Full Paper
doi.org/10.1002/chem.202101813
Chemistry—A European Journal
In order to clarify the mechanism of this photoredox-
catalyzed HDF, we conducted experimental studies mainly
focusing on the following two processes: SET from excited
photocatalyst 1* to fluoroarene 2 and regeneration of photo-
catalyst 1 in the absence of amine additives. Firstly we
examined the physicochemical properties of 1 to probe the SET
process (Figure 1). 1 exhibited intense absorption at 332 nm
and fluorescence at 403 nm in DMF (Figure 1a). The absorption
band spanned the near-UV regions up to 380 nm, which
enables photoexcitation of 1 by 365 nm LEDs. Cyclic voltamme-
try (CV) of 1 showed a reversible oxidation wave at +0.14 V vs.
SCE in DMF (Figure S1). From these results, the oxidation
potential of the lowest singlet excited state (S1) of 1 is estimated
to be À 2.94 V vs. SCE in DMF, which is slightly higher than the
reported value of 1* (À 3.25 V vs. SCE in MeCN).[16,22] This value is
almost equal to oxidation potentials of highly reducing alkali
metals,[9c] and is close to the reported value of the reduction
potential of fluorobenzene 2a (À 2.97 V vs. SCE in DMF).[3a] In
order to prove the SET step from 1* to fluoroarenes 2, Stern-
Volmer fluorescence quenching experiments were conducted
between 1 and arenes 2a, 2x, and 3a in DMF (Figure 1b). The
fluorescence intensity of 1 was diminished by the addition of
2a efficiently. Since no significant change of the absorption
spectra of 1 was observed in the presence of 2a, the interaction
between 1 and 2a in their ground states (e.g. forming a
charge-transfer complex) is negligible. Therefore, this result
clearly indicates that SET from the excited S1 state of 1 to 2a is
operative in the reaction conditions[23] and confirms the utility
of 1 as a powerful photoreductant. The fluorescence was also
quenched by the addition of electron-deficient fluoroarene 2o,
which did not undergo HDF at all. Considering the general
mechanism of hydrodehalogenation of haloarenes induced by
SET,[8] this defluorination reaction would proceed in a stepwise
*
fashion through SET from 1* to fluoroarene 2 to give 1+ and
*
fluoroarene anion radical 2À followed by elimination of the
fluoride anion to give an aryl radical (Figure 1c). In the case of
electron-deficient fluoroarenes, the elimination of the fluoride
anion would get slower since the electron density of anion
*
radical 2À on the aryl ring is decreased by electron-with-
*
drawing substituents. Instead, back electron-transfer from 2À
*
to 1+ would be preferred, leading to recovery of the
fluoroarenes 2. We also confirmed the HDF product 3a did not
influence the fluorescence intensity of 1 at all (Figure 1b). In
addition, the generation of phenyl radical from 2a was verified
by a radical trapping experiment with excess amounts of N-
methylpyrrole (7) (Figure 1d). Corresponding coupling product
8 between phenyl radical and 7 was obtained along with 3a
albeit in low yields. Other types of CÀ F bond functionalizations
via phenyl radical such as borylation proceeded similarly in
relatively low yields (Scheme S3).
We then turned our attention to the mechanism of the
regeneration of photocatalyst 1, the most intriguing process in
i
this HDF reaction (Scheme 2). First the role of Pr2NEt as a
i
sacrificial reductant was examined. The addition of Pr2NEt to
*
cation radical of 1 (1+ -PF6), which is known as a stable, isolable
organic radical,[24] led to facile formation of 1 in the dark
i
(Scheme 2a). Therefore, Pr2NEt plays a role of a single electron-
*
donor to 1+ to regenerate 1, although this SET process seems
to be thermodynamically somewhat unfavorable considering
*
their redox potentials (iPr2NEt: Eox = +0.52 V;[25] 1+ : Ered = +
0.14 V vs. SCE in DMF). The use of such an alkyl amine additives
as an electron-donor is quite common in photoredox-catalyzed
reductions.[7] In contrast, this HDF reaction proceeded catalyti-
i
cally to 1 in NMP even in the absence of Pr2NEt. To examine
the unconventional feature of this reaction, HDF of 2a in NMP
was monitored by 19F NMR analysis (Scheme 2b). We detected
5-fluoro-N-methyl-2-pyrrolidone (9)[26] in 23% yield along with
HF in 55% yield, which indicates the involvement of NMP as a
hydrogen atom donor to phenyl radical. It is worth noting that
significant amounts of 9 was observed even in the presence of
Figure 1. Mechanistic studies for the photo-induced SET step. a) Absorption
and fluorescence spectra (λex =332 nm) of 1 in DMF. b) Stern-Volmer
fluorescence quenching plots of 1 with addition of 2a (red), 2o (blue), and
3a (black) in DMF. c) Proposed mechanism for generation of aryl radical
from fluoroarene 2. d) Trapping experiment of phenyl radical with 7. The
yields of 8 and 3a were determined by 1H NMR and GC analyses,
respectively.
Chem. Eur. J. 2021, 27, 1–8
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