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International Edition: DOI: 10.1002/anie.201709571
German Edition:
DOI: 10.1002/ange.201709571

Synthetic Photochemistry

Radical-Based CÀC Bond-Forming Processes Enabled by the
Photoexcitation of 4-Alkyl-1,4-dihydropyridines
Luca Buzzetti, Alexis Prieto, Sudipta Raha Roy, and Paolo Melchiorre*
Abstract: We report herein that 4-alkyl-1,4-dihydropyridines
(alkyl-DHPs) can directly reach an electronically excited state
upon light absorption and trigger the generation of C(sp3)centered radicals without the need for an external photocatalyst. Selective excitation with a violet-light-emitting diode
turns alkyl-DHPs into strong reducing agents that can activate
reagents through single-electron transfer manifolds while
undergoing homolytic cleavage to generate radicals. We used
this photochemical dual-reactivity profile to trigger radicalbased carbon–carbon bond-forming processes, including
nickel-catalyzed cross-coupling reactions.

The chemical reactivity of electronically excited molecules

differs fundamentally from that in the ground state. This is the
underlying reactivity concept of photochemistry,[1] which has
traditionally allowed the development of unique chemical
transformations that are not achievable through conventional
ground-state pathways.[2] For example, an excited-state molecule is both a better electron donor (i.e., a better reductant)
and a better electron acceptor (i.e., a better oxidant) than in
the ground state.[3] This explains why the light excitation of
organic molecules can unlock unconventional reactivity
manifolds. In this context, our laboratory has been exploring
the potential of some chiral organocatalytic intermediates to
directly reach an electronically excited state upon visible-light
absorption to then switch on novel catalytic functions that are
unavailable to ground-state organocatalysis.[4] For example,
we showed that electron-rich enamines I, which act as
nucleophiles in the ground state, could become strong
reductants upon light excitation and trigger the formation of
radicals through single-electron transfer (SET) reduction of

electron-poor organic halides (Figure 1 a).[4a,b] In the same
vein, 1,4-dihydropyridine derivatives II (H-DHP, Figure 1 b)
are primarily understood as hydride (HÀ) sources in their
ground state.[5] Other researchers recently demonstrated that
exciting them with visible light affords strong photoreductants
that can productively engage in SET manifolds.[6]
These findings led us to wonder whether 4-alkyl-1,4dihydropyridines (alkyl-DHPs, 1), which closely resemble the
structure of H-DHP II, might unveil a similar reactivity
behavior upon excitation, becoming photoreductants in the
excited state (Figure 1 c). If so, the ensuing SET event with
a suitable electron acceptor would generate intermediate 1C+.
The radical cation 1C+, which is generally accessed by SET
oxidation of 1 using an external photoredox catalyst[7] or
a stoichiometric oxidant,[8] is prone to a fast homolytic
cleavage, eventually serving as a radical precursor.[7, 8] Overall,
the proposed photochemical mechanism would generate
C(sp3)-centered radicals, which could then trigger carbon–
carbon bond-forming processes under mild conditions and
only relying upon the photochemistry of readily available
alkyl-DHPs 1, which are prepared from aldehydes in one

[*] Prof. Dr. P. Melchiorre
ICREA—Catalan Institution for Research and Advanced Studies
Passeig Lluís Companys 23, 08010 Barcelona (Spain)
L. Buzzetti, Dr. A. Prieto, Dr. S. R. Roy, Prof. Dr. P. Melchiorre
ICIQ—Institute of Chemical Research of Catalonia the Barcelona
Institute of Science and Technology
Avenida Països Catalans 16, 43007 Tarragona (Spain)
E-mail: pmelchiorre@iciq.es
Homepage: http://www.iciq.org/research/research_group/profpaolo-melchiorre/
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201709571.
 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution Non-Commercial NoDerivs License, which
permits use and distribution in any medium, provided the original
work is properly cited, the use is non-commercial, and no
modifications or adaptations are made.
Angew. Chem. Int. Ed. 2017, 56, 15039 –15043

Figure 1. Radical generation strategies enabled by the excited-state
reactivity of organic molecules. a, b) Previous studies demonstrating
that light excitation turns enamines (a) and 1,4-dihydropyridines II
(H-DHP; b) into strong reductants and the resulting SET-based radical
generation mechanisms. c) The proposed strategy exploits the ability
of 4-alkyl-1,4-dihydropyridines (alkyl-DHPs, 1) to become both photoreductants and precursors of alkyl radicals upon excitation. EWG =
electron-withdrawing group; SET = single-electron transfer; filled dark
blue circle represents a bulky substituent on the chiral amine.

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step.[8, 9] Herein, we demonstrate how this idea was translated
into experimental reality.
To test the feasibility of our photochemical plan, we
initially studied the photophysical behavior of diethyl 4benzyl-1,4-dihydropyridine-3,5-dicarboxylate (Bn-DHP, 1 a),
which was selected as the model substrate. The optical
absorption spectrum of 1 a dissolved in CH3CN confirmed its
ability to absorb in the visible frequency region, up to 420 nm
(Figure 2 a, blue line). We found that 1 a was fluorescent when
excited at 405 nm, which allowed us to record the emission
spectrum (red dotted line in Figure 2 a, lmax = 470 nm) and to
measure the fluorescence lifetime in CH3CN using a picosecond pulsed laser.[10] The reconvolution fitting shows
a double exponential decay (Figure 2 b), affording the following values: t0 = 0.28 ns (below the instruments detection

Figure 2. a) Absorption of 1 a (blue line, lmax = 343 nm) and its emission (excitation at 405 nm, red dotted line, lmax = 470 nm) in CH3CN.
b) Fluorescence decay trace of 600 mm 1 a in CH3CN after picosecond
photoexcitation at 405 nm. c) EPR spectrum of the benzyl radical
generated from 1 a at 77 K after 10 minutes (orange line) and
60 minutes (green line) of light irradiation at 405 nm. d) Photochemical behavior of a solution of 1 a in CH3CN under 405 nm irradiation.
Yield measured by 1H NMR analysis using trichloroethylene as the
internal standard.

limit) and t1 = 4.1 ns. Interestingly, the excited-state lifetime
of Bn-DHP is comparable with the lifetime of unsubstituted
1,4-dihydropyridine of type II (H-DHP in Figure 1 b), which
was measured to be 320 ps.[6b] Considering the ability of HDHP to act as a strong reducing agent in the excited state,[6]
these photophysical studies encouraged us to evaluate the
photochemistry of the excited Bn-DHP (1 a*).
We conducted our experiments in CH3CN under irradiation by a high-power single light-emitting diode (HP single
LED, lmax = 405 nm) with an irradiance of 50 mW cmÀ2, as
controlled by an external power supply (full details of the
illumination set-up are reported in the Supporting Informa-

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tion). Direct illumination of a CH3CN solution of 1 a led to the
formation of toluene, 1,2-diphenylethane, and the corresponding pyridine (Figure 2 d). On repeating the reaction
in the presence of 2,2,6,6-tetramethyl-1-piperidinyloxy
(TEMPO, 1 equiv), a radical scavenger, we detected the
formation of the benzyl-TEMPO adduct in 55 % yield (details
in the Supporting Information). Overall, these experiments
indicate that the simple photoexcitation of 1 a can trigger the
formation of benzyl radicals. This notion was further corroborated by EPR studies of a solution of 1 a in MeCN,
conducted at 77 K and with the same illumination system used
in the experiments in Figure 2 c. The EPR spectrum showed
a strong isotropic X-band absorption and afforded a g-value
of 2.0035 (Figure 2 c), which is consistent with reported data
for the characterization of benzyl radicals.[11]
After establishing the ability of 1 a to generate C(sp3)centered radicals upon excitation, we evaluated its propensity
to act as a strong reducing agent in the excited state. To test
this possibility, we selected the aromatic ipso-substitution of
cyanoarenes as the model reaction.[7a, 12] Indeed, this process
classically proceeds through a radical coupling mechanism
that requires the concomitant formation of a C(sp3)-centered
radical and a persistent cyanoarene radical anion III
(Scheme 1 a). The ensuing CÀC bond formation affords
a cyclohexadienyl anion IV that is prone to rapid rearomatization through the elimination of cyanide to afford the
arene alkylation product. When mixing either 4-cyanopyridine (2 a, Scheme 1 b) or tetracyanobenzene (2 b, Scheme 1 c)
with 1 a under light irradiation, the corresponding products 3 a
and 3 b were generated in fairly good yields. These results are
consonant with the excited state of 1 a acting both as a strong
reducing agent, affording the cyanoarene radical anion of
type III upon SET reduction of substrates 2, and as a source of
a benzyl radical, generated upon homolytic cleavage of the
radical cation 1 aC+ that emerges from the SET. The reduction
potential of the excited Bn-DHP 1 a [Ered* (1 a*/1 aC+)] was
estimated as À2.0 V vs. Ag/Ag+ in CH3CN on the basis of
electrochemical and spectroscopic measurements. This establishes the thermodynamic feasibility of the SET reduction of
both 2 a (Ered = À1.87 V vs. SCE in CH3CN)[13a] and 2 b (Ered =
À0.64 V vs. SCE in CH3CN).[13b]

Scheme 1. a) Mechanism of the aromatic ipso-substitution of cyanoarenes, governed by the coupling of an alkyl radical and the persistent
cyanoarene radical anion III. Benzylation of 4-cyanopyridine 2 a (b) and
tetracyanobenzene 2 b (c) enabled by the direct photoexcitation of 1 a.

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These studies demonstrate that alkyl-DHPs 1 possess
a dual reactivity profile, since excitation with a violet-lightemitting diode turns them into strong reducing agents that are
able to activate reagents through SET manifolds while
undergoing homolytic cleavage to directly generate alkyl
radicals. We reasoned that this unique reactivity behavior
could be fully exploited in the framework of a photochemical
nickel-catalyzed cross-coupling method. Merging nickel catalysis[14] and photoredox catalysis[15] has recently emerged as
a versatile platform for developing highly enabling crosscoupling methods.[16] The success of this approach relies on
the ability of Ni0 complexes to undergo facile oxidative
addition with aromatic halides while engaging in radicalcapture mechanisms. This sequence results in the formation of
NiIII complexes, which afford the cross-coupled products
through rapid reductive elimination. At the same time, the
chemistry relies on the reactivity of visible-light-activated
photocatalysts that, upon excitation, can generate alkyl
radicals through SET activation of non-traditional crosscoupling nucleophiles. Crucially, the photoredox catalyst is
also involved in modulating the oxidation state of nickel
complexes by SET reduction, an essential step to restore the
original Ni0 catalyst. Since alkyl-DHPs 1 can act as both
photoreductants and precursors of alkyl radicals upon excitation, we surmised that their photochemistry could enable
a nickel-catalyzed C(sp2)ÀC(sp3) cross-coupling process without the need for any external photoredox catalyst.[17] The
proposed mechanism, depicted in Scheme 2, would start with

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DHPs 1* would complete the nickel catalytic cycle while
regenerating the C(sp3) radical.
Pleasingly, the proposed light-triggered cross-coupling
process turned out to be feasible at ambient temperature in
CH3CN using NiCl2·DME as the metal catalyst (5 mol %),
bipyridine (10 mol %) and 2,6-lutidine as the base (1 equiv).
Control experiments established the necessity of light irradiation (using high-powered LED,[19] lmax = 405 nm, with an
irradiance of 50 mW cmÀ2), with the process being completely
inhibited in the dark. As depicted in Figure 3, a variety of aryl

Figure 3. Survey of the aryl bromides 4 and the 4-alkyl-1,4-dihydropyridines 1 that can participate in the nickel-catalyzed cross-coupling
process enabled by the photochemical activity of 1. Reactions
performed on a 0.5 mmol scale using 1.5 equiv of 1 and an irradiance
of 50 mWcmÀ2. Yields refer to isolated products. [a] The NMR yield is
given, since the compound was isolated together with 10 % of
a byproduct. [b] Reaction time 72 hours. [c] 0.1 mmol scale reaction
performed over 16 hours (average of two runs).

Scheme 2. Proposed reaction mechanism for the nickel-catalyzed
C(sp2)ÀC(sp3) cross-coupling driven by the photochemical activity of
alkyl-DHPs 1.

the photoexcitation of alkyl-DHPs 1. In the first catalytic
cycle, the resulting excited-state intermediate 1* (Ered* (1 a*/
1 aC+) = À2.0 V vs. Ag/Ag+ in CH3CN) would reduce the NiII
precatalyst through two discrete SET events to afford the
active Ni0 intermediate (Ered NiII/Ni0 = À1.2 V versus SCE in
DMF).[18] The concomitant formation of the radical cation 1C+
would secure the formation of the alkyl radical, which is then
intercepted by the Ni0 complex. The emerging NiI organometallic species would then undergo oxidative addition with
the aryl bromide 4. This would lead to the NiIII intermediate,
which, after reductive elimination, forges the desired C(sp2)À
C(sp3) bond in the cross-coupled product 5. Finally, SET
reduction of the generated NiI complex by the excited alkylAngew. Chem. Int. Ed. 2017, 56, 15039 –15043

bromides 4 were successfully coupled with Bn-DHP 1 a to
afford the corresponding benzylated products 5 a–g in good to
high yields. The process tolerated a wide range of functional
groups, including cyano, aldehyde, ester, and chloride moieties. Moreover, a ortho substituent did not hinder the
reaction, yielding the coupling product 5 f in good yield. To
glean further insight into this process, we measured the
quantum yield (F) of the photochemical cross-coupling
reaction leading to adduct 5 a. A value of 0.0034 (l = 405 nm
in CH3CN, average of three runs) was determined, which is
consonant with the mechanism proposed in Scheme 2.
We then evaluated the possibility of using alkyl-DHP
substrates 1 bearing different alkyl fragments to benzyl at the
C4-position. Both linear and cyclic fragments could be
successfully coupled with aryl bromides (adducts 5 h–l). An
alkene, an acetal moiety, and an oxygen-based heterocycle
were all well-tolerated, leading to the corresponding products
5 h, 5 j and 5 k, respectively. In addition, a primary radical
successfully engaged in this coupling when adorned with
a stabilizing a-oxygen atom (adduct 5 l). These results are
mechanistically relevant because they demonstrate that

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Conflict of interest
The authors declare no conflict of interest.
Keywords: cross-coupling · dihydropyridines · nickel catalysis ·
photochemistry · synthetic methods
How to cite: Angew. Chem. Int. Ed. 2017, 56, 15039 – 15043
Angew. Chem. 2017, 129, 15235 – 15239

Figure 4. Survey of the acyl chlorides 6 and the 4-alkyl-1,4-dihydropyridines 1 that can participate in the nickel-catalyzed cross-coupling
process to afford ketones 7. Reactions performed on a 0.1 mmol scale
using 1.5 equiv of 1 and an irradiance of 50 mWcmÀ2. Yields refer to
isolated products (average of two runs). [a] 0.5 mmol scale reaction
performed over 24 hours.

different 4-alkyl-1,4-dihydropyridines 1 can be excited upon
visible-light irradiation and their photochemical activity
successfully integrated into a nickel catalytic cycle to enable
C(sp2)ÀC(sp3) cross-coupling processes.
We then wondered whether the same photochemical
method could be successfully translated to the nickelmediated cross-coupling of 1 with acyl chlorides 6.[20] As
shown in Figure 4, an array of dialkyl and aryl/alkyl ketones 7
could be efficiently prepared under mild conditions and
relying only on the visible-light excitation of alkyl-DHP
substrates 1.
In summary, we have demonstrated that 4-alkyl-1,4dihydropyridines can unlock rich photochemistry upon light
excitation. Selective absorption of violet light turns them into
strong reducing agents that activate reagents through singleelectron transfer manifolds while undergoing homolytic
cleavage to generate C(sp3)-centered radicals. This lighttriggered dual-reactivity profile was integrated into a nickel
catalytic cycle to enable C(sp2)ÀC(sp3) cross-coupling reactions without the need for an external photoredox catalyst.
We are conducting ongoing studies to further exploit the
photochemical activity of alkyl-DHPs in other radical-based
carbon–carbon bond-forming processes.

Acknowledgements
Financial support was provided by the Generalitat de
Catalunya (CERCA Program), MINECO (CTQ2016-75520P), and the European Research Council (ERC-2015-CoG
681840—CATA-LUX). L.B. thanks MINECO for a predoctoral fellowship (CTQ2013-45938-P). The authors thank Dr.
Fernando Bozoglian for assistance with photophysical studies
and Dr. Maria Dolores Gonzalez Candela for assistance with
EPR experiments.

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Manuscript received: September 15, 2017
Accepted manuscript online: October 6, 2017
Version of record online: October 24, 2017

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