Multicomponent Oxyalkylation of Styrenes Enabled by Hydrogen-Bond-Assisted Photoinduced Electron Transfer

Article abstract of DOI:10.1002/anie.201700049

Herein, we disclose a strategy for the activation of N-(acyloxy)phthalimides towards photoinduced electron transfer through hydrogen bonding. This activation mode enables efficient access to C(sp³)-centered radicals upon decarboxylation from bench-stable and readily available substrates. Moreover, we demonstrate that the formed alkyl radicals can be successfully employed in a novel redox-neutral method for constructing sp³?sp³ bonds across styrene moieties that gives straightforward access to complex alcohol and ether scaffolds.

Full text of DOI:10.1002/anie.201700049

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201700049
German Edition: DOI: 10.1002/ange.201700049
Photoredox Catalysis
Multicomponent Oxyalkylation of Styrenes Enabled by Hydrogen-
Bond-Assisted Photoinduced Electron Transfer
Adrian Tlahuext-Aca, R. Aleyda Garza-Sanchez, and Frank Glorius*
Abstract: Herein, we disclose a strategy for the activation of N-
(acyloxy)phthalimides towards photoinduced electron transfer
through hydrogen bonding. This activation mode enables
efficient access to C(sp3)-centered radicals upon decarboxyla-
tion from bench-stable and readily available substrates. More-
over, we demonstrate that the formed alkyl radicals can be
successfully employed in a novel redox-neutral method for
3
3
À
constructing sp sp bonds across styrene moieties that gives
straightforward access to complex alcohol and ether scaffolds.
V
isible-light-mediated radical decarboxylation of aliphatic
acids has emerged in recent years as a powerful tool to access
C(sp3)-centered radical intermediates.[1] The formation of
these species under mild conditions has led to the develop-
À
À
ment of impressive C C and C X bond forming transforma-
tions using cheap, abundant, and synthetically versatile
carboxylic functionalities.[2] From a fundamental point of
view, these strategies rely on photoinduced electron transfer
(PET) processes between carboxylic acids and strongly
oxidizing photoexcited catalysts, leading to the formation of
alkyl radicals after CO2 extrusion (Scheme 1a). Whereas this
PET event has been extensively explored in the vast majority
of photoredox-catalyzed transformations involving carboxylic
acids, electronically reversed light-driven electron transfers
where the carboxylic acid derivative is reduced before
undergoing radical decarboxylation have been less explored
(Scheme 1b).[3] If feasible, these new mechanistic pathways
would expand the scope of synthetic applications amenable to
visible-light-mediated decarboxylative methods, while taking
advantage of the efficiency of this radical generation strategy.
Motivated by the desire to investigate such an intriguing
mechanistic scenario and its potential applications in visible-
light-mediated organic synthesis, we were drawn to the
pioneering work of Okada on the use of N-(acyloxy)phtha-
limides as electrophilic substrates for radical decarboxyla-
tion.[4] Interestingly, despite to their ability to generate alkyl
radicals under visible-light-mediated conditions, their highly
negative reduction potentials (Er1e=d2 < À1.28 V vs. SCE in
MeCN)[5] hamper direct photoinduced electron transfers with
photoexcited catalysts. For these substrates, therefore, reduc-
tive conditions must be employed where the photoexcited
Scheme 1. a–c) Visible-light-mediated radical decarboxylative methods.
d,e) Development of an oxyalkylation method.
catalyst is converted into a highly reducing species prior to
engaging in SET with the corresponding N-(acyloxy)phthali-
mide (Scheme 1c).[6,7] This dramatically limits the scope of
accessible transformations to net reductive couplings. To
overcome these limitations and provide efficient access to
novel oxidative quenching processes for radical decarboxyla-
tion, we asked whether the ground-state redox properties of
N-(acyloxy)phthalimides could be tuned to provide a thermo-
dynamically feasible PET. Herein, we demonstrate that
hydrogen bonding, one of the most important intermolecular
interactions in biology, molecular self-assembly and non-
covalent synthesis, can be suitable to drive a direct PET
process using N-(acyloxy)phthalimides by means of increas-
ing their electron-acceptor strength (Scheme 1d).[8,9]
Moreover, we applied this strategy to the efficient
generation of a variety of alkyl radicals upon decarboxylation
from bench-stable and readily accessible substrates without
the need for sacrificial reductants. Under mild redox-neutral
conditions, we show that the formed alkyl radicals can be
further manipulated to achieve the oxyalkylation of styrenes.
These transformations are synthetically powerful since they
install two single bonds across an olefin, giving straightfor-
ward access to molecular complexity from simple materials
(Scheme 1e). Moreover, in contrast to other radical difunc-
[*] A. Tlahuext-Aca, R. A. Garza-Sanchez, Prof. F. Glorius
NRW Graduate School of Chemistry, Organisch-Chemisches Institut
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster
Correnstrasse 40, 48149 Mꢁnster (Germany)
E-mail: glorius@uni-muenster.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201700049.
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tionalization reactions such as oxytrifluoromethylation[10] or
oxyarylation,[11] oxyalkylation remains challenging and only
a limited number of methods are available for constructing
these important and versatile carbooxygenated building
blocks.[12]
We started our investigation by identifying conditions to
perform the radical hydroxyalkylation of 4-tert-butylstyrene
(1a) with 2a in the presence of water as a nucleophile and
hydrogen-bond donor. We screened five commonly employed
transition-metal-based photocatalysts (Table 1). To our
Table 1: Visible-light-mediated oxyalkylation of 1a.
Scheme 2. Scope with respect to the styrenes. Yields of isolated
product are given. See the Supporting Information for details.
Entry[a] Photocatalyst
E1/2 (M+/M*)[b] Yield [%][c]
1
2
3
4
fac-Ir(ppy)3
[Ir(ppy)2(dtbbpy)](PF6)
À1.73
À0.96
26
80 (75)
–
–
shown for 3ha–3ka, the reaction performs with similar
efficiency with substituents at the meta and ortho positions.
Styrenes featuring electron-withdrawing groups such as
trifluoromethyl or cyano groups, however, reacted sluggishly,
giving 3la and 3ma in moderate yields. While the 2-vinyl-
substituted naphthalene afforded moderate yield, other
alkenes such as a-substituted styrenes gave straightforward
access to the tertiary alcohols 3oa–3pa in good yields.
We next turned our attention to investigate whether other
N-(acyloxy)phthalimides can be employed as radical sources
using 1o as a substrate (Scheme 3). To our delight, a range of
primary, secondary, and tertiary alkyl moieties can be
successfully installed across the styrene unit with high yields
[Ir(dF(CF3)ppy)2(dtbbpy)](PF6) À0.89
[Ru(phen)3](PF6)2
À0.87
À0.81
À0.96
À0.96
À0.96
5
[Ru(bpy)3](PF6)2
–
6[d]
7[e]
8[f]
[Ir(ppy)2(dtbbpy)](PF6)
[Ir(ppy)2(dtbbpy)](PF6)
[Ir(ppy)2(dtbbpy)](PF6)
52
76
–
[a] Conditions: 1a (0.10 mmol), 2a (0.125 mmol), photocatalyst
(1.0 mol%), water (5.0 mmol) and MeCN (0.5 mL) under argon. [b] All
potentials are given in volts versus the saturated calomel electrode
(Ref. [1d]). [c] NMR yields using CH2Br2 as internal standard. [d] Water
(1.0 mmol). [e] 2a (0.10 mmol). [f] TEMPO (1.5 equiv). See the Sup-
porting Information for details. Yields of isolated product are given in
parentheses.
and good functional group tolerance. Moreover, as shown for
3
À
delight, 3aa was observed upon irradiation with 5 W blue
LEDs (lmax = 455 nm) in acetonitrile for 12 h when using
1 mol% of fac-Ir(ppy)3 or [Ir(ppy)2(dtbbpy)](PF6) in 26 and
80% yield as determined by NMR, respectively. These
reactivities are in agreement with the more reducing excited
states of these two iridium complexes compared to the other
photocatalysts tested. Following a short screen, conditions
were identified under which alcohol 3aa could be isolated in
75% yield when using [Ir(ppy)2(dtbbpy)](PF6) as a photo-
catalyst. Control experiments were then performed to dem-
onstrate the visible-light-mediated nature of this process and
the key role of water as an activator. Alcohol 3aa was not
observed in the absence of [Ir(ppy)2(dtbbpy)](PF6) or light
irradiation. Moreover, when performed in the absence of
water, no reactivity was observed and both 1a and 2a could be
recovered. Consistent with the radical nature of the devel-
oped oxyalkylation, the presence of TEMPO as a radical
scavenger completely inhibited the formation of 3aa, and the
corresponding 1-(cyclohexyloxy)-2,2,6,6-tetramethylpiperi-
dine adduct was characterized by mass spectrometry from
these assays (see the Supporting Information for details).
With the optimized conditions in hand, the substrate scope for
the hydroxyalkylation was assessed with a variety of styrenes
and 2a as a radical source. The reaction tolerates a variety of
electron-donating and -withdrawing groups at the para
position of the arene moiety (Scheme 2). Moreover, as
3od, 3oe, 3og, and 3oi, C(sp ) X and aliphatic alkene
moieties can easily be incorporated and can serve as reactive
centers for further functionalization processes.
As the next stage of our investigation, we sought to
demonstrate the generality of our approach in terms of the
use of other hydrogen-bond donors that can also be used as
nucleophiles in our oxyalkylation strategy (Scheme 4). Grat-
ifyingly, a variety of primary alcohols reacted smoothly to
Scheme 3. Scope with respect to the N-(acyloxy)phthalimides. [a] Iso-
lated as a 1:1 d.r. mixture. Yields of isolated product are given. See the
Supporting Information for details.
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Scheme 4. Scope with respect to the alcohols. [a] 2a (2.0 equiv), [Ir-
(ppy)2(dtbbpy)](PF6) (2.5 mol%). Isolated yields. See the Supporting
Information for details.
afford the tertiary ethers 4oa–6oa when employing 1o and 2a
as model substrates. 2-Propanol, however, gave only a mod-
erate yield of 7oa, even under higher loadings of photo-
catalyst and coupling partner.
Scheme 5. Proposed mechanism.
To gain insight into the mechanism of the visible-light-
mediated oxyalkylation of 1a with 2a and water, we studied
the kinetics of the photoexcited *[Ir(ppy)2(dtbbpy)]+ by
fluorescence techniques. A Stern–Volmer analysis revealed
that 2a does not quench the excited state of [Ir(ppy)2-
(dtbbpy)]+ in pure acetonitrile at room temperature. Con-
sistent with our proposed activation of 2a by hydrogen
bonding, however, the same assay in the presence of water
(2.8m) showed a marked decrease in the measured fluores-
cence of *[Ir(ppy)2(dtbbpy)]+. Under these conditions,
a Stern–Volmer constant (Ksv) of 60.8mÀ1 was measured.
Moreover, variation of the concentration of 2a and water
showed that the quenching processes exhibit a first-order
dependence on each component. Interestingly, a similar first-
order concentration dependence on other protic species such
as methanol was observed, however, due to its weaker ability
to engage in hydrogen bonding, the PET between *[Ir(ppy)2-
(dtbbpy)]+ and 2a is kinetically less favored when methanol is
used instead of water.[13] Finally, to provide further evidence
on the role of hydrogen bonding in the activation of 2a
towards PET, we performed a Stern–Volmer analysis using
N,N-dimethylformamide (DMF) as the solvent. Due to the
strong hydrogen-bond acceptor properties of DMF,[14] differ-
ent concentrations of 2a did not lead to any decrease in the
*[Ir(ppy)2(dtbbpy)]+ fluorescence in this solvent in the
presence of water (2.8m).
novel oxidative quenching manifold serves as a platform to
access a variety of alkyl radicals from bench-stable and
readily accessible carboxylic acid derivatives, and we hope it
will find further applications in visible-light-mediated organic
synthesis.
Acknowledgements
We thank M. Teders, C. Onneken, Dr. A. Gꢀmez-Suarꢁz, and
Dr. L. Candish for helpful discussions. Financial support by
the NRW Graduate School of Chemistry (A.T.A.) and the
Deutsche Forschungsgemeinschaft (Leibniz Award) is grate-
fully acknowledged.
Conflict of interest
The authors declare no conflict of interest.
Keywords: alkyl radicals · decarboxylation · hydrogen bonding ·
oxyalkylation · photoredox catalysis
A proposed mechanism for the oxyalkylation of alkenes is
depicted in Scheme 5. Upon visible-light irradiation, the
excited *[Ir(ppy)2(dtbbpy)]+ engages in single-electron trans-
fer with the hydrogen bond complex A to give the radical
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sv
MeOH
. See the Supporting Information
sv
we observed a K
> K
for further details.
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Manuscript received: January 3, 2017
Final Article published: && &&, &&&&
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Angewandte
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Communications
Photoredox Catalysis
A. Tlahuext-Aca, R. A. Garza-Sanchez,
F. Glorius*
&&&& — &&&&
Multicomponent Oxyalkylation of
Styrenes Enabled by Hydrogen-Bond-
Assisted Photoinduced Electron Transfer
OH, I see: An efficient strategy to activate
N-(acyloxy)phthalimides towards direct
photoinduced electron transfer with pho-
toexcited catalysts was developed. This
strategy enables efficient visible-light-
mediated formation of alkyl radicals that
can be used to achieve the challenging
oxyalkylation of styrenes under mild
redox-neutral conditions.
Angew. Chem. Int. Ed. 2017, 56, 1 – 5
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!