Angewandte Chemie International Edition
10.1002/anie.201905485
RESEARCH ARTICLE
Although
a
detailed spectroscopic study of transient
In conclusion, we have achieved the direct reduction of
aromatic compounds using visible-light photoredox catalysis. The
method allowed for the successful photoreduction of napthalenes,
larger aromatic hydrocarbons and heterocycles by using the
energy of two visible light photons. Such photoreductions are
valuable in synthesis. Triplet sensitizers that provide higher
energies are currently being investigated in order to expand the
scope of the reaction further.
intermediates would be necessary to prove the mechanistic
hypothesis fully, several experimental observations support our
mechanistic proposal already. UV-visible spectroscopy confirmed
that only photocatalyst 1 absorbs at λ = 455 nm (see SI, Fig. S4).
Therefore, a direct excitation of anthracene or naphthalene is
unlikely.[35] Furthermore, Stern-Volmer quenching studies
supported both the energy transfer from the excited photocatalyst
to anthracene and the electron transfer from DIPEA to the excited
photocatalyst (Fig. 3C). The energy transfer to anthracene and
the electron transfer are similar in rate, but not identical. An
excess of DIPEA does not interfere with the reaction (Table S6),
which indicates that the better overall performance with a ratio of
Experimental Section
General procedure for the reduction. A 5 mL crimp cap vial was
equipped with the substrate (0.2 mmol, 1 equiv.), DIPEA (0.2 mmol, 1
1
3 3 2 6
:10 may have its reason in other chemical steps involving DIPEA. equiv.), MeNH Cl (10 mol%), the photocatalyst Ir[dF(CF )ppy] (dtbpy)PF
(
1 mol%) and a stirring bar. After adding the solvent (2 mL DMF) via
Online UV-visible and EPR experiments provided evidence for the
syringe, the vial was capped under air. The reaction mixture was stirred
and irradiated using a 455 nm (± 10 nm) LED for 15-18 h at 25 °C. The
progress was monitored by TLC and GC analysis. The reaction mixture
was diluted with water (10 ml), extracted with ethyl acetate (3 x 20 ml),
II
formation of the Ir complex (Fig. 3D). The excitation mechanism
by an overall of two photons is supported by a quadratic
dependency of the product yield (determined by GC analysis) on
the irradiation intensity (Fig. 3E). Furthermore, substrate addition
2 4
washed with brine (1 x 20 ml) and dried over anhydrous Na SO . The
II
crude product was obtained by removing the solvents under reduced
pressure. Purification was performed by automated flash column
chromatography (silica, 0-100% EtOAc/PE).
to an independently generated Ir complex while stirring in the
dark resulted in no product formation (see SI). This confirmed that
an energy transfer step using another photon is crucial in
generating the radical anion of anthracene. To prove the presence
of a carbanion intermediate, a photochemical E1cB reaction was
designed (Fig. 3B). A photosensitized electron transfer, which
was followed by HAT, generated a carbanion intermediate of 5b′.
Subsequent leaving group elimination led to the alkene 5b′′ that
was then photoreduced to the corresponding alkane 4b′.
Acknowledgements
We thank German Science Foundation (DFG, KO 1537/18-1) and
the European Research Council (ERC) under the European
Union's Horizon 2020 research and innovation programme (grant
agreement No. 741623) for financial support. We thank Dr. Rudolf
Vasold, Regina Hoheisel, Julia Zach, Marsel Shafikov and Jenny
Phan for assistance with analytical measurements. We thank Prof.
Jimmie Weaver, Matthias Schmalzbauer, Dr. Indrajit Ghosh, Dr.
Stefano Crespi and Sascha Grotjahn for helpful discussions and
Ranit Lahmy for proof reading the manuscript.
Finally, experiments with deuterated DMF excluded HAT
and protonation processes that could involve the solvent.
Protonation occurs from the iminium ion of DIPEA, however,
MeNH
from MeNH
values of benzylic C-H and MeNH
respectively, in DMSO).[36-37] In addition, electrochemical data
suggested that added MeNH Cl lowers the reduction potential of
3
Cl also acts as an additional proton source as protonation
Cl to 5b′ is thermodynamically favourable (pKa
Cl are 30.1 and 11.1,
3
3
3
Keywords: photocatalysis • reduction • energy transfer •
the substrate by 0.10 V (see SI, Fig. S15), facilitating the electron
transfer and thereby accelerating the reaction (Table 1, entry 14).
electron transfer • dearomatization
The possibility of reducing the singlet-triplet gap (E
T
) of the
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substrate through a possible cation-π interaction[38] in the
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bathochromic shift in UV-vis and phosphorescence spectra could
be observed. Mechanisms involving triplet-triplet annihilation or a
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[
7
II
conPET process of Ir are unlikely under the reaction conditions.
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time-dependent UV measurements (Fig. S8) we can conclude
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that an Ir species is not stable for long in DMF under the reaction
[8]
II
conditions. A further excitation of Ir is therefore unlikely, which
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[
Conclusion
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1
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