the functionalization of such heterocycles and their
late stage amidation.14 (Table 3, entry 3). N-Aminoazoles are found
in bioactive compounds, 15 but their sensitive hydrazine group limits
a direct functionalization in synthesis.16 Visible light photocatalysis
allows the conversion with benzoyl azide 1b yielding the substituted
N-aminopyrrole 3p in 69%. Indoles, furan and thiophene gave the
expected amidation products in 35 – 61 % yield. Analogous to
benzoyl azides we investigated the reaction with alkenyl acyl azide
and sulfonylazide, which are cleanly converted to the corresponding
products in more than 50% yield (Table 3, entries 14 and 15).
12
13
14
61%
44%
55%
Table 3. Scope of heteroarenes undergoing photocatalytic C-H
15
52%
amidation.[a]
[a] Reaction conditions : 1a-k (0.34 mmol, 1 eq.), 2a-k (1.7 mmol, 5 eq.), H3PO4 (0.68
mmol, 2 eq.) and Ru(bipy)3Cl2.6 H2O (8.5 mol, 2.5mol%) in dry DMSO (0.09M) were
irradiated with blue light under N2. [b] Isolated yield, average of two reactions
Only benzofuran reacts with a significantly lower yield of 15%
(Table 3, entry 6), due to the formation of a side product (see
mechanistic proposal). To illustrate the potential of the new method
for synthesis, it was applied to the preparation of amide 6 having
analgesic and anti-inflammatory properties (Scheme 2).7b The
previously reported route used trifold substituted hydrazine 4, which
is cyclized to the indole core and acylated to give the desired product
6. This route is limited to symmetrical diarylhydrazines to avoid
product mixtures in the cyclisation step. The photocatalytic CH-
amidation gave the target compound in one step with a slightly
improved yield of 65%.
Entry
1
Benzoyl azide
Heteroarene
Product
Yield [%][b]
65%
2
3
72%
69%
4
5
64%
49%
6
7
8
15%
59%
46%
Scheme 2. Synthesis of compound 6 using photocatalytic C-H
amidation with benzoyl azide 1g
Mechanistic investigations
Benzoyl azides are known to loose dinitrogen upon sensitization
yielding nitrenes, which convert into isocyanates by the Curtius
rearrangement.17 In the presence of hydrogen donors benzamides are
obtained and the reaction with double bonds yields aziridines or
oxazolines.11,12,18 The ratio between isocyanate and nitrene formation
9
88%
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from the excited benzoyl azide is reported to be independent of the
presence and the amount of alkenes as trapping reagent;19 the
photophysical mechanism of sensitized and direct benzoyl azide
decomposition has been investigated in detail.20 Some recent
examples using azides in photocatalyzed reactions suggest a nitrene
intermediate from single electron transfer or by sensitization.21
Based on these findings and a series of control experiments, we
propose the following mechanism for the here described amidation
reaction: Ru(bipy)3Cl2 acts upon blue light irradiation as a triplet
sensitizer for benzoyl azides. The interaction of the excited
ruthenium complex and the azide was confirmed by a Stern-Volmer
experiment (see Supporting Information for data). An electron
transfer pathway is excluded, as benzoyl azides are insufficiently
electron deficient to be easily reduced by ruthenium photocatalysis:
The reduction potentials of [Ru(bipy)3Cl2]* and benzoyl azide 1b,
respectively, are -0.89 and -1.49 V versus the saturated calomel
electrode (SCE). Addition of TEMPO does not interfere with the
reaction, which also indicates the absence of radical intermediates in
this reaction. The energy transfer triggers the loss of dinitrogen
yielding the benzoyl nitrene, which converts in part via the Curtius
rearrangement to the corresponding isocyanate. GC-MS analysis of
the reaction mixture in the absence of acid clearly identified the
isocyanate. However, amidation products are only observed in the
presence of an acid, e.g. H3PO4, in the reaction mixture. As neither
the absorption of the ruthenium complex and the benzoyl azide, nor
the emission of the ruthenium complex and the reduction potential of
the benzoyl azide (from cyclic voltammetry) change by the addition
of the acid (see Supporting Information for data), the sensitization
step is not affected.22 The benzoyl nitrene may be protonated under
the strongly acidic conditions, as analogously reported for
substituted aryl nitrenes,23 giving electrophilic nitrenium ions, which
react with the electron rich heteroarene. The carbenium ion
intermediate reacts under a loss of a proton and rearomatization to
the amidation product.24 In the case of benzofurane and α-methyl-
styrene 7 the oxazoline formation25 is an alternative reaction
pathway. Benzoyl azide 1g reacts with α-methylstyrene 7 under blue
light irradiation yielding oxazoline 8 in 55% yield, while in the
reaction with benzofurane, oxazoline 9 was obtained as the major
product accompanied by benzofuran amide 3o in 15%. Isolated
oxazoline 9 does not convert into benzofuran 3o when exposed to
the reaction conditions, which indicates that its formation is
irreversible and it is not a reaction intermediate.
View Article Online
DOI: 10.1039/C4SC02365J
Scheme 4. Proposed mechanism of the photo CH-amidation of
benzofuran with benzoyl azide in the presence of acid, Ru(bipy)3Cl2
and blue light.
Conclusions
We have reported the C-H amidation of heterocycles with
benzoyl azides under very mild and practical conditions using visible
light. The only byproduct of this atom economic process is
dinitrogen. Heterocycles, such as pyrrols, indols, furan, benzofuran
or thiophene and substituted benzoyl azides give the corresponding
amide coupling products in moderate to good yields in a single step.
This first application of benzoyl azides in photocatalysis provides
CH amidation products of classic benzamides, which are typical
structural motifs of many bioactive compounds. The photoreaction
may therefore be a valuable alternative in the synthesis of bioactive
heterocycles to the previously reported use of phthalimides or
functionalized amines. A particular advantage of the new method is
the amidation under mild conditions allowing for a late stage
functionalization of complex heterocyclic molecules.
Experimental section
Standard procedure for the photocatalyzed CH amidation
reaction
In a 5 mL snap vial equipped with magnetic stirring bar
Ru(bipy)3Cl2,6H2O, (0.025 equiv), benzoyl azide (1 equiv),
H3PO4 (2 equiv) and the heteroarene (5 equiv) were dissolved
in dry DMSO (0.09 mmol/mL) and the resulting mixture was
degassed by “pump-freeze-thaw” cycles (×2) via a syringe
needle and filled with nitrogen. The vial was irradiated through
the vial’s plane bottom side using blue LEDs. After complete
conversion of the starting material as monitored by TLC, the
pressure in the vial is released by a needle and the reaction
mixture was transferred into a separating funnel, diluted with
ethyl acetate and washed with 15 mL of water. The aqueous
layer was washed three times with ethyl acetate. The combined
organic layers were dried over MgSO4, filtered and
concentrated in vacuum. Purification of the crude product was
achieved by flash column chromatography using petrol
ether/ethyl acetate as eluent.
Scheme 3. Isolated oxazolines from the photoreaction
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (GRK 1626,
Chemical Photocatalysis) for financial support.
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Germany, E-mail: Burkhard.Koenig@chemie.uni-regensburg.de
DOI: 10.1039/C4SC02365J
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Electronic
Supplementary
Information
(ESI)
available:
[characterization and synthesis of new compounds, general procedures,
proton and carbon NMR spectra, crystal structure analysis, gas
chromatographic analyses]. See DOI: 10.1039/b000000x/
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