Mechanism and Applications of the Photoredox Catalytic Coupling of Benzyl Bromides

Article abstract of DOI:10.1002/chem.201603517

The photoredox catalytic coupling of halomethyl arenes to bibenzyl derivatives has been demonstrated. The catalytic protocol employed the Hantzsch ester, potassium phosphate, and a photoactive cyclometalated Ir^IIIcomplex catalyst. A photochemical quantum yield as high as 20 % was obtained. The catalytic mechanism was investigated in detail by performing photophysical and electrochemical measurements, as well as by quantum chemical calculations. The results suggest that two-electron mediation might be responsible for the improved photon economy. The reaction protocol was compatible with halomethyl arenes that contain a variety of functional groups. Finally, the synthetic utility of our protocol was demonstrated by the preparation of a natural dihydrostilbenoid, brittonin A.

Full text of DOI:10.1002/chem.201603517

DOI: 10.1002/chem.201603517
Full Paper
&
Organic Chemistry
Mechanism and Applications of the Photoredox Catalytic
Coupling of Benzyl Bromides
Gyurim Park,[a] Seung Yeon Yi,[b] Jaehun Jung,[c] Eun Jin Cho,*[d] and Youngmin You*[e]
Abstract: The photoredox catalytic coupling of halomethyl
arenes to bibenzyl derivatives has been demonstrated. The
catalytic protocol employed the Hantzsch ester, potassium
phosphate, and a photoactive cyclometalated IrIII complex
catalyst. A photochemical quantum yield as high as 20%
was obtained. The catalytic mechanism was investigated in
detail by performing photophysical and electrochemical
measurements, as well as by quantum chemical calculations.
The results suggest that two-electron mediation might be
responsible for the improved photon economy. The reaction
protocol was compatible with halomethyl arenes that con-
tain a variety of functional groups. Finally, the synthetic utili-
ty of our protocol was demonstrated by the preparation of
a natural dihydrostilbenoid, brittonin A.
Introduction
generated in the presence of electron sources with very nega-
tive oxidation potentials. For instance, Zn metal is required to
promote the oxidative addition of benzyl bromides into a Ni
complex with a tridentate 2,6-bisoxazolylpyridine ligand.[22]
Therefore, the synthesis of bibenzyl without the use of strong
reducing agents still poses a great challenge.
Bibenzyl is the key motif found in a wide variety of dihydrostil-
benoids (Scheme 1).[1–3] The discovery of the biological actions
of dihydrostilbenoids, such as those of the combretastatin
series,[4] have sparked renewed research interest in the synthe-
sis of bibenzyl derivatives. The synthesis usually involves the
hydrogenation of stilbene. To expand synthetic routes to bi-
benzyls, novel methods that promote the Csp3–Csp3 coupling
of halomethyl arenes have been developed. Wurtz coupling is
one such traditional approach.[5] However, this method re-
quires stoichiometric use of excessive reducing reagents, such
as Li,[6,7] Mg,[8–14] Mn,[15,16] Fe,[17,18] Ni,[19] and Zn[20–24] metals, and
this limits its synthetic utility. The recently developed methods
of transition-metal-catalyzed Csp3–Csp3 coupling enable in-
creased control over the reaction conditions. The catalytic
mechanism involves the oxidative addition of benzyl halides
into low-valent complexes of Mn,[8,14,25–27] Fe,[9,10,28] Ni,[20,22,29]
Cu,[16–18,30,31] V,[32] Rh,[33] In,[34] Ti,[15,35] and Sm[36,37] that can be
[a] G. Park
Department of Food Science and Engineering
Ewha Womans University, Seoul 03760 (Republic of Korea)
[b] S. Y. Yi
Department of Chemistry and Nano Science
Ewha Womans University, Seoul 03760 (Republic of Korea)
Scheme 1. Structures of natural dihydrostilbenoids.
[c] J. Jung
Department of Applied Chemistry, Hanyang University
Ansan 15588 (Republic of Korea)
One promising alternative to the existing method is the cou-
pling of free benzyl radicals. Radical species can be derived
from benzyl halides through photoinduced reductive dissocia-
tion. This method offers significant advantages, as photoreduc-
tion can be executed by employing robust catalysts and very
stable electron donors under mild reaction conditions. The use
of coordinatively saturated late transition-metal complexes,
such as the complexes of RuII, IrIII, or PtII, as photocatalysts for
radical generation, is particularly appealing, because photon
[d] Dr. E. J. Cho
Department of Chemistry, Chung-Ang University
Seoul 06974 (Republic of Korea)
E-mail: ejcho@cau.ac.kr
[e] Dr. Y. You
Division of Chemical Engineering and Materials Science
Ewha Womans University, Seoul 03760 (Republic of Korea)
E-mail: odds2@ewha.ac.kr
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201603517.
Chem. Eur. J. 2016, 22, 1 – 11
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absorption generates long-lived triplet excited-state species ca-
pable of redox mediation. Moreover, these complexes are re-
sistant to degradation during repetitive photoredox cycles,
permitting high turnovers. Indeed, photoredox catalysis medi-
ated by the transition-metal complexes has been demonstrat-
ed in a wide range of organic transformations.[38–54] We have
also explored photoredox catalysis employing photofunctional
complexes of RuII, IrIII, and PtII, and established the photoelec-
trochemical mechanisms of the reductive actions of such com-
plexes.[55–60]
Results and Discussion
In our initial protocol for the synthesis of bibenzyl, an Ar-satu-
rated DMF solution containing 0.25m benzyl bromide (1a),
1.0 equivalents HEH, 1.0 equivalents diisopropylethylamine
(DIPEA), and 0.010 equivalents photocatalyst was photoirradiat-
ed with blue LEDs (7 W) for 4 h at room temperature. In this
protocol, HEH and DIPEA serve as a sacrificial electron donor
and a Brønsted base that scavenges HBr, respectively.[67,68] Sev-
eral photocatalysts were tested, including the homoleptic cy-
clometalated IrIII complexes, fac-[Ir(ppy)3] (ppy=2-phenylpyridi-
nato) and fac-[Ir(dfppy)3] (dfppy=2-(2,4-difluorophenyl)pyridi-
nato), and the cationic heteroleptic cyclometalated IrIII com-
plexes, [Ir(dfppy)2(phen)]PF6 (phen=1,10-phenanthroline)[55,59]
and [Ir(ppy)2(dtbbpy)]PF6 (dtbbpy=2,2’-[4,4’-di(tert-butyl)]bipyr-
idine). The conventional catalysts [Ru(bpy)3]Cl2 and
[Ru(phen)3]Cl2, and the cyclometalated PtII complex with ppy
and 2-acetylacetonato (acac) ligands[56] were also tested. As
summarized in Table 1, these reaction conditions afford the de-
sired bibenzyl product (2a) in yields ranging between 63 and
89% when the IrIII and PtII complexes are employed (Table 1,
entries 3–7). The RuII complexes do not produce bibenzyl, due
to the negative driving force for reductive quenching of the
excited-state species by HEH (entries 1 and 2; see the Support-
ing Information Figures S1 and S2 for the electrochemical po-
tentials of the RuII complexes). [Ir(ppy)2(dtbbpy)]PF6 was found
to be the best of the tested photoredox catalysts with regard
to yield and solubility.[69]
Taking advantage of our mechanistic understanding, we set
out to explore the utility of this photoinduced radical genera-
tion strategy in the bibenzyl synthesis. This strategy has previ-
ously been investigated: Tanaka and co-workers reported the
homocoupling of benzyl bromides upon the photoexcitation
of [Ru(bpy)3]2+ (bpy=2,2’-bipyridine) in the presence of 1-
benzyl-1,4-dihydronicotinamide (BNAH).[61] Sauvage and co-
workers found that [Cu(dap)]+ (dap=2,9-bis(p-anisyl)-1,10-phe-
nanthroline) was oxidatively quenched by benzyl bromide
which promoted the reductive cleavage of the CꢀBr bond.[62]
This protocol was recently re-examined by Reiser and co-work-
ers, and their study revealed that a triethylamine sacrificial
electron donor was unnecessary for the recovery of [Cu(dap)]+
[63]
.
Several other photosystems for radical-mediated bibenzyl
synthesis have also been reported. Decacarbonyldimanganese
[Mn2(CO)10] can undergo photolysis to afford two labile mono-
nuclear manganese complexes that promote the reductive
cleavage of benzyl bromides.[64] Semiconducting crystalline in-
organic materials, such as titania[65] and CdS quantum dots,[66]
have been examined for the synthesis of bibenzyls. These stud-
ies demonstrate that the photonic generation of benzyl radi-
cals with subsequent radical–radical coupling can serve as a val-
uable strategy for the preparation of bibenzyl motifs. However,
synthetic utility of the photoredox catalytic coupling of benzyl
halides has not been fully explored.
Table 1. Optimization of the reaction conditions for the photoredox cata-
lytic homocoupling of benzyl bromides[a]
To fully utilize the synthetic capability, it is essential to un-
derstand the mechanism of photoredox catalysis. Although the
groups of Tanaka and Sauvage investigated electron-transfer
behaviors in the bibenzyl syntheses,[61,62] many details of the
mechanism have yet to be fully established. For instance, little
is known about the potential role of bases, which may termi-
nate the catalytic cycle by hydrogen-atom transfer. In addition,
the electron stoichiometry has not been experimentally deter-
mined. Herein, we report the execution and mechanistic stud-
ies of the photoredox catalytic homocoupling of benzyl bro-
mides. In our study, we examined the photoredox catalysis
ability of a range of complexes of RuII, IrIII, and PtII. A variety of
bases were employed in order to investigate the electron-
transfer behaviors in the catalytic cycle. Our studies revealed
that the combined use of the Hantzsch ester (HEH) two-elec-
tron donor and potassium phosphate (K3PO4) Brønsted base
promotes a one-photon-induced two-electron catalytic cycle.
Finally, synthetic utility of our catalysis protocols has been fully
evaluated by using a variety of halmethyl arene substrates. Our
optimized protocols outperformed the previous catalysis em-
ploying [Ru(bpy)3]2+ and BNAH.[61]
Entry
Photocatalyst
Base
Yield [%][b]
1
2
3
4
5
6
7
8
[Ru(bpy)3]Cl2
[Ru(phen)3]Cl2
[Pt(ppy)(acac)]
fac-[Ir(ppy)3]
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
TEA
TMEDA
DBU
2,6-lutidine
K2CO3
Cs2CO3
K3PO4
K3PO4 (1.5 equiv)
–
K3PO4
K3PO4
trace
trace
63
70
89
78
84
54[c]
0[c]
26[c]
40
66
82
94
96
82
0
0
58[f]
fac-[Ir(dfppy)3]
[Ir(dfppy)2(phen)]PF6
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
no photocatalyst
[Ir(ppy)2(dtbbpy)]PF6
[Ir(ppy)2(dtbbpy)]PF6
9
10
11
12
13
14
15
16
17
18[d]
19[e]
K3PO4
[a] Reaction scale: 1a (0.1 mmol). [b] Yields were determined by perform-
ing gas chromatography with dodecane as the internal standard. [c] Am-
monium salts were formed. [d] No photoirradiation. [e] Aerobic condition.
[f] 20% of benzaldehyde was produced.
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The formation of considerable amounts of a toluene byprod-
uct was observed in the absence of HEH (data not shown).
This byproduct formation occurred through hydrogen-atom
abstraction from solvent (DMF) or a one-electron-oxidized spe-
[IrIII(ppy)2(dtbbpy)]+, undergoes a metal-to-ligand charge-trans-
fer (MLCT) transition under blue LED photoirradiation (i.e.,
dp(Ir)!p*(dtbbpy)), as inferred from the UV/Vis absorption
spectrum near the visible region (e(450 nm)ꢁ103 mꢀ1 cmꢀ1; see
Figure 1 and the Supporting Information Figure S3 for the
cies of DIPEA (i.e., DIPEA +). Sterically less hindered tertiary
C
amines, including triethylamine (TEA), N,N,N’,N’-tetramethyl-
ethylenediamine (TMEDA), and 1,8-diazabicyclo[5.4.0]undec-7-
ene (DBU), underwent nucleophilic substitution with the bro-
mide moiety in benzyl bromide to form ammonium salts
(Table 1, entries 8–10). In addition, the alkylamines quench the
photoexcited catalyst (vide infra). These observations indicate
that the commonly used alkylamines are not suitable for our
protocol. Thus, we chose to employ redox-innocent and non-
nucleophilic Brønsted bases, including K2CO3, Cs2CO3, and
K3PO4 (entries 12–14). It was found that K3PO4 produced a yield
as high as 96% (entries 14 and 15). Bibenzyl can be obtained
in modest yields even in the absence of K3PO4 (entry 16). This
production is most likely due to the deprotonation of the
spectra).
The
resulting
triplet
excited
state,
3
IV
+
Cꢀ
[Ir (ppy)2(dtbbpy) ] , is long-lived with a lifetime of 0.54 ms
3
IV
+
Cꢀ
(Supporting Information, Figure S4). [Ir (ppy)2(dtbbpy) ] has
an excited-state reduction potential (E*red) of 1.23 V versus SCE
in DMF, as determined by electrochemical and photophysical
methods (see the Supporting Information Figure S5 for the
voltammograms). This E*red value is more positive than the oxi-
dation potential (Eox) of HEH (0.89 V vs. SCE, DMF). Therefore,
reductive quenching by one-electron transfer from HEH to
3
IV
+
Cꢀ
[Ir (ppy)2(dtbbpy) ] is exergonic, with a positive driving
force of 0.34 eV, as determined by the Rehm–Weller equation.
The resulting one-electron-reduced photocatalyst,
III
Cꢀ 0
[Ir (ppy)2(dtbbpy) ] , donates its extra electron to benzyl bro-
strongly acidic, one-electron-oxidized HEH (HEH +; vide infra)
mide, which reductively cleaves the CꢀBr bond (Ered =ꢀ0.77 V
C
by the solvent. Control experiments performed in the absence
of the [Ir(ppy)2(dtbbpy)]PF6 photocatalyst or in the dark re-
vealed that photoirradiation and the photocatalyst are crucial
for the bibenzyl production (entries 17 and 18). The absence of
any reactivity in the solution devoid of photocatalysts suggests
that photoactive HEH[70–79] does not have the capacity to re-
ductively cleave benzyl bromides by oxidative quenching
(entry 17). Deoxygenation is crucial, as O2 equilibration leads to
the generation of a benzaldehyde byproduct (entry 19). This
result is consistent with a radical coupling mechanism. Finally,
photoredox catalytic coupling retains good reactivities in ace-
tonitrile and benzene, as well as DMF, thus exhibiting an insig-
nificant solvent dependence.
vs. SCE).
+
C
Since HEH is a strong acid, a Brønsted base readily deprot-
C
C
onates it to produce the HE radical (HE ). HE is a strong elec-
tron donor with an Eox value of ꢀ1.13 V versus SCE.[74] There-
C
fore, two electron-transfer paths are available for the HE : elec-
tron
donation
to
+
the
excited-state
photocatalyst,
3
IV
Cꢀ
[Ir (ppy)2(dtbbpy) ] (path A in Scheme 2), and direct elec-
tron transfer to benzyl bromide (path B in Scheme 2). Note
that both paths produce benzyl radicals. This catalysis mecha-
nism is thus characterized by a one-photon-induced two-elec-
tron process. This process is beneficial to the radical coupling
reaction, as it produces a second benzyl radical in the vicinity
of the site in which the first benzyl radical was generated from
the photocatalyst and HEH.
Scheme 2 depicts the proposed mechanism for the coupling
reaction of benzyl bromides. The ground-state photocatalyst,
Scheme 2. Proposed mechanism for the photoredox catalytic homocoupling of benzyl bromides.
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supra). This reductive electron transfer was investigated by
performing phosphorescence quenching experiments. At-
tempts to monitor the electron-transfer process by employing
transient photoluminescence techniques were hampered by
the inability of our laser instruments to selectively photoexcite
the Ir catalyst. Results from Stern–Volmer analyses of the
steady-state phosphorescence quenching data are displayed in
Figure 2. The phosphorescence spectra of the catalyst were ob-
tained at lex of 500 nm (e(500 nm)ꢁ30mꢀ1 cmꢀ1), the region at
which HEH does not absorb. A comparison between the pho-
toluminescence excitation spectra of [Ir(ppy)2(dtbbpy)]PF6 and
HEH reveals that the photoirradiation at 500 nm photoexcites
the photocatalyst selectively (Supporting Information, Fig-
ure S3). Figure 2a displays the phosphorescence titration spec-
tra obtained for 100 mm [Ir(ppy)2(dtbbpy)]PF6 with the addition
of HEH (0–10 mm).
Figure 1. Comparison of the photochemical action spectrum of the homo-
coupling reaction of benzyl bromide (~), and the UV/Vis absorption spectra
of [Ir(ppy)2(dtbbpy)]PF6 (c) and HEH (a). The dotted line and bars are
the simulated (TD-DFT, CAM-B3LYP/6-311+G(d,p):LANL2DZ//TD-CAM-B3LYP/
6-311+G(d,p): LANL2DZ) electronic absorption spectrum of [Ir(p-
py)2(dtbbpy)]PF6, and the calculated oscillator strengths, respectively. The
isodensity surface plots for the molecular orbitals that participate in the
MLCT transition of [Ir(ppy)2(dtbbpy)]PF6 (encircled in black) are shown un-
derneath. The UV/Vis absorption, phosphorescence, and photoluminescence
excitation spectra are illustrated in the Supporting Information, Figure S3.
Figure 2. Determination of the rate constant for photoinduced reductive
electron transfer from HEH to [Ir(ppy)2(dtbbpy)]PF6 in Ar-saturated DMF.
a) Phosphorescence titration spectra (lex =500 nm) of 100 mm [Ir(p-
py)2(dtbbpy)]PF6 obtained by increasing the HEH concentrations (0–10 mm).
b) Corresponding integrated phosphorescence intensity ratios (I0/I) as a func-
tion of the added HEH concentration. The grey line is a nonlinear least-
squares fit to I0/I=(1+Ka[HEH])·(1+kQt0[HEH]). Refer to the main text for the
definitions of the parameters.
We investigated the photoinduced redox processes of the
radical coupling reaction to assess the proposed catalysis
mechanism. In order to establish whether the reaction is
steered by photoexcitation of the catalyst, photochemical
quantum yields (PCQYs) for Ar-saturated reaction mixtures in
DMF were determined by ferrioxalate actinometry. Monochro-
matic photoexcitation wavelengths (lex) ranging between 340
and 500 nm were employed at 40 nm intervals. The photon
flux was kept as low as possible at 6.4–8.8ꢁ10ꢀ9 einsteinsꢀ1.
The maximum PCQY was determined to be 0.20 at lex =
460 nm. As illustrated in Figure 1, this photoexcitation wave-
length coincides with the singlet MLCT transition band of
[Ir(ppy)2(dtbbpy)]+. Quantum chemical calculations based on
time-dependent DFT (TD-DFT) support this spectral assignment
(see the circle in Figure 1). The PCQY values are very low in the
regions of HEH absorption and ligand-centered p–p* absorp-
tion of the catalyst. These two observations provide strong evi-
dence that the MLCT state of the catalyst (i.e.,
These phosphorescence spectra exhibit hypochromic shifts;
the extent of these changes are proportional to the concentra-
tion of added HEH. Phosphorescence quenching was quanti-
fied by determining the ratio of the integrated phosphores-
cence intensities (I0/I) in the presence (I) and absence (I0) of
HEH. Stern–Volmer analyses considering both associative and
diffusional quenching were performed by using the following
equation:
3
IV
+
Cꢀ
[Ir (ppy)2(dtbbpy) ] ) is responsible for the catalytic homo-
I0=I ¼ ð1 þ Ka½HEHꢂÞ ꢃ ð1 þ kQt0½HEHꢂÞ
ð1Þ
coupling.
UV/Visible absorption spectra of 50 mm [Ir(ppy)2(dtbbpy)]PF6
In this equation, Ka, kQ, t0, and [HEH] refer to the association
constant, the quenching rate constant, the phosphorescence
lifetime in the absence of HEH, and the molar concentration of
HEH, respectively. An iterative nonlinear least-squares fit of the
titration data to the equation yielded Ka and kQ values of 8.72ꢁ
10ꢀ15 mꢀ1 and 2.10ꢁ108 mꢀ1 sꢀ1, respectively. The very small Ka
value indicates the absence of an association between HEH
solutions in DMF were unaffected by the presence of excess
amounts of HEH (up to 5.0 mm; Supporting Information, Fig-
ure S6). This spectral behavior is indicative of a lack of ground-
state interactions, such as charge-transfer (CT) complex forma-
3
IV
+
Cꢀ
tion. Once the photoexcited catalyst, [Ir (ppy)2(dtbbpy) ] , is
generated, it undergoes reductive quenching by HEH (vide
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3
IV
+
Cꢀ
and [Ir (ppy)2(dtbbpy) ] (i.e., the formation of an exciplex).
The kQ value approaches the diffusion constant (kdiff, 8.25ꢁ
s
109 mꢀ1 ꢀ1) in DMF at 298 K, which can be estimated by em-
ploying the Einstein–Smoluchowski equation. In the presence
of 0.25m HEH, the reductive quenching rate amounts to 5.3ꢁ
107 sꢀ1. This quenching rate is one order of magnitude faster
than the inherent decay rate of the excited-state catalyst (i.e.,
1/t0 =1.9ꢁ106 sꢀ1). This implies that the generation of the one-
III
Cꢀ 0
electron-reduced catalyst ([Ir (ppy)2(dtbbpy) ] ) is efficient. The
3
IV
+
Cꢀ
kQ value for the reductive quenching of [Ir (ppy)2(dtbbpy) ]
by HEH is smaller than those by the widely employed electron
donors, sodium ascorbate (kQ =5.88ꢁ109 mꢀ1 ꢀ1; Supporting
Information, Figure S7) and DIPEA (kQ =3.50ꢁ108 mꢀ1 ꢀ1; Sup-
s
Figure 3. Photoinduced EPR measurements of Ar-saturated DMF solutions of
2.0 mm [Ir(ppy)2(dtbbpy)]PF6 and 100 mm HEH. g value=2.002.
s
porting Information, Figure S8). It should be noted, however,
that sodium ascorbate donates only one electron (vide infra),
and that the radical cation of DIPEA quenches the benzyl radi-
cal intermediate by the hydrogen-atom transfer (Table 1). Final-
3
IV
+
Cꢀ
Table 2. Optimization of the reaction conditions for the photoredox cata-
lytic homocoupling of benzyl bromides.[a]
ly, the phosphorescence lifetime of [Ir (ppy)2(dtbbpy) ] was
not altered in the presence of 5.0 mmK3PO4 or 5.0 mm benzyl
bromide. These results confirm the redox-innocent character of
K3PO4 and the absence of oxidative quenching by benzyl bro-
mide, respectively (Supporting Information, Figures S4 and S9).
As proposed in Scheme 2, the reductive quenching of the
catalyst by HEH yields HEH +. Photoinduced EPR spectra ob-
C
Entry
Hantzsch ester
[equiv]
K3PO4
[equiv]
Yield [%][b]
tained for Ar-saturated DMF solutions containing 2.0 mm
[Ir(ppy)2(dtbbpy)]PF6 and 100 mm HEH contained a weak EPR
signals with a g value of 2.002 (Figure 3), which demonstrated
generation of free radical species of HEH. Subsequent deproto-
nation by K3PO4 or solvents produces the highly reductive spe-
2a
0[c]
3a
1
2
3
4
5
6
7
8
–
1.5
1.5
1.5
–
1.5
1.5
–
0.5
1.0
1.5
2.0
0
2
3
5
3
6
5
4
4
3
3
0.5
0.7
0.7
1.0
1.5
1.0
1.0
1.0
1.0
1.0
49[d]
92
72
94
85
82
88
94
96
96
C
cies HE . This species can donate an additional electron for
benzyl radical generation. To examine this hypothesis, we per-
formed the photoredox catalytic homocoupling reaction with
various amounts of HEH or K3PO4. Table 2 illustrates that using
0.7 equivalents of HEH furnishes bibenzyl with a 92% yield
(Table 2, entry 3), and that further increasing the HEH concen-
tration does not improve the yield (entry 6). These results indi-
cate the occurrence of two-electron donation by HEH. In con-
trast, the influence of the amount of K3PO4 on the reaction
9
10
11
[a] Reaction scale: 1a (0.1 mmol). [b] Yields were determined by perform-
ing gas chromatography with dodecane as the internal standard. [c] Start-
ing material was recovered. [d] 40% of the starting material remained.
yields is not as strong as that of HEH (entries 7–11). This is be-
+
C
cause HEH can easily be deprotonated by solvents. Neverthe-
less, the presence of K3PO4 appears to be crucial in improving
the reaction yields and suppressing byproduct formation (i.e.,
toluene; entries 7–11).
electron donors, including sodium ascorbate and DIPEA (Sup-
porting Information, Figure S10).
The above results unambiguously demonstrate two-electron
+
C
More precise analyses of the stoichiometry of HEH were per-
formed by employing the method of continuous variations.
Figure 4 shows the Job’s plots of the integrated phosphores-
cence intensities of Ar-saturated DMF solutions containing
HEH, K3PO4, and [Ir(ppy)2(dtbbpy)]PF6. The HEH mole fractions
were continuously varied, while the total concentration of HEH
and [Ir(ppy)2(dtbbpy)]PF6 were kept constant (10 mm). In the
presence of K3PO4, the Job’s plot has a peak value at a HEH
mole fraction of 0.33. This indicates that the stoichiometry is
1:2 for HEH and the photocatalyst. In sharp contrast, the con-
trol experiments (Job’s analysis performed in the absence of
K3PO4) revealed a 1:1 stoichiometry. These experiments provide
firm evidence that HEH donates two electrons. This electron
economy is advantageous over those of other stoichiometric
donation through the consecutive processes HEH!HEH
!
C
HE !pyridium ion. GSMS spectrometry revealed generation of
the fully oxidized pyridium ion of HEH, supporting this notion
C
(Supporting Information, Figure S11). Once HE is produced, it
can donate an electron to benzyl bromide (path B in
IV
+
Cꢀ
Scheme 2), as well as to [Ir (ppy)2(dtbbpy) ] (path A in
Scheme 2). To examine the former radical-polar path, we per-
formed a dark-state homocoupling reaction of benzyl bromide.
Strongly oxidizing [(p-BrPh)3N]SbCl6 (Eox =1.12 V vs. SCE in
CH3CN)[80] was added into an Ar-saturated DMF solution con-
taining 0.25m benzyl bromide and equimolar concentrations
of HEH and K3PO4. It was envisaged that [(p-BrPh)3N]SbCl6
would oxidize HEH into HEH +, and that subsequent deproto-
C
C
nation by K3PO4 would yield HE without photocatalysis.
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14% and 2e, 14%). Note that the ratio for the product distri-
bution for 3/2a/2e was 2:1:1, a value obtainable for statistical
radical–radical coupling reactions.
To further explore the utility of our method, we attempted
the preparation of a natural product, brittonin A (Scheme 4).
The compound 5-(bromomethyl)-1,2,3-trimethoxybenzene
(1m) was converted into brittonin A (2m) in an 89% yield. This
demonstrates that the protocol may be useful for the synthesis
of a variety of bioactive dihydrostilbenoids.
Figure 4. Job’s plot analyses of the corrected integrated (lex =500 nm,
lems =510–750 nm) phosphorescence intensities of Ar-saturated DMF solu-
&
tions containing [Ir(ppy)2(dtbbpy)]PF6 and HEH in the presence ( ) and ab-
*
sence ( ) of K3PO4. The total concentration of [Ir(ppy)2(dtbbpy)]PF6 and HEH
was held constant at 10 mm.
Scheme 4. Photoredox catalytic synthesis of brittonin A.
Indeed, the dark reaction did produce the bibenzyl product, al-
though the yield was low (33%). This result suggests that both
paths B and A are operative in benzyl radical generation.
Conclusions
In the final phase of our study, we assessed the synthetic
utility of our photoredox catalysis protocol. As shown in
Table 3, photoredox catalysis is amenable to the homocoupling
of bromomethyl arenes that contain various functional groups.
High yields were obtained for benzyl bromides with o-(2b) or
p-methyl (2c), p-tert-butyl (2d), m-methoxy (2e), o-phenyl (2 f),
p-fluoro (2g), p-chloro (2h), p-trifluoromethyl (2i), o-pyrrolyl
(2j), ester (2k), and thioether (2l) substituents. However, the
presence of cyano and nitro abrogated the reactivity. Bromo-
methyl benzophenone mainly underwent hydrodebromination.
As expected, benzyl chloride with a high reduction potential
did not undergo the coupling reactions. The spectroscopic
To summarize, we have demonstrated the visible-light photore-
dox catalytic coupling of halomethyl arenes into bibenzyls. The
initial step in the catalytic cycle in our synthetic protocol in-
volves reductive quenching of the IrIII photocatalyst by HEH.
Subsequent deprotonation of the one-electron-oxidized spe-
+
C
C
cies of HEH (HEH ) by K3PO4 produces a highly reducing HE
radical species. Therefore, there are two reducing-species in
the catalytic cycle, which are the one-electron-reduced photo-
C
catalyst and HE . Both species are capable of producing benzyl
radical intermediates and contribute to high reaction yields for
the radical–radical coupling reaction. The 1:2 stoichiometry for
a photon to electron ratio has also been established. Finally,
the synthetic utility of this photoredox catalytic reaction was
demonstrated by the preparation of a natural dihydrostilbe-
noid, brittonin A.
1
identification data and the H and 13C NMR spectra of the cor-
responding bibenzyl products are summarized in the Experi-
mental Section and Supporting Information, respectively. The
photoredox catalysis is capable of cross-coupling the benzyl
radicals derived from two different bromomethyl arenes. As
shown in Scheme 3, photoirradiation of a DMF solution con-
taining equimolecular concentrations of benzyl bromide (1a)
and 3-(bromomethyl)anisol (1e) furnished a cross-coupled
product (3, 28%), along with homocoupling products (2a,
Experimental Section
General methods
NMR spectra were recorded on a Bruker 400 MHz instrument
(400 MHz for 1H NMR and 101 MHz for 13C NMR) or a Varian
600 MHz instrument (600 MHz for 1H NMR and 151 MHz for
1
13C NMR). Copies of H and 13C NMR spectra can be found in Figur-
1
es S12–S32 in the Supporting Information. H NMR experiments are
reported in units, parts per million (ppm), and were measured rela-
tive to residual chloroform (d=7.26 ppm) in the deuterated sol-
vent. 13C NMR spectra are reported in ppm relative to deutero-
1
chloroform (77.23 ppm), and all were obtained with H decoupling.
Photoredox catalytic homocoupling of benzyl bromide
An oven-dried re-sealable tube, equipped with a magnetic stir bar,
was
charged
with
benzyl
bromide
1
(1.0 mmol),
[Ir(ppy)2(dtbbpy)]PF6 (1.0 mol%), HEH (1.0 equiv), K3PO4 (1.5 equiv),
and DMF (4.0 mL). After degassing for 10 min by bubbling with
Scheme 3. Cross-coupling reaction of two bromomethyl arenes.
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Table 3. Scope of the photoredox catalytic homocoupling.[a]
Entry
1
Substrate
Product
Yield [%][b]
1a
1b
2a
2b
96
95
2
3
4
1c
2c
95
94
1d
2d
5
1e
2e
98
6
7
1 f
2 f
90
93
1g
2g
8
9
1h
1i
2h
2i
94
93
10
1j
2j
96
11
12
1k
1l
2k
2l
50[c]
90
[a] Reaction scale: 1a (0.1 mmol). [b] Isolated yields. [c] 20% of a hydrodebromination product was produced.
argon, the reaction mixture was placed under blue LED (7 W) irra-
diation at room temperature for 4 h, and the progress was moni-
tored by TLC or gas chromatography. The reaction mixture was
then diluted in dichloromethane and washed with water. The or-
ganic layer was dried over MgSO4, concentrated in vacuo, and puri-
fied with flash column chromatography, affording the bibenzyl
product 2.
Compound 2a: 1H NMR (400 MHz, CDCl3): d=7.34–7.30 (m, 4H),
7.26–7.21 (m, 6H), 2.96 ppm (s, 4H); 13C NMR (101 MHz, CDCl3): d=
141.99, 128.66, 128.54, 126.12, 38.16 ppm; Rf =0.40 (hexanes).
Compound 2b: 1H NMR (400 MHz, CDCl3): d=7.22–7.16 (m, 8H),
2.90 (s, 4H), 2.37 ppm (s, 6H); 13C NMR (101 MHz, CDCl3): d=
140.37, 136.11, 130.40, 129.06, 126.31, 126.25, 34.34, 19.48 ppm; IR
(neat): n˜ =2930, 1492, 1457, 755 cmꢀ1; Rf =0.50 (hexanes).
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Compound 2c: 1H NMR (400 MHz, CDCl3): d=7.20–7.10 (m, 8H),
2.87 (s, 4H), 2.33 ppm (s, 6H); 13C NMR (101 MHz, CDCl3): d=
139.07, 135.50, 129.21, 128.50, 37.86, 21.23 ppm; IR (neat): n˜ =
2917, 2851, 1514, 1453, 812 cmꢀ1; Rf =0.46 (hexanes).
Steady-state photoluminescence measurements
Phosphorescence spectra were obtained using a Quanta Master
400 scanning spectrofluorimeter at room temperature. The 10 or
50 mm solutions were thoroughly deaerated by bubbling Ar for at
least 30 min prior to performing the measurements.
Compound 2d: 1H NMR (400 MHz, CDCl3): d=7.40 (d, J=8.4 Hz,
4H), 7.25 (d, J=8.4 Hz, 4H), 2.96 (s, 4H), 1.40 ppm (s, 18H);
13C NMR (101 MHz, CDCl3): d=165.86, 162.24, 140.89, 123.03, 61.38,
24.96, 14.29 ppm; Rf =0.39 (hexanes).
Photoluminescence lifetime measurements
Ar-saturated 100 mm solutions in DMF were used for determination
of the photoluminescence lifetimes of samples. Photoluminescence
decay traces were acquired based on time-correlated single-
photon-counting (TCSPC) techniques by using a FluoTime 200 in-
strument (PicoQuant, Germany). A 377 nm diode laser (PicoQuant,
Germany) was used as the excitation source. The photolumines-
cence signals were obtained using an automated motorized mono-
chromator. Photoluminescence decay profiles were analyzed (Ori-
ginPro 8.0, OriginLab) using a single exponential decay model.
Compound 2e: 1H NMR (600 MHz, CDCl3): d=7.19 (dd, J=7.6,
7.4 Hz, 2H), 6.81–6.70 (m, 4H), 6.73 (s, 2H), 3.76 (s, 6H), 2.88 ppm
(s, 4H); 13C NMR (151 MHz, CDCl3): d=159.80, 143.54, 129.48,
121.03, 114.37, 111.47, 55.30, 38.02 ppm; Rf =0.62 (hexane/EtOAc=
8:1).
Compound 2 f: 1H NMR (600 MHz, CDCl3): d=7.39–7.32 (m, 6H),
7.23–7.18 (m, 4H), 7.17–7.11 (m, 6H), 7.00–6.96 (m, 2H), 2.73 ppm
(s, 4H); 13C NMR (151 MHz, CDCl3): d=142.14, 141.93, 139.35,
130.16, 129.49, 129.36, 128.21, 127.51, 126.94, 125.94, 35.05 ppm;
Rf =0.31(hexanes).
Determination of quantum yields (PCQYs)
The quantum yields for the coupling reactions were determined by
the standard ferrioxalate actinometry. A 0.0060m K3[Fe(C2O4)3] solu-
tion served as the chemical actinometer. 500 mL of the K3[Fe(C2O4)3]
solution was transferred to a 1 cmꢁ1 mm quartz cell, and the solu-
tion was photoirradiated with a monochromatized beam at 340,
380, 420, 460, and 500 nm for 30 s. Then same amount of 1% 1,10-
phenanthroline in sodium acetate buffer solution (4.09 g
CH3COONa dissolved in 18 mL of 0.5m H2SO4 and 32 mL of distilled
water) were added and stored in the dark for 1 h. The absorbance
changes at 510 nm were recorded. Inserting the value to Equa-
tion (2) returned the light intensity values of 6.4ꢁ10ꢀ9 einsteinsꢀ1
at 340 nm, 7.7ꢁ10ꢀ9 einsteinsꢀ1 at 380 nm, 8.9ꢁ10ꢀ9 einsteinsꢀ1
at 420 nm, 7.9ꢁ10ꢀ9 einsteinsꢀ1 at 460 nm, and 8.0ꢁ
10ꢀ9 einsteinsꢀ1 at 500 nm:
Compound 2g: 1H NMR (400 MHz, CDCl3): d=7.14 (dd, J=8.3,
J
HꢀF =5.7 Hz, 4H), 7.01 (dd, JHꢀF =8.5, J=8.3 Hz, 4H), 2.93 ppm (s,
4H); 13C NMR (101 MHz, CDCl3): d=161.55 (d, J=244.5 Hz), 132.18
(d, J=3.0 Hz), 130.02 (d, J=7.7 Hz), 115.22 (d, J=21.1 Hz),
37.28 ppm; 19F NMR (377 MHz, CDCl3): d=ꢀ116.72 ppm; IR (neat):
n˜ =2927, 2860, 1509, 1222, 1085, 833 cmꢀ1; Rf =0.39 (hexanes).
Compound 2h: 1H NMR (400 MHz, CDCl3): d=7.27 (d, J=8.4 Hz,
4H), 7.09 (d, J=8.4 Hz, 4H), 2.90 ppm (s, 4H); 13C NMR (101 MHz,
CDCl3): d=139.64, 131.80, 129.88, 128.49, 37.07 ppm; IR (neat): n˜ =
2925, 1490, 1088, 907, 824 cmꢀ1; Rf =0.47 (hexanes).
Compound 2i: 1H NMR (400 MHz, CDCl3): d=7.52 (d, J=2.0 Hz,
4H), 7.24 (d, J=2.0 Hz, 4H), 2.98 ppm (s, 4H); 13C NMR (101 MHz,
CDCl3): d=145.27, 128.99, 128.78 (q, J=32.62 Hz), 125.58 (q, J=
3.74 Hz), 124.53 (q, J=272.8 Hz), 37.44 ppm; 19F NMR (377 MHz,
CDCl3): d=ꢀ62.31 ppm; IR (neat): n˜ =2934, 2867, 1617, 1325, 1128,
908, 735 cmꢀ1; Rf =0.47 (hexanes).
Light intensity ðI0, einstein sꢀ1Þ ¼
ð2Þ
ðDAbsð510 nmÞ ꢄ VÞ=ðF ꢄ 11 050 mꢀ1 cmꢀ1 ꢄ DtÞ
In Equation (2), DAbs(510 nm), V, F, and Dt are the absorbance
change at 510 nm, volume (L), the quantum yield (1.1) of the fer-
rioxalate actinometer at 420 nm, and photoirradiation time (s), re-
spectively. Ar-saturated 4.0 mL DMF solutions containing 1.0 mmol
benzyl bromide, 1.0 mol% IrIII catalyst, 1.0 mmol Hantzsch ester,
and 1.5 mmolK3PO4 were photoirradiated under identical condi-
tions for 3 h. The bibenzyl products were quantitated by GCMS by
using dodecane as an internal standard, which were inserted into
Equation (3):
1
Compound 2j: H NMR (600 MHz, CDCl3): d=7.24 (ddd, J=7.2, 7.1,
1.6 Hz, 2H), 7.22 (ddd, J=7.2, 7.1, 1.6 Hz, 2H), 7.19 (dd, J=7.2,
1.6 Hz, 2H), 7.04 (dd, J=7.2, 1.6 Hz, 2H), 6.58 (d, J=2.3 Hz, 4H),
6.27 (d, J=2.3 Hz, 4H), 2.59 ppm (s, 4H); 13C NMR (151 MHz,
CDCl3): d=140.59, 137.85, 130.38, 128.07, 127.54, 126.98, 122.59,
108.96, 32.88 ppm; Rf =0.64 (hexane/EtOAc=4:1).
Compound 2l: 1H NMR (600 MHz, CDCl3): d=7.17 (d, J=8.2 Hz,
4H), 7.07 (d, J=8.2 Hz, 4H), 2.84 (s, 4H), 2.45 ppm (s, 6H); 13C NMR
(151 MHz, CDCl3): d=138.80, 135.63, 129.16, 127.20, 37.41,
16.43 ppm; Rf =0.58 (hexane/EtOAc=8:1).
PCQY ¼ ð½productꢂ ꢄ VÞ=ðI0 ꢄ DtÞ
ð3Þ
In Equation (3), [product] is the molar concentration of the biben-
zyl, Dt (s) is the photoirradiated time, V is the volume of the solu-
tion (L), and I0 is the light intensity obtained by Equation (2) (ein-
steinsꢀ1).
1
Compound 2m (brittonin A): H NMR (600 MHz, CDCl3): d=6.36 (s,
4H), 3.82 (s, 18H), 2.84 ppm (s, 4H); 13C NMR (151 MHz, CDCl3): d=
153.25, 137.52, 136.46, 105.71, 61.07, 56.27, 38.63 ppm; Rf =0.47
(hexane/EtOAc=1:1).
Electrochemical characterization
Cyclic voltammetry and differential pulse voltammetry experiments
were carried out using a CHI630B instrument (CH Instruments, Inc.)
using a three-electrode cell assembly. A Pt wire and a Pt microdisc
were used as the counter and the working electrodes, respectively.
A Ag/AgNO3 couple was used as a pseudo reference electrode.
Measurements were carried out in Ar-saturated DMF (2.0 mL) by
Steady-state UV/Vis absorption measurements
UV/Vis absorption spectra were collected on an Agilent Cary 300
spectrophotometer at 298 K. 10 or 50 mm DMF solutions were used
for the measurements unless otherwise mentioned.
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using a Gaussian 09 program. Geometry optimization and single-
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Y.Y. acknowledges a grant from the Ewha Womans University
(1-2015-0447-001-1) and W. J. Choi for technical assistance.
E.J.C. acknowledges grants from the National Research Founda-
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Keywords: bibenzyl · Hantzsch ester · organic chemistry ·
photochemistry · radical coupling
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Received: July 25, 2016
Published online on && &&, 0000
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FULL PAPER
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Organic Chemistry
G. Park, S. Y. Yi, J. Jung, E. J. Cho,* Y. You*
&& – &&
Mechanism and Applications of the
Photoredox Catalytic Coupling of
Benzyl Bromides
Radical coupling: Combined use of an
IrIII complex photocatalyst, the Hantzsch
ester, and K3PO4 enables photoredox
catalytic radical coupling of halomethyl
arenes (see scheme). A photochemical
quantum yield as high as 20% was ob-
tained and the catalytic mechanism was
investigated in detail by performing
photophysical and electrochemical
measurements, as well as by quantum
chemical calculations.
Chem. Eur. J. 2016, 22, 1 – 11
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11
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ