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International Edition: DOI: 10.1002/anie.201814497
German Edition:
DOI: 10.1002/ange.201814497

Synthetic Photochemistry

A Redox-Active Nickel Complex that Acts as an Electron Mediator in
Photochemical Giese Reactions
Thomas van Leeuwen, Luca Buzzetti, Luca Alessandro Perego, and Paolo Melchiorre*
Abstract: We report a simple protocol for the photochemical
Giese addition of C(sp3)-centered radicals to a variety of
electron-poor olefins. The chemistry does not require external
photoredox catalysts. Instead, it harnesses the excited-state
reactivity of 4-alkyl-1,4-dihydropyridines (4-alkyl-DHPs) to
generate alkyl radicals. Crucial for reactivity is the use of
a catalytic amount of Ni(bpy)32+ (bpy = 2,2’-bipyridyl), which
acts as an electron mediator to facilitate the redox processes
involving fleeting and highly reactive intermediates.

The venerable field of photochemistry is re-gaining a central

role in synthetic endeavors.[1] For example, the fast-moving
area of photoredox catalysis is providing modern chemists
with the possibility of generating radical species under mild
conditions and from bench-stable reagents.[2] A more classical
photochemical approach exploits the ability of substrates or
reaction intermediates to harvest the energy of the photons
and access an electronically excited state (direct photochemistry). The chemical reactivity of excited molecules[3] can then
be used to unlock reaction manifolds unavailable to conventional ground-state pathways. In this context, our laboratory
recently reported[4] how selective excitation with violet light
turns 4-alkyl-1,4-dihydropyridines (alkyl-DHPs, 1) into strong
reducing agents, which can activate reagents by singleelectron transfer (SET) manifolds while undergoing a homolytic cleavage to generate C(sp3)-centered radicals (Figure 1 a).[5] This dual photochemical reactivity was used to
trigger radical-based CÀC bond-forming processes.
[*] Prof. Dr. P. Melchiorre
ICREA—Catalan Institution for Research and Advanced Studies
Passeig Lluís Companys 23, 08010 Barcelona ( Spain)
Dr. L. Buzzetti, Prof. Dr. P. Melchiorre
IIT—Istituto Italiano di Tecnologia, Laboratory of Asymmetric
Catalysis and Photochemistry
via Morego 30, 16163 Genoa (Italy)
Dr. T. van Leeuwen, Dr. L. A. Perego, 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.201814497.
 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution-NonCommercial-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. 2019, 58, 4953 –4957

Figure 1. (a) The excited-state reactivity of 4-alkyl-1,4-dihydropyridines
(R-DHPs, 1): on excitation, they become both photoreductants and
precursors of alkyl radicals. (b) Proposed strategy to realize the Giese
reaction by combining the photochemistry of 1 and the action of
a catalytic nickel complex, which facilitates the redox processes by
acting as an electron mediator (EM); NiII rapidly oxidizes the shortlived, excited R-DHPs 1* while the ensuing NiI species reduces the
highly reactive intermediate A, which has a high tendency to undergo
side reactions. The overall sequence affords product 3.

In general, the direct excitation of substrates or intermediates can create new synthetic opportunities for radical
chemistry[3, 6] while bypassing the need for photoredox
catalysts, which are often based on precious transitionmetals. However, there are some challenges intrinsic to this
photochemical strategy. The excitation of organic molecules
provides strong oxidants and/or reductants. Thermodynamically speaking, this makes it easy to activate reagents by an
SET manifold.[7] However, these highly reactive excited-state
intermediates may suffer from short lifetimes and low
electron-transfer rates, thus rendering the SET interactions
with other reagents kinetically challenging.[8]
We recently wondered if electron mediators (EMs) could
be used to alleviate these limitations, thus expanding the
synthetic applicability of the direct photoexcitation approach.
Electron mediators have found widespread use in different
areas of chemistry, including dye-sensitized solar cells,[9] CO2
reduction,[10] and electrochemical synthesis.[11] They have also
been sporadically applied in photoredox catalysis.[12] This is
because, by acting as electron shuttles, they can facilitate

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exergonic redox processes that are kinetically slow. They can
also mitigate the occurrence of a back-electron transfer
(BET) and competitive side-reactivity.
However, to date, there has been little exploration of the
potential of EMs to accelerate a thermodynamically feasible
but kinetically unfavorable SET event involving the direct
excitation of organic intermediates. Herein, we demonstrate
the feasibility of this strategy. By combining the excited-state
reactivity of alkyl-DHPs 1 with the ability of a nickel complex
to act as an electron mediator, we have developed a Giese
addition process[13] without the need for an external photoredox catalyst.[14] Figure 1 b details the idea behind our
synthetic plan: photoexcitation turns 1 into a strong reductant
(1*). The EM facilitates the SET oxidation of this short-lived
electronically excited intermediate 1* to afford the radical
cation 1C+, which fragments into a carbon-centered radical
(RC). Interception by the electron-poor olefin 2 leads to
intermediate A, which is a highly reactive radical prone to
side-reactions, including b-scission and polymerization.[15] The
reduced form of the EM rapidly engages in a second SET,
reducing intermediate A to B while bringing the EM back to
the original oxidation state. The SET reduction of A is critical
for an effective Giese reaction leading to product 3. Overall,
the EM provides a catalytic manifold to secure an efficient
SET between 1* and intermediate A. The EM acts as
a reservoir for the electron donated by 1*, prolonging its
availability for the reduction of A. These redox processes
involve two short-lived intermediates, so they would be
problematic without EM.
To validate our proposal, we studied the Giese-type
radical conjugate addition to dimethyl fumarate (2 a) using 4isopropyl-DHP 1 a as the radical precursor (Table 1). We
conducted our experiments in CH3CN under irradiation by
a single high-power light-emitting diode (LED, lmax = 405 nm,
see the Supporting Information for details).
Performing the model reaction in the absence of an
electron mediator afforded product 3 a in low yield, while
a large amount of reagents 1 a and 2 a remained unreacted
(entry 1). We ascribed this low reactivity to the inefficiency of
Table 1: Optimization and control experiments.[a]

Entry

Conditions

Yield [%][b]

1
2
3
4
5
6

No mediator
p-Dicyanobenzene (0.2 equiv)
Methyl viologen dichloride (0.2 equiv)
Ni(bpy)3(BF4)2 (0.1 equiv)
Same as entry 4, no light, 50 8C
Same as entry 4, TEMPO (1 equiv)

32
57
41
85
<5
26[c]

[a] Reactions performed at ambient temperature on a 0.1 mmol scale
using 1.5 equiv of 1 a under illumination by a single high-power (HP)
LED (lmax = 405 nm) with an irradiance of 50 mWcmÀ2. [b] Yield
determined by 1H NMR analysis of the crude mixture using trichloroethylene as internal standard. [c] The TEMPO-trapped intermediate
(TEMPO-iPr) was obtained in 53 % NMR yield.

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the redox events underlying the overall process.[16] We then
tested our hypothesis that, by behaving alternately as a donor
and an acceptor, an EM could effectively shuttle electrons
between the key fleeting intermediates (1 a* and the radical of
type A, Figure 1 b). An effective EM must meet several
requirements: a) it should easily oxidize 1*; b) its reduced
form should selectively reduce intermediate A, but not
substrate 2 or the alkyl radical resulting from the fragmentation of 1*; and c) it should not strongly absorb at 405 nm to
avoid inner-filter effects.
In consonance with our design, the presence of organic
EMs (20 mol %) substantially improved the efficiency of the
Giese process. p-Dicyanobenzene (Ep/2 (EM/EMCÀ) =
À1.64 V vs. SCE)[17a] increased the yield of 3 a to 57 %
(entry 2), while another common redox mediator, methyl
viologen dichloride (E1/2 (EM2+/EMC+) = À0.41 V vs.
SCE),[17b] had only a negligible effect (entry 3). We then
turned our attention to transition metal complexes. Based on
the aforementioned criteria, Ni(bpy)3(BF4)2 was identified as
a promising EM.[18] This readily available and bench-stable
NiII complex, which is soluble in acetonitrile, is reduced at
Ep(NiII/NiI) = À1.35 V versus SCE,[19] and absorbs weakly at
405 nm. Gratifyingly, 10 mol % of Ni(bpy)3(BF4)2 increased
the yield of product 3 a significantly (up to 85 %, entry 4), and
secured full conversion of 1 a and 2 a. Further control
experiments showed that the reaction requires light and
does not proceed thermally (entry 5). The yield of 3 a was
greatly diminished in the presence of 2,2,6,6-tetramethyl-1piperidinyloxyl (TEMPO, 1 equiv), and we detected the
adduct generated upon isopropyl radical trap (53 % yield,
entry 6).
Having identified Ni(bpy)3(BF4)2 as a suitable EM, we
evaluated the synthetic potential of the photochemical Giese
reaction (Figure 2). We first demonstrated that a high efficiency was maintained when running the reaction on
a 2 mmol scale. This experiment required the use of commercially available LEDs (see section B5 in the Supporting
Information) and it afforded product 3 a in 78 % yield
(294 mg). Concerning the reaction scope, a variety of a,bunsaturated nitriles and esters can be successfully functionalized using 4-isopropyl-DHP 1 a as the radical precursor
(products 3 a–3 f). We then explored the possibility of using
substrates 1 bearing alkyl fragments other than isopropyl at
the C4-position. Secondary alkyl (3 g–h), benzyl (3 i,j), and aheterosubstituted primary radicals are tolerated well in this
transformation. Noteworthy examples include the addition of
a-oxygen (3 k–n) and a-nitrogen radicals (3 o–t), which bring
about the facile introduction of synthetically interesting
motifs such as benzotriazoles (3 s,t), phthalimides (3 o,p),
and dioxolanes (3 k,l). Appealing features of this protocol
include the need for a slight excess (1.5 equiv) of the radical
precursor 1. In contrast to other Giese addition protocols,
there is no need for stoichiometric amounts of additional
reagents, such as bases, reductants or hydrogen-atom donors,
or for photoredox catalysts.[14]
We next conducted extensive mechanistic investigations
to clarify the role played by the nickel catalyst and to establish
its ability to act as an EM. The first crucial step of the
mechanism proposed in Figure 1 b requires Ni(bpy)32+ to

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Figure 2. Survey of the electron-poor olefins 2 and the dihydropyridines 1 that can participate in the photochemical Giese addition. Reactions
performed at 0.1 mmol scale using 1.5 equiv of 1; yields refer to isolated products 3 after purification (average of two runs per substrate).
[a]
Yields of products obtained in the absence of Ni(bpy)3(BF4)2. [b] Conducted in a 1:1 CH3CN:CH2Cl2 (0.25 m) solvent mixture due to the low
solubility of the corresponding radical precursor 1.

oxidize the excited state of DHP (1 a*). Based on UV/Vis and
electrochemical data, the redox potential of 1 a* (E (1 aC+/
1 a*)) was estimated to be À1.9 V versus SCE according to the
Rehm–Weller approximation.[20] Therefore, the reduction of
NiII (Ep(NiII/NiI) = À1.35 V vs. SCE)[19] by 1 a* is thermodynamically feasible. We further investigated this photoreduction using the following experiment: direct illumination of
a CH3CN solution of 4-isopropyl-DHP 1 a at 405 nm in the
absence of the nickel(II) complex led to very low conversion
of 1 a into the corresponding pyridine (Scheme 1 a). The

Scheme 1. Mechanistic experiments; NMR yields are reported.
Angew. Chem. Int. Ed. 2019, 58, 4953 –4957

relative photostability of 1 a implies that the redox processes
underlying the generation of the isopropyl radical from 1 a*
are inefficient in the absence of an EM. The addition of
Ni(bpy)3(BF4)2 (10 mol %) greatly increased the degradation
of 1 a (Scheme 1 b). In this experiment, because of the lack of
an oxidation mechanism to regenerate the NiII complex, we
observed the formation of Ni0 particles, which otherwise were
never observed during the progress of the catalytic reaction.
When repeating the experiment (Scheme 1 c) in the presence
of dimethyl fumarate 2 a, which provides a mechanism for
regenerating the EM (see Figure 1 b), we recovered Ni(bpy)3(BF4)2 in 90 % yield.
The identity of the nickel(II) complex, which crashed out
from the reaction mixture upon dilution with Et2O, was
confirmed by UV/Vis and IR spectroscopy. Furthermore, the
recovered Ni(bpy)3(BF4)2 was successfully reused to promote
another Giese reaction, affording product 3 a in high yield
(Scheme 1 d).
Overall, the experiments detailed in Scheme 1 suggest
that Ni(bpy)3(BF4)2 can effectively mediate the SET oxidation
of 1 a* and that its regeneration is only possible in the
presence of olefin 2 (Schemes 1 c,d). The latter observation is
very relevant to the overall mechanism (Figure 1 b): to act as
an effective EM, it is essential that the NiI complex reduces
the highly reactive intermediate A, emerging from the radical

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Scheme 2. Redox potentials of the species involved in the proposed
mechanism.

addition to 2. The feasibility of this crucial redox step, which
turns back the nickel-based EM in the original oxidation state,
is supported by the redox potentials of the key intermediates
involved (Scheme 2). The data in Scheme 2 indicate that the
excited 1 a* can reduce Ni(bpy)32+. The resulting NiI complex
is not able to reduce typical radicals generated upon
fragmentation of 1 a* (for example E1/2 (BnC/BnÀ) =
À1.45 V[21] and Ecalc (iPrC/iPrÀ) = À2.42 V vs. SCE).[22] However, it can easily reduce radicals a to electron withdrawing
groups, such as intermediate A. This mechanistic possibility is
supported by the reduction potential of the radical resulting
from the addition of iPrC to 2 a, estimated by DFT calculations
(Ecalc (AC/AÀ) = À0.54 V vs. SCE).[22] An alternative scenario,
based on a self-propagating radical manifold, could be
envisaged in an SET reduction of A from the ground state
of 1 a, but this pathway can be excluded based on the redox
potential of 1 a (Ep/2 (1 a+C/1 a) =+ 1.04 V vs. SCE). The
quantum yield measured for the model reaction (F = 0.014)
is also incongruent with a radical chain process.
We also obtained evidence to support the intermediacy of
the anionic intermediate B, arising from the SET reduction of
A (Scheme 2). We performed deuteration experiments by
adding CD3OD to the reaction medium. A high deuterium
incorporation (> 80 %) was observed at the a-carbon of
product 3 a (Scheme 3). No deuterium incorporation took
place at the b-carbon, indicating that deuteration occurs
solely via the in situ-generated anion.
We deem it very unlikely that the reaction proceeds via
the intermediacy of organonickel species. The trapping of

Scheme 3. Deuteration experiments.

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photogenerated alkyl radicals with Ni complexes has been the
subject of recent studies.[23] While the majority of these
procedures employ a metal-to-ligand ratio of 1:1 or 1:1.5, the
present chemistry works when Ni is coordinatively saturated.
The addition of extra bpy (up to 10 equiv with respect to
Ni(bpy)32+) did not result in a significant drop of yield.
Additional observations are incongruent with the involvement of organometallic intermediates: a) Ni(bpy)3(BF4)2 can
be recovered almost quantitatively at the end of the reaction
and reused; b) in situ monitoring of the reaction by the Evans
NMR method (see Section C5 in the Supporting Information
for details) reveals no significant change in the magnetic
susceptibility of the reaction mixture, suggesting that the
resting state of the catalyst is Ni(bpy)32+; c) organic electron
mediators (Table 1, entries 2–3) promote the model reaction
in the absence of nickel.
We finally considered the possibility that Ni(bpy)32+ could
act as photoredox catalyst. The irradiation of the model
reaction at 530 nm, where only the Ni complex absorbs, did
not result in the conversion of 1 a or the formation of 3 a. This
result is not surprising, given that the electronic structure and
photophysical properties of Ni(bpy)32+ (a d8, high spin
complex, paramagnetic) are very different from those of
typical photoredox catalysts, including Ru(bpy)32+ (d6 low
spin, diamagnetic).[2a] This is why the sporadic applications of
nickel-based photoredox catalysts have required meticulous
engineering of the ligands to ensure a low-spin NiII centers in
a square planar coordination environment.[24] We also
observed that, in consonance with the notion that the reaction
is triggered by direct excitation of alkyl-DHP 1 and not of
Ni(bpy)32+,[25] the optimal irradiation wavelength changes
depending on the nature of the substrate. For example,
dihydropyridine 4 absorbs at significantly shorter wavelength
(lmax = 323 nm) than esters 1 (lmax = 335 nm for 1 a). Irradiation at 405 nm does not promote the formation of the Giese
addition product 5 at all, while a high yield is obtained using
a 365 nm LED (Scheme 4).

Scheme 4. Reactivity/wavelength correlation for substrate 4.

In conclusion, we have reported a photochemical strategy
to perform the Giese addition of C(sp3) radicals to a variety of
electron-poor olefins. Mechanistic investigations suggest that
readily available Ni(bpy)32+ acts as an electron mediator to
facilitate the redox processes between fleeting and highly
reactive intermediates. These findings could be relevant in the
design of other photochemical strategies based on the direct
excitation of organic molecules or intermediates. In addition,
they may have mechanistic implications in the context of the
combination of nickel and photoredox catalysis,[23] since
nickel complexes could be involved in kinetically enhancing
electron-transfer processes.

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Acknowledgements
Financial support was provided by MICIU (CTQ2016-75520P), the AGAUR (Grant 2017 SGR 981), and the European
Research Council (ERC-2015-CoG 681840—CATA-LUX).

Conflict of interest
The authors declare no conflict of interest.
Keywords: dihydropyridines · electron mediator ·
nickel catalysis · photochemistry · synthetic methods
How to cite: Angew. Chem. Int. Ed. 2019, 58, 4953 – 4957
Angew. Chem. 2019, 131, 5007 – 5011

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[25] For additional mechanistic discussion, including triplet quenching experiments, see the Supporting Information, Section C6.
Manuscript received: December 21, 2018
Revised manuscript received: February 11, 2019
Accepted manuscript online: February 12, 2019
Version of record online: March 6, 2019

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