Visible-Light/Photoredox-Mediated sp3 C-H Functionalization and Coupling of Secondary Amines with Vinyl Azides in Flow Microreactors

Article abstract of DOI:10.1002/chem.201504292

Structurally diverse imidazole derivatives were synthesized by a visible-light/[Ru(bpy)₃][(PF₆)₂]-mediated coupling of vinyl azides and secondary amines in flow microreactors. This operationally simple and atom-economical

Full text of DOI:10.1002/chem.201504292

DOI: 10.1002/chem.201504292
Communication
&
Photocatalysis
3
Visible-Light/Photoredox-Mediated sp CÀH Functionalization and
Coupling of Secondary Amines with Vinyl Azides in Flow
Microreactors
[a]
[b]
[c]
Dharmendra Kumar Tiwari,* Ram Awatar Maurya,* and Jagadeesh Babu Nanubolu
3
and the functionalization of sp CÀH bonds adjacent to the
[
6]
Abstract: Structurally diverse imidazole derivatives were
secondary nitrogen atom are rare. Presented in this commu-
nication are the results of a visible-light-mediated photoredox
synthesized by a visible-light/[Ru(bpy) ][(PF ) ]-mediated
3
6 2
3
coupling of vinyl azides and secondary amines in flow mi-
croreactors. This operationally simple and atom-economi-
cal protocol allows the formation of three new CÀN bonds
functionalization of sp CÀH bonds adjacent to the secondary
nitrogen atom, yielding structurally diverse imidazole deriva-
tives.
3
through the functionalization of sp CÀH bonds adjacent
Vinyl azides are very reactive species and have been em-
ployed as a pivotal three-atom synthon for the formation of di-
to the secondary nitrogen atom. In order to get mechanis-
tic insight of the coupling reaction, several control experi-
ments were carried out and discussed.
[
7]
verse N-heterocycles. We initially selected vinyl azide 1 for
[
7f]
our investigations as it can undergo nucleophilic attack
along with the liberation of N , thus providing an intermediate
2
suitable for the key step. We envisioned that vinyl azides
1
might be attacked by secondary amines 2 to generate terti-
Visible-light-mediated organic transformations have received
considerable attention in scientific community due to its sus-
ary amines 3, which should form nitrogen-centered radical
cation 5 under a visible-light-mediated photoredox catalysis,
eventually yielding imidazole derivatives 8 through a series of
cascade reactions (Scheme 1). Thus, the overall process might
[1]
tainable and green perspectives. Photochemical methods
provide remarkable reaction pathways, which are otherwise
[1,2]
3
difficult to achieve through conventional strategies.
Com-
be considered as the functionalization of sp CÀH bonds adja-
[3]
pared to UV-light-mediated transformations, visible-light-
mediated strategies are particularly attractive as visible light is
safer to handle and organic compounds are more stable in visi-
ble light towards photodecomposition. With the advent of the
commercially available and relatively stable transition-metal
photoredox catalysts, researchers enjoyed developing newer
carbon–carbon and carbon–heteroatom bond-forming reac-
cent to a secondary nitrogen atom.
In order to test the hypothesis, the reaction of vinyl azide
1a and 1,2,3,4-tetrahydroisoquinoline (2a) was taken as
a model reaction, and the effect of light, catalyst, oxidant, and
solvent was studied in a batch reactor (Table 1). The reaction
did not proceed in the absence of light or photocatalyst in
CH CN (entries 1–3). However, using [Ru(bpy) ][PF ] (1 mol%)
3
3
6 2
[1,2,4]
tions using visible light.
A large number of such reactions
and tert-butyl hydroperoxide (TBHP; 5 equiv) as an oxidant in
anhydrous acetonitrile led to the generation of a spot, which
was found to be 9a instead of 8a (entry 4). The structure of
the unexpected imidazole derivative 9a was confirmed by
single-crystal X-ray crystallography (Figure 1).
involve visible-light-mediated photoredox generation of a radi-
cal cation on a tertiary nitrogen atom as the key step in the
[5]
overall transformation. Examples of the visible-light-mediated
generation of a radical cation on a secondary nitrogen atom
Increasing the stoichiometry of TBHP did not increase the
product yield, but decreasing it led to a low yield of the imida-
zole 9a (Table 1, entries 5 and 6). It is worth mentioning that
the reaction was found to be moisture sensitive as it gave
poor yield of 9a (10%) along with several uncharacterized
products using [Ru(bpy) ]Cl ·6H O as the photocatalyst
+
[
a] Dr. D. K. Tiwari
Department of Inorganic and Physical Chemistry
CSIR-Indian Institute of Chemical Technology
Hyderabad-500007 (India)
E-mail: dktiwari.iict@gov.in
3
2
2
+
[
b] Dr. R. A. Maurya
(
entry 7). The reaction did not proceed with organic dyes, such
Department of Medicinal Chemistry and Pharmacology
CSIR-Indian Institute of Chemical Technology
Hyderabad-500007 (India)
as rose Bengal and eosin Y (entries 8 and 9). Exploring other
sources of oxidation, the reaction gave a poor yield of the imi-
dazole 9a (25%, entry 10). Moreover, employing H O as an ox-
E-mail: ramaurya.iict@gov.in
2
2
[
c] Dr. J. B. Nanubolu
idant was found unsuccessful and produced a very complex
mixture (entry 11). Next, we screened several solvents and
among them acetonitrile was found to be the best (entries 12
Centre for X-ray Crystallography
CSIR-Indian Institute of Chemical Technology
Hyderabad-500007 (India)
+
and 13). Furthermore, increasing the amount of [Ru(bpy) ][PF ]
[
] D. K. Tiwari and R. A. Maurya equally contributed to this work.
3
6 2
did not increase the product yield significantly whereas de-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504292.
creasing it lowered the product yield (entries 14 and 15).
Chem. Eur. J. 2016, 22, 526 – 530
526
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Despite optimizing various reaction conditions, the
yield of imidazole 9a was not very high in the batch re-
actor. Therefore, we decided to apply a flow microreactor
technology for our photochemical synthetic transforma-
tions. Due to a very large surface to volume ratio, high il-
lumination homogeneity, enhanced heat- and mass-trans-
fer capability, and reduced safety hazards, microreactors
are considered very suitable for photochemical reac-
[
8,9]
tions.
A flow microreactor for the synthesis of imida-
zole 9a by visible-light/[Ru(bpy) ][PF ] -mediated cou-
3
6 2
Scheme 1. Proposed coupling of vinyl azides 1 with secondary amines 2 to yield
imidazole derivatives 8 under a visible-light photoredox catalysis.
pling of vinyl azide 1a and amine 2a was easily assem-
bled by wrapping a visible-light-transparent perfluoroal-
koxy (PFA) capillary microreactor (ID=0.76 mm, Length=
1
0 m, and volume=4.53 mL) over a white LED light
[
a]
[9c]
Table 1. Evaluation of reaction conditions.
source (Figure 2).
Solutions of vinyl azide 1a and
amine 2a in acetonitrile, [Ru(bpy) ][PF ] in acetonitrile,
3
6 2
and TBHP in toluene were kept in three separate syringes
and connections were made through an X-junction. The
effect of residence time over the coupling of vinyl azide
1
a and 1,2,3,4-tetrahydroisoquinoline (2a) was studied in
the flow microreactor. The relative flow rates were man-
aged in such a way that the catalyst loading and oxidant
were 1 mol% and five equivalents, respectively, in the
final reaction mixture after X-junction. Full conversion of
[
b]
[c]
Entry
Light
Photocatalyst
Oxidant
Solvent
Yield[%]
[
h]
1
2
3
4
no
yes
no
–
–
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
ND
ND
ND
55
9
a (65% yield) was obtained in a residence time of
[Ru(bpy)
[Ru(bpy)
[Ru(bpy)
[Ru(bpy)
[Ru(bpy)
3
3
3
3
3
][PF
][PF
][PF
][PF
6
6
6
6
]
]
]
]
2
2
2
2
4
4 min (see Table S1 in the Supporting Information for
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
[
d]
e]
details). Improved yields of 9a (55% in batch versus 65%
in flow) and reduced reaction times (12 h in batch versus
5
6
7
8
9
1
54
35
10
ND
ND
25
ND
41
[
]Cl
2
·6H
2
O
4
4 min in flow) can be explained by considering a high
rose Bengal
eosin Y
photon flux, and increased illumination homogeneity in
the flow microreactor.
0
[Ru(bpy)
[Ru(bpy)
[Ru(bpy)
[Ru(bpy)
[Ru(bpy)
[Ru(bpy)
3
3
3
3
3
3
][PF
][PF
][PF6]2
][PF
][PF
][PF
6
]
]
2
2
O balloon
1
1
6
2
H
2
O
2
Having optimized flow microreactor conditions for the
visible-light-mediated coupling of vinyl azide 1a and
amine 2a in hand, we attempted to explore the scope of
this synthetic transformation. Vinyl azides used in our
study were prepared either from the Knoevenagel con-
densation of aldehydes with phenacyl azides or bromina-
tion followed by an azide substitution/elimination of styr-
enes (see the Supporting Information for details). Thus,
a number of structurally diverse vinyl azides were sub-
jected to the visible-light/[Ru(bpy) ][PF ] -mediated mi-
1
1
1
1
2
3
4
5
TBHP
TBHP
TBHP
TBHP
toluene
CHCl
6
6
6
]
2
]
2
]
2
3
32
56
45
[
f]
CH
CH
3
CN
CN
[
g]
3
[
(
(
1
[
[
a] Reaction condition: vinyl azide 1a (0.2 mmol), 1,2,3,4-tetrahydroisoquinoline
2a; 0.2 mmol), photocatalyst (1 mol%), oxidant (5 equiv), anhydrous solvent
10 mL), rt, stir, 12 h. [b] White LED (11 W) kept at a distance of approximately
0 cm from the reaction flask. [c] Isolated yield. [d] 10 equivalents of TBHP used.
e] 3 equivalents of TBHP used. [f] 2 equivalents of [Ru(bpy) ][PF used.
g] 0.5 equivalents of [Ru(bpy) ][PF used. [h] ND= not detected on TLC.
3
6 2
]
3
6 2
]
3
6 2
crofluidic conditions to synthesize imidazole derivatives
Figure 3). The reaction worked well with vinyl azides de-
(
rived from heteroaromatic aldehydes and aromatic aldehydes
containing halogen, and electron-donating and -withdrawing
Figure 1. ORTEP diagram of 9a with the atom-numbering scheme. Displace-
ment ellipsoids are drawn at the 30% probability level and H atoms are
shown as small spheres of arbitrary radius.
Figure 2. Coupling of vinyl azides 1 with amines 2 to yield imidazole deriva-
tives 9 mediated by visible light and a photocatalyst in a flow microreactor.
[
14]
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527
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Figure 3. Coupling of vinyl azides 1 with amines 2 to yield imidazole derivatives 9 mediated by visible light and [Ru(bpy)
picted in Figure 1; residence time=44 min.
3
][PF
6
]
2
in the flow microreactor de-
substituents (9a–m and 9o–v). The yields of imidazoles were
relatively low when vinyl azides derived from aromatic alde-
hydes containing an electron-donating group were used (9a,
d, f, i, j, q, r, t and v). It also worked well with vinyl azides de-
rived from aliphatic aldehydes (9n). The reaction was equally
successful with vinyl azides derived from phenacyl azides bear-
ing halogen, and electron-donating and -withdrawing substitu-
ents (9a–v). Good yields of imidazoles were also obtained
using styrene-derived vinyl azides (9z, a’and b’). The reaction
was compatible with cyclic secondary amines, such as 1,2,3,4-
tetrahydroisoquinoline (2a) and 6,7-dimethoxy-1,2,3,4-tetrahy-
droisoquinoline (2b). It is worth noting that the reaction also
worked with acyclic secondary amines, such as N-benzyl aniline
The evolution of the nitrogen under visible-light irradia-
tion in the presence of the photoredox catalyst indicated that
the decomposition of the vinyl azide occurred possibly in
[
10]
a photosensitized manner. In order to gain some mechanis-
tic insight into the reaction, vinyl azide 1b, which was care-
fully chosen in order to minimize the side reactions of 2H-
azirine and had both the ortho positions blocked by two
chlorine atoms, was treated with [Ru(bpy) ][PF ] (1 mol%) in
3
6 2
acetonitrile and irradiated with a white LED light in the ab-
sence of amines (Scheme 2). After workup, 2H-azirine 10a
(88%) was isolated under visible-light irradiation. However, no
conversion was observed in absence of the photocatalyst
in light or with the photocatalyst in dark. Furthermore,
vinyl azide 1b was converted into 2H-azirine 10a under
the thermal decomposition in 69% yield. These experiments
confirmed the visible-light/transition-metal-sensitized decom-
position of vinyl azides under our imidazole-forming proce-
dures.
(
(
(
2c, 9w), N-benzyl-p-anisidine (2d, 9x), and dibenzyl amine
2e, 9y), though the product yields were comparatively lower
35–58%). Simple secondary amines, such as pyrrolidine, piper-
idine, and morpholine, gave complex reaction mixtures, from
which desired imidazole products could not be separated.
Chem. Eur. J. 2016, 22, 526 – 530
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528
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Communication
Scheme 2. Visible-light/transition-metal-mediated decomposition of vinyl
azide 1b to 2H-azirine 10a.
Facile conversions of vinyl azide 1b to 2H-azirine 10a ex-
plains why the expected imidazole 8a was not obtained under
2
our reaction conditions. After 2H-azirine formation, the sp
Scheme 4. Control experiments: Coupling of 2H-azirine 10a with amine 2a
and imine 11 a under various conditions.
carbon directly attached to the azide (N ) group of the vinyl
3
azide 1 becomes highly electrophilic and is attacked by the
amine 2, leading to the formation of the imidazole 9.
Next, we performed a few experiments to check whether
radiation. The reaction of 10a with 2a in dark gave a complex
mixture, in which the desired imidazole 9o was not detected
on TLC.
[11]
[
Ru(bpy) ][PF ] oxidizes secondary amines to imines under
3 6 2
visible-light irradiation (Scheme 3). The oxidation of 1,2,3,4-tet-
rahydroisoquinoline (2a) to 3,4-dihydroisoquinoline (11 a) has
Based on these observations, the formation of imidazoles 9
can be explained by a plausible mechanism depicted in
Scheme 5. The 2H-azirine 10 is generated from vinyl azide
[
10,13]
1
by visible-light photosensitization, and imine 11 is gener-
ated from amine 2 by photoredox catalysis. Next, 10 and 11
react to yield an intermediate 12, which undergoes ring open-
ing and cyclization in a concerted manner to yield zwitterion
13. The zwitterion 13 rearranges to another intermediate 14,
which further oxidizes to imidazole 9. However, there may be
an alternative mechanism, in which the highly reactive 2H-azir-
ine 10 is attacked by the amine 2 to yield an intermediate 15,
which rearranges to compound 18 through intermediates 16
Scheme 3. Oxidation of 1,2,3,4-tetrahydroisoquinoline (2a) to 3,4-dihydroiso-
quinoline (11 a) under visible-light/transition-metal and conventional condi-
tions.
been successfully carried out using various catalysts under
[
12]
photoirradiation.
In a control experiment, 2a was treated
with [Ru(bpy) ][PF ] (1 mol%) and TBHP (2 equiv) in acetoni-
3
6 2
trile and irradiated with a white LED light (Scheme 3). Under
these conditions, 2a was rapidly oxidized to 11 a. Although, 2a
was slowly oxidized to 11 a in dark, no conversion was ob-
served in the absence of [Ru(bpy) ][PF ] either in light or in
3
6 2
dark. These experiments suggested that the secondary amines
used for the imidazole synthesis might have been oxidized to
imines through the generation of a radical cation on the nitro-
gen atom of the secondary amine.
Next, we reacted 2H-azirine 10a with 1,2,3,4-tetrahydroiso-
quinoline (2a) and 3,4-dihydroisoquinoline (11 a) separately to
study the imidazole synthesis (Scheme 4). The reaction of 2H-
azirine 10a with imine 11 a gave a high yield of imidazole 9o
(
88%) without photoirradiation, whereas the reaction of 10a
Scheme 5. Mechanistic rationalization for the visible-light/[Ru(bpy)
3 6 2
][PF ] -
with amine 2a gave 65% yield of 9o under the visible-light ir-
mediated coupling of vinyl azides 1 and amines 2 to yield imidazoles 9.
Chem. Eur. J. 2016, 22, 526 – 530
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529
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
and 17. Compound 18 has a tertiary nitrogen atom, which can
undergo photoredox catalysis under our reaction conditions
and oxidizes to imidazole 9.
In conclusion, we have developed a synthetic strategy for
structurally diverse imidazoles that relies on visible-light/
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[
3
6 2
oxidant in flow microreactors. The synthetic protocol devel-
3
oped utilizes visible light to functionalize sp CÀH bonds adja-
cent to a secondary amine. This atom-economical protocol
allows the formation of three new CÀN bonds in the overall
process.
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are thankful to the Department of Science and Technology,
Ministry of Science and Technology for financial support in the
form of INSPIRE Faculty and startup research grant for young
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Keywords: azides
· nitrogen heterocycles · oxidation ·
photochemistry · redox chemistry · visible light
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