Metal-Free, Visible Light-Photocatalyzed Synthesis of Benzo[b]phosphole Oxides: Synthetic and Mechanistic Investigations

Article abstract of DOI:10.1021/jacs.6b04069

Highly functionalized benzo[b]phosphole oxides were synthesized from reactions of arylphosphine oxides with alkynes under photocatalytic conditions by using eosin Y as the catalyst and N-ethoxy-2-methylpyridinium tetrafluoroborate as the oxidant. The reac

Full text of DOI:10.1021/jacs.6b04069

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Article
Metal-Free, Visible Light-Photocatalyzed Synthesis of
Benzo[b]phosphole Oxides: Synthetic and Mechanistic Investigations
Valentin Quint, Fabrice Morlet-Savary, Jean-François Lohier,
Jacques Lalevée, Annie-Claude Gaumont, and Sami Lakhdar
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b04069 • Publication Date (Web): 17 May 2016
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Metal-Free, Visible Light-Photocatalyzed Synthesis of
Benzo[b]phosphole Oxides: Synthetic and Mechanistic Investiga-
tions
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Valentin Quint,† Fabrice Morlet-Savary,‡ Jean-François Lohier,† Jacques Lalevée,‡ Annie-Claude
Gaumont,† and Sami Lakhdar*†
† Normandie Univ., LCMT, ENSICAEN, UNICAEN, CNRS, 14000-France
‡
Institut de Science des Matériaux de Mulhouse IS2M – UMR CNRS 7361 – UHA, 15, rue Jean Starcky, 68057 Mulhouse Cedex,
France
ABSTRACT: Highly functionalized benzo[b]phosphole oxides were synthesized from reactions of arylphosphine oxides
with alkynes under photocatalytic conditions by using eosin Y as the catalyst and N-ethoxy-2-methylpyridinium tetra-
fluoroborate as the oxidant. The reaction works under mild conditions and has a broad substrate scope. Mechanistic inves-
tigations have been undertaken and revealed the formation of a ground state electron donor acceptor complex (EDA) be-
tween eosin (the photocatalyst) and the pyridinium salt (the oxidation agent). This complex, which has been fully charac-
terized both in the solid state and in solution, turned out to exhibit a dual role i.e. the oxidation of the photocatalyst and
the formation of the initiating radicals, which undergoes an intramolecular reaction avoiding the classical diffusion between
the two reactants. The involvement of ethoxy and phosphinoyl radicals in the photoreaction has unequivocally been evi-
denced by EPR spectroscopy.
INTRODUCTION
Due to their unique photophysical and electronic prop-
erties, -conjugated phosphole molecules have become of
substantial importance in the burgeoning fields of organic
light-emitting diodes (OLEDs) and photovoltaic cells.1
While synthesis and electronic characterization of these
molecules have previously been reported,1 little is known
about benzo[b]phosphole oxides, which have recently
emerged as promising scaffolds for organic electronics and
bioimaging probes.2 Common synthetic approaches to this
type of molecules include treatment of prefunctionalized
substrates with strong bases3 or use of expensive transition
metal catalysts.4 Recently, the groups of Duan, Satoh and
Miura simultaneously showed that benzo[b]phosphole ox-
ides 3 are accessible through combination of secondary
phosphine oxides 1 with alkynes 2 in the presence of a stoi-
chiometric amount of AgOAc (2 equiv) or Mn salts at typ-
ically 100°C (Scheme 1).5 The reaction is depicted as pro-
ceeding through a radical pathway, where the phosphinoyl
radical 4 is generated upon oxidation of the secondary
phosphine oxide 1 with Ag(I) or Mn(III). Subsequent reac-
tion of 4 with the alkyne 2 affords the alkenyl radical 5.
Then, this radical undergoes intramolecular addition to
the aryl ring at the ortho position of the phosphorus atom.
This gives the cyclohexadienyl intermediate 6, which yields
the phosphorus heterocycle 3 upon oxidation and deproto-
nation. While this approach has proven valuable for the
synthesis of numerous benzo[b]phosphole oxides 3, it suf-
fers from disadvantages of requiring high temperature and
stoichiometric amounts of metal oxidants.
Scheme 1. Synthesis of benzo[b]phosphole oxides 3
through silver or manganese-mediated processes (previous
work) and visible photoredox approach (this work).
Based on previous kinetic studies by Turro et al., who
measured several bimolecular rate constants of the reac-
tions of phosphinoyl radicals with different substrates, typ-
ically ranging from 106 to 109 M–1 s–1,6 one can hypothesize
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that the addition of 4 to the alkyne 2 (step a in Scheme 1) sin Y 9c as a photocatalyst and N-ethoxy-2-methylpyri-
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and the subsequent steps can occur at room temperature.
It is therefore likely that the high temperature required in
the silver-mediated approach is only required for the for-
mation of the phosphinoyl radical 4.7 Thus, it should be
possible to develop an approach for generating phos-
phinoyl radicals under mild reaction conditions at room
temperature without requiring expensive transition-metal
catalysts or toxic reagents.
dinium 8b as an external oxidant delivered the desired
product in very high yield (95%) under visible light (green
LED, max = 525 nm, 5 W) irradiation (Entry 8). From the
solvents tested, dimethylformamide (DMF) was confirmed
to be the solvent of choice (Supporting Information).
Moreover, the use of a base was found to be crucial for the
reaction, not only to deprotonate the arenium intermedi-
ate 7 (Scheme 1), but also to deprotonate the two acidic
protons of 9c, affording the photophysically more active
dianionic eosin form, as in the absence of base only 24% of
the product was formed (Entry 11).9j,11 Bromotrichloro-
methane (BrCCl3), which has previously been employed by
Stephenson, König and others as an efficient oxidant for
photoredox functionalizations of –amino carbons,12 has
failed to deliver the desired product (entry 6). This can be
rationalized on the basis of previous kinetic investigation
by Turro et al., who showed that phosphinoyl radical can
react with BrCCl3 with rate constants close to the diffusion
limit (k ≈ 109 M-1s-1),12 to give the corresponding halogen-
ated phosphine oxides, which was identified by 31P NMR.
Furthermore, changing the base from NaHCO3 to NaOAc
resulted in dropping the conversion to about 40% (entry 9,
Table 1).
Because of the relatively low bond dissociation energies
of P–H bonds of phosphine oxides (BDE = 317 kJ/mol for
diphenylphosphine oxide),8 we anticipated that the use of
a visible-light photocatalyst (PC)9 in the presence of a well-
selected organic oxidant would, after a photon absorption,
reduce the oxidant to generate a radical along with the ox-
idized form of the photocatalyst (PC•+). Two parameters
are crucial for the success of the photoredox process: i) the
formed radical should be able to abstract the P–H hydro-
gen to form the corresponding radical, and ii) the redox
potential of (PC•+) should be high enough to oxidize the
cyclohexadienyl intermediate 6 (step c in Scheme 1) to the
corresponding arenium ion 7 and to regenerate ground-
state photocatalyst.
RESULTS AND DISCUSSION
Table 1. Identification of the Optimal Reaction Conditions
To test this hypothesis, we examined the reaction of di-
phenylphosphine oxide 1a with diphenylacetylene 2a in the
presence of organic oxidants, bases, and photocatalysts un-
der visible light irradiation. To keep the process as green as
possible, only organic photocatalysts, which are known to
be inexpensive, environmentally friendly and easy to han-
dle, have been tested.9j,k The choice of external oxidant was
based on their low redox potentials compared to those of
the excited forms of the photocatalysts.
Entry
PC
9b
9a
9c
9c
9a
9c
9c
9c
9c
–
Oxidant
8bc
Solvent
DMF
DMF
DMF
DMF
DMSO
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Conversion (%)b
Being recognized as efficient oxidants in photopolymer-
ization reactions, the onium salts 8a-c were good candi-
dates for our photoreaction.10 Indeed, apart from their in-
ertness to absorb in the visible region, 8a-c can easily be
reduced by common visible-light excited photocatalysts to
generate very energetic radicals thermodynamically capa-
ble to abstract hydrogen from phosphine oxides (Figure 1).
1
2
3
4
5
37
25c
43
62
70
0
75
95d
52e
4f
8bc
8cc
8ac
8bc
BrCCl3
8bc
6
7
8
9
10
11
12
8b
8bc
8bc
9c
9c
8bc
24g
0
–
[a] Heating caused by the LED lamp. [b] Determined from 31P
NMR spectroscopy using trioctylphosphine oxide as internal
standard. [c] Counterion: tetrafluoroborate. [d] Conditions used:
1a (2 equiv), 2a (1 equiv), 8b (2 equiv), NaHCO3 (1.2 equiv). [e]
NaOAc was used as a base. [f] Reaction time: 72h. [g] Without a
base.
Figure 1. Redox potentials of onium salts 8a–c and or-
ganic dyes 9a–c. a) Taken from Ref. [10].
Control experiments highlighted the essential role of the
photocatalyst, oxidant, and light in this photoreaction (en-
tries 10 and 11). While less than 4% of background reaction
was observed in the absence of 9c even after a long time
In line with our working hypothesis, we were pleased to
find that all organic dyes 9a–c catalyzed the reaction of 1a
with 2a in the presence of any of the three external oxi-
dants 8a–c (Entries 1–3). In particular, combination of eo-
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Remarkably, phosphine oxides bearing either dimethyl-
irradiation (72h); no product was detected in the absence
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of 8b after 24 h.
amino (1p) or trifluoromethyl (1q) groups at the para posi-
tions of the aromatic ring react with 2a to afford exclusively
the expected regioisomers 3p and 3q, respectively in fairly
good yields.
Similar to the report of Duan, Satoh and Miura,5 the de-
sired benzophospholes were formed, along with their regi-
oisomers, resulting from the aryl migration, were formed
for the reaction of the diphenylphosphine oxides bearing
electron donor or electron acceptor groups at the aromatic
rings (1u–x).13
With the optimized conditions in hand, we applied our
protocol for the synthesis of a large variety of benzophos-
phole oxides derived from different alkynes and phosphine
oxides. As depicted in Table 2, diphenylphosphine oxide 1a
reacts smoothly with various alkynes 2 under standard
conditions, affording 3 in good to excellent yields (43 to
91%). Remarkably, apart from the tolerance of alcohol and
ester groups, complete control of selectivity has been ob-
tained with the tested unsymmetrical alkynes. This regi-
oselectivity can be attributed to the ability of aryl groups
to stabilize the formed alkenyl radical (5).
Next, to evaluate the scalability of the reaction, we car-
ried out the reaction of 1a with 2a on a gram scale under
standard conditions. To our delight, 1.5 g of benzophos-
phole oxide 3a was isolated (81% yield), thus proving the
effectiveness of our protocol.
Good to excellent yields were also obtained when differ-
ent pentavalent phosphorus derivatives P(V) were com-
bined with diphenylacetylene 2a. Notably, in contrast to
silver-mediated dehydrogenative annulation, where the re-
action of diphenylphosphine sulfide 1r with diphenylacety-
lene 2a does not proceed at all,5b presumably due to the
high thiophilicity of silver, the same reaction proceeds
smoothly under the current protocol (3r, 74% yield).
In order to gain insights into the mechanism of this new
photoredox coupling, we have first recorded the electron
paramagnetic resonance (EPR) spectra of the reaction of
eosin Y (9c) and diphenylphosphine oxide 1a in the pres-
ence of the onium salt 8b and phenylbenzonitroxide (PBN)
10 as a radical trap in toluene. Under irradiation with
LED@530 nm of a mixture of eosin Y 9c (5 × 10-5 M) and N-
ethoxy-2-methylpyridinium 8b (5 × 10-2 M) in tert-bu-
tylbenzene, a radical characterized by the hyperfine cou-
pling constants hfc (aN = 13.6 G, aH = 1.9 G) for the PBN
adduct 11 is directly observed (Figure 2a); this radical has
been ascribed to EtO● in agreement with literature data.14
Table 2. Substrate Scope.
Remarkably, when diphenylphosphine oxide 1a is added
in this solution, the phosphinoyl radical 4a is unequivo-
cally identified along with remaining ethoxy radical (Fig-
ure 2b). The hyperfine coupling constants hfc of the phos-
phinoyl/PBN adduct 12 (aN = 14.18 G, aH = 3.03 G, and aP =
18.76 G) are in perfect agreement with those reported in
the literature,15 thus leaving no doubt as to the role of this
intermediate in this photoreaction
Reaction conditions: secondary phosphine oxide (1.0 mmol),
alkyne (0.5 mmol), N-ethoxy-2-methylpyridinium tetra-
fluoroborate (1.0 mmol), NaHCO3 (0.6 mmol), eosin Y (0.02
mmol) in 8 mL of DMF and under green light irradiation.
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Figure 2. (a) EPR spectra of spin adduct 11 generated in toluene in the presence of eosin Y 9c (5 × 10-5 M), N-ethoxy-2-
methylpyridinium 8b (5 × 10-2 M) and PBN 10 (2 × 10-2 M) at room temperature. (b) EPR spectra of spin adduct 12 generated
in toluene in the presence of eosin Y 9c (5 × 10-5 M), N-ethoxy-2-methylpyridinium 8b (5 × 10-2 M), diphenylphosphine oxide
(2 × 10-2M) and PBN 10 (5 × 10-2 M) at room temperature; the stars are related to the phosphinoyl radical.
Furthermore, UV-vis spectroscopy showed that the ad-
dition of different concentrations of N-ethoxy-2-
methylpyridinium 8b to a DMF solution of eosin gave rise
to the appearance of a red-shifted band (Figure 3). This can
be attributed to the formation of a ground state electron
donor acceptor (EDA) complex between eosin and the pyr-
idinium salt 8b. Similar complexes have previously been
isolated and characterized by Willner et al., who showed
that eosin and derivatives are good donor partners for the
formation of EDA complexes.16
Stokes shift of the emission peak of the eosin (9c) when
increasing concentration of the pyridium ion 8b are added
in DMF (Figure S7, Supporting Information).
More importantly, a higher quenching constant (kq = 1.8
× 1010 M-1 s-1) was derived from the Stern Volmer plot, thus
attesting of the efficiency of the excited state of the EDA
complex to reduce the pyridinium salt 8b (Figure S8, Sup-
porting Information). In line with this, when the EDA com-
plex (EY-8b) is exposed to a green irradiation at 530 nm, a
fast and strong photobleaching reaction is observed (Fig-
ure 4). This suggests the occurrence of an electron transfer
within the EDA complex giving rise to a very fast diffusion-
less generation of the ethoxy radical.
Figure 3. UV-visible spectra of eosin Y (9c = 1.0 × 10–5 M)
after addition of different concentration of pyridinium 8b
in DMF at 20 °C.
Figure 4. Changes in the UV-visible absorption spec-
trum of a DMF solution of EY-8b (5.9 × 10-6 M) under light
irradiation (max = 530 nm)–aerated solution.
Further evidence for the formation of an EDA complex
came from fluorescence spectroscopy, which showed a
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donor-acceptor ratio (Figure 5). The molecules adopt a
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slipped stacking motif and – interactions between the
pyridinium ring and an aromatic ring of an adjacent eosin
molecule (short C–C distances of 3.412(13) and 3.495(13) Å).
With a centroid-centroid separation of 4.366(9) Å, this in-
teraction is characteristic of an electron donor-acceptor
complex of this type.17
Insights into the structure of the eosin-pyridinium as-
sembly in solution are obtained by NMR spectroscopy. In-
deed, when isolated crystals of EY-8b are dissolved in deu-
terated methanol, only small changes have been detected
compared to the individual spectra. However, ROESY
NMR spectroscopy revealed through-space interactions
between H1 of the eosin and He, Hf and Hg of the pyri-
dinium, which indicates that the two components are in
close proximity one to another (Figure 6).
Figure 5. Asymmetric unit of EY-8b complex. Displace-
ment ellipsoids are drawn at the 50% probability level. For
the sake of clarity, H atoms and disordered pyridinium
moiety have been omitted.
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The most informative result has however been derived
from the isolation of stable dark-red crystals, suitable for
an X-ray diffraction analysis, grown by slow evaporation of
the solvent from a solution of eosin 9c and pyridinum 8b
in methanol at room temperature. As shown in Figure 5,
the X–ray structure confirms the formation of an eosin-
pyridinium complex (EY-8b) as a – complex with a 1:1
Figure 6. 1H NMR and 1D-ROESY spectra of the complex (EY-8b) in CD3OD at –15 °C, showing the irradiation of H1 and
subsequent correlations.
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Based on these investigations, a plausible mechanism for on the formation of a donor-acceptor ground state
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the formation of the heterocycle 3, is given in Figure 7. The
reaction begins with the formation of the EDA complex be-
tween the eosin 9c and the onium salt 8b. Upon absorption
of a green photon, the EY-8b complex exhibits a single
electron transfer event to generate the unstable ethoxy
radical, which undergoes a hydrogen abstraction from the
secondary phosphine oxide 1a to give rise to the corre-
sponding phosphinoyl radical 4a. This radical then reacts
with the alkyne 2a to generate the alkenyl radical 5a, which
subsequently attacks the phenyl ring of the phosphine ox-
ide to give the cyclohexadienyl radical 6a. With a redox po-
tential of about –0.1 V,18 6a is readily oxidized by EY•+ (E0(
EY•+/EY)= 0.70 V/SCE)9c to release the photocatalyst 9c and
generate the Wheland intermediate 7a. This species is im-
mediately rearomatized after deprotonation with NaHCO3
to yield the desired benzophosphole oxide 3a.
complex EY-8b between the photocatalyst and the oxidant,
which turned out to be highly reactive. The sustainable
generation of the phosphinoyl radical 4,20 which has
unambiguously been confirmed by EPR spectroscopy,
opens
a new avenue for the synthesis of various
phosphorus-heterocycles under very mild conditions.
Extension of this methodology toward the synthesis of new
phosphorus-based molecules is currently under
investigation in our laboratory and will be reported in due
course.
ASSOCIATED CONTENT
Supporting Information.
Supporting information is available free of charge via the In-
ternet at http://pubs.acs.org.
NMR spectra, experimental procedures, and mechanistic
studies.
AUTHOR INFORMATION
Corresponding Author
* sami.lakhdar@ensicaen.fr
Author Contributions
The manuscript was written through contributions of all au-
thors.
ACKNOWLEDGMENT
This paper is dedicated to Professor Paul Knochel. The authors
thank the CNRS, Normandie Université, and Labex Synorg
(ANR-11-LABX-0029) for financial support. V.Q. is grateful to
the “Ministère de l’enseignement supérieur et de la recherche“
for a fellowship. The authors are indebted to Prof. Jean-Claude
Daran (University of Toulouse) for helping with the determi-
nation of the X-ray structure of complex EY-8b, and to Dr.
Rémi Legay and Mr. Hussein El Siblani for assistance with
NMR experiments. Prof. Herbert Mayr, Dr. Biplab Maji, Mr.
Hannes Erdmann, and Dr. Guillaume Berionni are acknowl-
edged for helpful discussions.
Figure 7. Postulated mechanism for the synthesis of the
benzo[b]phosphole oxides 3a via photoredox catalysis. EY:
eosin Y.
Taking into account the fact that many photoredox pro-
cesses can involve radical chains,19 one can hypothesize
that the external oxidant 8b can be reduced by 6a to form
7a inducing a chain propagation (Figure 7). Although this
scenario can thermodynamically be excluded on the basis
of redox potentials of 6a and 8b, we proceeded with the
measurement of the quantum yield (QY) of the reaction of
1a with 2a under the optimized photocatalytic conditions
to confirm this hypothesis. By employing a protocol previ-
ously described by Jacobi von Wangelin et al.,11c we meas-
ured a QY of 0.27, which implies that a chain propagation
is unlikely and that the photocatalytic pathway is popu-
lated.
REFERENCES
[1] For selected reviews on synthesis and reactivity of phospholes
and derivatives, see: a) Mathey, F. Angew. Chem. Int. Ed.
2003, 42, 1578–1604; Angew. Chem. 2003, 115, 1616–1643;
b) Baumgartner, T.; Réau, R. Chem. Rev. 2006, 106, 4681–
4727; c) Crassous, J.; Réau, R. Dalton Trans. 2008, 6865–6876;
d) Matano, Y.; Imahori, H. Org. Biomol. Chem. 2009, 7, 1258–1271;
e) Baumgartner, T. Acc. Chem. Res. 2014, 47, 1613–1622;
f) Stolar, M.; Baumgartner, T. Chem. Asian J. 2014, 9, 1212–1225.
[2] a) Tsuji, H.; Sato, K.; Sato, Y.; Nakamura, E. J. Mater. Chem.
2009, 19, 3364–3366; b) Tsuji, H.; Sato, K.; Sato, Y.; Nakamura, E.
Chem. Asian J. 2010, 5, 1294– 1297; c) Yamaguchi, E.; Wang, C. G.;
Fukazawa, A.; Taki, M.; Sato, Y.; Sasaki, T.; Ueda, M.; Sasaki, N.;
Higashiyama, T.; Yamaguchi, S. Angew. Chem. Int. Ed. 2015, 54,
4539–4543; Angew. Chem. 2015, 127, 4622–4626.
CONCLUSIONS
In conclusion, we have described the first photocatalytic
method for the synthesis of benzo[b]phosphole oxides
through an oxidative C-H/P-H functionalization reaction
of secondary phosphine oxides with alkynes. The reaction
proceeds smoothly under metal-free conditions and has a
broad substrate scope. The efficiency of the process relies
[3] a) Tsuji, H.; Sato, K.; Ilies, L.; Itoh, Y.; Sato, Y.; Nakamura, E.
Org. Lett. 2008, 10, 2263–2265; b) Sanji, T.; Shiraishi, K.; Kashiwa-
bara, T.; Tanaka, M. Org. Lett. 2008, 10, 2689–2692; c) Fukazawa,
A.; Hara, M.; Okamoto, T.; Son, E.-C.; Xu, C.; Tamao, K.; Yamagu-
chi, S. Org. Lett. 2008, 10, 913–916; d) Fukazawa, A.; Yamada, H.;
Yamaguchi, S. Angew. Chem. 2008, 120, 5664–5667; Angew. Chem.
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Int. Ed. 2008, 47, 5582– 5585; e) Fukazawa, A.; Ichihashi, Y.;
[11] a) Neumann, M.; Fuldner, S.; König, B.; Zeitler, K. Angew.
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31
32
3
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37
38
39
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4
45
46
47
48
49
50
51
52
53
54
5
56
57
58
59
60
Kosaka, Y.; Yamaguchi, S. Chem. Asian. J. 2009, 4, 1729– 1740.
[4] For seminal contributions, see: a) Baba, K.; Tobisu, M.;
Chatani, N. Angew. Chem. 2013, 125, 12108–12111; Angew. Chem. Int.
Ed. 2013, 52, 11892–11895; b) Berger, O.; Petit, C.; Deal, E. L.; Mont-
champ, J. -L. Adv. Synth. Catal. 2013, 355, 1361–1373; c) Wu, B.;
Santra, M.; Yoshikai, N. Angew. Chem. Int. Ed. 2014, 53, 7543–
7546; Angew. Chem. 2014, 126, 7673– 7676; d) Wu, B.; Chopra, R.;
Yoshikai, N. Org. Lett., 2015, 17, 5666– 5669; e) Zhou, Y.; Gan, Z.;
Su, B.; Li, J.; Duan, Z.; Mathey, F. Org. Lett. 2015, 17, 5722−5724.
[5] a) Chen, Y. R.; Duan, W. L. J. Am. Chem. Soc. 2013, 135, 16754 –
16757; b) Unoh, Y.; Hirano, K.; Satoh, T.; Miura, M. Angew. Chem.
Int. Ed. 2013, 52, 12975 – 12979; Angew. Chem. 2013, 125, 13213–13217;
c) Ma, W.; Ackermann, L. Synthesis 2014, 46, 2297–2304; d) Fisher,
H. C.; Berger, O.; Gelat, F.; Montchamp, J. -L. Adv. Synth. Catal.
2014, 356, 1199–1204; e) during the preparation of this manuscript,
Tang et al. reported on the synthesis of benzophospholes by using
a copper (2 mol%)/tert-butyl hydroperoxide (2 equiv.) catalyst
system; see: Zhang, P.; Gao, Y.; Zhang, L.; Li, Z.; Liu, Y.; Tang, G.;
Zhao, Y. Adv.Synth. Catal. 2016, 358,138 –142
[6] Sluggett, G. W.; Tuorro, C.; George, M. W.; Koptyug, I. V.;
Turro, N. J. J. Am. Chem. Soc. 1995, 117, 5148–5153.
[7] Liu et al. have shown that diphenylphosphinoyl radical can be
formed at 110 °C, when diphenyl phosphine oxide is combined
with AgF in the presence of 5,5-dimethyl-1-1-pyrroline N-oxide as
a radical spin trap see: Li, Z.; Fan, F.; Zhang, Z.; Xia, Y.; Liu, D.;
Liu, Z-Q. RSC. Adv. 2015, 5, 27853–27856.
[8] For a recent comprehensive review on phosphorus–centered
radical reactions; see: Pan, X.-Q.; Zou, J.-P.; Yi, W.-B.; Zhang, W.
Tetrahedron 2015, 71, 7481-7529, and references therein.
[9] For selected reviews on visible-light photoredox catalysis; see:
a) Zeitler, K. Angew. Chem. Int. Ed. 2009, 48, 9785–9789; Angew.
Chem. 2009, 121, 9969–9974; b) Narayanam, J. M.; Stephenson, C.
R. J.; Chem. Soc. Rev. 2011,40, 102–113; c) Xuan, J.; Xiao, W.-J. An-
gew. Chem. Int. Ed. 2012, 51, 6828– 6838; Angew. Chem. 2012, 124,
6934–6944; d) Shi, L.; Xia, W. Chem. Soc. Rev. 2012, 41, 7687–7697;
e) Reckenthaeler, M.; Griesbeck, A. G. Adv. Synth. Catal. 2013, 355,
2727–2744; f) Ravelli, D.; Fagnoni, M.; Albini, A. Chem. Soc. Rev.
2013, 42, 97–113; g) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C.
Chem. Rev. 2013, 113, 5322–5363; h) Schultz, D. M.; Yoon, T. P.
Science 2014, 343, 985; i) Koike, T.; Akita, M. Inorg. Chem. Front.
2014, 1, 562–576; j) Hari, D. P.; König, B. Chem. Commun. 2014, 50,
6688–6699; k) Nicewicz, D. A.; Nguyen, T. M. ACS Catal. 2014, 4,
355–360; l) Pitre, S. P.; McTiernan, Ch. D.; Scaiano, J. C. Acc. Chem.
Res. 2016, 10.1021/acs.accounts.6b00012.
Chem. Int. Ed. 2011, 50, 951–954; Angew. Chem. 2011, 123, 981–985;
b) Hari, D. P.; Schroll, P.; König, B. J. Am. Chem. Soc. 2012, 134,
2958–2961; c) Majek, M.; Filace, F.; Jacobi von Wangelin, A.
Beilstein J. Org. Chem. 2014, 10, 981–989.
[12] Sluggett, G. W.; Macgary, P. F.; Koptyug, I. V.; Turro, N. J. J.
Am. Chem. Soc. 1996, 118, 7367-7372.
[13] For a review on radical aryl migration reactions, see: Chen, Z.
-M.; Zhang, X. -M.; Tu, Y. -Q. Chem. Soc. Rev. 2015, 44, 5220–5245.
[14] Jenkins, C. A.; Murphy, D. M.; Rowlands, C. C.; Egerton, T.
A. J. Chem. Soc., Perkin Trans. 2 1997, 2479−2485.
[15] a) Morlet-Savary, F. ; Klee, J. E. ; Pfefferkon, F. ; Fouassier, J.
P. ; Lalevée, J. Macromol. Chem. Phys. 2015, 216, 2161–2170; b) Lale-
vée, J. ; Morlet-Savary, F. ; Tehfe, M. A. ; Graff, B. ; Fouassier, J. P.
Macromolecules 2012, 45, 5032–5039.
[16] a) Willner, I.; Eichen, Y.; Rabinovitz, M.; Hoffman, R.;
Cohen, S. J. Am. Chem. Soc. 1992, 114, 637–644; b) Willner , I.;
Marx , S.; Eichen, Y. Angew. Chem. Int. Ed. Engl. 1992, 31, 1243–
1244 Angew. Chem. 1992, 104, 1255–1256.
[17] for selectd examples on the use of electron donor−acceptor
complexes in organic synthesis; see: a) Lima, C. G. S.; Lima, T. de
M.; Duarte, M.; Jurberg, I. D.; Paixꢀo, M. W. ACS Catal. 2016, 6,
1389−1407; b) Arceo, E.; Jurberg, I. D.; ꢁlvarez-Fernꢂndez, A.;
Melchiorre, P. Nat. Chem. 2013, 5, 750−756; c) Nappi, M.; Bergon-
zini, G.; Melchiorre, P. Angew. Chem., Int. Ed. 2014, 53, 4921−4925;
Angew. Chem. 2014, 126, 5021–5025; d) for the elucidation of the
structure of an electron donor-acceptor complex in the solid state;
see: Kandukuri, S. R.; Bahamonde, A.; Chatterjee, I.; Jurberg, I. D.;
Escudero-Adan, E. C.; Melchiorre, P. Angew. Chem., Int. Ed. 2015,
54, 1485−1489; Angew. Chem. 2015, 127, 1505 –1509.
[18] Bahtia, K.; Schuler, R. H. J. Phys. Chem. 1974, 78, 2335–2338.
[19] a) Cismesia, M. A.; Yoon, T. P. Chem. Sci. 2015, 6, 5426–5434;
b) Kärkäs, M. D.; Matsuura, B. S.; Stephenson, C. R. J. Science 2015,
349, 1285–1286; c) Studer, A.; Curran, D. P. Angew. Chem. Int. Ed.
2016, 55, 58–102; Angew. Chem. 2016, 128, 58–102.
[20] For recent contributions on the generation of phosphorus
centered radical under photoredox conditions, see: He, Y.; Wu,
H.; Toste, F. D. Chem. Sci. 2015, 6, 1194– 1198, (b) Xuan, J.; Zeng,
T.-T.; Chen, J.-R.; Lu, L.-Q.; Xiao, W.-J. Chem. - Eur. J. 2015, 21,
4962–4965.
[10] Fouassier, J. P.; Lalevée, J. Photoinitiators for Polymer
Synthesis: Scope, Reactivity and Efficiency, Wiley-VCH,
Weinheim, 2012.
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