Visible-Light-Driven Synthesis of 1,3,4-Trisubstituted Pyrroles from Aryl Azides

Article abstract of DOI:10.1021/acs.orglett.9b02731

The synthesis of 1,3,4-trisubstituted pyrroles via visible-light mediated photoredox catalyzed condensation of arylazides and aldehydes has been reported herein. The methodology avoids the use of stoichiometric oxidants and provides the corresponding N-co

Full text of DOI:10.1021/acs.orglett.9b02731

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Visible-Light-Driven Synthesis of 1,3,4-Trisubstituted Pyrroles from
Aryl Azides
Yang Liu,†,§ Adriano Parodi,†,§ Simone Battaglioli,† Magda Monari,† Stefano Protti,*,‡
and Marco Bandini*,†
†Department of Chemistry “G. Ciamician”, Alma Mater Studiorum, University of Bologna, via Selmi 2, 40126 Bologna, Italy
‡PhotoGreen Lab, V. Le Taramelli 10, 27100 Pavia, Italy
S
* Supporting Information
ABSTRACT: The synthesis of 1,3,4-trisubstituted pyrroles via visible-light mediated photoredox catalyzed condensation of
arylazides and aldehydes has been reported herein. The methodology avoids the use of stoichiometric oxidants and provides the
corresponding N-containing arenes in good yields (up to 78%) and mild conditions. Mechanistic rationale is provided via a
dedicated and combined spectroscopic/experimental investigations.
The advent of visible-light photoredox catalysis brought new
ideas and possibility for the generation of high-energy chemical
species, such as radicals as well as charged intermediates, under
mild reaction conditions.5 Such an approach has been
extensively employed also in the site-selective functionalization
of arenes6 and, though to a lesser extent, in the direct
formation of (hetero)aromatic compounds7 with limitations
related to intramolecular metal cocatalyzed methodologies.8
In continuation to our ongoing interests in the chemistry of
aromatic compounds9 and visible-light assisted functionaliza-
tion of arenes10,11 we present herein an unprecedented mild
protocol for the preparation of densely substituted pyrroles by
means of photoredox production of the key carbon centered
radical intermediate.
In order to avoid concomitant oxidative processes involving
the aniline reaction partner, we decided to adopt azides as
nitrogen-based reagents12 that are known to produce the
corresponding aniline counterparts via photoreductive SET
protocol under protic conditions.13 Concomitantly, the in situ
generated [Ru(III)] acts as the oxidant for the α-carbonyl
radical formation (Scheme 1). In the present working plan, the
arylazide would play the double role of nitrogen source to be
incorporated into the heteroaromatic core and “formal”
stoichiometric oxidant of the whole process.
he construction of (hetero)arenes, together with their
Tsite-selective functionalization and dearomatization pro-
cesses represents one of the current milestones in chemical
synthetic methodology related to aromatic compounds.1
In this context, pyrroles are key building blocks for the
realization of a wide range of functional materials as well as
bioactive compounds,2 however their preparation is not always
a trivial synthetic task due to their peculiar and extraordinary
reactivity profile. Therefore, mild experimental conditions
should always be targeted during the design of a new synthetic
route to pyrrolyl derivatives.3 Among the most performing
synthetic sequences available for pyrroles, the condensation of
aldehydic compounds with anilines under oxidative con-
ditions4a is gaining growing credits with the development of
catalytic,4b metal-free4c and ball-milling mediated4d approaches
(Figure 1).
In all cases, however, the employment of a stoichiometric
oxidant is required in order to form the α-formyl or α-imino
radical derivatives (see inset Figure 1).
This protocol would enable the realization of an
unprecedented stoichiometric oxidant-free synthesis of site-
selective substituted pyrroles under desirable mild conditions.
At the outset of the optimization stage, we identified in the
arylazide 1a and phenylacetaldehyde 2a the model substrates
to be condensed under photocatalytic regime. Upon an
Figure 1. State-of-the-art synthesis of pyrroles via oxidative
condensation of anilines with aldehydes.
Received: August 3, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02731
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Additionally, the impact of the presence and nature of the
photosensitizer (PS) on the reaction profile was evaluated
through dedicated experiments (entries 10−14), highlighting
the role of [Ru(4,4′-(tBu)2bpy)3](PF6)2 as the elective
photocatalyst for the reaction. Finally, the replacement of 1a
with p-anisidine caused the failure of the protocol; this
evidence suggests that the photocatalyzed reduction of the
arylazide is critical for generating high energy radical species in
the reaction machinery (vide infra).
Scheme 1. Working Plan of the Present Visible-Light-
Assisted Synthesis of Pyrroles
With the optimal conditions in hands, the scope and the
versatility of the methodology toward the realization of a
library of pyrrole derivatives were assessed. To this aim, two
sets of experiments namely: condensation of different
arylazides (1b-k) with phenylacetaldehyde 2a and p-MeO-
phenylazide (1a) with a range of aldehydes (2b−m) were
carried out and the results reported in the Table 2 and Scheme
2, respectively.
extensive survey of reaction conditions involving solvent, light
source, photosensitizer and additives (see SI for a complete list
of attempts), the concomitant employment of [Ru(4,4′-
(tBu)2bpy)3](PF6)2 (2 mol %), 4,4′-(tBu)2bpy (10 mol %)
and AcOH (2 equiv) in DMF enabled the isolation of the N-
PMP-3,4-diphenyl pyrrole 3aa in 74% yield and 87:13 ratio
with respect to the isomeric N-PMP-2,4-diphenyl pyrrole
3aa’14 (Table 1, entry 1, vide infra for mechanistic
considerations).
a
Table 2. Scope of the Reaction: Azide
a
Table 1. Optimization of the Reaction Conditions
b
c
run
Ar
C6H5 (3ba)
yield 3/3′ (%)
3/3′
1
2
3
4
5
6
7
8
9
51
78
74
50
75
47
63
62
45
53
90/10
84/16
80/20
83/17
83/17
81/19
88/12
85/15
80/20
80/20
4-CH3C6H4 (3ca)
3-CH3C6H4 (3da)
2-OCH3C6H4 (3ea)
4-nOBuC6H4 (3fa)
3,4-(CH3)2C6H3 (3ga)
2,4-(CH3)2C6H3 (3ha)
4-FC6H4 (3ia)
b
c
run
deviation from optimal
dark
blue LED (1 W)
with morpholine (0.5 equiv)
with DIPEA (0.5 equiv)
acid free
yield 3/3′ (%)
3/3′
1
2
3
4
5
6
74
NR
62
73
65
87/13
85/15
87/13
86/14
84/16
--
87/13
84/16
ND
84/16
85/15
84/16
84/16
3-BrC6H4 (3ja)
4-BrC6H4 (3ka)
10
a
37
Reaction conditions: 1 (0.14 M). 1/2a: 1/2.5. Under anhydrous
b
d
conditions and degassed solvent. Determined after flash chromatog-
7
no degassing
traces
24
c
raphy as a 3/3′ mixture. Determined on the reaction crude by GC−
8
9
no 4,4′-(tBu)2bpy
DMSO as the solvent
no [Ru(II)]
[Ir(III)] (2 mol %)
Eosin-Y
[Ru(bpy)3](PF6)2
[Ru(4,4′-(tBu)2bpy)3]Cl2
p-anisidine instead of 1a
MS.
55
12
67
18
65
68
NR
10
11
12
13
14
15
Variously substituted arylazides were effectively employed in
the synthesis of pyrroles regardless the phenyl substitution
pattern. In particular, ortho, meta and para substitutions were
efficiently tolerated as well as the presence of both electron-
rich (1c-h) and electron-deficient (1i-k) functional groups did
not affect the visible-light assisted protocol, delivering the
corresponding N-aryl-diphenyl-pyrroles 3 in good yields (up to
78%) and moderate to good regioselectivity (3:3′ up to
90:10). Moreover, mono and disubstitutions (1g and 1h) at
the ortho, meta and para positions of the aromatic ring of the
aldehydic compound were fully compatible with the protocol.
In addition, the orthogonal electronic properties of func-
tional groups located at the arenes of the phenylacetaldehyde
(EDG: 2b-f and EWG: 2h-i) were adequately tolerated
(Scheme 2). The protocol was also suitable for ortho-
substituted compounds (yield up to 78% in the case of 2e).
Finally, aliphatic linear aldehydes were also tested under best
conditions. Although a moderate yield was observed (30%
yield), the corresponding 3,4-dialkyl-pyrroles 3al and 3am
were isolated in excellent regioselectivity (up to 94:6).
a
Reaction conditions: 1a (0.14 M). 1a/2a: 1/2.5. Under anhydrous
b
conditions and degassed solvent. Determined after flash chromatog-
c
raphy as a 3/3′ mixture. Determined on the reaction crude by GC−
d
MS. With reagent grade DMF. PMP: 4-MeOC6H4. [Ir(III)]:
(Ir[dF(CF3)ppy]2(dtbpy))PF6. NR: no reaction. ND: not deter-
mined.
From the data collected in Table 1 some conclusions can be
drawn. First of all, light irradiation proved to be essential
(entries 1 vs 2/3). In particular, no reaction was observed in
the dark and a slight drop in chemical yield was obtained when
using a 1 W blue-LED irradiation (62%, entry 3). The addition
of an external reductant such as morpholine or DIPEA (entries
4,5) did not affect significantly the reaction course. In contrast,
the absence of protic source (entry 6), the presence of oxygen
(entry 7) and removal of 4,4′-(tBu)2bpy as an additive (entry
8) were proven detrimental for the whole protocol.15
Mechanistically, the catalytic process represented in the
Scheme 3 (upper part) is postulated. Stern−Volmer quenching
B
DOI: 10.1021/acs.orglett.9b02731
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 2. Scope of the Reaction: Aldehyde
Scheme 3. Mechanistic Hypotheses for the Formation of 3
(Top) and Minor Isomer 3′ (Middle); Examples of
Chemical Manipulation of Pyrrole 3aa (Bottom)
a
Reaction conditions: 1 (0.14 M). 1a/2: 1/2.5. Under anhydrous
conditions and degassed solvent. The 3:3′ ratio is shown in
parentheses. PMP: 4-MeOC6H4. The X-ray structure of 3ag is also
reported (see the SI).
experiments were performed to investigate the interaction of
photoexcited catalyst with the substrates and pointed out the
role of arylazide 1 in the process (see SI).
Once generated under visible light irradiation, the photo-
excited [Ru(II)]* complex undergoes oxidative quenching (E
(Ru(II)*/Ru(III) = −1.20 V vs SCE)16a by the arylazide (E =
−0.39 V vs SCE for p-NO2-phenylazide),16b leading to
formation of an anilino radical16c that would be subsequently
protonated to radical cation A.16d Then, transient [Ru(III)] (E
(Ru(III)/Ru(II) = 2.16 Vs SCE),16a species could act as a SET
oxidant for the enolized aldehyde affording, after deprotona-
tion, the α-carbonyl-radical B and restoring the catalytically
active [Ru(II)] adduct. The presence of B was proved by
carrying out the reaction in the presence of TEMPO (2.5
equiv), where the adduct E17 was isolated in (66% yield, see SI
for further details).
pyrrolyl-phenol 4aa and α,α’-Br2 pyrrole 5aa were isolated in
good yields (88% and 60%, respectively, Scheme 3 lower).
In conclusion, an unprecedented visible-light photoredox
catalyzed route to trisubstituted pyrroles is documented by
condensing enolizable aldehydes and arylazides. The method-
ology by means of catalytic amounts of [Ru(II)] as the
photocatalyst and 7 W blue-LED irradiation enables the
replacement of classic stoichiometric oxidants (i.e., TBHP,
Mn(III) salts) for the realization of α-carbonyl radicals B with
the concomitant photoinduced reduction of azides.
Therefore, trapping of B with a second molecule of aldehyde
would deliver the α-hydroxy-radical C18 that undergoes
oxidation to the corresponding 1,4-dialdehyde D by A.19
Final condensation of D with the in situ formed aniline would
afford the pyrrole 3 via Paal-Knorr-type condensation. In the
Scheme 3 (lower part), a tentative reaction picture to account
the formation of the minor 2,4-disubstituted pyrroles 3′ has
been presented. In this regard, the in situ aldol adduct F could
undergo acid catalyzed dehydration and consequent enamine
formation (G) with acid catalyzed condensation with aniline.20
Annulation reaction (C−N bond formation)21 followed by
SET oxidation and proton loss from the C(2)-site would result
into the formation of 3′.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02731.
■
S
General methods, experimental data, and crystallo-
graphic data (PDF)
NMR spectra (PDF)
Accession Codes
CCDC 1945119−1945120 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Finally, the synthetic manipulability of pyrroles 3 was
undisclosed by subjecting compound 3aa to site-selective OMe
cleavage (BBr3, DCM, - 78 °C) and bromination (NBS, 0 °C,
DMF). Under very mild conditions, the corresponding
C
DOI: 10.1021/acs.orglett.9b02731
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: marco.bandini@unibo.it.
*E-mail: stefano.protti@unipv.it.
ORCID
Stefano Protti: 0000-0002-5313-5692
Marco Bandini: 0000-0001-9586-3295
Author Contributions
§Y.L. and A.P. contributed equally to this work.
Notes
́
Piscitelli, S.; Chiarucci, M.; Fabrizi, G.; Goggiamani, A.; Ramon, R. S.;
Nolan, S. P.; Bandini, M. Angew. Chem., Int. Ed. 2012, 51, 9891−9895.
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Bottoni, A.; Miscione, G. P.; Bandini, M. Chem. - Eur. J. 2015, 21,
18445−18453. (i) Romano, C.; Jia, M.; Monari, M.; Manoni, E.;
Bandini, M. Angew. Chem., Int. Ed. 2014, 53, 13854−13857. (j) An, J.;
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Acknowledgement is made to University of Bologna for
financial support. Y.L. thanks the Chinese Scholarship Council
(No. 201609120008) for funding support.
Parodi, A.; Monari, M.; Castineira Reis, M.; Silva Lopez, C.; Bandini,
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M. Chem.Eur. J. 2017, 23, 2442−2449. (k) Favaretto, L.; An, J.;
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